Supervolcano “Rosetta Stone” Discovered in Alps 

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The Yellowstone caldera. Courtesy of USGS, Smith and Siegel

A fossil supervolcano has been discovered in the Italian Alps’ Sesia Valley, where rare uplift of the Earth’s crust reveals for the first time the actual “plumbing” from the surface to the source of the magma deep within the Earth. Geologists are referring to it as the “Rosetta Stone” of supervolcanoes.

Historically called calderas, supervolcanoes are enormous craters tens of kilometers in diameter. These giant pockmarks in the Earth’s surface are produced by rare and massive explosive eruptions that rank among nature’s most violent events, producing devastation on a regional scale, and possibly triggering climatic and environmental effects at a global scale. Their eruptions are sparked by the explosive release of gas from molten rock, or magma, as it pushes its way to the Earth’s surface.

The uplift in Sesia Valley reveals the tracks and trails of the magma as it moved through the Earth’s crust to an unprecedented depth of 25 kilometers. It is more than 13 kilometers in diameter. The discovery scientists say Sesia Valley’s caldera erupted during the Permian geologic time period.

Calderas erupt hundreds to thousands of cubic kilometers of volcanic ash. Explosive events occur every few hundred thousand years. Supervolcanoes have spread lava and ash vast distances, and scientists believe they may have set off catastrophic global cooling events at different periods in the Earth’s past.

“Bishop Tuff” at Long Valley, from a volcanic event that erupted 140 cubic miles of magma 760,000 years ago. Courtesy of U.S. Geological Survey

A team, led by James E. Quick, a geology professor at Southern Methodist University (SMU), has reported the discovery in a new research article. Their findings will help to advance scientific understanding of active supervolcanoes, like Yellowstone — the second-largest supervolcano in the world, which last erupted 630,000 years ago.

“What’s new is to see the magmatic plumbing system all the way through the Earth’s crust,” says Quick, who previously served as program coordinator for the Volcano Hazards Program of the U.S. Geological Survey. “Now we want to start to use this discovery. We want to understand the fundamental processes that influence eruptions: Where are magmas stored prior to these giant eruptions? From what depth do the eruptions emanate?”

Sesia Valley’s unprecedented exposure of magmatic plumbing provides a model for interpreting geophysical profiles and magmatic processes beneath active calderas. The exposure also serves as direct confirmation of the cause-and-effect link between molten rock moving through the Earth’s crust and explosive volcanism.

“It might lead to a better interpretation of monitoring data and improved prediction of eruptions,” says Quick, lead author of the research article.

Calderas, which typically exhibit high levels of seismic and hydrothermal activity, often swell, suggesting movement of fluids beneath the surface.

“We want to better understand the tell-tale signs that a caldera is advancing to eruption so that we can improve warnings and avoid false alerts,” Quick explains.

To date, scientists have been able to study exposed caldera “plumbing” from the surface of the Earth to a depth of only five kilometers. Scientific understanding has been limited to geophysical data and analysis of erupted volcanic rocks. Quick likens the relevance of Sesia Valley to seeing bones and muscle inside the human body for the first time after previously envisioning human anatomy on the basis of a sonogram only.

“We think of the Sesia Valley find as the ‘Rosetta Stone’ for supervolcanoes because the depth to which rocks are exposed will help us to link the geologic and geophysical data,” Quick says. “This is a very rare spot. The base of the Earth’s crust is turned up on edge. It was created when Africa and Europe began colliding about 30 million years ago and the crust of Italy was turned on end.”

Scientists have documented fewer than two dozen caldera eruptions in the last 1 million years. Besides Yellowstone, other monumental explosions have included Lake Toba on Indonesia’s Sumatra island 74,000 years ago, which is believed to be the largest volcanic eruption on Earth in the past 25 million years. Described as a massive climate-changing event, the Lake Toba eruption is thought to have killed an estimated 60 percent of humans alive at the time.

Long Valley in California is a caldera that remains active. It erupted about 760,000 years ago and spread volcanic ash for 600 cubic kilometers. The ash blanketed the southwestern United States, extending from California to as far west as Nebraska.

“There will be another supervolcano explosion,” Quick says. “We don’t know where. Sesia Valley could help us to predict the next event.”

The article, “Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km.,” appears in the journal Geology.

Quick is a professor in the SMU Roy M. Huffington Department of Earth Sciences as well as SMU associate vice president for research and dean of graduate studies. Co-authors of the report are Silvano Sinigoi, Gabriella Peressini and Gabriella Demarchi, all of the Universita di Trieste; John L. Wooden, Stanford University; and Andrea Sbisa, Universita di Trieste.