Scientists have scales to measure the strength of natural phenomena like earthquakes and hurricanes. But what about the eruptive power of volcanoes? For that, geologists use the Volcanic Explosivity Index. Here’s how it works.
The Volcanic Explosivity Index (VEI) was first proposed in 1982 by Christopher Newhall of the U.S. Geological Survey and Stephen Self of the University of Hawaii. Their intention was to develop a scale to estimate the explosive magnitude of historical volcanoes.
To that end, they came up with an incrementing logarithmic scale to measure the magnitude of past explosive eruptions, which Newhall described as a “semiquantitative compromise between poor data and the need in various disciplines to evaluate the record of past volcanism.”
But establishing the parameters for a useful scale proved easier said than done. Unlike earthquakes or hurricanes, there are different types of volcanoes, and they produce different products, like massive plumes of ejected rock and ash, or molten lava flows.
Moreover, and as scientists later learned, volcanoes also churn-out varying degrees of sulfur dioxide at rates irrespective of eruptive power. It’s for that reason that the VEI had to be rejected as a way to measure an eruption's potential impact on the climate. Today, it’s used exclusively to measure the explosive power of both historic and new eruptions.
How the Scale Works
Similar to the Richter scale, the VEI uses a numerical index ranging from 0 to 8. Each increment represents an 10-fold increase in explosivity.
Factors that are taken into account include the volume of pyroclastic material (including ashfall, pyroclastic flows, and other ejecta), the height of the eruption, duration in hours, and a number of other qualitative measurements.
So, given that the scale is primarily driven by the volume ejected, it goes like this:
- VEI 0: eruptions that produce less than 0.0001 cubic kilometer of ejecta (small events that typically produce flowing lava, which is called an effusive eruption)
- VEI 1: eruptions that produce between 0.0001 and 0.001 cubic kilometers of ejecta
- VEI 2: eruptions that produce between 0.001 and 0.01 cubic kilometers of ejecta
- VEI 3: eruptions that produce between 0.01 and 0.1 cubic kilometers of ejecta
And so on until we get to VEI 8.
So, a VEI 5 is roughly 100 times more explosive than a VEI 3, and a VEI 8 is a million times more powerful than a VEI 2. Sometimes a + symbol is added to account for the wide degree of variation between each number in the scale.
The VEI doesn’t go past 8, but that doesn’t mean a VEI 9 isn’t impossible. Scientists may still uncover evidence of such an event buried somewhere in the geological record.
Eruptions Throughout History
And indeed, by using the VEI, geologists have been able to document the Earth’s tumultuous volcanic past. Here’s a graphic put together by Geology.com that shows the relative impact of notable historical eruptions:
Shockingly, as this graphic shows, a volcanic eruption like the one at Mt. St. Helens — a cataclysmic event that blew a huge chunk off the side of the mountain — is absolutely dwarfed in comparison to some of the larger ones. The size of the Yellowstone eruption is also disturbing given that the caldera could erupt at any time.