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, defined as less than 10,000 m3 (350,000 cu ft) of tephra ejected; and 8 representing a mega-colossal explosive eruption that can eject 1,000,000,000,000 m3 (3.5×1013 cu ft) of tephra and have a cloud column height of over 50 km (31 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.
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Scientists indicate how powerful volcanic eruptions are using the VEI. It records how much volcanic material is thrown out, to what height, and how long the eruption lasts; the scale goes from 0 to 8. The scale is logarithmic; an increase of 1 indicates an eruption that is 10 times more powerful.
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 (350,000 to 35,000,000 cu ft) while the factor is 10 between all higher indices.
| VEI | Ejecta volume | Classification | Description | Plume | Frequency | Tropospheric injection | Stratospheric injection[1] | Examples |
|---|---|---|---|---|---|---|---|---|
| 0 | < 10,000 m³ | Hawaiian | effusive | < 100 m | constant | negligible | none | Kilauea, Piton de la Fournaise |
| 1 | > 10,000 m³ | Hawaiian/Strombolian | gentle | 100–1000 m | daily | minor | none | Stromboli, Nyiragongo (2002) |
| 2 | > 1,000,000 m³ | Strombolian/Vulcanian | explosive | 1–5 km | weekly | moderate | none | Galeras (1993), Mount Sinabung (2010) |
| 3 | > 10,000,000 m³ | Vulcanian/Peléan | severe | 3–15 km | few months | substantial | possible | Nevado del Ruiz (1985), Soufrière Hills (1995) |
| 4 | > 0.1 km³ | Peléan/Plinian | cataclysmic | 10–25 km | ≥ 1 yr | substantial | definite | Mount Pelée (1902), Eyjafjallajökull (2010) |
| 5 | > 1 km³ | Plinian | paroxysmal | 20–35 km | ≥ 10 yrs | substantial | significant | Mount Vesuvius (79 CE), Mount St. Helens (1980) |
| 6 | > 10 km³ | Plinian/Ultra-Plinian | colossal | > 30 km | ≥ 100 yrs | substantial | substantial | Krakatoa (1883), Mount Pinatubo (1991) |
| 7 | > 100 km³ | Ultra-Plinian | super-colossal | > 40 km | ≥ 1,000 yrs | substantial | substantial | Thera (Minoan Eruption), Tambora (1815) |
| 8 | > 1,000 km³ | Supervolcanic | mega-colossal | > 50 km | ≥ 10,000 yrs | substantial | substantial | Yellowstone (640,000 BP), Toba (74,000 BP) |
A total of 47 eruptions of VEI 8 magnitude or above, ranging in age from Ordovician to Pleistocene, have been identified, of which 42 occurred in the past 36 million years. The most recent is Lake Taupo's Oruanui eruption, 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.[2] There have been at least 5 identified Holocene eruptions with a VEI of 7. There are also 58 plinian eruptions, and 13 caldera-forming eruptions, of large, but unknown magnitudes. There are likely many other eruptions that are not identified.