Volcanoes (1.1.4)

Volcanoes occur on constructive and destructive plate boundaries where plates melt and magma erupts through a plate. Alternatively, they may occur on hotspots too.

Volcanoes form when magma (molten rock from beneath the Earth's crust) reaches the surface. The magma erupts to form lava.

Hotspots: areas of volcanic activity where hot magma plumes from the mantle rise and burn through weaker parts of the crust, (possibly) creating volcanoes and islands. The plume stays in place as the plate continues to move, sometimes causing a chain of islands (e.g. Hawaii).

Structure Of Volcanoes

magma chamber - where the molten rock is stored beneath the ground
main vent - channel through which magma travels to reach the Earth's surface
secondary vent - some magma may escape through the side of the volcano, particularly if the main vent becomes blocked
crater - found at the top of the volcano, where the magma erupts from

Volcanic Effects

Ash from large volcanoes has been known to affect global climates

geothermal energy (heat within the Earth used to generate electricity) can be generated in areas where magma lies close to the surface → good for increasing renewable energy usage
ash ejected by volcano acts as a good fertiliser for soils
volcanoes attract many tourists
volcanoes are dangerous - can kill people and damage property
economic activity can suffer as it's hard for businesses to operate post-eruption
habitats and landscapes are damaged by lava flows

Explosivity Of Volcanoes

If a volcano has been dormant for a long time, it would be more explosive as there has been a long build-up of pressure. In contrast, if a volcano erupts frequently, it will be less explosive as there wouldn't be as much pressure (e.g. volcanoes in Hawaii).

Volcanoes are more explosive where the Earth's crust is weaker and thinner or where there are more fractures.

Volcanoes that contain basaltic magma are more explosive as it as a low viscosity (than rhyolitic magma). Basaltic magma also contains less gas so there is less pressure.

The more magma, the more explosive the volcano can be.

The bigger the volcano, the bigger the eruption, the less frequent the eruption.

Earthquakes can trigger volcanoes.

Volcanoes happen more on a destructive margin:

the plate that's being subducted melts (oceanic), feeding into the magma chamber. More magma in the magma chamber = increased pressure
more build-up of pressure at destructive margins
in contrast, no pressure at conservative margins, because the crust isn't being destroyed/weakened so the molten magma can't reach the surface

Volcanic Hazards

lava flows - silica makes lava viscous and slow, which is common in explosive eruptions
- viscosity: resistance to flow
lahars (mudflows) - caused by various reasons, usually by melting ice at high latitudes
glacial floods - when temperatures are high from magma, glaciers or ice sheets at high temperatures quickly melt and a large amount of water is discharged
tephra - any type of rock that is ejected by a volcano
toxic gases - released during some eruptions, even CO₂ can be toxic as it can replace oxygen since it's heavier
acid rain - caused when gases like sulfur dioxide are released into the atmosphere
pyroclastic flows - clouds of burning hot ash and gas that collapses down a volcano at high speeds (average speed of 60mph but can reach 430mph)
volcanic bombs - large blocks of hot rocks thrown from a volcano

Environmental Volcanic Hazards

- ecosystems damaged through various volcanic hazards (primary effects)

- wildlife killed

- water acidified from acid rain (secondary effects)

- volcanic gases contribute to greenhouse effect (global warming)

Economic Volcanic Hazards

- businesses and industries destroyed/disrupted (primary effects)

- jobs lost (secondary effects)

- profit from tourism industry

Social Volcanic Hazards

- people killed (primary effects)

- homes destroyed from lava/pyroclastic flows

- fires can start putting lives at risk (secondary effects)

- mudflows/floods

- trauma

- homelessness

Political Volcanic Hazards

- government buildings and other important areas destroyed/disrupted (primary effects)

- conflicts concerning government response, food shortages, insurance etc. (secondary effects)

The magnitude of volcanoes is measured using the Volcanic Explosivity Index (VEI) - the more powerful, the more explosive.

Volcanoes are classed as either active, dormant or extinct. Usually, a higher frequency eruption means they are effusive, whereas low frequency means they are explosive (due to build up and pressure).

effusive = calmer, lower magnitude eruptions
explosive = intense high magnitude eruptions

Volcanic eruptions are normally regular, in the sense that the eruptions on each type of boundary are similar (e.g. eruptions on destructive boundaries will regularly be explosive).

The regularity of eruptions can help estimate when eruptions will take place (i.e. every 10 years). Seismic activity, gases releasing, elevation etc. can all indicate an imminent eruption, but there are no definite predictions to a volcanic eruption.

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Geography Note on Paper 1: Living with the physical environment.