Geologists from Brigham Young University, Berkeley Geochronology Center and the Pacific Northwest National Laboratory have found evidence of twenty ancient supervolcanoes near the Utah-Nevada border.

Supervolcanoes are giant volcanoes that blast out more than 1,000 cubic km of volcanic material when they erupt. They are different from the more familiar stratovolcanoes because they aren’t as obvious to the naked eye and affect enormous areas.

“Supervolcanoes as we’ve seen are some of Earth’s largest volcanic edifices, and yet they don’t stand as high cones. At the heart of a supervolcano instead, is a large collapse. Those collapses in supervolcanoes occur with the eruption and form enormous holes in the ground in plateaus, known as calderas,” said Dr Eric Christiansen of Brigham Young University, who is a co-author of two papers published in the journal Geosphere (paper 1 & paper 2).

The newly discovered supervolcanoes aren’t active today, but 30 million years ago more than 5,500 cubic km of magma erupted during a one-week period near a place called Wah Wah Springs.

“In southern Utah, deposits from this single eruption are 4 km thick. Imagine the devastation – it would have been catastrophic to anything living within hundreds of miles,” Dr Christiansen said.

Dinosaurs were already extinct during this time period, but what many people don’t know is that 25-30 million years ago, North America was home to rhinos, camels, tortoises and even palm trees.

Dr Christiansen with colleagues measured the thickness of the pyroclastic flow deposits. They used radiometric dating, X-ray fluorescence spectrometry, and chemical analysis of the minerals to verify that the volcanic ash was all from the same ancient super-eruption.

The scientists found that the Wah Wah Springs eruption buried a vast region extending from central Utah to central Nevada and from Fillmore on the north to Cedar City on the south. They even found traces of ash as far away as Nebraska.

The team also found evidence of 15 super-eruptions and 20 large calderas – the so-called Indian Peak-Caliente caldera complex.

These supervolcanoes have diameters up to 60 km and are filled with intracaldera tuff and breccias. They have been hidden in plain sight for millions of years despite their enormous size.

“The ravages of erosion and later deformation have largely erased them from the landscape, but our careful work has revealed their details. The sheer magnitude of this required years of work and involvement of dozens of students in putting this story together,” Dr Christiansen said.

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Best MG et al. 2013. The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions. Geosphere, vol. 9, no. 6, pp. 1562-1636; doi: 10.1130/GES00945.1

Best MG et al. 2013. The 36–18 Ma Indian Peak–Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions. Geosphere, vol. 9, no. 4, pp. 864-950; doi: 10.1130/GES00902.1