Volcanoes (DP IB Geography)
Revision Note
Characteristics of a Volcano
A volcano is formed when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust
The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI):
The scale is open-ended but the highest in recorded human history was an 8 (Tambora 1815)
All volcanoes have features in common, as shown in the diagram below:
Volcanic eruptions may include a range of features such as the ones listed in the table below
Features of an eruption
Feature | Characteristics |
---|---|
Lava | When magma erupts to the surface it is known as lava. The lava can be thin and runny or thick and slow moving. This depends on the composition of the magma |
Ash | Ash is pulverised solid lava that measures less than 2mm in diameter. It is ejected into the atmosphere and can travel thousands of kilometres |
Earthquakes | Magma rising to the surface through the vents in the volcano increases pressure on the Earth's crust, leading to earth tremors |
Volcanic bombs | These are fragments of molten rock that are ejected from the volcano. They are between 60mm and 5m in diameter |
Due to the different types of eruption and lava, the formation of a volcano varies
Composite volcanoes
Composite (strato-volcanoes):
Steep-sided
Sticky (viscous) lava
More explosive eruptions
Formed from alternating layers of ash and lava
Tend to form on convergent (destructive) plate boundaries
Shield volcanoes
Shield volcanoes:
Gently sloping sides
Runny/thin lava
Less explosive – gentle eruptions
Tend to form on divergent (constructive) plate boundaries or hot spots
Frequent eruptions
Cinder volcanoes
Cinder volcanoes are:
Circular and cone shaped
Usually less than 1000 feet high
Composed of hardened ash, tephra and lava
The lava is forcefully ejected from the volcano and cools whilst in the air, falling as cinder fragments
Secondary Hazards of Volcanoes
There are a range of secondary hazards associated with volcanic eruptions
Lahars – mudflows that occur when tephra mixes with water, either from rainfall or from melted snow and ice
They are fast-flowing and destroy everything in their path
E.g. the lahars that occurred when Mount Pinatubo erupted in the Philippines in 1991 caused extensive damage and disruption
Lahars are usually associated with composite volcanoes
Pyroclastic flow – a super-heated mixture of gas and tephra that flows at speeds of up to 700km per hour:
This is one of the deadliest volcanic hazards as the pyroclastic flow can travel long distances and destroy everything in its path
E.g. when Fuego volcano erupted in Guatemala in 2018, pyroclastic flows destroyed several nearby towns
Pyroclastic flows are usually associated with composite volcanoes
Landslides – the mass movement of materials down a slope occurring when the rock is no longer able to resist the force of gravity:
This can occur during volcanic eruptions due to the movement of the crust
Landslides can also cause a tsunami if the debris falls into the sea
Examiner Tip
You may be asked in the exam to examine the relationship between secondary hazards and the type of volcano. For example, pyroclastic flows are usually the result of highly explosive eruptions from composite volcanoes.
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