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.

Geology.com elaborates on the jaw-dropping power of megavolcanoes:

Forty-seven eruptions have been rated VEI 8 because they are thought to have produced an amazing 1,000 cubic kilometers or more of ejecta. This would be a mass of uncompacted ejecta ten kilometers in length, ten kilometers in width and ten kilometers deep. Eruptions at Toba (74,000 years ago), Yellowstone (640,000 years ago) and Lake Taupo (26,500 years ago) are three of the 47 VEI 8 sites that have been identified.

The VEI 8 eruption with the greatest volume of ejecta known to have occurred in the United States, and possibly in the world, was at La Garita Caldera in southwestern Colorado about 28 million years ago. There, the Fish Canyon Tuff has an original estimated volume of about 5,000 cubic kilometers!

Eruption(s) at the Paraná and Etendeka traps igneous province had an eruptive volume of over 2.6 million cubic kilometers. However, these are thought to be effusive eruptions producing fluid basalt lava rather than explosive eruptions producing ejecta. The Paraná and Etendeka eruption(s) occurred about 128 to 138 million years ago. Their lava flows span from eastern Brazil onto the western portions of Namibia and Angola. They occurred when Africa and South America were connected.

Historically speaking, as the value of VEI increases, the frequency of that type of eruption decreases.

Other notable eruptions (via the U.S. Geological Survey):

• The Mono-Inyo Craters Volcanic Chain, California, during the past 5,000 years, ranging from VEI 1 to 3
• The 1815 eruption of Tambora, Indonesia, had a VEI of 7 and a volume in excess of 100 km3

Top image: Volcanogeek.