The Volcanic Explosivity Index
What an eruption’s VEI number actually tells you
One number for an eruption
The Volcanic Explosivity Index (VEI) is a 0-to-8 scale for the size of an explosive eruption, proposed by Chris Newhall and Steve Self in 1982 so that eruptions across history could be compared on a single measure. It is the volcanic equivalent of an earthquake magnitude.
A VEI is assigned from whatever evidence survives: the volume of erupted material (tephra — ash, pumice and rock fragments), the height of the eruption column, the style and duration of the eruption, and eyewitness or geological accounts. Volume is the primary criterion for well-studied eruptions.
The scale
Most recorded eruptions are VEI 1 to 3. A VEI 4 happens somewhere on Earth roughly once a year; a VEI 5 about once a decade; a VEI 6 once or twice a century. A VEI 7 is rare — the 1815 Tambora eruption was the last, and before that the 1257 Samalas eruption. No VEI 8 has occurred in recorded human history.
| VEI | Description | Tephra volume | Plume height | Example |
|---|---|---|---|---|
| 0 | Non-explosive (effusive) | < 10,000 m³ | < 100 m | Kīlauea (typical) |
| 1 | Gentle | > 10,000 m³ | 100 m – 1 km | Stromboli (typical) |
| 2 | Explosive | > 1,000,000 m³ | 1–5 km | Whakaari / White Island 2019 |
| 3 | Severe | > 0.01 km³ | 3–15 km | Nevado del Ruiz 1985; Soufrière Hills 1995– |
| 4 | Cataclysmic | > 0.1 km³ | > 10 km | Eyjafjallajökull 2010; Mont Pelée 1902 |
| 5 | Paroxysmal | > 1 km³ | > 10 km | Mount St. Helens 1980; Vesuvius AD 79 |
| 6 | Colossal | > 10 km³ | > 20 km | Krakatau 1883; Pinatubo 1991; Novarupta 1912 |
| 7 | Super-colossal | > 100 km³ | > 20 km | Tambora 1815; Samalas 1257; Thera ~1600 BC |
| 8 | Mega-colossal (“super-eruption”) | > 1,000 km³ | > 20 km | Toba ~74,000 yr ago; Yellowstone; Taupō (Ōruanui) |
Why each step is ten times bigger
From VEI 2 upward, the scale is defined so that each whole step is a tenfold increase in erupted volume. A VEI 6 (Pinatubo 1991, about 10 km³) produced roughly ten times the material of a VEI 5 (Mount St. Helens 1980, about 1 km³) and a hundred times that of a VEI 4.
This means the visual difference between, say, a VEI 3 and a VEI 6 on a chart badly understates reality — the VEI 6 moved a thousand times more rock. The circle sizes on the map follow the VEI, but even they compress the true range.
What the VEI leaves out
The VEI measures explosivity, not danger, and not everything a volcano does:
- Lava flows. A purely effusive eruption that quietly pours out several cubic kilometres of basalt scores a low VEI. The 1783–84 Laki eruption in Iceland rates VEI 4, yet its gas cloud killed a fifth of Iceland’s population and disrupted weather across the Northern Hemisphere.
- Death toll. The VEI 4 eruption of Mont Pelée in 1902 killed about 28,000 people because a pyroclastic flow reached a city. Many VEI 6 eruptions in remote areas have killed no one.
- Gas and climate. Sulfur dioxide output, which drives the cooling effect of large eruptions, is not part of the VEI. Pinatubo (VEI 6) had an outsized climate effect because it was unusually sulfur-rich.
For volume specifically, some volcanologists now prefer the continuous eruption magnitude and intensity scales of Pyle (2000), which are calculated directly from erupted mass. The VEI remains the standard because it can be estimated for prehistoric eruptions where little is left to measure.
Sources
- Newhall, C. G. & Self, S. (1982). “The Volcanic Explosivity Index (VEI): an estimate of explosive magnitude for historical volcanism.” Journal of Geophysical Research 87(C2), 1231–1238.
- Global Volcanism Program, Smithsonian Institution — “Volcanic Explosivity Index (VEI)”, Volcanoes of the World.
- Pyle, D. M. (2000). “Sizes of volcanic eruptions.” In Encyclopedia of Volcanoes, Academic Press.
- U.S. Geological Survey — “VEI: Volcanic Explosivity Index.”