“Supervolcanoes” and Super Eruptions: What VEI 8 Means, and Yellowstone’s Two

A "supervolcano" is a volcanic center that has produced an eruption at the top of the Volcanic Explosivity Index (VEI), level 8. The VEI runs from 0 to 8 on a logarithmic scale, much like earthquake magnitude, and each level has a name, from "effusive" at 0 to "apocalyptic" at 8.

The Yellowstone caldera from the Washburn Range, reaching to the Red Mountains (upper right); the rim of the Grand Canyon of the Yellowstone in front. National Park Service.

What a VEI 8 eruption is

• It ejects more than 200 cubic miles (1,000 cubic km) of material as pyroclastic fragments and ash.
• Its column rises more than 16 miles (25 km), about 85,000 ft, into the stratosphere, and winds can carry the ash around the planet. Ash from Yellowstone's Lava Creek eruption, 640,000 years ago, is found as far away as the Mississippi River in Louisiana.
• In the traditional styles it is ultra-Plinian, larger than the eruption of Vesuvius in 79 CE that Pliny witnessed.

So much magma comes out that the roof of the chamber caves in, forming a caldera. These resurgent calderas are enormous but do not build towering mountains: Yellowstone's caldera is about 53 by 28 miles (85 by 45 km), yet its walls rise only about 1,600 ft (500 m). The landform is a broad volcanic plateau ringed by low cliffs, with a resurgent dome pushed up in the middle by magma moving below. Such systems also erupt many times, in many sizes, before and after the climax: both Yellowstone and Valles Caldera, the two resurgent calderas in national park sites, produced lava flows, domes and pyroclastic deposits.

"Super eruption," not "supervolcano"

The word supervolcano is popular but less used in science. Some volcanologists, including the scientist-in-charge of the Yellowstone Volcano Observatory, dislike it, preferring to call the eruptions "super" rather than the volcanoes, since the same centers also have much smaller eruptions.

Super eruptions are rare: the bigger an eruption, the less often it happens. About 40–50 volcanoes are erupting worldwide at any time, and 1,350 have erupted during the Holocene, but none in a VEI 8 eruption. The most recent was at Taupō in New Zealand, about 27,000 years ago.

Yellowstone's two

The volumes of several eruptions compared; the first and third Yellowstone eruptions were super eruptions. National Park Service.

Sources

• National Park Service, "“Super Volcanoes”"; rewritten in hubnx's own words. Photograph and illustration: National Park Service.