Volcanic Explosivity Index

USGS PHOTO GLOSSARY OF VOLCANIC TERMS
The Volcanic Explosivity Index, or VEI, was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions. It is a 0-to-8 index of increasing explosivity. Each increase in number represents an increase around a factor of ten. The VEI uses several factors to assign a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms.
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In the figure, the volumes of several past explosive eruptions and the corresponding VEI are shown. Numbers in parentheses represent total volume of erupted pyroclastic material (tephra, volcanic ash, and pyroclastic flows) for selected eruptions; the volumes are for uncompacted deposits. Each step increase represents a ten fold increase in the volume of erupted pyroclastic material.A series of small to moderate explosive eruptions from Mono-Inyo Craters Volcanic Chain, California, during the past 5,000 years ranged from VEI of 1 to 3. The 18 May 1980 eruption of Mount St. Helens was a VEI 5 with an erupted volume of about 1 km3. The 1991 eruption of Mount Pinatubo had a volume of about 10 km3 and a VEI of 5 to 6. The 1815 eruption of Tambora, Indonesia, had a VEI of 7 and a volume in excess of 100 km3. The eruption of Long Valley Caldera about 760,000 years ago had a VEI of 7 and a volume of 600 km3 of material. The largest explosive eruption on the figure occurred at Yellowstone about 600,000 years ago with a VEI of 8 and a volume of about 1,000 km3 of material.
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Did you know?
The VEI has some similarities to the Richter magnitude scale used to measure earthquakes. It has a simple numerical index of increasing magnitude of explosivity, with each interval representing an increase of about a factor of 10 in the volume of erupted tephra.
There are no known explosive events with a VEI larger than 8.
Volcanologists developed the VEI in order to help estimate the climatic impact of volcanic eruptions. They soon learned, however, that the amount of sulfur dioxide gas injected high into the atmosphere (which is not necessarily related to the size of an eruption) was a critical factor in determining the climatic impact of volcanic eruptions. Today, the VEI is primarily used to estimate the relative size of an explosive eruption.
Large explosive eruptions occur much less frequently than small ones. Through 1994, the record of volcanic eruptions in the past 10,000
years maintained by the Global Volcanism Program of the Smithsonian Institution shows that there have been 4 eruptions with a VEI of 7, 39 of VEI 6, 84 of VEI 5, 278 of VEI 4, 868 of VEI 3, and 3,477 explosive eruptions of VEI 2.
References
Newhall, C.G., and Self, S., 1982, The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism, Journal of Geophysical Research, v. 87, p. 1231-1238.
Simkin, Tom, and Siebert, Lee, 1994, Volcanoes of the World -- A regional directory, gazetteer, and chronology of Volcanism during the last 10,000 years: Tuscon, Geoscience Press, Inc., p. 349.

Glossary of volcanic terms. - University of Wisconsin Oshkosh. . 2001.

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