The Volcanic Explosivity Index: how scientists rank eruptions
When scientists say an eruption was a "VEI 5" or "the biggest VEI 7 since Tambora," they are using the Volcanic Explosivity Index — a logarithmic scale that ranks eruptions by the volume of material they ejected and how high their plumes reached. Here is what each level means, with examples and limits.
What VEI measures
The VEI combines several physical properties of an eruption: volume of pyroclastic material erupted, height of the eruption column, duration, and a qualitative description of style. Volumes range from less than 10,000 cubic metres at VEI 0 to more than 1,000 cubic kilometres at VEI 8. The scale is logarithmic — each step is roughly ten times bigger than the previous one.
VEI 0: non-explosive
Effusive eruptions, mainly lava flows. Hawaiian-style eruptions at Kīlauea and Mauna Loa are typically VEI 0 or 1. The Icelandic fissure eruptions of Holuhraun in 2014 were technically among the largest VEI 1 eruptions on record by volume, but the explosivity was low.
VEI 1: small, gentle eruptions
Small puffs of ash, small lava flows, low eruption columns. Most days at Stromboli or Sakurajima register here. Few outside the immediate vicinity ever notice them.
VEI 2: moderate
Small explosive eruptions. Plumes 1–5 kilometres. Galeras 1993 and Etna 2002 were in this range. Most of the world's frequent volcanic activity sits at this level.
VEI 3: larger
Plumes 3–15 kilometres. The 2010 Eyjafjallajökull eruption that shut down European airspace was a VEI 3 to low VEI 4. By volume small, but explosive enough to inject ash high into the troposphere and stratosphere.
VEI 4: large
Plumes 10–25 kilometres. Galunggung 1982, Soufrière Hills in Montserrat 1995–1997, Eyjafjallajökull's peak phase. Capable of serious regional damage and worldwide aviation disruption.
VEI 5: very large
Plumes more than 25 kilometres. Mount St. Helens 1980 and Vesuvius AD 79 sit in this category. A VEI 5 is the threshold at which an eruption begins to affect global climate detectably for months.
VEI 6: colossal
Even larger. Pinatubo 1991, Krakatoa 1883 and Novarupta 1912 are the canonical examples. A VEI 6 injects enough sulphur into the stratosphere to cool the planet by tenths of a degree for years.
VEI 7: super-colossal
Tambora 1815 is the only VEI 7 in modern history; it caused the "year without a summer" in 1816 across the Northern Hemisphere. Earlier VEI 7 events include Samalas 1257 and possibly several others recorded in ice cores.
VEI 8: mega-colossal
Reserved for caldera-forming supereruptions: Toba (74,000 years ago), Yellowstone (640,000 years ago and earlier), Taupo (several events in the last 30,000 years). No VEI 8 has occurred in human history. The next one, whenever it comes, will reshape civilisation.
The limits of the scale
VEI is useful but imperfect. It is biased toward explosive, silica-rich eruptions and undercounts large basaltic flows. Laki's 1783 fissure eruption in Iceland, which killed a quarter of the population, is only VEI 6 by volume but had global climate effects rivalling much higher VEI events. The Magnitude and Intensity scales developed in the 2000s partly address this.
Why the scale matters
The VEI is the common language scientists, governments and journalists use to communicate eruption size. Hazard plans, insurance models and climate-impact studies all rely on it. When a new eruption begins, the first estimate of likely VEI is the single most important number for planning the response.
How to read it for planning
If you live within a hundred kilometres of an active volcano, look at the highest VEI in its known history. A volcano whose worst recorded eruption is a VEI 3 is a very different proposition from one with a VEI 6 in its past, even if both have been quiet for centuries.
See them on the map
Filter the map to volcanoes with major historic eruptions and the geography of the VEI becomes visible — the arc volcanoes of subduction zones produce most of the high-VEI events, while the basaltic shields and rifts contribute the most volume in lower-VEI flows.