Vulcanicity and Tectonics (AQA A Level Geography)

• 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

Global distribution of active volcanoes

• 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

Volcanic Explosivity Index