Earth’s natural processes has lead to the formation of major adverse weather events known as “natural disasters”. They bring nothing but destruction, catastrophe, bloodshed, death tolls, and heartbreak for those affected. Every year, an average of 60,000 people are killed by natural disasters globally every year. In the past decade, they were responsible for 0.01%-0.4% of deaths. Historically, the most fatal natural disasters are droughts and floods, but now this title has been overtaken by earthquakes in recent history. Natural disasters tend to inflict the heaviest toll on poverty-stricken countries, which meant highest death tolls were associated with disasters affecting low-to-middle income countries that lack the infrastructure to defend firmly and respond swiftly to such events.
https://en.wikipedia.org/wiki/List_of_countries_by_natural_disaster_risk
The United Nations University Institute for Environment and Human Security (UNU-EHS) calculated the World Risk Index (WRI) for most countries in the 2016 World Risk Report. A country’s WRI indicates its vulnerability and exposure to natural hazards. According to 2017 sources, the top 10 countries with the highest WRI include:
- (1) Vanuatu
- (2) Tonga
- (3) Philippines
- (4) Guatemala
- (5) Bangladesh
- (6) Solomon Islands
- (7) Costa Rica
- (8) Brunei
- (9) Cambodia
- (10) Papua New Guinea
On the other hand, the top 10 countries with the lowest WRI include:
- (1) Malta
- (2) Qatar
- (3) Saudi Arabia
- (4) Barbados
- (5) Grenada
- (6) Iceland
- (7) Bahrain
- (8) Kiribati
- (9) United Arab Emirates
- (10) Sweden
What are the different types of natural disasters?
(A) Geological Disasters
i. Avalanches
https://en.wikipedia.org/wiki/Avalanchehttps://en.wikipedia.org/wiki/List_of_avalanches_by_death_toll
Avalanches (snow slides) involve a large snow slab, initially lying upon a weaker layer of snow fractures, sliding down a steep slope of a snowy mountainside. They may be beautiful to witness from a distance, but their intensity and unpredictability makes it fatal.
There are different types of avalanches:
- Slab avalanches = They form in snow that were deposited, or redeposited in wind. They characteristically have a block (slab) of snow severed from its surroundings due to fractures. Elements include:
— A crown fracture at the top of the start zone.
— Flank fractures on the sides of the start zone.
— A staunchwall fracture at the bottom.
Since the crown and flank fractures are vertical walls in the snow, this separates the snow entrained in the avalanche from the snow remaining on the slope. Slabs vary in thickness from a few centimetres to about 3 metres.
- Powder snow avalanches = They are large avalanches that form turbulent suspension currents, which consist of a powder cloud. These avalanches can slide up to 300 km/h (190 mph), carrying an immeasurable exact amount of snow.
- Wet snow avalanches = They are a low velocity suspension of snow and water, with the flow confining to the track surface. Friction between the sliding surface of the track and the saturated flow of the water accounts for the low speed (10-40km/h). However, their large mass and density can make wet snow avalanches destructive, meaning they can plough through soft snow, and scour boulders, earth, trees and other vegetation. They are triggered by either loose snow propagation or slab propagation, occurring in snow packs consisting of saturated water and is isothermally equilibrated to the melting point of water.
- Ice Avalanche = It involves a large chunk of ice, such as from a calving glacier, falling onto ice, triggering a motion of broken ice chunks, e.g. Khumbu Icefall. The motion shares similarities with a rock slide or a landslide.
What is the avalanche pathway?
The avalanche pathway down a slope depends on the slope’s degree of steepness and the volume of snow/ice involved in the mass movement.
- The origin of the avalanche is called “the Starting Point”, which usually occurs on a 30–45 degree slope.
- The body of the pathway is called “the Track of the avalanche”, which typically occurs on a 20–30 degree slope.
- When the avalanche loses momentum, it eventually stops once it approaches “the Runout Zone”, which usually occurs on <20 degree slope.
The most deadly avalanches recorded are:
https://www.thoughtco.com/important-information-about-avalanches-1435309
How often do avalanches occur?
Around 90% of avalanches are triggered by human intervention, with as many as 40 avalanche fatalities in North America and around 150 worldwide every year. A majority of avalanche victims are snow climbers, skiers, or snowmobilers. Globally, it's estimated as many as 1 million natural avalanches occur every year, with approximately 100,000 of them occurring in the United States, and 1000s of them occurring in British Columbia and Canada. Usually, the popular winter holiday tourist destinations in the Alpine countries such as France, Austria, Switzerland, Italy and United States (esp. Colorado, Alaska & Utah) experience the highest number of avalanches and the greatest loss of life annually.
How does an avalanche kill?
The speed of avalanche slides varies depending on the angle of slope snow falls off, the mass and volume of the slab, with estimates ranging from 10 km/h to as fast as 320 km/h. This means avalanches have the energy to destroy any obstacle (such as house, cabin and shack) or unsecured infrastructure (such as roads and railway lines). Furthermore, avalanches are capable of killing humans through trauma because the sheer amount of force is sufficient to break bones and cause internal bleeding. If you somehow survive the initial impact, but you are unable to dig yourself out of the snow or fail to be rescued by emergency workers within 15 mins, you're certain to die from suffocation due to a buildup of carbon dioxide.
Seconds from Disaster episode on Galtür avalanche:
https://www.youtube.com/watch?v=UzGdyt0iavw
ii. Landslides
https://en.wikipedia.org/wiki/Landslide
https://en.wikipedia.org/wiki/List_of_landslides
A landslide is the motion of rock, earth, or debris along a slope, mainly driven by gravity. They occur when the slope experiences geological changes that destabilises its condition.
This is due to the decrease of the shear strength of the material on the slope, an increase in the shear stress borne by the material, or both. A slope's stability can alter due to a number of factors acting in unison or independently. The natural causes of landslides include:
Landslides can be exacerbated by human activities, such as:
Where and when were the deadliest landslides?
https://www.worldatlas.com/articles/deadliest-landslides-in-recorded-history.html
Prehistoric landslides include:
Historical landslides include:
https://en.wikipedia.org/wiki/List_of_landslides
A landslide is the motion of rock, earth, or debris along a slope, mainly driven by gravity. They occur when the slope experiences geological changes that destabilises its condition.
This is due to the decrease of the shear strength of the material on the slope, an increase in the shear stress borne by the material, or both. A slope's stability can alter due to a number of factors acting in unison or independently. The natural causes of landslides include:
- Soil saturation by rain water infiltration, snow melting, or glaciers melting.
- Increase in groundwater levels or pore water pressure (e.g. because of acquifer recharge in rainy seasons, or by rain water infiltration).
- Increase in hydrostatic pressure within cracks and fissures
- Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire - a fire in forests lasting for 3-4 days).
- Erosion of the toe of a slope by rivers or ocean waves.
- Physical and chemical weathering (e.g. by sustained freezing and thawing, heating and cooling, salt leaking in the groundwater or mineral dissolution)
- Ground shaking caused by earthquakes, which directly destabilises the slope (e.g. by inducing soil liquefaction) or weaken the material and cause cracks to generate landslide.
- Volcanic eruptions.
Landslides can be exacerbated by human activities, such as:
- Deforestation, cultivation and construction
- Vibrations from machinery or traffic.
- Blasting and mining
- Earthwork (e.g. changing the slope's shape, or inflicting new loads)
- Removal of deep-rooted vegetation that bound colluvium to bedrock in shallow soils.
- Agricultural or forestry activities
Where and when were the deadliest landslides?
https://www.worldatlas.com/articles/deadliest-landslides-in-recorded-history.html
Prehistoric landslides include:
Historical landslides include:
iii. Mudslides
https://en.wikipedia.org/wiki/Mudflow
https://www.nationalgeographic.com/news/2014/3/140324-mudslides-natural-disasters-geology-science/
Also known as mudflows, mudslides are a type of rapid landslide that moves along a channel, such as a river. It is a form of slope movement that consists of a rapid surging flow of partially or fully liquified debris due to the sheer volume of water. Mudslides also comprise of clay giving the debris its fluid properties, increasing its distance traversed and across shallower slope regions.
What triggers mudslides?
Heavy rainfall, snowmelt, or high levels of groundwater that flow through cracked bedrock can trigger motion of soil or sediments. Heavy rainfall on hills or mountain slopes causes extensive soil erosion, which would trigger mudslides. Studies found mudslides are comprised of at least 50% silt and clay-sized materials and up to 30% water. Sufficiently large mudslides have the momentum and energy to flatten villages and countrysides.
 |
| Before and after pictures of devastation caused by a mudslide in Stava, Italy on 19 July, 1985 |
https://www.youtube.com/watch?v=yNShw5LsXbk
The areas at the highest risk of a fatal mudslide include:
- Areas where wildfires or human modification of the land have destroyed vegetation
- Areas with a history of landslides
- Steep slopes and areas at the foot of slopes or canyons
- Slopes that were transformed for building or road construction
- Channels along streams and rivers
- Areas where surface runoff is directed
iv. Debris flow (Lahar)
https://en.wikipedia.org/wiki/Debris_flow
Debris flows involve water-laden masses of soil and fragmented rock rushing down mountain slopes, funnelling into stream channels, entraining objects in their path, and forming dense, muddy deposits on valley grounds. Around 40-50% of a debris flow consists of volumetric sediments, with the remaining portion being water. Debris includes sediment grains with sizes ranging from microscopic clay particles to giant boulders.
They can be triggered by intense rainfall or snowmelt, by dam-break or glacial outburst floods, or by landslides. The main factors of debris flow include:
Gravity accelerates the debris flows down the mountain slope, following steep mountain channels that debouche onto alluvial fans or floodplains. The front of a debris flow tend to contain copious amounts of coarse material such as boulders and logs that yield friction. Behind it is a lower-friction, liquefied flow body which typically comprises of sand, silt and clay, which help retain high pore-fluid pressures responsible for augmenting debris-flow mobility. In certain cases, the tail of a debris flow tends to highly concentrated in water. Debris flows typically move in a series of pulses, or discrete surges, wherein each pulse or surge has a distinctive head, body and tail.
What are the different types of debris flows?
1. Lahar
https://en.wikipedia.org/wiki/Lahar
Gerrard (1990) defined the lahar as a volcanic mudflow or debris flow. The word 'lahar' derives from the Javanese, introduced by Berend George Escher in 1922. A 2012 study found the physical properties (e.g. consistency, viscosity and approximate density) of a lahar is similar to that of wet concrete (i.e. fluid when moving, solid at rest). Around 5600 years ago, the Osceola Lahar produced by Mount Rainier (Washington) culminated in a wall of mud 140 metres (160 ft) deep in the White River canyon, blanketing an area of over 330 square kilometres (130 sq. mi.), for a total volume of 2.3 cubic kilometres (0.5 cu. mi.).
What are the causes of lahar?
Which places are at risk of lahar?
Examples of lahar
2. Jökulhlaup
Derived from the Icelandic word for "glacial run", a jökulhlaup is a type of glacial outburst flood. It was originally referred to the subglacial outburst floods from Vatnajökull, Iceland, which are often triggered by geothermal heating and rarely triggered by a volcanic subglacial eruption.
Since jökulhlaups emerge from hydrostatically-sealed lakes with floating levels above the threshold, their peak discharge can be considerably immense compared to the marginal or extra-maginal lake burst. The hydrograph of a jökulhlaup usually either rises over a few weeks with the largest flow near the end, or rises significantly over several hours. Björnsson (2002) suggested these patterns reflected channel melting, and sheet flow under the front, respectively.
Describe the jökulhlaup process
1. Subglacial water generation
Studies in 2008 found meltwater can be generated on the glacier surface (supraglacially), below the glacier (basally), or in both locations. Whereas, surface pooling occurs due to ablation (surface melting). Basal melting occurs due to geothermal heat flux out of the earth's surface, and friction heating caused by movement of ice over the surface. Piotrowski (1997) used the basal meltwater production rates to calculate the annual production of subglacial water from a typical northwestern German catchment during the last Weichselian glaciation, which yielded 642 x 106 m3.
2. Supraglacial and subglacial water flow
Piotrowski (1997) found the water collects in surface or subglacial ponds or lakes when the rate of production exceeds the rate of loss through the aquifer. It is also known that the signatures of supra glacial and basal water flow changes according to the passage zone. In all surface environments, supraglacial glow shares similarities with stream flow, by which water flows from higher areas to lower areas due to gravity. However, the water in basal flow is either generated through melting of ice at the base or pulled downward from the surface by gravity, before it collects at the base of the glacier in ponds and lakes in a region laminated with ice. If a surface drainage path is absent, then water from surface melting flows downward and pool in crevices within the ice. On the other hand, water from basal melting pools beneath the glacier. Each source can lead to formation of a subglacial lake. Studies in 2008 stated that the hydraulic head of the water accumulated in a basal lake increases as water drains through the ice until the pressure sufficiently expands to the point of either forcing a path through the ice or floating the ice above it.
Debris flows involve water-laden masses of soil and fragmented rock rushing down mountain slopes, funnelling into stream channels, entraining objects in their path, and forming dense, muddy deposits on valley grounds. Around 40-50% of a debris flow consists of volumetric sediments, with the remaining portion being water. Debris includes sediment grains with sizes ranging from microscopic clay particles to giant boulders.
They can be triggered by intense rainfall or snowmelt, by dam-break or glacial outburst floods, or by landslides. The main factors of debris flow include:
- Slopes steeper than 25 degree
- Availability of abundant loose sediment, soil, or weathered rock
- Ample amounts of water to completely saturate loose material.
Gravity accelerates the debris flows down the mountain slope, following steep mountain channels that debouche onto alluvial fans or floodplains. The front of a debris flow tend to contain copious amounts of coarse material such as boulders and logs that yield friction. Behind it is a lower-friction, liquefied flow body which typically comprises of sand, silt and clay, which help retain high pore-fluid pressures responsible for augmenting debris-flow mobility. In certain cases, the tail of a debris flow tends to highly concentrated in water. Debris flows typically move in a series of pulses, or discrete surges, wherein each pulse or surge has a distinctive head, body and tail.
What are the different types of debris flows?
1. Lahar
https://en.wikipedia.org/wiki/Lahar
Gerrard (1990) defined the lahar as a volcanic mudflow or debris flow. The word 'lahar' derives from the Javanese, introduced by Berend George Escher in 1922. A 2012 study found the physical properties (e.g. consistency, viscosity and approximate density) of a lahar is similar to that of wet concrete (i.e. fluid when moving, solid at rest). Around 5600 years ago, the Osceola Lahar produced by Mount Rainier (Washington) culminated in a wall of mud 140 metres (160 ft) deep in the White River canyon, blanketing an area of over 330 square kilometres (130 sq. mi.), for a total volume of 2.3 cubic kilometres (0.5 cu. mi.).
A lahar can vary in size, with widths ranging from a few metres to several 100 metres, and depths ranging from a few centimetres to 10s of metres. They also vary in velocity, ranging from a few metres per second to several 10s of metres per second, which outruns top human sprint speed. On steeper slopes, a lahar can flow as fast as 200 km/hr (120 mph). Hoblitt, Miller & Scott (1987) modelled a lahar flowing more than 300 km would lead to devastation in its path.
What are the causes of lahar?
- During a volcanic eruption, lava or pyroclastic surges melt snow and glaciers.
- Lava erupting from open vents and combining with wet soil, mud or snow on the volcano slope, forming a viscous, intense lahar.
- Floods caused by glaciers, lake breakouts, or heavy rainfalls.
- Water from a crater lake, coalescing with volcanic material in an eruption.
- Heavy rainfall on unconsolidated pyroclastic deposits.
- Volcanic landslides amalgamated with water.
- Melting snow and glaciers during mild to hot weather events.
- Earthquakes situating beneath or adjacent to the volcano loosens debris, which triggers a lahar avalanche.
- Rainfall causes stationary slabs of solidified mud to slide down the slopes at more than 30 km/h (20mph).
Which places are at risk of lahar?
- Mount Rainier, USA
- Mount Ruapehu, New Zealand
- Merapi & Galunggung, Indonesia
- Towns in the Puyallup River valley, Washington e.g. Orting, Sumner, Puyallup, Fife, & Port of Tacoma
- Mount Pinatubo, Philippines
Examples of lahar
 |
| 1985 Nevado del Ruiz, Armero tragedy: This lahar wiped out the town of Armero in Columbia. |
 |
| 1991 Mount Pinatubo eruption: Before (A) and after (B) photographs of a river valley filled in by lahars from Mount Pinatubo. |
2. Jökulhlaup
Derived from the Icelandic word for "glacial run", a jökulhlaup is a type of glacial outburst flood. It was originally referred to the subglacial outburst floods from Vatnajökull, Iceland, which are often triggered by geothermal heating and rarely triggered by a volcanic subglacial eruption.
Since jökulhlaups emerge from hydrostatically-sealed lakes with floating levels above the threshold, their peak discharge can be considerably immense compared to the marginal or extra-maginal lake burst. The hydrograph of a jökulhlaup usually either rises over a few weeks with the largest flow near the end, or rises significantly over several hours. Björnsson (2002) suggested these patterns reflected channel melting, and sheet flow under the front, respectively.
Describe the jökulhlaup process
1. Subglacial water generation
Studies in 2008 found meltwater can be generated on the glacier surface (supraglacially), below the glacier (basally), or in both locations. Whereas, surface pooling occurs due to ablation (surface melting). Basal melting occurs due to geothermal heat flux out of the earth's surface, and friction heating caused by movement of ice over the surface. Piotrowski (1997) used the basal meltwater production rates to calculate the annual production of subglacial water from a typical northwestern German catchment during the last Weichselian glaciation, which yielded 642 x 106 m3.
2. Supraglacial and subglacial water flow
Piotrowski (1997) found the water collects in surface or subglacial ponds or lakes when the rate of production exceeds the rate of loss through the aquifer. It is also known that the signatures of supra glacial and basal water flow changes according to the passage zone. In all surface environments, supraglacial glow shares similarities with stream flow, by which water flows from higher areas to lower areas due to gravity. However, the water in basal flow is either generated through melting of ice at the base or pulled downward from the surface by gravity, before it collects at the base of the glacier in ponds and lakes in a region laminated with ice. If a surface drainage path is absent, then water from surface melting flows downward and pool in crevices within the ice. On the other hand, water from basal melting pools beneath the glacier. Each source can lead to formation of a subglacial lake. Studies in 2008 stated that the hydraulic head of the water accumulated in a basal lake increases as water drains through the ice until the pressure sufficiently expands to the point of either forcing a path through the ice or floating the ice above it.
3. Episodic releases
When meltwater stockpiles to an extent, this leads to episodic discharges beneath both the continental ice sheets and Alpine glaciers. As water accumulates, the overlying ice elevates, and the water flows outward in a pressurised layer or a growing under-ice lake, leading to discharge. Since areas with thinner overlying ice sheets are lifted first, water can flow up the terrain underlying the glacier. Wingham (2006) stated that a release path is unveiled as the water accumulates and ice lifts.
If a pre-existing channel is lacking, the water would initially discharge in a broad-front jökulhlaup, before it spreads out in a thin front. Over time, the flow erodes the underlying materials and the overlying ice, giving rise to a tunnel valley channel. Both the overlying ice thickness and gradient of the underlying earth determine the direction of the channel, which tend to be uphill since the pressure of the ice forces the water to regions of low ice coverage before emerging at a glacial face. Studies in 2008 pointed out that these tunnel valleys generally mapped out the glacier thickness during its development.
The coarse rocks and boulders discovered deep inside and at the mouth of tunnels indicates that the rapid, high-volume discharge is highly erosive. Shaw, Pugin & Young (2008) found this erosive environment contributed to the formation of tunnels over 400 m deep and 2.5 km wide in the Antarctic.
Piotrowski (1997) proposed a detailed analytic model of this process that predicts a cycle.
- Meltwater is produced as a result of geothermal heating from below. Since surface ablation water is minimal at the glacial maximum, it doesn't penetrate more than 100 metres into a glacier.
- Meltwater initially drains through subglacial aquifers.
- When the hydraulic transmissivity of the substratum is exceeded, subglacial meltwater amasses in basins.
- Water gather abundantly to open the ice blockage in the tunnel valley, which accumulated after the previous discharge.
- The tunnel valley discharges the meltwater excess, which causes turbulent flows to melt out or erode the excess ice and valley floor.
- As the water level sinks, the pressure reduces until the tunnel valley close with ice and water flow ceases once more.
Examples of jökulhlaup
A few studies reported jökulhlaups occurring in locations outside of Vatnajökull, including the Laurentian ice sheet and the Scandinavian ice sheet during the Last Glacial Maximum.
A. Iceland
- Mýrdalsjökull = This place is subject to large jökulhlaups during the eruption of the subglacial volcano Katia, approximately every 40 to 80 years. The 1755 Katia volcanic eruption is estimated to have had a peak discharge of 200,000 to 400,000 m3/s.
- Grímsvötn = This volcano frequently generates largejökulhlaups from Vatnajökull. The 1996 eruption generated a peak flow of 50,000 m3/s, which lasted for several days.
- Eyjafjallajökull = In 2010, this volcano generated ajökulhlaup with a peak flow that ranged from 2000 to 3000 m3/s.
B. North America
- British Columbian Coast Mountains = In July 1996, a jökulhlaup resulted from an ice-dammed surface lake that was drained via a subglacial tunnel through Goddard Glacier. The flood surge of from 100 to 300 m3/s flowed 11km through Farrow Creek to terminate in Chilko Lake.
- Glacial Lake Iroquoise = This drains to the Atlantic in the Hudson Valley.
- St. Lawrence Valley = When the glaciers in this location melts, the Glacial Lake Candona drains to the North Atlantic
- Lake Agassiz = This lake is located in the centre of North America.
- Columbia River Gorge ("Missouri Floods")
What are the theories and models of debris flows?
- Rheologically based models applicable to mud flows interpret debris flows as single-phase homogeneous materials, e.g. Bingham, viscoplastic, Bagnold-type dilatant fluid, thixotropic, etc.
- Dam break wave - Proposed by Hunt (1982) and Chanson et al. (2006)
- Roll wave - Proposed by Takahashi (1981) and Davies (1986)
- Progressive wave - Proposed by Hungr (2000)
- A type of translating rock dam - Proposed by Coleman (1993).
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011JF002186
 |
| A general two-phase debris flow model, Pudasaini, 2012. The virtual mass force (C) can substantially change the dynamics of two-phase debris flow. Solid and fluid phases are represented by the solid and the dashed lines, respectively. The thin red lines (C 1/4 0) are without the virtual mass and the thick blue lines (C 1/4 0:5) are with the virtual mass (t = 7s). Other simulation parameters are as in Figure 2. With the virtual mass force, the solid particles bring more fluid mass with them, fluid is pumped to the front, the front is led by fluid flood followed by the main debris surge. The solid mass loses some inertia, so it is pushed back. |
Originally proposed by Iverson in 1997, the mixture theory treats debris flows as two-phase solid-fluid mixtures.
In real two-phase (debris) mass flows, a firm coupling exists between the solid and the fluid momentum transfer. Buoyancy decreases the solid's normal stress, which subsequently lessens the frictional resistance, hence increases flow mobility (longer travel distances). Moreover, buoyancy force diminishes the solid lateral normal stress, as well as the basal shear stress (thus frictional resistance) by a factor (1 - γ), where γ is the density ratio between the fluid and the solid phases.
If the flow is neutrally buoyant, i.e. γ = 1, the debris mass becomes more fluid and travels longer distances. McArdell, Bartelt & Kowalski (2007) found this effect occurs in highly viscous natural debris flows. For neutrally buoyant flows, the parameters that vanish include Coulomb friction, the lateral solid pressure gradient and the basal slope effect on the solid phase, and the drag coefficient is zero. In this limiting case, the only remaining solid force is caused by gravity, which corresponds with buoyancy. Under these conditions, the debris mass is fully fluidised and is able to flow economically, increasing the distance travelled.
If the solid and fluid phases shift together, the debris bulk mass becomes increases in fluidity. This implies the front moves significantly further, while the tail lags behind, thus the overall flow height decreases. When γ = 0, the flow experiences zero buoyancy force. This means the effective frictional shear stress for the solid phase is that of pure granular flow. In this case, the force due to the press gradient is transformed, the drag is high and the effect of the virtual mass disappears in the solid momentum, which leads to slower debris flow.
v. Earthquakes
https://en.wikipedia.org/wiki/Earthquake
When the surface of the Earth oscillates, this causes a sudden burst of energy in the Earth's lithosphere, generating seismic waves. This is known as an earthquake (quake, tremor, temblor).
https://en.wikipedia.org/wiki/Gutenberg%E2%80%93Richter_law
Using current instrumentation, around 500,000 earthquakes occur worldwide every year. Weak earthquakes occur frequently worldwide in places such as:
In seismology, the Gutenberg-Richter Law (GR Law) is a relationship between the magnitude and the total number of earthquakes in any given region and time period of at least that magnitude:
-- a & b = Constants, i.e. they are the same for all values of N and M.
In 1956, Charles Francis Richter and Beno Gutenberg were the first to propose the relationship between earthquake magnitude and frequency.
In seismically active regions, the parameter b (or b-value) is often close to 1.0. Hence, for a given frequency of magnitude 4.0+ events, there will be 10 times as many magnitude 3.0+ tremors and 100 times as many magnitude 2.0+ tremors. Bhattacharya et al. noted variation of b-values fell in the approximate range of 0.5 to 2 depending on the region's source environment.
Factors that are often alluded to explain these variations in b-values include:
The b-value apparently decreases for small magnitude event ranges in all records of earthquakes, which is referred to "roll-off" of the b-value. This is attributable to the plot of the logarithmic version of the GR law smoothing out at the low magnitude end of the plot. Since small tremors are different to detect and characterise, it suggests the roll-off of the b-value is caused by incomplete data sets. Since there are insufficient stations detecting and recording small tremors due to decreasing instrumental signals to noise levels, many low-magnitude earthquakes are not recorded. Bhattacharya et al. argued some models of earthquake dynamics can predict a physical roll-off of the earthquake size distribution.
Since the a-value represents the total seismicity rate of the region, the GR law can be expressed in terms of the total number of events:
Ntot = 10a
-- The total number of events.
Since 10a is the total number of tremor events, 10-b*M must be the probability of those tremor events occurring.
New models have shown to generalise the original GR law. Costa & Posadas presented such models in 2004, while Silva et al. presented a modified form in 2006.
-- N = Total number of events
-- a = A proportionality constant
-- q = Non-extensivity parameter followed by Constantino Tsallis that characterises systems not explained by the Boltzmann-Gibbs statistical form for equilibrium physical systems.
Sarlis, Skordas & Varotsos (2010) suggested that above some magnitude threshold, this equation simplifies to the original Gutenberg-Richter form expressed as:
Maslov & Anokhin (2012) obtained another generalisation from the solution of the generalised logistic equation. They evaluated values of parameter b for events recorded in Central Atlantic, Canary Islands, Magellan Mountains and the Sea of Japan. Burud & Chandra Kishen applied the generalised logistic equation to the acoustic emission in concrete. They demonstrated the b-value yielded from the generalised logistic equation monotonically increases with damage and is denoted as a damage compliant b-value.
Sanchez & Vega-Jorquera (2018) used Bayesian statistical techniques to generalise GR law, from they presented an alternative form for parameter b. They applied their model to powerful earthquakes occurring in Chile from 2010 to 2016.
Mortality counts from earthquakes worldwide 1990-2019
Earthquake events of magnitude 5.0 or larger worldwide from 1990 to 2019
Since 1931, the number of seismic stations has increased from about 350 to the many thousands today. Along with advancements in seismic technology, more earthquakes have been detected compared to the past, but it doesn't necessarily indicate an increase in the number of earthquake events.
The United States Geological Survey estimated an average of 18 major earthquakes (magnitude 7.0-7.9) and 1 earthquake of magnitude 8.0+ occurs annually since 1900. Most of the world's earthquakes (90%, and 81% of the largest) occur in the 40,000 km-long (25,000 mi), horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which mostly bounds the Pacific Plate. James Jackson (2006) noted powerful earthquakes also occur along other plate boundaries, e.g. along the Himalayan Mountains.
https://en.wikipedia.org/wiki/Seismic_magnitude_scales
https://www.gfz-potsdam.de/en/gshap/
Seismic magnitude scales are used to describe to overall strength or "size" of an earthquake. The earthquake's magnitude is usually determined from a seismogram's measurements of its seismic waves.
What is earthquake magnitude and ground-shaking intensity?
Tectonic forces puts mechanical stress on the Earth's crust. If this stress is sufficient to fissure the crust, or to overcome the friction that impedes one block of crust from slipping past another, it unleashes a burst of energy in the form of seismic waves that shake the ground.
Earthquake magnitude is defined as the estimate of the relative "size" or strength of an earthquake, and therefore its potential to shake the ground. Bormann, Wendt & Di Giacomo (2013) described it as "approximately related to the released seismic energy".
Earthquake intensity is defined as the strength or force of shaking at a given location, which associates with the peak ground velocity.
An isoseismal map of the observed intensities can allow a seismologist to estimate an earthquake's magnitude from both the maximum intensity observed, and from the extent of the area where the earthquake was perceived.
The intensity of local ground-shaking depends on several factors:
What are magnitude scales?
Since 2005, the International Association of Seismology and Physics of the Earth's Interior (IASPEI) standardised the measurement procedures and equations for the principal magnitude scales, ML , MS , mb, mB and mbLg.
(a) "Richter" magnitude scale
https://en.wikipedia.org/wiki/Richter_magnitude_scale
Developed by Charles F. Richter in 1935, the Richter scale is a measure of the strength (magnitude) of earthquakes.
This scale is determined by a logarithm of the amplitude of waves recorded by seismographs. The original formula is:
-- A0 = This empirical formula depends only on the epicentral distance of the station (δ)
Practically speaking, readings from all observing stations are averaged after adjusting for station-specific corrections to yield the ML value.
This formula shows that with each number increase in magnitude, it represents a 10-fold increase in measured amplitude. Furthermore, this corresponds to an increase of energy by a factor of 31.6.
This table describes the typical effects and magnitude values of earthquakes near the epicentre. Note that intensity and thus ground effects are determined by multiple factors:
- Magnitude of the earthquake
- Distance to the epicentre
- Depth of the earthquake's focus beneath the epicentre
- Location of the epicentre
- Geological conditions
What are the magnitude empirical formulae?
The following formulae for Richter magnitude (ML) are alternatives for Richter correlation tables based on Richter standard seismic event (ML = 0, A = 0.001 mm, D = 100 km). Below Δ is the epicentral distance in km.
The Lillie empirical formula is:
For distances (D) less than 200 km,
-- D = Distance (km)
The Bisztricsany (1958) empirical formula for epicentral distances between 4˚ to 160˚.
-- Δ = Degrees
-- ML = Between 5 and 6
The Tsumura empirical formula:
-- ML = Between 3 and 5.
The Tsuboi, University of Tokyo, empirical formula:
(b) Other "local" magnitude scales
(c) Body-wave magnitude scales
Researchers estimated approximately 10% or less of an earthquake's total energy is diffused as seismic energy. The remaining 90% or more of an earthquake's total energy either powers the earthquake fracture growth or converts into heat generated by friction. Thus, according to Spence, Sipkin & Choy (1989), earthquakes decreases the Earth's available elastic potential energy and increases in temperature, though these effects are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.
There are 3 main types of earthquake faults: normal, reverse (thrust) and strike-slip.
Normal and reverse faults are dip-slip types, where the displacement along the fault is in the direction of dip and one surface moves vertically along the fault line. An oblique slip is a combination of dip-slip and strike slip faults.
(a) Normal faults
(b) Reverse faults
(c) Strike slip faults
The maximum earthquake magnitude on a fault is determined by the available width, which varies by a factor of 20. A 2011 study measured the dip angle of the rupture plane along converging plate margins as about 10 degrees. Therefore, the width of the plane within the top brittle crust ranges from 50 to 100 km (31 to 62 mi) (Japan, 2011; Alaska, 1964), which theoretically produce the most powerful earthquakes.
A 2011 catalog found that strike-slip faults were typically oriented almost virtually, which culminate in an approximate width of 10 km (6.2 mi) within the brittle crust. Hence, it is not possible for earthquake magnitudes larger than 8 to occur. Since many faults are located along spreading centres, where the thickness of the brittle layer is about 6 km (3.7 mi), it limits the maximum earthquake magnitudes.
Schorlemmer, Wiemer & Wyss (2005) identified a hierarchy of stress level in the 3 fault types. The highest stress levels cause thrust faults, intermediate stress levels cause strike-slip and lowest stress levels cause normal faults. Consider the direction of the greatest principal stress, and the direction of the force pushing the rock mass during the fault rupture.
- Normal faults: The rock mass is pushed down vertically, hence the pushing force (greatest principal stress) equals the weight of the rock mass itself.
- Thrust faults: The rock mass escapes in the direction of the least principal stress (i.e. upwards), which elevates the rock mass, and hence, the overburden equals the least principal stress.
- Strike-slip faults: It is an intermediate between the 2 faults types described above.
A volcano tectonic earthquake is caused by movement of the magma beneath the Earth's surface, which results in pressure changes where the rock surrounding the magma stresses until it disintegrates or shifts.
What are the causes?
A fracture mechanics approach is used to model rupture propagation, which associates the rupture with a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy within the region around the crack tip, which increases as fracture energy decreases. Theoretically, the velocity of rupture propagation is significantly faster than the displacement velocity across the fault. Independent of its size, earthquake ruptures usually propagate at velocities within the range 70-90% of the S-wave velocity. A subset of earthquake ruptures are thought to propagate at speeds greater than the S-wave velocity, which would make these super-shear earthquakes observable during large strike-slip events. In the case of the 2001 Kunlun earthquake, the unusually wide zone of co-seismic damage caused by this tremor attributes to the effects of the sonic boom generated by such earthquakes. Slower earthquakes have ruptures travelling at unusually low velocities, which can cause devastating tsunami earthquakes, as in the 1896 Sanriku earthquake.
When the surface of the Earth oscillates, this causes a sudden burst of energy in the Earth's lithosphere, generating seismic waves. This is known as an earthquake (quake, tremor, temblor).
How often do earthquakes occur?
https://www.usgs.gov/natural-hazards/earthquake-hazards/lists-maps-and-statisticshttps://en.wikipedia.org/wiki/Gutenberg%E2%80%93Richter_law
Using current instrumentation, around 500,000 earthquakes occur worldwide every year. Weak earthquakes occur frequently worldwide in places such as:
- USA: California, Alaska
- El Salvador
- Mexico
- Guatemala
- Chile
- Peru
- Indonesia
- Philippines
- Iran
- Pakistan
- Portugal: Azores
- Turkey
- New Zealand
- Greece
- Italy
- Nepal
- Japan
In seismology, the Gutenberg-Richter Law (GR Law) is a relationship between the magnitude and the total number of earthquakes in any given region and time period of at least that magnitude:
log10(N) = a - b*Mor
N = 10a-b*M-- N = Number of events having a magnitude > N
-- a & b = Constants, i.e. they are the same for all values of N and M.
 |
| Gutenberg-Richter law for b = 1 |
In 1956, Charles Francis Richter and Beno Gutenberg were the first to propose the relationship between earthquake magnitude and frequency.
In seismically active regions, the parameter b (or b-value) is often close to 1.0. Hence, for a given frequency of magnitude 4.0+ events, there will be 10 times as many magnitude 3.0+ tremors and 100 times as many magnitude 2.0+ tremors. Bhattacharya et al. noted variation of b-values fell in the approximate range of 0.5 to 2 depending on the region's source environment.
 |
| GR law plotted for various b-values |
- Stress applied to the material
- Depth
- Focal mechanism
- Strength heterogeneity of the material
- Proximity of macro-failure
The b-value apparently decreases for small magnitude event ranges in all records of earthquakes, which is referred to "roll-off" of the b-value. This is attributable to the plot of the logarithmic version of the GR law smoothing out at the low magnitude end of the plot. Since small tremors are different to detect and characterise, it suggests the roll-off of the b-value is caused by incomplete data sets. Since there are insufficient stations detecting and recording small tremors due to decreasing instrumental signals to noise levels, many low-magnitude earthquakes are not recorded. Bhattacharya et al. argued some models of earthquake dynamics can predict a physical roll-off of the earthquake size distribution.
 |
| Roll-off compared to ideal GR law with b = 1. |
N = Ntot*10-b*Mwhere
Ntot = 10a
-- The total number of events.
Since 10a is the total number of tremor events, 10-b*M must be the probability of those tremor events occurring.
New models have shown to generalise the original GR law. Costa & Posadas presented such models in 2004, while Silva et al. presented a modified form in 2006.
-- a = A proportionality constant
-- q = Non-extensivity parameter followed by Constantino Tsallis that characterises systems not explained by the Boltzmann-Gibbs statistical form for equilibrium physical systems.
Sarlis, Skordas & Varotsos (2010) suggested that above some magnitude threshold, this equation simplifies to the original Gutenberg-Richter form expressed as:
b = 2*(2 - q)/(q - 1)
Maslov & Anokhin (2012) obtained another generalisation from the solution of the generalised logistic equation. They evaluated values of parameter b for events recorded in Central Atlantic, Canary Islands, Magellan Mountains and the Sea of Japan. Burud & Chandra Kishen applied the generalised logistic equation to the acoustic emission in concrete. They demonstrated the b-value yielded from the generalised logistic equation monotonically increases with damage and is denoted as a damage compliant b-value.
Sanchez & Vega-Jorquera (2018) used Bayesian statistical techniques to generalise GR law, from they presented an alternative form for parameter b. They applied their model to powerful earthquakes occurring in Chile from 2010 to 2016.
Mortality counts from earthquakes worldwide 1990-2019
Earthquake events of magnitude 5.0 or larger worldwide from 1990 to 2019
Since 1931, the number of seismic stations has increased from about 350 to the many thousands today. Along with advancements in seismic technology, more earthquakes have been detected compared to the past, but it doesn't necessarily indicate an increase in the number of earthquake events.
The United States Geological Survey estimated an average of 18 major earthquakes (magnitude 7.0-7.9) and 1 earthquake of magnitude 8.0+ occurs annually since 1900. Most of the world's earthquakes (90%, and 81% of the largest) occur in the 40,000 km-long (25,000 mi), horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which mostly bounds the Pacific Plate. James Jackson (2006) noted powerful earthquakes also occur along other plate boundaries, e.g. along the Himalayan Mountains.
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| Earthquake epicentres occur mostly along tectonic plate boundaries, especially on the Pacific Ring of Fire. |
How are earthquakes detected and measured?
https://en.wikipedia.org/wiki/Seismic_magnitude_scales
https://www.gfz-potsdam.de/en/gshap/
Seismic magnitude scales are used to describe to overall strength or "size" of an earthquake. The earthquake's magnitude is usually determined from a seismogram's measurements of its seismic waves.
 |
| Global occurrence of earthquakes worldwide, Oluwafemi et al. (2018). |
Tectonic forces puts mechanical stress on the Earth's crust. If this stress is sufficient to fissure the crust, or to overcome the friction that impedes one block of crust from slipping past another, it unleashes a burst of energy in the form of seismic waves that shake the ground.
Earthquake magnitude is defined as the estimate of the relative "size" or strength of an earthquake, and therefore its potential to shake the ground. Bormann, Wendt & Di Giacomo (2013) described it as "approximately related to the released seismic energy".
Earthquake intensity is defined as the strength or force of shaking at a given location, which associates with the peak ground velocity.
An isoseismal map of the observed intensities can allow a seismologist to estimate an earthquake's magnitude from both the maximum intensity observed, and from the extent of the area where the earthquake was perceived.
 |
| Isoseismal map of 2011 earthquake of Pacific coast of Tōhoku... was a magnitude 9.0 (Mw) undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST (05:46 UTC) on Friday March 11 2011, with the epicentre approximately 70 km (43 mi) east of the Oshika Peninsula of Tōhoku and the hypocentre at an underwater depth of approximately 30 km (19 mi). |
- Magnitude of the earthquake
- Soil conditions: Thick layers of soft soil (such as fill) amplify seismic waves, while sedimentary basins tend to resonate, which increases the duration of tremors.
- Geological structures: Seismic waves passing under the sound end of San Francisco Bay bounce back off the base of the Earth's crust towards San Francisco and Oakland.
What are magnitude scales?
Since 2005, the International Association of Seismology and Physics of the Earth's Interior (IASPEI) standardised the measurement procedures and equations for the principal magnitude scales, ML , MS , mb, mB and mbLg.
(a) "Richter" magnitude scale
https://en.wikipedia.org/wiki/Richter_magnitude_scale
Developed by Charles F. Richter in 1935, the Richter scale is a measure of the strength (magnitude) of earthquakes.
 |
| Charles F. Richter circa 1970 (1900 - 1985) |
ML = log10(A) - log10(A0(δ)) = log10[A/A0(δ)]-- A = Maximum excursion of the Wood-Anderson seismograph
-- A0 = This empirical formula depends only on the epicentral distance of the station (δ)
Practically speaking, readings from all observing stations are averaged after adjusting for station-specific corrections to yield the ML value.
This formula shows that with each number increase in magnitude, it represents a 10-fold increase in measured amplitude. Furthermore, this corresponds to an increase of energy by a factor of 31.6.
This table describes the typical effects and magnitude values of earthquakes near the epicentre. Note that intensity and thus ground effects are determined by multiple factors:
- Magnitude of the earthquake
- Distance to the epicentre
- Depth of the earthquake's focus beneath the epicentre
- Location of the epicentre
- Geological conditions
What are the magnitude empirical formulae?
The following formulae for Richter magnitude (ML) are alternatives for Richter correlation tables based on Richter standard seismic event (ML = 0, A = 0.001 mm, D = 100 km). Below Δ is the epicentral distance in km.
The Lillie empirical formula is:
ML = log10(A) - 2.48 + 2.76*log10(Δ)-- A = Amplitude (maximum ground displacement) of the P-wave (μm) measured at 0.8 Hz.
For distances (D) less than 200 km,
ML = log10(A) + 1.6*log10(D) - 0.15and for distances between 200 and 600 km,
ML = log10(A) + 3.0*log10(D) - 3.38-- A = Seismograph signal amplitude (mm)
-- D = Distance (km)
The Bisztricsany (1958) empirical formula for epicentral distances between 4˚ to 160˚.
ML = 2.92 + 2.25*log10(τ) - 0.001*Δ˚-- τ = Duration of the surface wave (seconds)
-- Δ = Degrees
-- ML = Between 5 and 6
The Tsumura empirical formula:
ML = -2.53 + 2.85*log10(F - P) + 0.0014*Δ˚-- (F - P) = Total duration of oscillation (seconds)
-- ML = Between 3 and 5.
The Tsuboi, University of Tokyo, empirical formula:
ML = log10(A) + 1.73*log10(Δ) - 0.83-- A = Amplitude (μm)
(b) Other "local" magnitude scales
- Japanese Meterological Agency magnitude scale (MJMA, MJMA, or MJ) = Used for shallow (depth < 60 km) earthquakes within 600 km.
(c) Body-wave magnitude scales
- mB scale = Originally developed by Gutenberg (1945), then by Gutenberg & Richter (1956), it overcame the distance and magnitude limitations of the ML scale inherent in the use of the surface waves.
- mb scale = Uses only P-waves measured in the first few seconds on a particular model of short-period seismograph. It was introduced in the 1960s coinciding with the establishment of the World-Wide Standardised Seismograph Network. A 2013 study explained the function of this scale in short periods improved its detection of smaller events, which helps differentiate tectonic earthquakes from underground nuclear explosions.
- mbLg scale= Developed by Nuttli (1973) in an attempt to solve the problem involving all of North America east of the Rocky Mountains. It measured the amplitude of short-period (~ 1 second) Lg waves, which was a complex form of the Love wave. Although Lg waves attenuate quickly along any oceanic path, they propagate strongly through the granitic continental crust.
(d) Surface-wave magnitude scales (Ms, MS, and Ms)
- This scale was inspired by a process developed by Beno Gutenberg in 1942, which measured shallow earthquakes stronger or more distant than the limits of Richter's original scale. It primarily measured the amplitude of surface waves (Rayleigh waves or Love waves) for a period of 20 seconds.
- The proposal of the "Moscow-Prague formula" in 1962, which was recommended by the IASPEI in 1967, became the basis of the standardised Ms20 scale.
- A "broad-band" variant (Ms_BB, Ms(BB)) measures the largest velocity amplitude in the Rayleigh-wave for periods up to 60 seconds.
- The MS7 scale used in China is a variant of Ms calibrated for use with the Chinese-made "type 763" long-period seismograph.
(e) Moment magnitude and energy magnitude scales
- Developed by Kanamori (1977) and Hanks & Kanamori (1979), the moment magnitude scale is based on the earthquake's seismic moment M0.
- It measures how much work an earthquake does as one patch of rock slides past another patch of rock. The SI unit of seismic moment is Newton-metres (N*m, Nm).
- Nevertheless, a large proportion of an earthquake's total energy dissipates as friction, which heats up the crust.
- The energy magnitude scale, ME, measures the comparatively small portion of energy radiated as seismic waves. The proportion of total energy radiated as seismic waves varies depending on the focal mechanism and tectonic environment. However, this scale is generally not used due to difficulties in estimating the radiated seismic energy.
(f) Energy class (K-class) scales
- Developed by Soviet seismologists in the remote Garm (Tajikistan) region of Central Asia in 1955, K (from the Russian word класс meaning "class") is a measure of the earthquake magnitude in the energy class or K-class system.
- Similar to Richter-style magnitudes, K values are logarithm, but have a different scaling and zero point. Bindi et al. (2011) evaluated that the K values ranges between 12 and 15, which correspond to about magnitude ranges between 4.5 and 6.
(g) Tsunami magnitude scales
- Earthquakes that rupture relatively slowly at sea, hence delivers more energy for longer time (lower frequencies), produce tsunamis.
- The tsunami magnitude scale, Mt, is based on a 1979 correlation by Abe of earthquake seismic moment (M0 ) with the amplitude of tsunami waves using measurements form tidal gauges.
- It was originally intended to estimate the magnitude of historic earthquakes where only the tidal data was available. Blackford (1984) stated the correlation can be reversed to predict tidal height from earthquake magnitude.
- Abe (1989) evaluated that, under low-noise conditions, tsunami waves as short as as 5 cm can be predicted, corresponding to a magnitude 6.5 earthquake.
- Bormann, Wendt & Di Giacomo (2013) developed the mantle magnitude scale, Mm, which measured the Rayleigh waves penetrating into the Earth's mantle, without the knowledge of other parameters such as the earthquake's depth.
(h) Duration and Coda magnitude
- Md scales estimate the magnitudes of earthquakes based on the duration or length of a portion of the seismic wave-train. They are used to measure local or regional earthquakes, as well as weaker earthquakes.
- Mc scales measure the duration or amplitude of a portion of the seismic wave, known as the coda. These scales are useful for short distances (less than ~ 100 km), since it provides a rapid estimate of the earthquake's magnitude prior to knowing its exact location.
(i) Macroseismic magnitude scales
- In the absence of seismic wave records, magnitudes are estimated from reports of the macroseismic events described by intensity scales. The process developed by Gutenberg and Richter in 1942 associated the maximum intensity observed (I0) to the magnitude. Labels for this scale include Mw(I0), or Mms.
- Frankel (1994) and Johnston (1996) portrayed an isoseismal map to illustrate the areas of differing intensities perceived. Labels for this scale include M0(An), Mfa, MLa or MI.
- Makris & Black (2004) defined Peak ground velocity (PGV) and peak ground acceleration (PGA) as measures of the force that causes destructive ground shaking. In Japan, there is a network of strong-motion accelerometers that collects PGA data to be correlated with different magnitude earthquakes. This correlation is then inverted to estimate the severity of ground shaking at a particular location due to an earthquake of a specified magnitude at a specified distance.
(j) Other magnitude scales
- Mh ("magnitude determined by hand") scales are used for minuscule magnitudes or poor data when determining the local magnitude, or multiple shock waves or cultural noises complicating the records.
What causes earthquakes?
When sufficient stored elastic strain energy drives fracture propagation along a fault line, it produces a tectonic earthquake, which can occur anywhere in the earth. When the sides of a fault slide past each other smooth and aseismically, this indicates a lack of irregularities or asperities along the surface that act to increase the frictional resistance. If asperities along the fault surfaces are present, this leads to stick-slip phenomena. Once the fault locks, relative motion continues between the plates, increasing stress and thence, stored strain energy in the volume around the fault surface. Ohnaka (2013) expounded that as stress accumulates to the point of bursting through the asperity, the plates rapidly slide over the locked portion of the fault, which subsequently releases the stored energy. Vassiliou & Kanamori (1982) identified this released energy is an amalgamation of radiated elastic strain seismic waves, frictional heating of the fault surfaces, and cracking of the rock, which produces an earthquake. This process is known as the elastic-rebound theory.Researchers estimated approximately 10% or less of an earthquake's total energy is diffused as seismic energy. The remaining 90% or more of an earthquake's total energy either powers the earthquake fracture growth or converts into heat generated by friction. Thus, according to Spence, Sipkin & Choy (1989), earthquakes decreases the Earth's available elastic potential energy and increases in temperature, though these effects are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.
What are the characteristics of earthquakes?
An earthquake radiates energy in the form of seismic waves, which reflect the nature of both the rupture and the earth's crust the waves propagate through. Seismologists first identify specific kinds of seismic waves on a seismograph in order to determine the earthquake's magnitude. Then they measure all the characteristics of a seismic wave, including its timing, amplitude, frequency, orientation, and duration. In some cases, adjustments are implemented to account for distance, the type of crust, and the characteristics of the seismograph that recorded the seismogram. |
| A typical seismogram |
- The compressive P-waves (red lines) are the fastest seismic waves and are detected first, usually in about 10 seconds for an earthquake around 50 km away.
- The sideways-shaking S-waves (green lines) are detected several seconds after the P-waves, which travel slightly over half the speed of the P-waves. This delay directly indicates the seismograph's distance to the epicentre of the tremor. S-waves usually take about an hour to reach a certain point 1000 km away from the epicentre.
- Both of these waves are categorised as body-waves, which transcend directly through the earth's crust.
- The S-waves are subsequently followed by surface waves, e.g. Love waves and Rayleigh waves, which travel only at the earth's surface.
- Surface waves are minor for deeper earthquakes, which interact minimally with the surface.
- On the other hand, surface waves are more powerful for shallow earthquakes less than approximately 60km deep, and has a duration of several minutes. They carry most of the tremor's energy, and cause a tremendous amount of damage.
What are the different type of earthquakes?
A. Fault types |
| 3 types of faults: A. Strike-slip B. Normal C. Reverse |
(a) Normal faults
- Normal faults typically occur in areas where the crust extends such as a divergent boundary.
- Earthquakes associated with these faults are generally less than magnitude 7.
(b) Reverse faults
- Reverse faults typically occur in areas wherein the crust contracts such as a convergent boundary.
- The most powerful tremors, called megathrust earthquakes, are attributed to reverse faults, especially along convergent boundaries, which were at least magnitude 8.
(c) Strike slip faults
- Strike-slip faults involve 2 surfaces slipping horizontally past each other, e.g. transform boundaries.
- They produce major earthquakes up to magnitude 8.
For every unit increase in magnitude, it is estimated the energy unleashed by the earthquake increases by a factor of 30. Geoscience Australia calculated that the energy released by an 8.6 magnitude earthquake is equivalent to about 10,000 WWII atomic bombs.
In 1979, Wyss found the energy released by an earthquake, as well its magnitude, is proportional to the area of the fault ruptures and the stress drop. Hence, if both the length and width of the faulted area increases, the magnitude of the resulting tremor also increases. As the topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates descend into the hot mantle, they store elastic energy before being released in fault ruptures. Sibson (2002) found that since rocks warmer than 300°C (572°F) flow in response to stress, they don't rupture in earthquakes. The largest observed lengths of ruptures and mapped faults are roughly 1000 km (620 mi), such as Alaska (1957), Chile (1960), and Sumatra (2004), all of which are in subduction zones. The longest earthquake ruptures on strike-slip faults, such as the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939), and the Denali Fault in Alaska (2002), are about 1/3 to 1/2 as long as the lengths along subducting plate margins, and those along normal faults are still shorter.
 |
| Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles |
A 2011 catalog found that strike-slip faults were typically oriented almost virtually, which culminate in an approximate width of 10 km (6.2 mi) within the brittle crust. Hence, it is not possible for earthquake magnitudes larger than 8 to occur. Since many faults are located along spreading centres, where the thickness of the brittle layer is about 6 km (3.7 mi), it limits the maximum earthquake magnitudes.
Schorlemmer, Wiemer & Wyss (2005) identified a hierarchy of stress level in the 3 fault types. The highest stress levels cause thrust faults, intermediate stress levels cause strike-slip and lowest stress levels cause normal faults. Consider the direction of the greatest principal stress, and the direction of the force pushing the rock mass during the fault rupture.
- Normal faults: The rock mass is pushed down vertically, hence the pushing force (greatest principal stress) equals the weight of the rock mass itself.
- Thrust faults: The rock mass escapes in the direction of the least principal stress (i.e. upwards), which elevates the rock mass, and hence, the overburden equals the least principal stress.
- Strike-slip faults: It is an intermediate between the 2 faults types described above.
B. Intraplate earthquake
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| Comparison of the 1985 and 2017 earthquakes on Mexico City, Puebla and Michoacán/Guerrero |
Along plate boundaries within the continental lithosphere, the deformation of the rock mass propagates over a widespread area than the plate boundary itself. For example, in the San Andreas fault continental transform, numerous tremors occur away from the plate boundary and associate with strains that develop within the broader zone of deformation due to major irregularities in the fault trace (e.g. the "Big Bend" region). e.g. An oblique convergent plate boundary between the Arabian and Eurasian plates stretches across the northwest part of the Zagros Mountains. Talebian & Jackson (2004) found this deformation along this plate boundary creates thrust sense movements directed perpendicular to the boundary over a wide zone in a southwest direction and strike-slip motion along the Main Recent Fault adjacent to the actual plate boundary itself.
Nettles & Ekström (2010) discovered all tectonic plates generate internal stress fields produced by their interactions with neighbouring plates and sedimentary loading or unloading (e.g., deglaciation). Noson and Thorsen (1988) suggested these stresses sufficiently lead to failures along the existing fault planes, leading to intraplate earthquakes.
C. Shallow-focus and deep focus
The majority of tectonic earthquakes originate at the ring of fire with epicentres around 10s and 1000s of kilometres. Earthquakes depth's of less than 70 km (43 mi) are classified as "shallow-focus" earthquakes, while those with a focal depth between 70 and 300 km (43 and 186 mi) are "mid-focus" or "intermediate-depth" earthquakes. A 2005 study implied deep-focus earthquakes may occur at depths between 300 and 700 km (190 and 430 mi) in subduction zones, where older and colder oceanic crust slide under another tectonic plate. These seismically active areas of subduction are known as Wadati-Benioff zones. If the subducted lithosphere can't be any more brittle, due to the high temperature and pressure, this generates deep-focus earthquakes. Greene II and Burnley (1989) proposed a mechanism behind deep-focus earthquakes, which involves faulting caused by olivine undergoing a phase transition into a spinel structure.
D. Volcano tectonic earthquake
https://en.wikipedia.org/wiki/Volcano_tectonic_earthquakeA volcano tectonic earthquake is caused by movement of the magma beneath the Earth's surface, which results in pressure changes where the rock surrounding the magma stresses until it disintegrates or shifts.
 |
| Nevado del Ruiz during the 1985 eruption. This eruption was one where seismic activity was monitored in order to determine that an eruption was imminent. |
What are the causes?
Tectonic subduction zones is one possible cause for a volcano tectonic earthquake. Schmincke (2004) explained that the compression of plates at the subduction zones pushes magma under them to move, since magma can't move through newly compressed crust. Magma tends to pool in magma chambers underneath the Earth's surface and between the converging tectonic plates. It's known many of the well known volcanoes situate along those boundaries, including the Ring of Fire. As the plates move, underground magma flow in and out of these chambers. Since the earth is already unstable, any magma movement would cause the earth around it to cave in or shift, which causes seismic activity.
E. Rupture dynamics
When a point on the fault surface initially ruptures, it creates a tectonic earthquake, by which process is known as nucleation. Available evidence of the rupture dimensions of the smallest earthquakes estimate the scale of the nucleation zone is smaller or larger than 100 m (330 ft). Since about 40% of earthquakes are preceded by foreshocks, nucleation may involve a preparation process. Once the fault rupture occurs, it propagates along the fault surface. However, the process is not well understood, due to the difficulty of recreating the high sliding velocities in a laboratory. The US National Research Council noted that the effects of powerful ground motion increases the difficulty of recording information near a nucleation zone.A fracture mechanics approach is used to model rupture propagation, which associates the rupture with a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy within the region around the crack tip, which increases as fracture energy decreases. Theoretically, the velocity of rupture propagation is significantly faster than the displacement velocity across the fault. Independent of its size, earthquake ruptures usually propagate at velocities within the range 70-90% of the S-wave velocity. A subset of earthquake ruptures are thought to propagate at speeds greater than the S-wave velocity, which would make these super-shear earthquakes observable during large strike-slip events. In the case of the 2001 Kunlun earthquake, the unusually wide zone of co-seismic damage caused by this tremor attributes to the effects of the sonic boom generated by such earthquakes. Slower earthquakes have ruptures travelling at unusually low velocities, which can cause devastating tsunami earthquakes, as in the 1896 Sanriku earthquake.
F. Tidal triggers
https://en.wikipedia.org/wiki/Tidal_triggering_of_earthquakes
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| Amplitude of the ocean tide at Golden Gate Bridge for 5 weeks in 1970. Brackets indicate seismic window periods as defined by Jim Berkland. |
Tidal triggering of earthquakes is an idea that tidal forces induce seismic activity.
Wilcock (2009) defined the concept of syzygy as the combined tidal effects of the sun and and moon - either directly as earth tides in the crust itself, or indirectly by hydrostatic loading due to ocean tides. These effects are sufficient to trigger tremors in the severely stressed rock that is on the verge of fracturing, which corresponds with a higher proportion of tremors occurring at times of maximal tidal stress, such as the new and full moons.
Studies suggest that seismicity tend to occur at low tides, particularly for reverse faults, due to the fault being unclamped by the unloading, hence minimising friction. Wilcock noted that ocean loading had no effect on strike-slip faults. Thomas, Nadeau & Bürgmann (2009) demonstrated a significantly correlation between small tidally induced forces and non-volcanic tremor activity. Volcanologists calibrate and test sensitive volcano deformation monitoring instruments by using the regular, predictable Earth tide movements.
Wilcock (2009) defined the concept of syzygy as the combined tidal effects of the sun and and moon - either directly as earth tides in the crust itself, or indirectly by hydrostatic loading due to ocean tides. These effects are sufficient to trigger tremors in the severely stressed rock that is on the verge of fracturing, which corresponds with a higher proportion of tremors occurring at times of maximal tidal stress, such as the new and full moons.
Studies suggest that seismicity tend to occur at low tides, particularly for reverse faults, due to the fault being unclamped by the unloading, hence minimising friction. Wilcock noted that ocean loading had no effect on strike-slip faults. Thomas, Nadeau & Bürgmann (2009) demonstrated a significantly correlation between small tidally induced forces and non-volcanic tremor activity. Volcanologists calibrate and test sensitive volcano deformation monitoring instruments by using the regular, predictable Earth tide movements.
G. Clusters:
A 2009 study outlined most earthquakes follow a sequence of events, in relation to each other in terms of location and time. The U.S. Geological Survey found most earthquake clusters consist of minor tremors that cause little to no damage, recurring frequently.
- Aftershocks
https://en.wikipedia.org/wiki/Aftershock
An aftershock is a minor earthquake that follows a major earthquake acting in the same area of the main tremor, caused by the adjustment of displaced crust being affected by the main tremor.
Most aftershocks situate over the region of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with main tremor. They are usually detected within a distance equal to the rupture length away from the fault plane.
a. Omori's law:
This law states the frequency of aftershocks decreases approximately with the reciprocal of time after the main tremor. This empirical law was first described by Fusakichi Omori in 1894 and is namely called Omori's law, which is expressed as:
In 1961, Utsu proposed a modified version of Omori's law:
These equations interpret a rapid decrease in the rate of aftershocks over time. In other words, the rate of aftershocks is inversely proportional to the inverse of time. Quigley (2012) stated the magnitude of the main tremor and this relationship help estimate the probability of future aftershocks occurring. Mathematically speaking, the probability of aftershocks 2 days after a major earthquake is half of the probability of the first day after the same main tremor, then 1/3 of that on the 3rd day, 1/4 of that on the 4th day, and so on (i.e. Zipf's Law).
Although these patterns match the statistical nature of aftershocks, the actual times, numbers and locations of the aftershocks are stochastic, while tending to follow these patterns. The values of the constants are determined by fitting to data after a major earthquake has occurred, which implies no specific physical mechanism in any given case. Guglielmi (2016) asserted the solution of the differential equation describes the evolution of the aftershock activity, which its interpretation is based on the concept of faults deactivating in the vicinity of the epicentre. In addition, Shaw (1993) noted that the Utsu-Omori Law was obtained from a nucleation process. The results indicated the spatial and temporal distribution of aftershocks can be separated into a dependence on space and a dependence on time. Sánchez and Vega (2018) applied a fractional solution of the reactive differential equation to a double power law model to demonstrate the number density decay in numerous ways.
b. Båth's law:
This law states the difference in magnitude between a main tremor and its largest aftershock is approximately constant, independent of the main shock magnitude, typically 1.1 - 1.2 on the Moment magnitude scale.
c. Gutenberg-Richter law:
Aftershock sequences typically follow the Gutenberg-Richter law of size scaling. See above for details of this law.
What are the effects of aftershocks?
Due to their unpredictability in terms of timing and magnitude, aftershocks can be as devastating as a strong earthquake, which would cause buildings that initially survived the first tremor to topple over. Earthquakes of larger magnitude are followed by frequent and stronger aftershocks and the sequences can last at least 7 years especially if such an earthquake situates in a seismically quiet area. For instance, the New Madrid Seismic Zone, 1811-12 tremors. When the rate of seismicity dwindles to a background level, the aftershock sequence is considered to be ceased.
Gardner (2013) reported land movement around the New Madrid Zone to be no more than 0.2 mm (0.0079 in) a year. Meanwhile, Wallace (2007) measured the movement of the San Andreas Fault to be, on average, 37 mm (1.5 in) a year across California. In 1899, Austrian geologist Josef Knett studied a swarm of 100s of tremor events in western Bohemia / Vogtland that occurred early 1824. He coined the term
 |
| During the 2008 Sichuan earthquake, the aftershock distribution demonstrates that the epicentre (where the rupture began) situates to one end of the final area of the slip, which suggests the rupture propagates asymmetrically. |
An aftershock is a minor earthquake that follows a major earthquake acting in the same area of the main tremor, caused by the adjustment of displaced crust being affected by the main tremor.
Most aftershocks situate over the region of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with main tremor. They are usually detected within a distance equal to the rupture length away from the fault plane.
a. Omori's law:
This law states the frequency of aftershocks decreases approximately with the reciprocal of time after the main tremor. This empirical law was first described by Fusakichi Omori in 1894 and is namely called Omori's law, which is expressed as:
n(t) = k/(c + t)-- k & c = Constants that vary between earthquake sequences.
In 1961, Utsu proposed a modified version of Omori's law:
n(t) = k/(c + t)p-- p = 3rd constant that modifies the decay rate and typically falls in the range of 0.7 - 1.5.
These equations interpret a rapid decrease in the rate of aftershocks over time. In other words, the rate of aftershocks is inversely proportional to the inverse of time. Quigley (2012) stated the magnitude of the main tremor and this relationship help estimate the probability of future aftershocks occurring. Mathematically speaking, the probability of aftershocks 2 days after a major earthquake is half of the probability of the first day after the same main tremor, then 1/3 of that on the 3rd day, 1/4 of that on the 4th day, and so on (i.e. Zipf's Law).
Although these patterns match the statistical nature of aftershocks, the actual times, numbers and locations of the aftershocks are stochastic, while tending to follow these patterns. The values of the constants are determined by fitting to data after a major earthquake has occurred, which implies no specific physical mechanism in any given case. Guglielmi (2016) asserted the solution of the differential equation describes the evolution of the aftershock activity, which its interpretation is based on the concept of faults deactivating in the vicinity of the epicentre. In addition, Shaw (1993) noted that the Utsu-Omori Law was obtained from a nucleation process. The results indicated the spatial and temporal distribution of aftershocks can be separated into a dependence on space and a dependence on time. Sánchez and Vega (2018) applied a fractional solution of the reactive differential equation to a double power law model to demonstrate the number density decay in numerous ways.
b. Båth's law:
This law states the difference in magnitude between a main tremor and its largest aftershock is approximately constant, independent of the main shock magnitude, typically 1.1 - 1.2 on the Moment magnitude scale.
c. Gutenberg-Richter law:
Aftershock sequences typically follow the Gutenberg-Richter law of size scaling. See above for details of this law.
 |
| Magnitude of the Central Italy earthquake of August 2016 (red dot) and aftershocks (orange dots) (which continued to occur after the period shown here. |
What are the effects of aftershocks?
Due to their unpredictability in terms of timing and magnitude, aftershocks can be as devastating as a strong earthquake, which would cause buildings that initially survived the first tremor to topple over. Earthquakes of larger magnitude are followed by frequent and stronger aftershocks and the sequences can last at least 7 years especially if such an earthquake situates in a seismically quiet area. For instance, the New Madrid Seismic Zone, 1811-12 tremors. When the rate of seismicity dwindles to a background level, the aftershock sequence is considered to be ceased.
Gardner (2013) reported land movement around the New Madrid Zone to be no more than 0.2 mm (0.0079 in) a year. Meanwhile, Wallace (2007) measured the movement of the San Andreas Fault to be, on average, 37 mm (1.5 in) a year across California. In 1899, Austrian geologist Josef Knett studied a swarm of 100s of tremor events in western Bohemia / Vogtland that occurred early 1824. He coined the term
- Swarms
https://en.wikipedia.org/wiki/Earthquake_swarm
An earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period of time. The time a swarm lasts varies from days to years.
Examples
https://en.wikipedia.org/wiki/Earthquake_swarm
An earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period of time. The time a swarm lasts varies from days to years.
 |
| Chronology of the 2003-04 Ubaye earthquake swarm |
What are the effects of earthquakes?
1. Shaking and rupturing of the ground
Since ground rupture breaks and displaces the Earth's surface along the trace of the fault, it poses a major risk to large engineering structures such as dams, bridges, nuclear power stations. The severity of the shaking and ground rupture depends on several factors, such as earthquake magnitude, distance from the epicentre, and local geological and geomorphological conditions.
2. Soil Liquefaction
Earthquakes weakens the water-saturated granular material in the soil and transforms it from a solid to a liquid, known as soil liquefaction. This causes rigid structures, such as buildings and bridges, to tilt or sink into the liquefied deposits. e.g. The 1964 Alaskan earthquake caused by buildings to sink into the ground, eventually collapsing upon themselves, due to soil liquefaction.
3. Human Impacts
Earthquakes are known to injure or kill living organisms, damage infrastructure, damage general property, and collapse or destabilise buildings. The aftermath may result in disease, lack of basic necessities, mental toll such as panic attacks, depression to survivors, and higher insurance premiums.
4. Landslides
Earthquakes can destabilise hill slopes, which result in landslides. After the tremors have dissipated, the danger of future landslides persists while emergency personnel are rescuing survivors.
5. Fire
5. Fire
Earthquakes can damage electrical power or gas lines, which significantly increase the risk of fire. If the water mains rupture, this drastically increases the difficulty of preventing the spread of fire. e.g. A majority of fatalities in the 1906 San Francisco earthquake were caused by resulting fire than by the tremors themselves.
6. Tsunami
6. Tsunami
When earthquakes occur at sea, it generates large sea waves that are produced by the sudden or abrupt movement of large volumes of water, called a tsunami. In the open ocean, the distance between wave crests is estimated to exceed 100 km (62 mi), and the wave periods varies from 5 mins to 1 hr. Depending on its depth, such tsunamis can travel between 600 and 800 km/hr (between 373 and 497 miles per hour).
7. Flood
If earthquakes damage dams, they can trigger devastating floods. A 2011 study stated earthquakes can cause landslips to dam rivers, thence collapse and cause floods. e.g. If an earthquake damaged Usoi Dam on the Sarez Lake in Tajikistan, approximately 5 million people are at risk from a catastrophic flood.
Most deadliest earthquakes by death toll
Where and when will the next major earthquakes strike next in the future?
Despite seismological methods being developed in predicting the time and place in which future earthquakes will strike next, it is extremely difficult to produce scientifically reliable predictions. Some sources suggest the San Andreas fault or the Rodgers Creek fault in the San Francisco Bay region before 2032, which prompted citizens living in California to practice earthquake drills in the case of such a devastating event occurs. Only time will time.
Kobe 1995 Earthquake:
https://www.youtube.com/watch?v=-k6W3hkMGYQ
Fukushima 2011 Earthquake:
https://www.youtube.com/watch?v=U-lhiTnJgdE
Loma Prieta, San Francisco Bay 1989 Earthquake:
https://www.youtube.com/watch?v=NSPpjc5iqaE
7. Flood
If earthquakes damage dams, they can trigger devastating floods. A 2011 study stated earthquakes can cause landslips to dam rivers, thence collapse and cause floods. e.g. If an earthquake damaged Usoi Dam on the Sarez Lake in Tajikistan, approximately 5 million people are at risk from a catastrophic flood.
When were the deadliest earthquakes?
- The deadliest earthquake in recorded history was the (23 January) 1556 Shaanxi earthquake, which killed more than 830,000 Chinese people. A majority of the victims were crushed by their houses (or yaodongs), which were dwellings carved out of loess hillsides.
- The 1976 Tangshan earthquake killed between 240,000 and 655,000 Chinese people, which was the most fatal in the 20th century.
- The most powerful earthquake ever recorded on a seismograph was a 9.5 magnitude earthquake near Cañete, Chile on 22 May 1960.
 |
| Earthquakes (M6.0+) between 1900 and 2017 |
 |
| This graph illustrates earthquakes of magnitude 8.0+ from 1900 to 2018. The apparent 3D volumes of the bubbles are linearly proportional to their respective fatalities.The colour indicates the continent, and the legend counts the number of quakes for each. Notice the absence of Africa. |
Most powerful earthquakes by magnitude
Most deadliest earthquakes by death toll
Despite seismological methods being developed in predicting the time and place in which future earthquakes will strike next, it is extremely difficult to produce scientifically reliable predictions. Some sources suggest the San Andreas fault or the Rodgers Creek fault in the San Francisco Bay region before 2032, which prompted citizens living in California to practice earthquake drills in the case of such a devastating event occurs. Only time will time.
Kobe 1995 Earthquake:
https://www.youtube.com/watch?v=-k6W3hkMGYQ
Fukushima 2011 Earthquake:
https://www.youtube.com/watch?v=U-lhiTnJgdE
Loma Prieta, San Francisco Bay 1989 Earthquake:
https://www.youtube.com/watch?v=NSPpjc5iqaE
vi. Sinkholes
https://en.wikipedia.org/wiki/Sinkhole
https://en.wikipedia.org/wiki/List_of_sinkholes
https://www.nationalgeographic.com/environment/sinkhole/
Also known as a cenote, sink, swallet, swallow hole, or doline, a sinkhole is defined as a depression or hole in the ground caused by a collapse of the surface year.
Sinkholes forming in thick layers of homogenous limestone are caused by increased groundwater flow due to high rainfall. Examples of such sinkholes include the Nakanaï Mountains, and on the New Britain island in Papua New Guinea.
The largest known sinkholes are the:
- Xiaozhai Tiankeng (Chongqing, China) = 662 metres (2172 ft) deep
- Giant sótanos in Querétaro and San Luis Potosi states in Mexico
https://en.wikipedia.org/wiki/List_of_sinkholes
https://www.nationalgeographic.com/environment/sinkhole/
Also known as a cenote, sink, swallet, swallow hole, or doline, a sinkhole is defined as a depression or hole in the ground caused by a collapse of the surface year.
How and where do sinkholes often occur?
Tills (2013) explained that sinkholes occurred in karst landscapes, which is a type of topography formed from the dissolution of soluble rocks such as dolomite, gypsum, and limestone. Thousands of sinkholes occur within a small region of karst landscapes, which yields a pock-marked landscape. They drain water from its vicinity, with only the subterranean rivers remaining in these regions. Examples of karst landscapes that are home to numerous deep sinkholes include Khammouan Mountains (Laos) and Mamo Plateau (Papua New Guinea). The largest known sinkholes formed in sandstones are Sima Humboldt and Sima Martel in Venezuela.Sinkholes forming in thick layers of homogenous limestone are caused by increased groundwater flow due to high rainfall. Examples of such sinkholes include the Nakanaï Mountains, and on the New Britain island in Papua New Guinea.
The largest known sinkholes are the:
- Xiaozhai Tiankeng (Chongqing, China) = 662 metres (2172 ft) deep
- Giant sótanos in Querétaro and San Luis Potosi states in Mexico
 |
| Xiaozhai 'Tiankeng" (collapse of a river cave roof), viewed from the southern rim; provincial-level municipally of Chongqing (before 1997 part of Province Sichuan). It is 511 deep from the lowest point of the vertical wall's rim. Its upper section is 600 m in diameter. The lower section is 300 m across, with vertical cliffs over 300 m high round its entire perimeter, except for a steep fan of collapsed debris banking against its northern wall. |
Unusual processes have lead to the formation of cavernous sinkholes of Sistema Zacatón in Tamaulipas (Mexico), where more than 20 sinkholes and other karst landscapes were shaped by volcanically heated, acidic groundwater.
In North America, Florida is known for having frequent sinkhole collapses, especially in the central part of the state, due to the underlying limestone that has existed from 15 to 25 million years.
Sinkholes are formed from natural processes of erosion or gradual removal of soluble bedrock (such as limestone) through percolation of water, collapsing of a cave roof, or a lowering of the water table. They also form through suffosion, which involves loose soil, loess, or other non-cohesive material above sitting on a limestone substratum containing fissures and joints, before being gradually washed through them by rain or surface water. Ultimately, this depresses the karst landscape. For instance, groundwater can dissolve the carbonate cement that stabilise the sandstone particles, and then transport the lax particles, before forming a void.
Sinkholes commonly situate at areas where the rock composition below the surface is mainly limestone or carbonate rock, salt beds, or soluble rocks, such as gypsum, as well as sandstone or quartzite. As the rock dissolves, this leads to spaces and caverns developing underground, which cause the land surface to collapse dramatically.
In urban areas, sinkholes occur when the water main breaks or sewer collapses due to old pipes' structure unable to resist the weight bearing down on it. They can also occur after alterations to the land surface due to the development of industrial and runoff-storage ponds. When a substantial amount of new material sits on top of the roof, it triggers its collapse to open up a void or cavity in the subsurface, leading to a sinkhole.
They form due to water dissolving the limestone beneath a soil sheet. This dissolution distends natural openings in the rock such as joints, fractures, and bedding planes. A 2019 U.S. Geological Survey found the soil settles down into the distended openings that form a small depression at the ground surface.
b. Cover-subsidence sinkholes
They form where voids in the underlying limestone settle the soil to produce larger surface depressions.
c. Cover-collapse sinkholes
Also known as "dropouts", they form where excess soil settles down into voids in the limestone to the point of the ground surface collapsing, which can occur abruptly. They can also form when man alters the natural water-drainage patterns in karst areas.
d. Pseudokarst sinkholes
Although they resemble karst sinkholes, they form from processes other than the natural dissolution of rock.
e. Human accelerated sinkholes
 |
| El Zacatón, a group of unusual karst features located in Aldama Municipality near the Sierra de Tamaulipas in the northeastern state of Tamaulipas, Mexico. This water-filled sinkhole is 339 m (1112 ft) deep. |
What causes sinkholes?
Sinkholes are formed from natural processes of erosion or gradual removal of soluble bedrock (such as limestone) through percolation of water, collapsing of a cave roof, or a lowering of the water table. They also form through suffosion, which involves loose soil, loess, or other non-cohesive material above sitting on a limestone substratum containing fissures and joints, before being gradually washed through them by rain or surface water. Ultimately, this depresses the karst landscape. For instance, groundwater can dissolve the carbonate cement that stabilise the sandstone particles, and then transport the lax particles, before forming a void.
Sinkholes commonly situate at areas where the rock composition below the surface is mainly limestone or carbonate rock, salt beds, or soluble rocks, such as gypsum, as well as sandstone or quartzite. As the rock dissolves, this leads to spaces and caverns developing underground, which cause the land surface to collapse dramatically.
In urban areas, sinkholes occur when the water main breaks or sewer collapses due to old pipes' structure unable to resist the weight bearing down on it. They can also occur after alterations to the land surface due to the development of industrial and runoff-storage ponds. When a substantial amount of new material sits on top of the roof, it triggers its collapse to open up a void or cavity in the subsurface, leading to a sinkhole.
What are the different types of sinkholes?
a. Solution sinkholesThey form due to water dissolving the limestone beneath a soil sheet. This dissolution distends natural openings in the rock such as joints, fractures, and bedding planes. A 2019 U.S. Geological Survey found the soil settles down into the distended openings that form a small depression at the ground surface.
b. Cover-subsidence sinkholes
They form where voids in the underlying limestone settle the soil to produce larger surface depressions.
c. Cover-collapse sinkholes
Also known as "dropouts", they form where excess soil settles down into voids in the limestone to the point of the ground surface collapsing, which can occur abruptly. They can also form when man alters the natural water-drainage patterns in karst areas.
d. Pseudokarst sinkholes
Although they resemble karst sinkholes, they form from processes other than the natural dissolution of rock.
e. Human accelerated sinkholes
 |
| Man-made activities and land alterations that cause water-level fluctuations accelerate cover-collapse sinkholes. |
Karst experts claimed that man's activities accelerate the formation of karst sinkholes to within a few years compared to karst sinkholes forming under natural conditions that eroded over 1000s of years. Sowers (1996) implied the sinkhole collapse hazards to life and property are caused by collapses in soil cavities, which is facilitated by fluctuating water levels. As water levels descend, the softened soil fragments seep deeper into rock cavities. As water flow in karst conduits, they carry the soil away to continue the process. Most human-induced sinkholes are caused by concentrated surface water rather than natural diffused recharge. Activities that facilitate sinkhole collapse include timber removal, ditching, laying pipelines, sewers, water lines, storm drains, drilling etc. Newton (1987) asserted these activities increase the downward movement of water to the point of exceeding the natural rate of groundwater recharge. Benson & Yuhr (2015) pointed out the increased runoff from the impervious surfaces of roads, roofs and parking lots can facilitate human-induced sinkhole collapses.
Other names for sinkholes
- Aven = Pit cave in occitan in the south of France
- Black holes = A group of unique, round, water-filled pits in the Bahamas. They are dissolved in carbonate mud from above, by the sea water. The water is dark due to the layer of phototrophic microorganisms concentrated in a dense, purple coloured layer 15 - 20 m (49 - 66 ft) deep, which absorbs the light. Moreover, metabolism within this layer can increase the temperature of the water, e.g. Black Hole of Andros.
- Blue holes = Initially defined as deep underwater sinkholes of the Bahamas, then used to describe any deep water-filled pits formed in carbonate rocks. The term is derived from the deep blue colour of the water in these sinkholes, produced by the water's clarity and sinkhole's depth.
- Cenotes = Refers to the characteristic water-filled sinkholes in the Yucatán Peninsula, Belize and other regions. They tend to form in limestone that were deposited in shallow seas generated by the Chicxulub meteorite's impact.
- Sótanos = Large pits in several states of Mexico
- Tiankengs = Extremely large sinkholes, usually deeper and wider than 250 m (820 ft), with mostly vertical walls, often created by the collapse of caverns. In Chinese, it means 'sky holes'.
- Tomo = Refers to New Zealand karst country to describe pot holes.
- Crown hole = A type of subsidence due to subterranean human activity, such as mining and military trenches. e.g. WWI trenches in Ypres, near mines in Nitra, Slovakia, limestone mine in Dudley, England, above an old gypsum mine in Magheracloone, Ireland.
Where are the deepest sinkholes?
vii. Volcanic Eruptions
https://en.wikipedia.org/wiki/Volcano
https://en.wikipedia.org/wiki/Types_of_volcanic_eruptions
https://en.wikipedia.org/wiki/List_of_largest_volcanic_eruptions
https://www.nationalgeographic.com/environment/natural-disasters/volcanoes/
Thoughty2 video about a volcano:
https://www.youtube.com/watch?v=CmwJ_SAahR8&ab_channel=Thoughty2
Volcanoes are ruptures in the crust of a planetary-mass object (e.g. Earth) that spews hot lava, volcanic ash, and gases from a magma chamber below the Earth's mantle. The word 'volcano' is derived from Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn derives from Vulcan, the god of fire in Roman mythology.
Volcanoes created more than 80% of Earth's landmass, laying the foundation for life to thrive on it. Its explosive nature erected mountains and craters, its lava rivers trickle into bleak landscapes. Over time, the elements erode these volcanic rocks, releasing nutrients into the soil, fertilising them and allow civilisations to flourish. Volcanoes exist on every continent, including Antarctica, with roughly 1500 still potentially active worldwide today. About 161 of them situate within the boundaries of USA.
Volcanic activity varies at different volcanoes due to the geological processes driving the flow of molten rock beneath the Earth's surface.
Volcanoes can form along different plate boundaries:
A. Divergent plate boundaries
https://en.wikipedia.org/wiki/Divergent_boundary
In plate tectonics, a divergent plate boundary (or constructive / extensional boundary) is a linear feature that involves 2 tectonic plates shifting away from each other. Divergent boundaries located within continents initially produce rifts, eventually forming rift valleys. The most active of such boundaries occur between oceanic plates and exist as mid-oceanic ridges. They also form volcanic islands, due to molten lava rising to fill in the gaps produced by the diverging plates.
Divergent boundaries are incarnated in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys, including the East African Great Valley. Moreover, they generate tremendous fault zones in the oceanic ridge system. Since the spreading of plates is not uniform, the rates at which adjacent ridge blocks spread differ as massive transform faults occur. These are the fracture zones responsible for the submarine earthquakes.
Examples of divergent plate boundaries are located at:
- Mid-Atlantic Ridge
- Red Sea Rift
- Baikal Rift Zone
- East African Rift
- East Pacific Rise
- Gakkel Ridge
- Galapagos Rise
- Explorer Ridge
- Juan de Fuca Ridge
- Pacific-Antarctic Ridge
- West Antarctic Rift System
- Great Rift Valley
B. Convergent plate boundaries
https://en.wikipedia.org/wiki/Convergent_boundary
A convergent boundary is a feature on Earth where 2 or more lithospheric tectonic plates collide, before one plate eventually slides beneath the other creating a subduction zone. The subduction zone can be defined by a plane where many tremors occur known as the Wadati-Benioff zone. Plate collisions occur for millions of years, which account for numerous volcanism, earthquake, orogenesis, lithosphere destruction, and deformation events. They occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere.
Plate tectonics are driven are convection cells in the mantle, caused by the heat generated by radioactive decay of elements in the mantle escaping to the surface and the sinking of cool materials from the surface down into the mantle. These convection cells carry hot material from the mantle to the surface along spreading centres that generate new crust. As this new crust is driven away from the spreading centre by the formation of newer crust, it decreases in temperature, width and increases in density. When this high density crust converges with low density crust, this instigates subduction. Gravitational force pushes the subducting slab into the mantle, as well as increases the plate velocity. As the relatively cool subducting slab submerges into the mantle, it then warms up to dehydrate hydrous materials. This releases water into the warmer asthenosphere, which partially melts the asthenosphere and causes volcanism. Kearey (2009) found both dehydration and partial melting occurs along the 1000°C isotherm, typically at depths between 65 and 130 km (40 and 81 mi).
How does it cause volcanism and volcanic arcs?
The oceanic crust contains hydrated minerals such as the amphibole group, e.g. Tremolite. As the oceanic lithosphere enters the subduction zone, it increases in temperature and metamorphoses, which then dehydrates the hydrated minerals contained within basalts. This releases water into the asthenosphere, which leads to partial melting, which elevates more buoyant, hot material, causing volcanism at the surface and pluton emplacement at the subsurface.
When magma approaches the surface, they produce volcanic arcs, which form as island arc chains or as arcs on continental crust. Kearey (2009) outlined 3 series of volcanic rocks that generally form arcs:
- Tholeiitic = Low iron basalts
- Calc-alkaline = Moderately enriched in potassium and incompatible elements
- Alkaline = Highly enriched in potassium
Examples:
- Collision between the Eurasian Plate & Indian Plate to form the Himalayas.
- Collision between the Australian Plate & Pacific Plate to form the Southern Alps in NZ.
- Subduction of the northern part of the Pacific Plate and the NW North American Plate to form the Aleutian Islands.
- Subduction of the Nazca Plate beneath the South American Plate to form the Andes.
- Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate to form the New Zealand to New Guinea subduction/transform boundaries.
- Collision of the Eurasian Plate and the African Plate to form the Pontic Mountains in Turkey.
- Subduction of the Pacific Plate beneath the Mariana Plate to form the Mariana Trench.
- Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range.
C. Hotspots
https://en.wikipedia.org/wiki/Hotspot_(geology)
https://en.wikipedia.org/wiki/Types_of_volcanic_eruptions
https://en.wikipedia.org/wiki/List_of_largest_volcanic_eruptions
https://www.nationalgeographic.com/environment/natural-disasters/volcanoes/
Thoughty2 video about a volcano:
https://www.youtube.com/watch?v=CmwJ_SAahR8&ab_channel=Thoughty2
Volcanoes are ruptures in the crust of a planetary-mass object (e.g. Earth) that spews hot lava, volcanic ash, and gases from a magma chamber below the Earth's mantle. The word 'volcano' is derived from Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn derives from Vulcan, the god of fire in Roman mythology.
Volcanoes created more than 80% of Earth's landmass, laying the foundation for life to thrive on it. Its explosive nature erected mountains and craters, its lava rivers trickle into bleak landscapes. Over time, the elements erode these volcanic rocks, releasing nutrients into the soil, fertilising them and allow civilisations to flourish. Volcanoes exist on every continent, including Antarctica, with roughly 1500 still potentially active worldwide today. About 161 of them situate within the boundaries of USA.
Volcanic activity varies at different volcanoes due to the geological processes driving the flow of molten rock beneath the Earth's surface.
How and where do volcanoes form?
A majority of the volcanoes form along the boundaries of Earth's tectonic plates, which are enormous sections of Earth's lithosphere that continuously shift, colliding into one another. When 2 neighbouring tectonic plates impact each other, one tend to slide under the other in a subduction zone. When landmass descends deep into the Earth, it increases underground temperatures and pressures, forcing the release of water from the rocks. This water decreases the melting point of the overlying rocks, producing magma that flows upwards to the Earth's surface, which may reactivate a dormant volcano.Volcanoes can form along different plate boundaries:
A. Divergent plate boundaries
https://en.wikipedia.org/wiki/Divergent_boundary
In plate tectonics, a divergent plate boundary (or constructive / extensional boundary) is a linear feature that involves 2 tectonic plates shifting away from each other. Divergent boundaries located within continents initially produce rifts, eventually forming rift valleys. The most active of such boundaries occur between oceanic plates and exist as mid-oceanic ridges. They also form volcanic islands, due to molten lava rising to fill in the gaps produced by the diverging plates.
Divergent boundaries are incarnated in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys, including the East African Great Valley. Moreover, they generate tremendous fault zones in the oceanic ridge system. Since the spreading of plates is not uniform, the rates at which adjacent ridge blocks spread differ as massive transform faults occur. These are the fracture zones responsible for the submarine earthquakes.
Examples of divergent plate boundaries are located at:
- Mid-Atlantic Ridge
- Red Sea Rift
- Baikal Rift Zone
- East African Rift
- East Pacific Rise
- Gakkel Ridge
- Galapagos Rise
- Explorer Ridge
- Juan de Fuca Ridge
- Pacific-Antarctic Ridge
- West Antarctic Rift System
- Great Rift Valley
B. Convergent plate boundaries
https://en.wikipedia.org/wiki/Convergent_boundary
A convergent boundary is a feature on Earth where 2 or more lithospheric tectonic plates collide, before one plate eventually slides beneath the other creating a subduction zone. The subduction zone can be defined by a plane where many tremors occur known as the Wadati-Benioff zone. Plate collisions occur for millions of years, which account for numerous volcanism, earthquake, orogenesis, lithosphere destruction, and deformation events. They occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere.
Plate tectonics are driven are convection cells in the mantle, caused by the heat generated by radioactive decay of elements in the mantle escaping to the surface and the sinking of cool materials from the surface down into the mantle. These convection cells carry hot material from the mantle to the surface along spreading centres that generate new crust. As this new crust is driven away from the spreading centre by the formation of newer crust, it decreases in temperature, width and increases in density. When this high density crust converges with low density crust, this instigates subduction. Gravitational force pushes the subducting slab into the mantle, as well as increases the plate velocity. As the relatively cool subducting slab submerges into the mantle, it then warms up to dehydrate hydrous materials. This releases water into the warmer asthenosphere, which partially melts the asthenosphere and causes volcanism. Kearey (2009) found both dehydration and partial melting occurs along the 1000°C isotherm, typically at depths between 65 and 130 km (40 and 81 mi).
How does it cause volcanism and volcanic arcs?
The oceanic crust contains hydrated minerals such as the amphibole group, e.g. Tremolite. As the oceanic lithosphere enters the subduction zone, it increases in temperature and metamorphoses, which then dehydrates the hydrated minerals contained within basalts. This releases water into the asthenosphere, which leads to partial melting, which elevates more buoyant, hot material, causing volcanism at the surface and pluton emplacement at the subsurface.
When magma approaches the surface, they produce volcanic arcs, which form as island arc chains or as arcs on continental crust. Kearey (2009) outlined 3 series of volcanic rocks that generally form arcs:
- Tholeiitic = Low iron basalts
- Calc-alkaline = Moderately enriched in potassium and incompatible elements
- Alkaline = Highly enriched in potassium
Examples:
- Collision between the Eurasian Plate & Indian Plate to form the Himalayas.
- Collision between the Australian Plate & Pacific Plate to form the Southern Alps in NZ.
- Subduction of the northern part of the Pacific Plate and the NW North American Plate to form the Aleutian Islands.
- Subduction of the Nazca Plate beneath the South American Plate to form the Andes.
- Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate to form the New Zealand to New Guinea subduction/transform boundaries.
- Collision of the Eurasian Plate and the African Plate to form the Pontic Mountains in Turkey.
- Subduction of the Pacific Plate beneath the Mariana Plate to form the Mariana Trench.
- Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range.
C. Hotspots
https://en.wikipedia.org/wiki/Hotspot_(geology)
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| This diagram shows a cross section through the Earth's lithosphere (in yellow) with magma rising from the mantle (in red). Lower diagram illustrates a hotspot track caused by their relative movement. |
In geology, hotspots are volcanic areas of the Earth's mantle from which hot plumes rise upward, erecting volcanoes on the overlying crust.
In 1963, J. Tuzo Wilson first postulated the concept of hotspots based on his theories of the Hawaiian Islands materialising as a result of the tectonic plate's sluggish movement across a hot region underneath the Earth's surface. A 1999 US Geological Survey suggested that hotspots were augmented by narrow streams of hot mantle rising from the Earth's core-mantle boundary in a mantle plume. Geologists estimate about 20 to many thousands of hot spots are augmented by mantle plumes. The hypothesis applies to a few of the most active volcanic regions in Hawaii, Réunion, Yellowstone, Galápogas, and Iceland.
In 1963, J. Tuzo Wilson first postulated the concept of hotspots based on his theories of the Hawaiian Islands materialising as a result of the tectonic plate's sluggish movement across a hot region underneath the Earth's surface. A 1999 US Geological Survey suggested that hotspots were augmented by narrow streams of hot mantle rising from the Earth's core-mantle boundary in a mantle plume. Geologists estimate about 20 to many thousands of hot spots are augmented by mantle plumes. The hypothesis applies to a few of the most active volcanic regions in Hawaii, Réunion, Yellowstone, Galápogas, and Iceland.
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| This schematic diagram illustrates the physical processes inside the Earth that lead to the generation of magma. Partial melting begins above the fusion point. |
What are hotspot volcanoes composed of?
Most hotspot volcanoes are mainly made of basalt (e.g. Hawaii, Tahiti), which makes them less explosive than subduction zone volcanoes. Continental hotspots release basaltic magma through the continental crust, which melt to form rhyolites. e.g. Yellowstone Caldera. If a rhyolite completely erupts, it is followed by eruptions of basaltic magma spewing through the same lithospheric fissures. Holbek (1983) found the Ilgachuz Range in British Columbia was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.
What are hotspot volcanic chains?
The joint mantle plume / hotspot hypothesis proposes the feeder structures being fixed relative to one another, with the continents and seafloor drifting overheard. It also predicts that time-progressive chains of volcanoes develop on the surface. e.g. Yellowstone is located at the end of a chain of extinct calderas, which progressively aged to the west.
Geologists use hotspot volcanic chains in an attempt to track the movement of the Earth's tectonic plates. Since many chains aren't time-progressive (e.g. the Galápagos) and fixed relative to one another (e.g. Hawaii and Iceland), this effort hasn't yielded much data.
I'll delve into the details of plate tectonics and all tectonic boundaries in another post.
Examples of postulated hotspot locations
- Hawaii = Hawaiian-Emperor seamount chain
- Louisville = Louisville Ridge
- Gough and Tristan = Walvis Ridge
- Bowie = Kodiak-Bowie Seamount chain
- Cobb = Cobb-Eickelberg Seamount chain
- New England = New England Seamounts, Great Meteor hotspot track
- Anahim = Anahim Volcanic Belt
- Mackenzie = Mackenzie dike swarm
- Saint Helena = St. Helena Seamount Chain-Cameroon Volcanic Line
- Réunion = Southern Mascarene Plateau-Chagos-Maldives-Laccadive Ridge
- Kerguelen = Ninety East Ridge
- Easter = Tuamotu-Line Island chain
- Macdonald = Austral-Gilbert-Marshall
- Juan Fernández = Juan Fernández Ridge
- Tasmantid = Tasmantid Seamount Chain
What are the effects of volcanoes?
A. Volcanic gases
Most hotspot volcanoes are mainly made of basalt (e.g. Hawaii, Tahiti), which makes them less explosive than subduction zone volcanoes. Continental hotspots release basaltic magma through the continental crust, which melt to form rhyolites. e.g. Yellowstone Caldera. If a rhyolite completely erupts, it is followed by eruptions of basaltic magma spewing through the same lithospheric fissures. Holbek (1983) found the Ilgachuz Range in British Columbia was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.
What are hotspot volcanic chains?
The joint mantle plume / hotspot hypothesis proposes the feeder structures being fixed relative to one another, with the continents and seafloor drifting overheard. It also predicts that time-progressive chains of volcanoes develop on the surface. e.g. Yellowstone is located at the end of a chain of extinct calderas, which progressively aged to the west.
Geologists use hotspot volcanic chains in an attempt to track the movement of the Earth's tectonic plates. Since many chains aren't time-progressive (e.g. the Galápagos) and fixed relative to one another (e.g. Hawaii and Iceland), this effort hasn't yielded much data.
I'll delve into the details of plate tectonics and all tectonic boundaries in another post.
Examples of postulated hotspot locations
- Hawaii = Hawaiian-Emperor seamount chain
- Louisville = Louisville Ridge
- Gough and Tristan = Walvis Ridge
- Bowie = Kodiak-Bowie Seamount chain
- Cobb = Cobb-Eickelberg Seamount chain
- New England = New England Seamounts, Great Meteor hotspot track
- Anahim = Anahim Volcanic Belt
- Mackenzie = Mackenzie dike swarm
- Saint Helena = St. Helena Seamount Chain-Cameroon Volcanic Line
- Réunion = Southern Mascarene Plateau-Chagos-Maldives-Laccadive Ridge
- Kerguelen = Ninety East Ridge
- Easter = Tuamotu-Line Island chain
- Macdonald = Austral-Gilbert-Marshall
- Juan Fernández = Juan Fernández Ridge
- Tasmantid = Tasmantid Seamount Chain
What are the effects of volcanoes?
A. Volcanic gases
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| Schematic of volcano injection of aerosols and gases |
The most abundant gases released by a volcano are water vapour (H2O), carbon dioxide (CO2) and sulfur dioxide (SO2) , as well as hydrogen sulfide (HS), hydrogen chloride (HCl), and hydrogen fluoride (HF). Minor and trace gases found in volcanic emissions include Hydrogen (H2), Carbon Monoxide (CO), Halocarbons, organic compounds, and volatile metal chlorides.
Along with the aforementioned gases, explosive volcanic eruptions also release ash (pulverised rock and pumice) into the stratosphere up to 16-32 km (10-20) mi high. Sulfur dioxide converts to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the Earth's albedo, which reflects more radiation from the Sun back into outer space, hence cools the Earth's lower atmosphere
or troposphere. Nonetheless, the aerosols also absorb heat radiating up from the Earth's surface, thereby warming up the stratosphere. Volcanic eruptions that occurred during the 20th century decreased the average temperature at the Earth's surface by half a degree Fahrenheit (-17.5 degrees Celsius) for 1 to 3 years. Davis (2008) theorised the sulfur dioxide released from the Huaynaputina eruption may have contributed to the Russian famine of 1601-1603.
B. Significant consequences
(i) Prehistorical
A 2009 ScienceDaily report discussed a volcanic winter that occurred around 70,000 years ago after the Lake Tobe supereruption on Sumatra Island in Indonesia. The global aftermath of the Toba eruption killed most humans at the time and created a population bottleneck impacting the genetic inheritance of all humans today.
One of the largest known volcanic events that occurred in the last 500 million years of Earth's geological history, responsible for the formation of the Siberian Traps, lasted for at least a million years. Historians theorised this eruptive event may have likely caused the "Great Dying" about 250 million years ago, which killed an estimated 90% of species that existed at the time.
(ii) Historical
de Boers and Sanders (2002) implied the 1815 eruption of Mount Tambora contributed to global climate anomalies known as the "Year Without a Summer" due to the North American and European weather. Oppenheimer (2003) stated the failure of agricultural crops and death of livestock in much of the Northern Hemisphere, which culminated in one of the worst famines of the 19th century. Ó Gráda (2009) suggested a volcanic eruption may have been responsible for the freezing winter of 1740-41, which led to widespread famine in northern Europe.
C. Acid rain
Sulfate aerosols trigger complex chemical reactions on their surfaces that transform chlorine and nitrogen chemical species in the stratosphere. In combination with increased stratosphere chlorine levels from chlorofluorocarbon (CFC) pollution, this effect generate chlorine monoxide (ClO), which disintegrates ozone (O3). As the aerosols expand and coagulate, they settle down into the upper troposphere, where they become nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) dissolve in water droplets in the eruption cloud and swiftly plummet to the ground as acid rain. The injected ash also falls quickly from the stratosphere, with a large proportion of it removed with several days to a few weeks. A 1997 study found explosive volcanic eruptions release the greenhouse gas carbon dioxide, providing an abundant source of carbon for biogeochemical cycles.
A 2007 US Geological Survey estimated between 130 and 230 teragrams (145 million and 255 million short tons) of carbon dioxide each year. Larger injections of aerosols into the Earth's atmosphere may cause visual effects (such as atypical colourful sunsets) and impact global climate.
D. Volcanic hazards
https://en.wikipedia.org/wiki/Volcanic_hazards
A. Fissure vents
https://en.wikipedia.org/wiki/Fissure_vent
A fissure vent (or volcanic fissure, eruption fissure) is a linear volcanic vent where lava erupts, usually without any volcanic activity. Their dimensions are typically a few metres
wide and many kilometres long. They can cause large flood basalts to run in lava channels, and subsequently in lava tubes, which builds up to create a spatter cone. Fissure vents are connected to dikes that are several kilometres deep, which are linked to deeper magma reservoirs, typically under volcanic centres. Fissures are usually found in or along rifts and rift zones such as Iceland and the East African Rift.
Iceland:
The radial fissure vents of Hawaiian volcanoes produced "curtains of fire" as lava fountains erupted along a portion of a fissure, which built up low ramparts of basaltic spatter on both sides of the fissure.
B. Shield volcanoes
https://en.wikipedia.org/wiki/Shield_volcano
https://en.wikipedia.org/wiki/List_of_shield_volcanoes
This type of volcano is composed of mainly fluid lava flows. It is named as it has a low profile, which resembles a warrior's shield lying on the ground. The term 'shield volcano' is derived from the German term Schildvulkan, coined by the Australian geologist Eduard Suess in 1888. This term was subsequently calqued into English by 1910.
Shield volcanoes are caused by gentle effusive eruptions of highly fluid lava that produce a broad, gently sloped "shield" over a period of time. Examples of pyroclastic shields include:
- Billy Mitchell volcano, Papua New Guinea
- Purico complex, Chile
- Ilgachuz Range, British Columbia, Canada
Where are the shield volcanoes?
Around the world, shield volcanoes form over hotspots, such as the Hawaiian-Emperor seamount chain and the Galágapos Islands, or over conventional rift zones, such as the Icelandic shields and the shield volcanoes of East Africa. e.g. Tamu Massif, in an ocean basin.
i. Hawaiian Islands
ii. Galápagos Islands
iii. Iceland
iv. East Africa
v. Extraterrestrial volcanoes
C. Lava Domes
https://en.wikipedia.org/wiki/Lava_dome
https://en.wikipedia.org/wiki/List_of_lava_domes
Lava domes are a circular mound-shaped protrusion produced by the slow extrusion of viscous lava from a volcano. The geochemistry of lava domes consists of mainly basalt or rhyolite and occasionally dacite and andesite. The characteristic dome shape attributes to the high viscosity that minimises the distance lava can flow freely due to high levels of silica in the magma, or fluid magma degassing.
ii. Lava spine
https://en.wikipedia.org/wiki/Lava_spine
A lava spine or lava spire is a vertical growth of solid lava that is propelled from a volcanic vent. They can either be created by viscous lava gradually being forced out of the vent, or by magma that has solidified within the vent before being expelled. e.g. Soufrière Hill Volcano on Montserrat, formed in 1997 (image above).
iii. Lava coulées
Along with the aforementioned gases, explosive volcanic eruptions also release ash (pulverised rock and pumice) into the stratosphere up to 16-32 km (10-20) mi high. Sulfur dioxide converts to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the Earth's albedo, which reflects more radiation from the Sun back into outer space, hence cools the Earth's lower atmosphere
or troposphere. Nonetheless, the aerosols also absorb heat radiating up from the Earth's surface, thereby warming up the stratosphere. Volcanic eruptions that occurred during the 20th century decreased the average temperature at the Earth's surface by half a degree Fahrenheit (-17.5 degrees Celsius) for 1 to 3 years. Davis (2008) theorised the sulfur dioxide released from the Huaynaputina eruption may have contributed to the Russian famine of 1601-1603.
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| This solar radiation graph (1958-2008) illustrates how the radiation decreases after major volcanic eruptions. |
 |
| Sulfur dioxide concentration over the Sierra Negra Volcano, Galapagos Islands, during an eruption in October 2005. |
B. Significant consequences
(i) Prehistorical
A 2009 ScienceDaily report discussed a volcanic winter that occurred around 70,000 years ago after the Lake Tobe supereruption on Sumatra Island in Indonesia. The global aftermath of the Toba eruption killed most humans at the time and created a population bottleneck impacting the genetic inheritance of all humans today.
One of the largest known volcanic events that occurred in the last 500 million years of Earth's geological history, responsible for the formation of the Siberian Traps, lasted for at least a million years. Historians theorised this eruptive event may have likely caused the "Great Dying" about 250 million years ago, which killed an estimated 90% of species that existed at the time.
(ii) Historical
de Boers and Sanders (2002) implied the 1815 eruption of Mount Tambora contributed to global climate anomalies known as the "Year Without a Summer" due to the North American and European weather. Oppenheimer (2003) stated the failure of agricultural crops and death of livestock in much of the Northern Hemisphere, which culminated in one of the worst famines of the 19th century. Ó Gráda (2009) suggested a volcanic eruption may have been responsible for the freezing winter of 1740-41, which led to widespread famine in northern Europe.
 |
Comparison of major United States supereruptions (VEI 7 and 8) with major historical volcanic eruptions in the 19th and 20th century. Left to right: Yellowstone 2.1 Ma; Yellowstone 1.3 Ma; Long Valley 6.26 Ma; Yellowstone 0.64 Ma 19th century eruptions: Tambora 1815, Krakatoa 1883 20th century eruptions: Novarupta 1912, St. Helens 1980, Pinatubo 1991 |
C. Acid rain
Sulfate aerosols trigger complex chemical reactions on their surfaces that transform chlorine and nitrogen chemical species in the stratosphere. In combination with increased stratosphere chlorine levels from chlorofluorocarbon (CFC) pollution, this effect generate chlorine monoxide (ClO), which disintegrates ozone (O3). As the aerosols expand and coagulate, they settle down into the upper troposphere, where they become nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) dissolve in water droplets in the eruption cloud and swiftly plummet to the ground as acid rain. The injected ash also falls quickly from the stratosphere, with a large proportion of it removed with several days to a few weeks. A 1997 study found explosive volcanic eruptions release the greenhouse gas carbon dioxide, providing an abundant source of carbon for biogeochemical cycles.
A 2007 US Geological Survey estimated between 130 and 230 teragrams (145 million and 255 million short tons) of carbon dioxide each year. Larger injections of aerosols into the Earth's atmosphere may cause visual effects (such as atypical colourful sunsets) and impact global climate.
D. Volcanic hazards
https://en.wikipedia.org/wiki/Volcanic_hazards
A volcanic hazard is defined as the probability of a volcanic eruption or related geophysical event occurring in a given geographic area and within a specified window of time. The risk associated with a volcanic hazard depends on the proximity and vulnerability of an asset or a population of people adjacent to a probable volcanic event occurring.
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| This schematic diagram illustrates some of the many ways that volcanoes can cause problems for those nearby. |
Lava flows normally follow the topography, descending down depressions and valleys
as it flows down the volcano. The kinetic energy of a lava flow is sufficient to bury roads, farmlands, and other forms of personal property, as well as demolish homes, cars and lives lying in its path. However, lava flows are slow, which provides people adequate time to evacuate out of the immediate areas.
Tephra is defined as the debris or pyroclastic material launched out of a volcano during an eruption. This material is categorised according to its size:
i. Dust = < 1/8 mm
- Coats cars to render them inoperable since it clogs the engine
- Coats homes to add weight to the roof causing it to collapse.
- Causes long-term respiratory issues
ii. Ash = 1/8 - 2 mm
- Causes long-term respiratory issues
iii. Cinders = 2 - 64 mm
- Flaming pieces of ejected volcanic material that can incinerate homes and wooded areas.
iv. Bombs & blocks = > 64 mm
- Strikes various objects and humans within range of volcano with substantial force.
- Projectiles known to travel many metres in the air and found several kilometres away from the initial eruption point.
- A pyroclastic flow is a fast-moving (up to 700 km/h) scorching (~1000°C) mass of air and tephra that charge down the sides of a volcano during an explosive eruption.
- When pyroclastic materials combine with water from a nearby stream or river, it transforms the watercourse into a fast moving mudflow, called a lahar.
- Volcanic activity that induce stress changes in solid rock due to magma injecting or withdrawing causes volcano tectonic earthquakes.
- When magma is instantly forced into the surrounding rocks, as a precursor to the major eruption, it triggers long period earthquakes.
What are the different types of volcanoes?
Most volcanoes pertained a conical shape, emitting lava and poisonous gases from a crater to its summit. Nonetheless, there are many other types of volcanoes.A. Fissure vents
https://en.wikipedia.org/wiki/Fissure_vent
A fissure vent (or volcanic fissure, eruption fissure) is a linear volcanic vent where lava erupts, usually without any volcanic activity. Their dimensions are typically a few metres
wide and many kilometres long. They can cause large flood basalts to run in lava channels, and subsequently in lava tubes, which builds up to create a spatter cone. Fissure vents are connected to dikes that are several kilometres deep, which are linked to deeper magma reservoirs, typically under volcanic centres. Fissures are usually found in or along rifts and rift zones such as Iceland and the East African Rift.
Iceland:
- Parallel to rift zones at the diverging boundary of the Eurasian and the North American Plate lithosphere plates, as part of the Mid-Atlantic Ridge.
- Laki fissures, part of the Grímsvötn volcanic system. It produced one of the biggest effusive eruptions on earth in historical times, causing a flood basalt of 12-14 km3 of lava in 1783.
- Between 934-40 A .D., the Eldgjá effusive fissure eruption occurred in the Katla volcanic system in South Iceland, releasing ~18.7 km3 of lava
- In September 2014, the Holuhraun fissure eruption occurred as part of an eruption series in Bároarbunga volcanic system.
Hawaii:
The radial fissure vents of Hawaiian volcanoes produced "curtains of fire" as lava fountains erupted along a portion of a fissure, which built up low ramparts of basaltic spatter on both sides of the fissure.
Other examples of fissure vents:
- Ray Mountains, Canada
- Cordón Caulle, Chile
- Manda-Inakir, Eritrea
- Alu, Ethiopia
- Hertali, Ethiopia
- Banda Api, Indonesia
- Singu Plateau, Burma
- Estelií, Nicaragua
- Pagan, Northern Mariana Islands
- Tor Zwar, Pakistan
- São Jorge Island, Portugal
- Tolbachik, Russia
- Lanzarote, Spain
B. Shield volcanoes
https://en.wikipedia.org/wiki/Shield_volcano
https://en.wikipedia.org/wiki/List_of_shield_volcanoes
This type of volcano is composed of mainly fluid lava flows. It is named as it has a low profile, which resembles a warrior's shield lying on the ground. The term 'shield volcano' is derived from the German term Schildvulkan, coined by the Australian geologist Eduard Suess in 1888. This term was subsequently calqued into English by 1910.
Shield volcanoes are caused by gentle effusive eruptions of highly fluid lava that produce a broad, gently sloped "shield" over a period of time. Examples of pyroclastic shields include:
- Billy Mitchell volcano, Papua New Guinea
- Purico complex, Chile
- Ilgachuz Range, British Columbia, Canada
- Active shield volcanoes experience near-continuous eruptive activity over long periods of time, causing a gradual accumulation of edifices that can reach massive dimensions.
- Mauna Loa = Summit of the world's largest subaerial volcano, 4169 m (13678 ft) above sea level, over 100 km (60 mi) wide at its base, consists of roughly 80,000 km3 (19,000 mi3)
- Shield volcanoes contain a gentle (~ 2-3 degrees) slope that gradually steepens with elevation (~ 10 degrees) before eventually flattening near the summit, forming an upwardly convex shape.
- Typical shield volcanoes situated in California and Oregon are about 5-6 km (3-4 mi) in diameter and 500-600 m (1500-2000 ft) in height.
- Shield volcanoes in the central Mexican Michoacán-Guanajuano volcanic field are measured to be on average 340 m (1100 ft) high and 4,100 m (13,500 ft) wide, with an average slope angle of 9.4 degrees, and an average volume of 1.7 km3 (0.4 mi3)
- In Hawaii, a majority of fissure venting eruptions initiate with a "wall of fire" along a major fissure line before centralising to a few points. Compared to Icelandic volcanoes, by which eruptions are predominately summit calderas, Hawaiian volcanoes are asymmetrical, which causes the lava to flow evenly and symmetrically.
What are the eruptive characteristics of shield volcanoes?
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| Diagram of a Hawaiian eruption: 1. Ash plume 2. Lava fountain 3. Crater 4. Lava lake 5. Fumaroles 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike |
- The Hawaiian eruptions are slow-moving and effusive, which are typical of shield volcanism. Since they are the calmest of volcanic events, these eruptions are characterised by the effusive emission of highly fluid basaltic lavas with low gaseous content. Studies measured these lava flows travel greater distances before solidifying, forming broad but relatively thin magmatic sheets often less than 1 m (3 ft) thick. Topinka (2005) implied the characteristically low, broad profile of a mature shield volcano is constructed by low volumes of such lavas layered over a long period of time.
- Hawaiian eruptions often occur at decentralised fissure vents, initiating with large "curtains of fire" that swiftly subside and concentrate at specific locations on the volcano's rift zones.
- Meanwhile, central-vent eruptions often take the form of large lava fountains (both continuous and sporadic), which can reach heights of at least 100s of metres. The particles from lava fountains usually cool in the air before landing on the ground, which clump together into cindery scoria fragments.
- When the air is thick with clasts, they can't cool down fast enough due to the surrounding heat, and contact the ground still hot, stockpiling into spatter cones.
- A 2014 study noted Pu'u 'Ō'ō is a cinder cone of Kīlauea that erupt continuously from January 3, 1983 until April 2018.
- Pāhoehoe lava is relatively smooth and flows with a viscous texture. They flow in more conventional sheets, or by advancing lava "toes" in snaking lava columns. A 2010 study found that if viscosity increases on the section of the lava or shear stress increases on the section of local topography, it morphs a pāhoehoe flow into an 'a'ā flow, however the reverse never occurs.
- 'a'ā flows are denser, and more viscous (thus flows slower) and lumpy, which are 2 - 20 m (10 - 70 ft) thick. They flow through pressure after which the general mass behind it moves forward. The partially solidified front of the flow steepens formed by the mass of the flowing lava behind it until it becomes detached. As the top of the flow rapidly cools down, the molten underbelly of the flow is mitigated by the solidifying rock above it, which allows the 'a'ā flow movement to be sustained for long periods of time.
- Studies conducted by the University of San Diego and University of North Dakota found shield volcanoes consisted of cinder cones on their flanks, resulting from tephra ejections common during incessant activity and markers of currently and formerly active sites on the volcano.
- Since Hawaiian shield volcanoes aren't located near any plate boundaries, their volcanic activity is attributed to the movement of oceanic plate over an upwelling of magma, known as a hotspot. Over millions of years, the tectonic movement generated long volcanic trails across the seafloor. The lava spewed by the Hawaiian and Galápagos island shield volcanoes mainly consist of Sodium, Potassium, and Aluminium.
- A common feature of shield volcanism is a lava tube, which are cave-like volcanic strengths formed by the solidifying of overlaying lava. Topinka (2002) implied these structures promote the propagation of lava, as the walls of the tube insulates the lava within. A 2010 study claimed lava tubes account for a considerable portion of shield volcanic activity, e.g. an estimated 58% of the lava forming Kīlauea originates from lava tubes.
- In certain shield volcanic eruptions, basaltic lava gush out of a long fissure instead of a central vent, and envelop the landscape with a long band of volcanic material in the form of a broad plateau. Such plateaus are located in Iceland, Washington, Oregon, and Idaho, with the most prominent ones situating along the Snake River in Idaho and the Columbia River in Washington and Oregon, which were measured to over thicker than 2 km (1 mi).
- Another common feature of shield volcanoes is a caldera, which form and reform over a volcano's lifespan. A 2010 study found a caldera fills up by future eruptions, with the cycle of collapse and regeneration occurring throughout the volcano's lifespan.
- When water and lava combine in shield volcanoes, the eruptions become hydrovolcanic. A 2007 study described hydrovolcanic eruptions as explosive and drastically different from the usual shield volcanic activity, which are particularly prevalent at the Hawaiian Isles.
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| 'A'a flows over solidified pāhoehoe on Kīlauea, Hawai'i |
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| A pāhoehoe lava fountain on Kīlauea erupts |
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| A lava lake in the caldera of Erta Ale, an active shield volcano in Ethiopia |
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| Pu'u 'Ō'ō is a parasitic cinder cone on Kīlauea, lava fountaining at dusk in June 1983, near the start of its current eruptive cycle. |
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| The Thurston lava tube on Hawai'i island. |
Where are the shield volcanoes?
Around the world, shield volcanoes form over hotspots, such as the Hawaiian-Emperor seamount chain and the Galágapos Islands, or over conventional rift zones, such as the Icelandic shields and the shield volcanoes of East Africa. e.g. Tamu Massif, in an ocean basin.
i. Hawaiian Islands
- A chain of largest and most prominent shield (hotspot) volcanoes in the Pacific Ocean.
- Caused by the movement of the Pacific Plate over the Hawaii hotspot.
- A long chain of volcanoes, atolls, and seamounts 2,600 km (1,616 mi) long with a total volume of over 750,000 km3 (179,935 mi3).
- At least 43 major volcanoes, and Meiji Seamount at its terminus near the Kuril-Kamchatka Trench is dated to be around 85 million years old.
- Mauna Loa = 2nd largest volcano on Earth, stands 4,170 m (13,680 ft) above sea level, reaches a further 13 km (8 mi) below the waterline and into the crust, roughly 80,000 km3 (19,000 mi3) of rock.
- Kīlauea = One of the most active volcanoes on Earth, eruptions ongoing since January 1983.
- An isolated set of volcanoes, containing shield volcanoes and lava plateaus, about 1,100 km (680 mi) west of Ecaudor.
- Caused by the Galápagos hotspot, dated between roughly 4.2 million and 700,000 years of age.
- Isabela Island, the largest island, consists of 6 coalesced shield volcanoes, each delineated by a large summit caldera.
- Española, the oldest island, and Fernandina, the youngest are also shield volcanoes.
- Perched on a large lava plateau known as the Galápagos platform, which creates a shallow water depth of 360 to 900 m (1,181 to 2,953 ft) at the base of the islands, stretching over a 280 km (174 mi)-long diameter.
- Blue Hill is a shield volcano on the southwestern section of Isabela Island, with the last eruption occurring between May and June 2008.
- La Cumbre is an active shield volcano on Fernandina Island that has been erupting since April 11, 2009.
iii. Iceland
- This country is the site of 130 volcanoes of various types, which is located over the Mid-Atlantic Ridge.
- They were dated around the Holocene age, between 5,000 and 10,000 years old, except for the island of Surtsey, a Surtseyan shield.
- Narrowly distributed, and occurred in 2 bands in the West and North Volcanic Zones.
- Icelandic shield volcanoes are mainly small (~ 15 km3 (4 mi3)), symmetrical, and characterised by eruptions from summit calderas. They are composed of either tholeiitic olivine or picritic basalt.
iv. East Africa
- A majority of volcanic activity was generated by the East African Rift, a developing plate boundary in Africa.
- Most active shield volcano in Africa is Nyamuragira.
- During the 20th century, lava flows extended down the flanks more than 30 km (19 mi) from the summit, arriving at Lake Kiku.
- Other active shield volcanoes include Erta Ale in Ethiopia, Menangai, and Mount Marsabit.
v. Extraterrestrial volcanoes
- Volcanoes can also exist on any rocky planet or moon sufficiently large or active to contain a molten core.
- Shield volcanoes have been found on Mars, Venus, and Io; cryovolcanoes on Triton; and subsurface hotspots on Europa.
- Martian volcanoes are variable in size up to 23 km (14 mi) in height and 595 km (370 mi) in diameter.
- Olympus Mons, the highest Martian volcano, is the tallest known mountain on any planet in the solar system.
- Venus has over 150 shield volcanoes that are flatter and larger in surface area than those on Earth, some of which are more than 700 km (430 mi) in diameter.
C. Lava Domes
https://en.wikipedia.org/wiki/Lava_dome
https://en.wikipedia.org/wiki/List_of_lava_domes
Lava domes are a circular mound-shaped protrusion produced by the slow extrusion of viscous lava from a volcano. The geochemistry of lava domes consists of mainly basalt or rhyolite and occasionally dacite and andesite. The characteristic dome shape attributes to the high viscosity that minimises the distance lava can flow freely due to high levels of silica in the magma, or fluid magma degassing.
- The non-linear dynamics caused by crystallisation and outgassing of the highly viscous lava in the dome's conduit makes the evolution of lava domes unpredictable. Domes are subjected to processes such as growth, collapse, solidification, and erosion.
- Lava domes are thought to expand by endogenic or exogenic growth. Fink and Anderson (2001) explained the former implies the enlargement of a lava dome caused by the influx of magma into the dome interior, and the latter implies the discrete lobes of lava positioned upon the surface of the dome.
- Since the lava's high viscosity prevents it from flowing markedly from the vent where it originally extrudes, it produces a dome-like shape of sticky lava that cools slowly in-situ.
- Domes can be several 100 metres high, or can grow gradually for months, years or centuries to reach considerable heights.
- Erupting domes can often experience events of explosive eruptions over time because of the intermittent buildup of gas pressure. Parfitt & Wilson (2008) suggested the pyroclastic flows are created by a section of a lava dome collapsing and exposing pressurised magma.
- Other hazards associated with lava domes include the destruction of property from lava domes, forest fires, and lahars triggered by re-mobilisation of ash and debris.
- The characteristics of lava dome eruptions include shallow, prolonged and hybrid seismicity, due to the excess fluid pressures in the contributing vent chamber. Sparks (1997) listed other characteristics such as hemispherical dome shape, cycles of dome growth over time, and sudden onsets of violent explosive activity.
i. Cryptodomes
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| Evolution of the intrusive-extrusive rhyolite cryptodome complex at Pálháza, Tokaj Mts. (modified after Németh et al., 2008). |
A cryptodome (from the Greek κρυπτός, kryptos, "hidden, secret") is a dome-shaped structure produced by a build-up of viscous magma at a shallow depth, causing the surface to bulge. e.g. 1980 Mount St. Helens eruption.
ii. Lava spine
A lava spine or lava spire is a vertical growth of solid lava that is propelled from a volcanic vent. They can either be created by viscous lava gradually being forced out of the vent, or by magma that has solidified within the vent before being expelled. e.g. Soufrière Hill Volcano on Montserrat, formed in 1997 (image above).
iii. Lava coulées
 |
| Chao dacite coulée flow-domes (left centre), northern Chile, viewed from Landsat 8. |
Lava coulées are domes that flow away from their original position, resembling both lava domes and lava flows.
e.g. The world's largest known dacite flow is the Chao dacite coulée complex, which is formed between 2 volcanoes in northern Chile. The flow is over 14 km (8.7 mi) long, contains common features such as pressure ridges, and a flow front 400 m (1,300 ft) tall.
E. Volcanic / Cinder Cones
https://en.wikipedia.org/wiki/Volcanic_cone
https://en.wikipedia.org/wiki/Cinder_cone
Volcanic cones are created by ejecta expelled from a volcanic vent, which accumulate around the vent in the shape of a cone with a central crater. Depending on the nature and size of the fragments ejected during the eruption, there are different types of volcanic cones.
i. Stratovolcanoes (Composite Cones)
https://en.wikipedia.org/wiki/Stratovolcano
https://en.wikipedia.org/wiki/List_of_stratovolcanoes
Stratovolcanoes or composite cones are a conical volcano created by numerous layers (strata) of hardened lava, tephra, pumice, and ash. They are characterised by their steep profile with a summit crater and frequent periods of explosive eruptions and effusive eruptions. The lava flowing from such volcanoes normally cools and hardens before strewing long distances, due to high viscosity. The magma forming this lava is usually felsic, with medium-to-high levels of silica (i.e. rhyolite, dacite, or andesite), and low levels of less-viscous mafic magma.
e.g. The world's largest known dacite flow is the Chao dacite coulée complex, which is formed between 2 volcanoes in northern Chile. The flow is over 14 km (8.7 mi) long, contains common features such as pressure ridges, and a flow front 400 m (1,300 ft) tall.
E. Volcanic / Cinder Cones
https://en.wikipedia.org/wiki/Volcanic_cone
https://en.wikipedia.org/wiki/Cinder_cone
Volcanic cones are created by ejecta expelled from a volcanic vent, which accumulate around the vent in the shape of a cone with a central crater. Depending on the nature and size of the fragments ejected during the eruption, there are different types of volcanic cones.
i. Stratovolcanoes (Composite Cones)
https://en.wikipedia.org/wiki/Stratovolcano
https://en.wikipedia.org/wiki/List_of_stratovolcanoes
Stratovolcanoes or composite cones are a conical volcano created by numerous layers (strata) of hardened lava, tephra, pumice, and ash. They are characterised by their steep profile with a summit crater and frequent periods of explosive eruptions and effusive eruptions. The lava flowing from such volcanoes normally cools and hardens before strewing long distances, due to high viscosity. The magma forming this lava is usually felsic, with medium-to-high levels of silica (i.e. rhyolite, dacite, or andesite), and low levels of less-viscous mafic magma.
 |
| Mount Vesuvius, near the city of Naples in Italy, violently erupted in 79 AD. Back then, it ruined the Roman cities of Pompeii and Herculaneum. The last eruption of this stratovolcano occurred in March 1944. |
How are stratovolcanoes formed?
How do they affect the climate and atmosphere?
- Stratovolcanoes are commonly located at sub location zones, creating chains and clusters along plate tectonic boundaries where oceanic crust subducts continental crust (e.g. Cascade Range, Andes, Campania) or another tectonic plate (island arc volcanism, e.g. Japan, Philippines, Aleutian Islands).
- The magma ascends as water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is discharged into mantle rock of the asthenosphere above the sinking oceanic slab, known as "dewatering". This phenomenon occurs at specific pressures and temperatures for each mineral, as the plate descends to considerable depths.
- The water released from the rock lowers the melting point of the overlying mantle rock, which subsequently undergoes partial melting and ascends due to its lighter density relative to the surrounding rock. This water also coalesces temporarily at the base of the lithosphere.
- The magma then ascends through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition. When the magma reaches the uppermost surface, it coalesces in a magma chamber within the crust below the stratovolcano.
What are the hazards of stratovolcanoes?
- Subduction-zone stratovolcanoes, such as Mount St. Helens, Mount Etna and Mount Pinatubo, usually erupt with explosive force, since the magma is too stiff to allow volcanic gases to escape. Consequently, the volcanic gases are trapped in the magma under tremendous internal pressures. Once the vent is breached and the crater is opened, the magma degasses explosively, spewing out magma and volcanic gases at high speed.
- Most deaths are caused by pyroclastic flows and lahars, which are common hazards following explosive eruptions of subduction-zone stratovolcanoes. e.g. Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean.
- In March to April 1982, 3 explosive eruptions of El Chichón in the State of Chiapas in southeastern Mexico lead to the worst volcanic disaster in Mexico's history. Pyroclastic flows destroyed villages within 8 km (5 mi) of the volcano, and killed more than 2,000 people.
- In 1991, Philippines’ Mount Pinatubo (Central Luzon) spewed an ash cloud 40 km (25 mi) into the air and produced enormous pyroclastic surges and lahar floods into the surrounding areas. In the same year, Japan’s Unzen Volcano (Kyushu) erupted after being dormant for approximately 200 years to create a new lava dome at its summit.
- In 79 AD, Mount Vesuvius eruption completely enveloped the nearby ancient cities of Pompeii and Herculanuem with thick deposits of pyroclastic surges and lava forms.
- Ash = Volcanic clouds from explosive eruptions poses a dangerous hazard to aviation safety. E.g. the 1982 eruption of Galunggung (Java) caused temporary engine failure and structural damage to British Airways Flight 9. Fortunately the plane landed safely.
- Volcanic bombs = They are extrusive igneous rocks with variable sizes, which are explosively expelled from stratovolcanoes during their climatic eruptive phases. They can fly over 20 km (12 mi) away from the volcano, putting adjacent buildings and people at risk.
- Lahar = They are mixtures of volcanic debris and water, which originate from rainfall or the melting of snow and ice by hot volcanic materials, such as lava.
- Lava = Its high viscosity, hence slow flow, is due to its high silica content. However, some stratovolcanoes, such as Nyiragonga, erupt fluid lava that can flow quickly and threaten property, as well as melt down ice and glaciers.
How do they affect the climate and atmosphere?
- The June 1991 Mount Pinatubo eruption slightly decreased global temperatures, with particulates in the stratosphere responsible for the brilliant sunsets and intense sunrises. It also dissipated aerosols produced from sulfur dioxide, carbon dioxide, etc.
- When sulfur dioxide combined with water, it produced droplets of sulfuric acid. The sulfur dioxide clouds blocked a portion of the sunlight from reaching the troposphere and ground.
- The April 1815 eruption of Mount Tambora on Sumbawa island in Indonesia also produced similar climatic and atmospheric effects. This lead to 1816 being famous for “the Year without a Summer”, causing significant agricultural crises and famines worldwide.
ii. Spatter cones
- It is a low, steep-sided hill or mound that is composed of welded lava fragments, called spatter, which forms around a lava fountain emitting from a central vent.
- They are about 3-5 metres (9.8-14.6 ft) high.
- In the case of a linear fissure, lava fountains produce wide embankments of spatter, called spatter ramparts, along both sides of the fissure.
- Spatter cones are roughly circular and cone shaped, while spatter ramparts are linear wall-like features.
- Studies found both spatter cones and spatter ramparts are normally formed by lava fountaining associated with mafic, highly fluid lava, such as those erupted in the Hawaiian Islands. As spatter erupts into the air by a lava fountain, it doesn’t cool down quickly enough before contacting the ground, hence remaining as a liquid.
- As the spatter lands and binds to the underlying spatter, this creates a cone composed of spatter either agglutinated or welded to each other.
Diamond Head tuff cone, Oahu, Hawaii, USA
- Occasionally called an ash cone, a tuff cone is a small monogenetic volcanic cone produced by phreatic (hydrovolcanic) explosions that emit magma through a conduit from a deep-seated magma reservoir.
- They have high rims with a maximum relief of 100-800 metres (330-2620 ft) above the crater floor and slopes steeper than 25 degrees.
- They typically have a rim-to-rim diameter of 300-5000 metres (980-16400 ft), and comprises of thick-bedded pyroclastic flow and surge deposits produced by eruption-fed density currents and bomb-scoria beds originated from fallout from its eruption column.
- Tuff cones often change, palagonitise, by either its interaction with groundwater or its warm and wet deposition.
- Tuff rings are an associated type of small monogenetic volcano created by phreatic explosions, which is directly correlated with magma emitted to the surface through a conduit from a deep-seated magma reservoir.
- They have rims with low, broad topographic profiles and topographic slopes with angles of 25 degrees or less.
- The maximum thickness of pyroclastic debris composed of the rim of a typical tuff ring is less than 50-100 m (160-330 ft) thick. The pyroclastic materials that constitutes the rim is composed of relatively fresh, raw, distinctly and thin-bedded volcanic surge and air fall deposits, as well as local bedrock gushed out of the crater.
- Both tuff cones and tuff rings are produced by explosive eruptions from a vent where magma interacts with either groundwater and/or shallow body of water within a lake or sea.
- When magma, expanding steam and volcanic gases combine, this produces fine-grained pyroclastic debris called ash.
- The volcanic ash (forming the tuff cone) accumulates either as fallout from eruption columns, from low-density volcanic surges and pyroclastic flows, or a combination of both.
- Tuff cones are linked to volcanic eruptions within shallow bodies of water, whereas tuff rings are linked to eruptions within either water saturated sediments and bedrock or permafrost.
https://en.wikipedia.org/wiki/Cinder_cone
- A cinder cone is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, and/or cinder accumulating around a volcanic vent.
- The pyroclastic fragments are produced by explosive eruptions or lava fountains from a single, cylindrical vent.
- As the gas-charged lava is blasted powerfully into the air, it disintegrates into smaller fragments that solidify and fall as either clinkers, cinders, or scoria around the vent to produce a typically symmetrical cone with a 30-40 degree slopes.
 |
| Cross-section diagram of a cinder cone or scoria cone |
Where do cinder cones occur?
- They are commonly discovered on the flanks of shield volcanoes, stratovolcanoes, and calderas. e.g. Around 100 cinder cones on the flanks of Mauna Kea, a shield volcano on Hawaii.
- In 1943, Paricutin appeared in a corn field in Mexico from a new vent. It was 424 metres (1391 ft) tall, spewed lava flows 25 km2 (9.7 mi2) in area, and erupted for 9 years.
- Cerro Negro, Nicaragua - Part of a group of 4 young cinder cones NW of Las Pilas volcano. Since 1850, there had been more than 20 eruption events.
- Satellite images may have uncovered cinder cones on other terrestrial bodies in the solar system. e.g. On the flanks of Pavonis Mons in Tharsis, in the region of Hydraotes Chaos on Coprates Chasma, in the volcanic field Ulysses Colles, or Marius Hills.
v. Rootless cones
https://en.wikipedia.org/wiki/Rootless_cone
F. Supervolcanoes
https://en.wikipedia.org/wiki/Supervolcano
What are large igneous provinces (LIP)?
https://en.wikipedia.org/wiki/Rootless_cone
 |
| Rootless cone in Lake Myvatn, Iceland |
- Former called a pseudocrater, a rootless cone is a volcanic landform resembling a typical volcanic crater, but is different from a typical volcanic vent from where lava erupted from. They lack a magma conduit that links to the magma chamber below the Earth’s surface.
- They are formed by steam explosions as hot lava flows over a wet surface, such as a swamp, a lake, or a pond.
- e.g. Icelandic craters in the lake Mývatn (Skútustaðagígar), the Rauðhólar in the region of Reykjavík, the Landbrotshólar of South-Iceland's Katla UNESCO Global Geopark near Kirkjubæjarklaustur.
- Also found in the Athabasca Valles region of Mars
 |
| “Rootless Cones” on Mars - due to lava flows interacting with water (MRO, January 4, 2013) (21.965o N 197.807o E) |
F. Supervolcanoes
https://en.wikipedia.org/wiki/Supervolcano
- Supervolcanoes are large volcanoes with eruptions of a Volcanic Explosivity Index (VEI) of 8, the largest ever recorded. This denotes the volume of volcanic deposits is greater than 1000
km3 (240 mi3). - The term “supervolcano” was first used in 1949, originating in an early 20th-century scientific debate about the geological history and features of the Three Sisters volcanic region of Oregon, USA.
- In 1925, Edwin T. Hodge thought a large volcano named Mount Multnomah once existed in the Three Sisters region before it was destroyed by violent volcanic explosions.
- The term “supervolcano” was popularised by the BBC popular science television program Horizon in 2000.
- The term “megacaldera” is occasionally used for caldera supervolcanoes, such as the Blake River Megacaldera complex in the Abitibi greenstone belt of Ontario and Quebec, Canada.
 |
| Map of large Flood Basalt igneous provinces worldwide |
- Large igneous provinces are immense regions of basalts that result from flood basalt eruptions. e.g. Iceland, the Siberian Traps, Deccan Traps, and the Ontong Java Plateau.
- Keller (2014) hypothesised the Réunion hotspot produced the Deccan Traps around 66 million years ago, which coincided with the Cretaceous-Paleogene extinction event. Although the scientific consensus is the extinction event was the result of a meteor impact, the volcanic activity may have been a result of environmental stresses on extant species up to the Cretaceous-Paleogene boundary.
- Furthermore, the largest flood basalt event (the Siberian Traps) occurred around 250 million years ago, which coincided with the largest mass extinction in history (i.e. the Permian-Triassic extinction event. However, it is unknown whether it was merely responsible for the extinction event.
G. Underwater volcanoes
https://en.wikipedia.org/wiki/Submarine_volcano
H. Subglacial volcanoes
https://en.wikipedia.org/wiki/Subglacial_volcano
https://en.wikipedia.org/wiki/Submarine_volcano
- Also known as submarine volcanoes, underwater volcanoes are vents or fissures beneath the sea surface in the Earth’s surface from which magma erupts. Martin et al. (2013) estimated about 75% of Earth’s magma output originated from volcanoes situated at mid-ocean ridges.
- e.g. Kolumbo submarine volcano in the Aegean Sea near the island of Santorini.
 |
Scheme of a submarine eruption: 1) Water vapour cloud; 2) Water; 3) Stratum; 4) Lava flow; 5) Magma conduit; 6) Magma chamber; 7) Dike; 8) Pillow lava |
- Water influences the characteristics of underwater volcanic eruptions and its subsequent explosions.
- Logically, water rapidly cools and solidifies magma, transforms it into volcanic glass. When lava contacts lava, it forms a solid crust around it with advancing lava trickling into it.
- A majority of submarine volcanoes are seamounts, which are extinct volcanoes erupting abruptly from a seafloor of 1,000 - 4,000 meters depth.
- There are approximately more than 30,000 seamounts worldwide, with many more yet to be discovered.
- There are 2 types of submarine eruptions: (1) Formed by the slow release and burst of large lava bubbles, or (2) formed by a rapid explosion of gas bubbles.
https://en.wikipedia.org/wiki/Subglacial_volcano
 |
| This diagram illustrates a small volcano erupting under an ice sheet, the major rock types formed, meltwater lake and principal meltwater escape routes. |
Also known as glaciovolcanoes, subglacial volcanoes form by eruptions under the surface of a glacier of ice sheet that is subsequently melted into a lake by the rising lava.
- As the subglacial volcano erupts, the lava’s heat melts the overlying ice, producing water.
- The water rapidly cools the lava, resulting in pillow lava shapes.
- When the pillow lava breaks off and flow down the volcano slopes, pillow breccia, tuff breccia, and hyacloclastite form.
- If the meltwater is released from beneath the ice layer, it results in a large glacial lake outburst flood.
- Subglacial volcanoes are commonly found in Iceland, Antarctica, as well as British Columbia and Yukon Territory, Canada.
- Subglacial eruptions often cause jökulhlaups, or tremendous floods of water. e.g. In November 1996 the eruption of the Grímsvötn Volcano beneath the Vatnajökull ice sheet severely damaged bridges and subsequent jökulhlaup covered more than 750 km2 (290 sq mi).
- In 2008, Corr & Vaughan suggested a subglacial volcano occurred beneath the Antarctic ice sheet around 2200 years ago.
I. Mud volcanoes
https://en.wikipedia.org/wiki/Mud_volcano
(1) >63% silica, felsic lava
(2) 52-63% silica, intermediate lava
— A’a = A rough surface and a typical texture of viscous lava flows.
— Pāhoehoe = Smooth and viscous surface, formed from more fluid lava flows.
https://en.wikipedia.org/wiki/Mud_volcano
- A mud volcano or mud dome is a landform produced by the eruption of mud or slurries, water or gasses.
- They can be caused by a piercement structure produced by a pressurised mud diapir that ruptures the Earth’s surface or ocean bottom.
- The temperature of the mud volcano may be as low as the freezing point of the ejected materials, since venting associates with the production of hydrocarbon clathrate hydrate deposits. Moreover, mud volcanoes are linked to petroleum deposits, as well as tectonic subduction zones and orogenic belts.
- Close mud domes also emit incombustible gases including helium, whereas lone mud domes are emit methane.
- Approximately 1100 mud volcanoes have been identified on Earth on land and in shallow water. Researchers estimated over 10,000 mud volcanoes may exist on continental slopes and abyssal plains.
Their features include:
— Gryphon = A steep-sided cone shorter than 3 metres that emits mud.
— Mud cone = A high cone shorter than 10 metres that emits mud and rock fragments.
— Scoria cone = Formed by heating of mud deposits during fires.
— Salse = Water-dominated pools with gas seeps.
— Spring = Water-dominated outlets smaller than 0.5 metres.
— Mud shield
Their emissions include:
— In 2002, Dimitrov, and Etiope & Klusman estimated about 10-20 Tg/yr of of methane is emitted from onshore and shallow offshore mud volcanoes.
— Milkov et al. (2003) estimated the global gas flux to be around 33 Tg/yr (15.9 Tg/yr during quiescent periods plus 17.1 Tg/yr during eruptions), and 6 Tg/yr of greenhouse gases from onshore and shallow offshore mud volcanoes. Furthermore, deep water sources emit roughly 27 Tg/yr.
— Kopf (2003) estimated 1.97×1011 to 1.23×1014 m³ of methane is emitted by all mud volcanoes per year, of which 4.66×107 to 3.28×1011 m³ by surface volumes. This is equivalent to 141k - 88k Tg/yr from all mud volcanoes, of which 0.033–235 Tg is from surface volcanoes.
What do volcanoes discharge?
A. Lava composition(1) >63% silica, felsic lava
- Felsic lava such as dacites or rhyolites are more viscous and are erupted as domes or short, stubby flows. They tend to form stratovolcanoes or lava domes. e.g. Lassen Peak in California.
- Since siliceous magma is viscous, it traps volatiles, resulting in a catastrophic magma eruption, forming stratovolcanoes.
- Pyroclastic flows (ignimbrites) are hazardous products of such volcanoes, as they comprise of molten volcanic ash that is too heavy to be carried into the atmosphere by the air. e.g. Alaska’s Valley of Ten Thousand Smokes, created by the eruption of Novarupta near Katmai in 1912.
(2) 52-63% silica, intermediate lava
- Andesitic volcanoes generally occur above subduction zones e.g. Mount Merapi, Indonesia.
- They are typically created at convergent boundary margins of tectonic plates.
- Higher proportions of magnesium (Mg) and iron (Fe), or basaltic.
- Less viscous than rhyolitic lavas, depending on their eruption temperature.
- Hotter than felsic lavas
- They occur at mid-ocean ridges, shield volcanoes and continental flood basalts.
- Also known as komatiites, they are extremely rare.
- Not many have erupted at the Earth's surface since the Proterozoic Era, back when the planet’s heat flow was higher. B. Lava texture
— A’a = A rough surface and a typical texture of viscous lava flows.
— Pāhoehoe = Smooth and viscous surface, formed from more fluid lava flows.
What are the different classifications of volcanic activity?
A. Active- There is no consensus among volcanologists on the definition of an “active” volcano. The matter of fact is the volcano’s lifespan ranges from months to several million years.
- Although many of Earth’s volcanoes haven’t demonstrated signs of eruption in recent times, they have erupted numerous times in the past few thousand years.
- Suggested definitions include “erupted in the last 10,000 years” (Holocene times) and historical time (or recorded history).
- The European Space Agency stated most active volcanoes are situated on the Pacific Ring of Fire.
- The International Association of Volcanology estimated more than 500 active volcanoes worldwide. e.g. In China and the Mediterranean, the history of active volcanoes date back approximately 3000 years, whereas in the Pacific Northwest of the USA and Canada, its history dates back to less than 300 years, and around 200 years in Hawaii and New Zealand.
— Kīlauea, Hawaii
— Piton de la Fournaise, Réunion
— Mount Yasur
— Mount Etna
— Stromboli
— Santa María
— Sangay
— Mount Nyiragongo & its neighbour, Nyamuragia in Africa
— Erta Ale in the Afar Triangle
— Mount Erebus, Antarctica
— Mount Merapi
— Whakaari, White Island
— Ol Doinyo Lengai
— Ambrym
— Arenal Volcano
— Pacaya
— Klyuchevskaya Sopka
— Sheveluch
B. Dormant / Inactive / Reactivated
They are defined as “not erupted for thousands of years, but may erupt again in the future”.
They are defined as “not erupted for thousands of years, but may erupt again in the future”.
— Lake Yellowstone = A recharge period of around 700,000 years
— Lake Toba = A recharge period of around 380,000 years.
— Vesuvius volcano
— Mount Pinatubo
— Soufrière Hills volcano on the island of Montserrat
— Fourpeaked Mountain in Alaska
C. Extinct
They are defined as “unlikely to erupt again due to the lack of a magma supply”.
Examples of extinct volcanoes include:
— Those on the Hawaiian-Emperor seamount chain in the Pacific Ocean
— Hohentwiel, Germany
— Shiprock, New Mexico
— Zuidwal volcano, Netherlands
— Monte Vulture, Italy
— One beneath Edinburgh Castle, Scotland
Where are the most dangerous volcanoes?
— Avachinsky-Koryaksky, Kamchatka, Russia
— Nevado de Colima, Jalisco and Colima, Mexico
— Mount Etna, Sicily, Italy
— Galeras, Nariño, Colombia
— Mauna Loa, Hawaii, USA
— Mount Merapi, Central Java
— Mount Nyiragonga, Democratic Republic of the Congo
— Mount Rainier, Washington, USA
— Sakurajima, Kagoshima Prefecture, Japan
— Santa Maria / Santiaguito, Guatemala
— Santorini, Cyclades, Greece
— Taal Volcano, Luzon, Philippines
— Teide, Canary Islands, Spain
— Ulawun, New Britain, Papua New Guinea
— Mount Unzen, Nagasaki Prefecture, Japan
— Vesuvius, Naples, Italy
B. Hydrological Disasters
i. Floods
https://en.wikipedia.org/wiki/Flood
https://en.wikipedia.org/wiki/List_of_deadliest_floods
https://www.youtube.com/watch?v=4PXj7bOD7IY&ab_channel=NationalGeographic
Floods are defined as overflows of water that immerse usually dry land. The word “flood” originates from the Old English flod, derived from Germanic.
What causes floods?
i. Upslope factors
- The amount, location and timing of water flowing towards a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow of water at downstream regions.
- Precipitation vents such as evaporation, percolation through the soil, sequestering as snow or ice, rapid runoff from surfaces such as pavement, rock, rooftops, and saturated or frozen ground.
- The frequency of a precipitation threshold of interest is determined by the the number of measurements that exceed such threshold values within the total time period from which observations are noted.
- Each data point of measured depth is divided by the period of time between observations, this gives the intensity. This parameter would be less than the actual peak intensity if the duration of the rainfall event lasted less than the fixed time interval for which measurements are reported.
- Land area of the the watershed upstream of the area of interest is an important upslope factor in determining flood magnitude.
- For watersheds of less than approximately 80 km2 (30 mi2), rainfall intensity and the main channel slope have to be considered.
- Time of Concentration (TC) is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel that controls flooding of the area of interest. Urquhart (1959) defined TC as the critical duration of peak rainfall for the area of interest
- Downstream conditions that limit coastal flooding lands include the ocean or coastal flooding bars forming natural lakes.
- In flooding low lands, elevation changes such as tidal fluctuations influence the level of coastal and estuarine flooding.
- Less predictable events such as tsunamis and storm surges.
- The flow channel’s geometry, depth of channel, speed of flow and amount of sediments within the water all influence of elevation of flowing water.
- Flow channel restrictions such as bridges and canyons may limit the water elevation above such restrictions.
- Growth of vegetation, accumulation of ice or debris, or construction of bridges, buildings, or levees within the flood channel may influence effective flood channel geometry.
- Unusually intense, warm rainfall melting heavy snow packs that obstruct channels from floating ice, and release small impoundments like beaver dams are coincidental events that lead to extreme flood events.
- Coincident events occur more frequently than anticipated from simplistic statistical prediction models, since such models consider only precipitation runoff flowing within unobstructed drainage channels.
- Using measurements from the 2010-11 Queensland floods, Brown et al. (2011) demonstrated any criterion merely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by velocity and water depth fluctuations. Since they ignore the subsequent risks associated with large debris entrained by the flow motion.
What are the effects of floods?
i. Primary effects
- Loss of life and damage to buildings and other infrastructure such as bridges, sewerage systems, roadways and canals.
- Damage to power transmission and power generation, plus knock-on effects.
- Loss of drinking water treatment and water supply, leading to a loss of drinking water or severe water contamination.
- Loss of sewage disposal facilities.
- Lack of clean water combined with human sewage in the flood waters increases the risk of waterborne diseases, such as typhoid, giardia, cryptosporidium, and cholera etc.
- Damage to roads and transport infrastructure.
- Inundation of farm land, making the land unworkable and preventing farmers from planting or harvesting crops. This leads to food shortages for both humans and farm animals.
- Economic hardship due to a temporary decline in tourism, rebuilding costs or food shortages, leading to price hikes.
- Subsequent psychological damage to those affected, as well as property damage.
- Waterlogged houses, leading to the development of indoor mould and adverse health effects, particularly respiratory symptoms.
- Economic hardships such as reduced property values by 10-25%.
- Forced closures of 40% of affected businesses following a flood.
- In the USA, insurance is available against flood damage to both homes and businesses.
iii. Benefits
- Recharging ground water, increasing the fertility and amount of nutrients in soils.
- Provide water resources in arid and semi-arid regions of uneven distribution of precipitation throughout the year.
- Eliminates pests in farmlands.
- Maintains ecosystems in river corridors, as well as floodplain biodiversity.
- Spreads nutrients to lakes and rivers, increasing biomass and improving fisheries.
- Provides suitable locations for spawning fish species with few predators and elevated levels of of nutrients or food.
- Some fish species, such as weather fish, swim along flood waters to approach new habitats. The increased spawning of fish also makes it an abundant food source for birds.
- Improved viability of hydropower, a renewable source of energy.
What are the different types of floods?
i. Areal
- If water accumulates in flat or low-lying areas due to heavy rainfall or snowmelt that is more rapid that the normal rate of infiltration or running off, floods occur.
- When surface soil saturates, it effectively stops infiltration. This is due to the water table being shallow, or from intense rain from one or a series of storms.
- Areal flooding starts in flatter areas such as floodplains and in local depressions not linked to a stream channel.
- Jones (2000) thought endorheic basins experience areal flooding during periods during which precipitation exceeds evaporation.
- From the smallest ephemeral streams in humid zones to dry channels in barren climates to the world’s largest rivers, floods can occur in all types of river and stream channels.
- On tilled lands, overland flow can cause muddy floods where sediments are hoisted by run off and carried as suspended matter or bed load.
- Drainage obstructions such as landslides, ice, debris, or beaver dams can exacerbated localised flooding.
- Slow-rising floods often occur in large rivers with large catchment areas. They may be caused by sustained rainfall, fast snow melt, monsoons, or tropical cyclones.
- Rapid flooding events, including flash floods, often occur on smaller rivers, rivers with steep valleys, rivers flowing over impermeable terrain, or dry channels. This may be a result of localised convective precipitation (intense thunderstorms) or an instantaneous release of water from an upstream impoundment generated behind a dam, landslide, or glacier.
- Hjalmarson (1984) measured the flow rate of a flood increased from approximately 50 to 1500 ft
3/s (1.4 to 42 m3/s) in 1 minute.
iii. Coastal & Estuarine
- In estuaries, flooding occurs as a result of a combination of storm surges caused by winds and low barometric pressure and gigantic waves meeting high upstream river flows.
- In coastal areas, flooding occurs due to storm surges combined with high tides and tremendous wave events situated at sea. This leads to waves cascading flood defences, and in worst cases by tsunami or tropical cyclones.
iv. Urban floods
- Inundation of land or property in a built environment, especially in more densely populated areas. This is a result of heavy rainfall that overwhelms the capacity of drainage systems, such as storm sewers.
- A 2013 report by Center for Neighborhood Technology, Chicago IL, defined urban flooding as a condition that is characterised by its repetitive and systemic impacts on communities. This can occur regardless of the communities’ location relative to the designated floodplains or any body of water.
- Besides from potential overflow of rivers and lakes, substantial snowmelt, stormwater or water released from damaged water mains may accumulate on property and in public rights-of-way, leak through building walls and floors, or enter buildings through sewer pipes, toilets and sinks.
- In urban areas, existing paved streets and roads can increase the speed of flowing water.
- Impervious surfaces can block rainfall from infiltrating into the ground, thus resulting in a higher surface run-off that exceeds local drainage capacity.
- e.g. Nîmes, France (1998); Vaison-la-Romaine, France (1992); New Orleans, USA (2005), Rockhampton, Bundaberg, Brisbane in Queensland, Australia (2010-11)
v. Catastrophic
Associated with major infrastructure failures such as dam collapses, or drainage channel modifications from a landslide, earthquake, or volcanic eruption, as well as tsunamis.
When and where were the deadliest floods?
Flood models:
https://www.youtube.com/watch?v=HPhLJhhOiMs&ab_channel=ImbaPixel
ii. Tsunamis
https://en.wikipedia.org/wiki/Tsunami
https://www.youtube.com/watch?v=Wx9vPv-T51I
What causes tsunamis?
Haugen et al. (2005) stated the tsunami’s principal generation mechanism is the displacement of a substantial volume of water or perturbation of the sea. Studies accounted this displacement of water to either earthquakes, landslides, volcanic eruptions or glacier calvings.
https://www.youtube.com/watch?v=Wx9vPv-T51I
- Coming from the Japanese word tsunami 津波, meaning “harbour wave”, it is a series of waves in a body of water caused by the displacement of a large volume of water, often in an ocean or a large lake.
- They are occasionally referred to as ‘tidal waves’. Although both tsunamis and tidal waves involve inland movement of water in a wave-like pattern, the impact of tsunamis are significantly greater. The term ‘tidal wave’ has decreased in usage by scientists, since the causes of tsunamis are unrelated to tides. Note that tides are created by the gravitational pull of the moon and sun rather than the displacement of water.
- When a tsunami follows an earthquake situating in the ocean, the term seismic sea wave is occasionally used since tsunami waves are generated by seismic activity such as earthquakes.
Haugen et al. (2005) stated the tsunami’s principal generation mechanism is the displacement of a substantial volume of water or perturbation of the sea. Studies accounted this displacement of water to either earthquakes, landslides, volcanic eruptions or glacier calvings.
https://www.youtube.com/watch?v=KB-TO5kq5Aw
https://www.youtube.com/watch?v=B7UULBTArLY&ab_channel=Ridddle
https://www.youtube.com/watch?v=SlwZzbGh7Cw&ab_channel=SamuelR45%3ATheWebUnlocked
https://www.youtube.com/watch?v=Wx9vPv-T51I&t=1s&ab_channel=TED-Ed
i. Seismicity
https://www.youtube.com/watch?v=B7UULBTArLY&ab_channel=Ridddle
https://www.youtube.com/watch?v=SlwZzbGh7Cw&ab_channel=SamuelR45%3ATheWebUnlocked
https://www.youtube.com/watch?v=Wx9vPv-T51I&t=1s&ab_channel=TED-Ed
i. Seismicity
- Tsunamis are often generated by the sudden deformation of the sea floor and vertical displacement of the overlying water. When a tectonic earthquake occurs underwater, the water above the deformed area displaces from its equilibrium position.
- When thrust faults related to convergent or destructive plate boundaries move rapidly, this leads to significant water displacement vertically, creating a tsunami.
- Studies found geological movements on normal faults also lead to seabed displacement, with only the largest causing devastating tsunamis e.g. 1977 Sumba and 1933 Sanriku.
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| 1. Drawing of tectonic plate boundary before earthquake. 2. Over-riding plate bulges under strain, causing tectonic uplift. 3. Plate slips, causing subsidence and releasing energy into water. 4. The energy released produces tsunami waves. |
- A 2013 study by Australian Geographic observed tsunamis have shorter wave heights offshore, and long wavelengths (~ 100+ km).
- As they approach shallower water, the wave grows taller, a process known as wave shoaling.
Examples:
— 8.6 Magnitude Aleutian Islands earthquake-triggered tsunami that struck Hilo on the island of Hawaii.
— Storegga (~ 8000 years ago)
— Grand Banks (1929)
— Papua New Guinea (1998)
ii. Landslides
- In 1958, a tremendous landslide in Lituya Bay, Alaska, triggered the largest tsunami ever recorded, with a height of 524 m (1719 ft). However, it didn’t travel long distances since it reached land virtually immediately.
- In 1963, a landslide in Monte Toc triggered a tsunami in the reservoir behind the Vajont Dam, Italy. The wave swept over the 262-metre (860 ft)-high dam by 250 metres (820 ft) and destroyed the adjacent towns, killing around 2,000 people.
- Although landslides can displace water in shallower regions of the coastline, there haven’t been any recorded events of large landslides triggering a transoceanic tsunami.
iii. Meteorological
- Some meteorological conditions, such as sharp changes in barometric pressure, can displace adequately copious amounts of water to trigger tsunamis, albeit with lower potential energies.
- When these so-called meteotsunamis reach the shore, resonance can amplify its impact to cause localised damage and threaten lives.
- Monserrat et al. (2006) listed the following examples of devastating meteotsunamis:
— 15 June 2006 Menorca
iv. Man-made or triggered tsunamis
- In World War II, the New Zealand Military Forces initiated Project Seal, which attempted to use explosives to generate small tsunamis in a location, today known as Shakespear Regional Park. Unfortunately, the project was deemed a failure.
- In the Pacific Proving Ground, an American nuclear test programme called Operation Crossroads detonated two 20 kilotonnes of TNT (84 TJ) bombs, 1 in the air and 1 underwater, above and below the shallow waters of the Bikini Atoll lagoon, 50 m (160 ft) deep. After its detonation around 6 km (3.7 mi) from the nearest island, the maximum height of the waves were around 3–4 m (9.8–13.1 ft) at the shoreline.
- Glasstone & Dolan (1977) reported the effects of shallow and deep underwater explosions doesn’t generate adequate energy to assimilate ocean tsunami waveforms. They found a large proportion of the energy produces steam, resulting in vertical fountains above the water, thus generating compressional waveforms.
What are the characteristics of tsunamis?
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| This diagram shows the wave slows down and its amplitude (height) increases as it enters shallow water. |
- Tsunamis have 2 damaging mechanisms: the forceful destruction of high-speed water, and the devastating power of a sheer volume water draining off the land that carries a significant amount of debris with it.
- Tsunamis in the deep ocean have wavelengths of up to 200 km (120 mi), travelling at faster than 800 km/h (500 mph). At any given point, the wave oscillation takes 20 or 30 mins to complete 1 cycle and its amplitude is about 1 metre (3.3 ft).
- The velocity of a tsunami is evaluated through the square root of the water’s depth (h) multiplied by the gravitational acceleration (g). vt = (√h)*g
- In fluid dynamics, Green’s law states that as a tsunami approaches the coast and shallower waters, wave shoaling compresses the wave as well as decelerates it to below 80 km/h (50mph). Furthermore, its wavelength shortens to less than 20 km (12 mi) and its amplitude increases tremendously.
- As the tsunami’s wave peak approaches the shore, it is known as the ‘run up’, which is measured in metres above a reference sea level.
- All waves have a positive and negative peak, i.e. a ridge and a trough, respectively.
- If the ridge arrives at the shore first, an enormous breaking wave or flash flooding would be the most immediate event impacting on land.
- If the trough arrives at the shore first, a drawback occurs as the shoreline recedes, which exposes usually submerged areas.
The first scales used to measure the intensity of tsunamis were the Sieberg-Ambraseys (1962) scale, used in the Mediterranean Sea, and the Imamura-Iida intensity scale (1963), used in the Pacific Ocean. The latter scale was modified by Soloviev (1972) that calculated the tsunami intensity "I" according to the formula:
I = 0.5 + log2(Hav)
— Hav = Average tsunami height along the nearest coastline (m)
— This scale is referred to as the Soloviev-Imamura tsunami intensity scale.
The first scale that calculated a tsunami’s magnitude was proposed by Murty & Loomis based on the potential energy a wave carried. However, there was difficulty in calculating the tsunami’s potential energy, which rendered the scale unused. Abe introduced the tsunami magnitude scale (Mt), using the formula:
Mt = a*log(h) + b*log(R) + D
— h = Maximum tsunami-wave amplitude (m) measured by a tide gauge at a distance R from the epicentre.
— R = Distance from the epicentre
— a, b & D = Constants used to make the Mt scale to be as closely as possible the moment magnitude scale.
How is the tsunami height measured?
2011 Tōhoku earthquake Tsunami
- Amplitude, Wave Height, Or Tsunami Height = Height relative to the normal sea level.
- Run-up, or Inundation Height = Tsunami’s max height on the ground above sea level.
- Flow Depth = Tsunami’s height above ground, regardless of the height of the location or sea level.
- (Maximum) Water Level = Maximum height above sea level as viewed from trace or water mark.
Videos of tsunamis:
2004 Boxing Day Indian Ocean Tsunami
2004 Boxing Day Indian Ocean Tsunami
https://www.youtube.com/watch?v=86ThCibkHQw&ab_channel=Rachid82UK
1963 Vajont Dam, Italy Tsunami
iii. Limnic Eruptions
Also known lake overturn, a limnic eruption is a natural disaster that involves dissolved carbon dioxide (CO2) abruptly erupting from deep lake waters, producing a gas cloud that may suffocate wildlife, livestock, and humans.
What are the causes of limnic eruptions?
- So far, the causes of limnic eruptions are poorly understood. Scientists theorise earthquakes, volcanic activity, and other explosive events may be possible triggers for limnic eruptions.
- Researchers suggested a balance between volcanic heat flux and atmospheric cooling, as well as rain and evaporation are required for limnic eruptions to occur.
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What are the effects of limnic eruptions?
- A limnic eruption creates a CO2 cloud above the lake and disperse to the surrounding region.
- Since CO2 is denser than air, it falls down towards the ground, and simultaneously displace the breathable air, leading to CO2 poisoning, hence asphyxia.
- Other reported symptoms include skin blisters (possibly due to low blood oxygen levels).
- Lake Nyos (1986)
- Lake Kivu hasn’t experienced a limnic eruption yet, however researchers have found higher rates of methane dissociation and an increasing surface temperature by about 0.12 degrees C per decade.
Part 2 will cover cyclonic storms and thunderstorms.
