How Do Scientists Compare the Sizes of Volcanic Eruptions?

The Volcanic Explosivity Index Rates Explosive Eruptions From 0 to 8

Geologists cannot hold a tape measure to an eruption. As the USGS puts it, eruption size cannot be determined by instruments [1]. Instead, in 1982 two volcanologists, Chris Newhall of the U.S. Geological Survey and Stephen Self of the University of Hawaii, proposed a scale called the Volcanic Explosivity Index, or VEI [3]. Their paper in the Journal of Geophysical Research calls it a semi-quantitative estimate: part measurement, part judgment. They offered it because the records of past eruptions are often patchy and rarely contain the data needed to measure explosive size directly [2].

The scale runs from 0 to 8, and the rule is the surprise. In most places, each step up means roughly ten times more erupted material (the USGS makes an exception for the lowest steps, 0 to 2) [1]. Scientists call a scale like this logarithmic. It lets a single page show a tiny eruption and a gigantic one side by side. It caught on: the original paper reports that it had been assigned to more than 8,000 historic and prehistoric eruptions [2].

One Step Up the Scale Means About Ten Times More Erupted Material

What is being measured is mostly tephra, the ash and rock fragments blasted into the air. The USGS gives these reference points [1]:

• VEI 0: non-explosive, with less than 10,000 cubic meters of tephra (350,000 cubic feet).
• VEI 4: about 0.1 cubic kilometers (0.024 cubic miles). Example: La Soufrière on the island of St. Vincent, 2021.
• VEI 5: about 1 cubic kilometer (0.24 cubic miles). Example: Mount St. Helens, 1980.
• VEI 6: about 10 cubic kilometers (2.4 cubic miles). Example: Mount Pinatubo in the Philippines, 1991.
• VEI 8: more than 1,000 cubic kilometers (240 cubic miles), and an eruption column higher than 20 km (12 miles). The USGS lists the Yellowstone caldera-forming eruption of 631,000 years ago here.

A simple way to picture it: if a VEI 5 eruption were one swimming pool of ash, a VEI 6 would be about ten pools and a VEI 7 about a hundred. The pools are only a picture for the ratio, not a measurement from the sources.

The ratio is a rule of thumb, not a precise fact about every pair of eruptions. Tambora in 1815 is rated VEI 7, two steps above Mount St. Helens, so you might expect 100 times more ash. The USGS says it ejected about 30 to 80 times more [4]. That is because every step covers a range of volumes, and an eruption can sit near either end of its step.

The same USGS page says eruptions of VEI 5 or more happen worldwide about once every 20 years on average, so the top of the scale is rare [4].

A VEI Number Combines Ash Volume, Cloud Height and a Description

Volume is the main ingredient, but it is not the only one. The USGS says volume, the height of the eruption cloud, and plain-language descriptions ranging from “gentle” to “mega-colossal” are used together [1]. For a modern eruption, scientists can look at the deposits and also at what observers saw. For a prehistoric one, they have only the deposits.

There is a rough shortcut for the volume part. The reference points above fit a counting rule: write the ash volume in cubic meters, count how many zeros it has, and subtract 4. One cubic kilometer is a billion cubic meters (1 followed by 9 zeros), so 9 − 4 gives VEI 5. Ten cubic kilometers has 10 zeros, giving VEI 6. This is my own way of reading a few USGS reference points, not an official method. It cannot assign a VEI by itself: the real index also weighs cloud height and descriptions, and the edges of each step (for example, exactly 1,000 cubic kilometers) are not settled by this arithmetic.

Why Paricutin Rates a 4 Even Though It Produced as Much Ash as a 5

If volume decided everything, the story would end here. But in 1943 a volcano named Paricutin appeared in a Mexican cornfield and kept erupting for nine years. It ejected about 1.3 cubic kilometers (0.31 cubic miles) of ash and rock, which alone would suggest VEI 5. The USGS classifies it as VEI 4 because that material came out slowly over nine years and the activity was never violently explosive [1].

The lesson is that the index measures explosiveness, not only how much came out. An eruption that spreads its output over years is a different event from one that blasts it out in hours.

What the VEI Does Not Measure: Lava, Climate and Cloud Height Alone

The VEI is a good ruler for one job, comparing explosive eruptions, and a poor one for others.

• Lava is not counted. The USGS notes that lava was left out of Paricutin’s tally, though it produced a similar volume [1]. A volcano that mostly oozes lava can look small on this scale.
• A tall cloud can mislead. Cloud height depends on water content and on latitude. The USGS says the January 2022 Hunga Tonga–Hunga Ha’apai eruption had a very tall plume but an ash volume more like a smaller eruption, and it calls applying the VEI there “clearly problematic” [1].
• Climate follows sulfur, not ash. Ash falls out of the upper atmosphere within days to weeks and has little lasting climate effect. The main driver of cooling is sulfur dioxide, which turns into tiny droplets that reflect sunlight [5]. Pinatubo, a VEI 6, sent about 20 million metric tons of sulfur dioxide high into the atmosphere [5]. The USGS fact sheet reports that sulfur dioxide from the eruption temporarily lowered global temperatures by about 0.5°C (about 1°F) from 1991 through 1993 [6]. A VEI number alone would not have told you that.

So the VEI is a ruler for the size of the blast, not a measure of how far the effects travel.

What Scientists Still Cannot Say About the Biggest Eruptions

The top of the scale is the least certain. No VEI 8 or 9 eruption has ever been observed, so nobody knows how long one lasts [1]. The USGS rates the Toba eruption in Indonesia, about 74,000 years ago, as VEI 8 and says it may possibly be a 9. It also notes that some of the giant Yellowstone eruptions may each have been several events separated by gaps of up to decades, and that more field work is needed [1]. Because we only have the deposits, even the size of the scale’s biggest steps comes with question marks.

Try the Counting Rule on an Eruption of Your Own

With an adult and a calculator, try this. Imagine a volcano that put out 100 cubic kilometers of ash. One cubic kilometer is 1,000,000,000 cubic meters. How many zeros does 100 cubic kilometers have in cubic meters, and what does the rough rule give you? (Answer: 11 zeros, minus 4, gives 7.) Now try 1,000 cubic kilometers, and notice that the rule is only a rough check.

Then pick one of the eruptions described in this note: Mount St. Helens, Pinatubo or Paricutin. Find what the USGS pages below say about it and ask three questions: how much ash, how high the cloud, and how long it lasted. Do the three clues agree with the VEI it is given? When they do not, as with Paricutin, you have found a reason why the index is only half measurement and half judgment. The Smithsonian Global Volcanism Program also keeps eruption records for individual volcanoes. I could not open its pages when writing this note, so I have not used them as a source.

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