Volcanic explosivity index

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 eruptions in history given a magnitude of 8. A value of 0 is given for non-explosive eruptions, defined as less than 10,000 m³ (350,000 cu ft) of tephra ejected; and 8 representing a supervolcanic eruption that can eject 1.0×10¹² m³ (240 cubic miles) of tephra and have a cloud column height of over 20 km (66,000 ft). The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed ejecta criteria, with the exception of between VEI-0, VEI-1 and VEI-2.

Classification

With indices running from 0 to 8, the VEI associated with an eruption is dependent on how much volcanic material is thrown out, to what height, and how long the eruption lasts. The scale is logarithmic from VEI-2 and up; an increase of 1 index indicates an eruption that is 10 times as powerful. As such, 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 one hundred, from 10,000 to 1,000,000 m³ (350,000 to 35,310,000 cu ft), while the factor is ten between all higher indices. For ejecta volumes above 0.001 km³, the VEI may be calculated as

${\displaystyle VEI=\lfloor lo{g}_{10}(v)\rfloor +5}$

where ${\displaystyle v}$ is the volume of ejecta in km³. In the following table, the frequency of each VEI indicates the approximate frequency of new eruptions of that VEI or higher.

About 40 eruptions of VEI-8 magnitude within the last 132 million years (Mya) have been identified, of which 30 occurred in the past 36 million years. Considering the estimated frequency is on the order of once in 50,000 years, there are likely many such eruptions in the last 132 Mya that are not yet known. Based on incomplete statistics, other authors assume that at least 60 VEI-8 eruptions have been identified. The most recent is Lake Taupō's Oruanui eruption, more than 27,000 years ago, which means that there have not been any Holocene eruptions with a VEI of 8.

There have been at least 10 eruptions of VEI-7 in the last 11,700 years. There are also 58 Plinian eruptions, and 13 caldera-forming eruptions, of large, but unknown magnitudes. By 2010, the Global Volcanism Program of the Smithsonian Institution had cataloged the assignment of a VEI for 7,742 volcanic eruptions that occurred during the Holocene (the last 11,700 years) which account for about 75% of the total known eruptions during the Holocene. Of these 7,742 eruptions, about 49% have a VEI of 2 or lower, and 90% have a VEI of 3 or lower.