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

• 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