Classic view wrong, scientists say, huge pots of magma not brewing under most volcanoes

About 75,000 years ago, some scientists say, the last truly colossal volcanic eruption on Earth came close to wiping out all the primates, including humans. That eruption occurred when the Toba volcano in Indonesia exploded in an almost unbelievably shattering display.

Other people with a flare for the dramatic warn that a supervolcano underlying Yellowstone National Park could erupt in the not-so-distant future and push humanity to the verge of extinction. University of North Carolina at Chapel Hill scientists say not to worry, especially anytime soon.

“It’s not hyperbole to say that the biggest eruptions could bring an end to civilization,” said Dr. Allen F. Glazner, professor of geologic sciences at UNC. “Such eruptions are evident in the geologic record, and the classic textbook picture of volcanoes implies that huge pots of magma are brewing under most active volcanoes today.”

Happily, that traditional view is wrong, according to Glazner’s latest research — work conducted jointly with UNC assistant geology professor Dr. Drew S. Coleman and Dr. John M. Bartley of the University of Utah.

In two studies appearing in April issue of GSA Today and the May issue of Geology, the scientists present new insights into the potential for volcanoes to produce gigantic eruptions — explosions thousands of times larger than the 1980 eruption of Mount Saint Helens.

“Although evidence for such massive eruptions is found throughout the geologic record, our investigation of magmas frozen below long-extinct volcanoes in California’s Sierra Nevada led us to conclude that the largest eruptions are significantly less likely than many people believed,” Glazner said.

In their investigation, team members studied magma bodies that cooled beneath the land’s surface. Those bodies, called “plutons” after Pluto, the Greek god of the underworld, are the chief building blocks of the Earth’s crust, he said. Vast pieces of formerly molten rock, they contain many known rock and mineral resources.

“Much of Chapel Hill, for example, lies on the Chapel Hill Granite pluton and its associated volcanic rocks,” the geologist said. “Most scientists picture plutons as solidifying from enormous underground blobs of molten rock known as magma that feed overlying volcanoes.”

Typically, plutons are hundreds to thousands of cubic kilometers in volume. For that reason, geologists long assumed that huge stores of magma are commonplace active volcanoes, Glazner said. They also reasoned that the potential for truly catastrophic eruptions exists in many volcanically active areas.

“Our new work casts doubt on the assumption that gigantic eruptions should be relatively common,” he said.

Glazner, Coleman and Bartley combined observations of the deep Earth provided by seismic waves produced during earthquakes with mathematical modeling of magma cooling and precise dating and field mapping Sierra Nevada plutons.

Because small percentages of liquid in a rock slow seismic waves dramatically, the waves are sensitive probes for the tiniest volumes of molten rock, Glazner said.

“However, even under active volcanoes, seismic waves show little evidence for big blobs of magma,” Coleman said. “Our mathematical models indicate that if big magma chambers existed, they should solidify in less than a million years, but new high-precision age determinations completed here at UNC indicate that plutons can take up to 10 million years to form.”

New field mapping demonstrated that plutons once thought to be thousands of cubic kilometers of homogeneous rock that cooled from a single magma reservoir preserve subtle evidence of a much slower, piecemeal assembly, he said.

The results suggest that plutons are likely to be built by a multitude of small molten intrusions over millions of years and that plutons are not like a closed can of food waiting to explode when heated, Coleman said.

“We conclude that volcanoes are more prone to chugging along, producing many small — though still dangerous — eruptions such as the 1980 eruption of Mount Saint Helens, rather than huge civilization-destroying eruptions,” he said.

Former UNC College of Arts and Sciences students Walt Gray and Ryan Z. Taylor, now with the Southwest Research Institute and the U.S. Forest Service, respectively, contributed to the new work. The National Science Foundation supported it.