Volcanoes (SL IB Geography)

• 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:

Volcano characteristics

• 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

Composite volcano

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

Shield volcano

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

• There are a range of secondary hazards associated with volcanic eruptions

Volcanic hazards

• 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

Map showing the extent of lahars in an eruption of Mount Pinatubo

• 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