Vulcanicity and Tectonics (AQA A Level Geography)
Revision Note
Written by: Rhiannon Molyneux
Reviewed by: Bridgette Barrett
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
Examiner Tips and Tricks
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
Examiner Tips and Tricks
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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