Vulcanicity and Tectonics (AQA A Level Geography)

Revision Note

Distribution of Volcanic Hazards

  • Most active volcanoes occur at or near plate boundaries

  • Many (about 75%) occur around the 'Ring of Fire' surrounding the Pacific Ocean

  • Vulcanicity (volcanic activity) is focused on constructive and destructive plate margins

    • At constructive margins, eruptions tend to be small and >effusive 

    • The eruptions are usually basaltic lava:

      • Low gas content

      • Low viscosity

      • Higher temperature

    • At destructive margins, eruptions tend to be explosive as the magma is forcing its way to the surface

    • These eruptions are often andesitic or rhyolitic lava:

      • High gas content

      • High viscosity

      • Lower temperature

  • Volcanoes can also be found at hot spots in the middle of plates, like Hawaii in the central Pacific

distribution-of-volcanic-eruptions
Global distribution of active volcanoes

Examiner Tip

It is important that you are confident to describe distribution.

  • What is the general pattern?

  • Does the pattern relate to anything else for example the location of plate boundaries?

  • Are they near the equator or further away?

  • Are they inland or coastal?

Use map features to help with your description - place names, compass rose, latitude and longitude

Magnitude of Volcanic Hazards

  • Some volcanic eruptions are gentle and effusive while others are hugely explosive

    • This is dependent on the magma composition

  • The Volcanic Explosivity Index (VEI) is used to measure the magnitude of an eruption

  • This is a logarithmic scale from 0-8 

  • It cannot be measured on a scientific instrument so is calculated based on a series of measurements and observations

    • These include:

      • Height of material ejected into the atmosphere

      • Volume of material

      • Duration of the eruption

vei
Volcanic Explosivity Index

Examiner Tip

Make sure that you can explain the difference between volcanoes at destructive and constructive margins. 

Volcanoes found at destructive margins erupt less frequently and more explosively. This is due to more viscous, andesitic magma which prevents gas from escaping and leads to a greater build-up of pressure. 

Volcanoes found at constructive margins erupt more frequently and effusively. This is due to less viscous, basaltic magma that allows gas to escape easily, preventing the build-up of pressure.

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