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

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