Vocabulaire FormaTerre: Instruments: Volcanic Explosivity Index

Volcanic Explosivity Index

First proposed by Newhall and Self (1982), a now widely used, open-ended semi-quantitative classification scheme - based principally on the height of the eruption column and the erupted volume of tephra - to describe the size of explosive eruptions. The largest historical eruption (Tambora, Indonesia, in 1815) is assigned a VEI = 7; for comparison, the 18 May 1980 eruption of Mount St. Helens was ranked as VEI = 5. However, much larger explosive eruptions in the geologic past (e.g., voluminous caldera-forming events at Yellowstone volcanic system, Wyoming) can be assigned VEI = 8. Non- explosive eruptions are VEI = 0, regardless of size. As with earthquakes, small to moderate-size explosive volcanic events (< VEI = 5) occur much more frequently than large events (> VEI = 5); Simkin and Siebert (1994) have assigned VEI estimates for all the world’s known eruptions during the last 10,000 years. In terms of the energy involved, the energy of the 1980 Mount St. Helens (VEI = 5) eruption has been estimated to be about 2x1016 J, which is roughly the amount of seismic energy released by an Mw = 8 earthquake. The energy of the 1815 Tambora eruption (VEI = 7) has been estimated to be about 1x1019 J, and is thus comparable to the seismic energy of the 1960 Chilean earthquake (Mw = 9.5), the largest instrumentally recorded earthquake so far.

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