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Volcano Explosivity Index 

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VEI Volcanic xplosivity Index USGSThe Volcano Explosivity Index or VEI has 8 levels of volcanic eruption each level being 10x greater than the last, similar to the Richter Scale, it is a measure of the size of volcanic eruption and is based on the volume of erupted pyroclastic material (ashfall, pyroclastic flows), height of eruption column and how long the eruption lasts. The scale shows how some known volcanic eruptions appear on the scale. Even our largets modern times eruptions are small compared to pre-history eruptions.

The eruption of Mt St Helens in 1980 has a VEI of 4 , a Yellowstone or Toba eruption would be an 8 - 1000 times greater.