Volcanic explosivity index

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VEI and ejecta volume correlation

The volcanic explosivity index (VEI) was devised by Chris Newhall of the US Geological Survey and Stephen Self at the University of Hawaii 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 1.0×10¹² m³ (240 cubic miles) 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 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 100 from 10,000 to 1,000,000 m³ (350,000 to 35,310,000 cu ft) while the factor is 10 between all higher indices.

VEI	Ejecta volume	Classification	Description	Plume	Frequency	Tropospheric injection	Stratospheric injection	Examples
0	< 10,000 m³	Hawaiian	Effusive	< 100 m	constant	negligible	none	Kīlauea, Piton de la Fournaise, Erebus
1	> 10,000 m³	Hawaiian / Strombolian	Gentle	100 m–1 km	daily	minor	none	Nyiragongo (2002), Raoul Island (2006), Stromboli Island - (continuous since Roman times to present))
2	> 1,000,000 m³	Strombolian / Vulcanian	Explosive	1–5 km	weekly	moderate	none	Unzen (1792), Cumbre Vieja (1949), Galeras (1993), Sinabung (2010)
3	> 10,000,000 m³	Vulcanian / Peléan	Catastrophic	3–15 km	few months	substantial	possible	Nevado del Ruiz (1985), Soufrière Hills (1995), Nabro (2011)
4	> 0.1 km³	Peléan / Plinian	Cataclysmic	10–25 km	≥ 1 yr	substantial	definite	Mayon (1814), Pelée (1902), Eyjafjallajökull (2010)
5	> 1 km³	Plinian	Paroxysmic	20–35 km	≥ 10 yrs	substantial	significant	Vesuvius (79), Fuji (1707), Mount Tarawera (1886), St. Helens (1980), Puyehue (2011)
6	> 10 km³	Plinian / Ultra-Plinian	Colossal	> 30 km	≥ 100 yrs	substantial	substantial	Veniaminof (c. 1750 BC), Huaynaputina (1600), Krakatoa (1883), Novarupta (1912), Pinatubo (1991), Laki (1783)
7	> 100 km³	Ultra-Plinian / Supervolcanic	Mega-colossal	> 40 km	≥ 1,000 yrs	substantial	substantial	Mazama (c. 5600 BC), Thera (c. 1620 BC), Samalas (Mount Rinjani) (1257), Tambora (1815), Taupo (180)
8	> 1,000 km³	Supervolcanic	Apocalyptic	> 50 km	≥ 10,000 yrs	substantial	substantial	Yellowstone (640,000 BC), Toba (74,000 BC), Taupo (24,500 BC), La Garita Caldera (26.3 Ma)

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. There have been at least five 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.

Limitations of VEI

Under the VEI, ash, lava, lava 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.

Although VEI is quite suitable for classifying the explosive magnitude of eruptions, the index is not as significant as sulphur dioxide emissions in quantifying their atmospheric and climatic impact, as a 2004 paper by Georgina Miles, Roy Grainger and Eleanor Highwood points out.

“Tephra, or fallout sediment analysis, can provide an estimate of the explosiveness of a known eruption event. It is, however, not obviously related to the amount of SO₂ emitted by the eruption. The volcanic explosivity index (VEI) was derived to catalogue the explosive magnitude of historical eruptions, based on the order of magnitude of erupted mass, and gives a general indication as to the height the eruptive column reached. The VEI itself is inadequate for describing the atmospheric effects of volcanic eruptions. This is clearly demonstrated by two eruptions, Agung (1963) and El Chichón (1982). Their VEI classification separates them by an order of magnitude in explosivity, although the volume of SO₂ released into the stratosphere by each was measured to be broadly similar, as shown by the optical depth data for the two eruptions.”

Lists of large eruptions

Clickable imagemap of notable volcanic eruptions. The apparent volume of each bubble is linearly proportional to the volume of tephra ejected, colour-coded by time of eruption as in the legend. Pink lines denote convergent boundaries, blue lines denote divergent boundaries and yellow spots denote hotspots.

• List of large volcanic eruptions of the 19th century (≥ VEI 4)
• List of large volcanic eruptions of the 20th century (≥ VEI 4)
• List of large volcanic eruptions in the 21st century (≥ VEI 4)
• Timetable of major worldwide volcanic eruptions (mostly VEI 6, within 2 kya)
• List of large volcanic eruptions (mostly VEI 6-8, within 50 Mya)
• List of largest volcanic eruptions (VEI 7-8, within 1600 Mya)

See also

• Supervolcano
• Lists of volcanoes
• List of natural disasters by death toll

External links

• VEI glossary entry from a USGS website
• How to measure the size of a volcanic eruption, from The Guardian
• The size and frequency of the largest explosive eruptions on Earth, a 2004 article from the Bulletin of Volcanology
• List of Large Holocene Eruptions (VEI > 4) from the Smithsonian Global Volcanism Program