The Volcanic Explosivity Index (VEI) was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaiʻi in 1982 to provide a relative measure of the explosiveness of volcanic eruptions.
Volume of products, eruption cloud height, and qualitative observations (using terms ranging from "gentle" to "mega-colossal") are used to determine the explosivity value. The scale is open-ended with the largest volcanoes in history given magnitude 8. A value of 0 is given for non-explosive eruptions (less than 104 cubic metres of tephra ejected) with 8 representing a mega-colossal explosive eruption that can eject 1012 cubic metres of tephra and have a cloud column height of over 25 km (16 mi). The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed eruption criteria (exception: between VEI 0 and VEI 1).
Note that ash, volcanic bombs, and ignimbrite are all treated alike. Density and vesicularity (gas bubbling) of the volcanic products in question is not taken into account. In contrast, the DRE (Dense-Rock Equivalent) is sometimes calculated to give the actual amount of magma erupted. Another weakness of the VEI is that it does not take into account the power output of an eruption, which makes it extremely difficult to determine with prehistoric or unobserved eruptions.
Scientists indicate how powerful volcanic eruptions are using the VEI. It records how much volcanic material is thrown out, how high the eruption goes, and how long it lasts. The scale goes from 0 to 8. An increase of 1 indicates a 10 times more powerful eruption.
Note: There is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of 100 from 10,000 to 1,000,000 m³ while the factor is 10 between all higher indices.
*Count of VEI 2 and VEI 3 eruptions in the last 10,000 years are based on 1994 figures maintained by the Global Volcanism Program of the Smithsonian Institution. Count of eruptions greater than VEI 3 in the last 10,000 years are based on its 2010 figures. There are also 58 plinian eruptions, and 13 caldera-forming eruptions, of large, but unknown magnitudes.
A total of 47 eruptions of VEI–8 magnitude or above, ranging in age from Ordovician to Pleistocene, are identified, of which 42 eruptions are known from the past 36 million years. The most recent one is Lake Taupo's Oruanui eruption, occurring 26,500 years ago, which means that there have not been any Holocene (within the last 10,000 years) eruptions with a VEI of 8.
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