Crystals help scientists reconstruct ancient eruption near Yellowstone National Park
U.S. Geological Survey/Flickr (CC BY 2.0)
Massive supervolcanic eruptions can be triggered much more quickly than previously thought, scientists report July 21 in Geology.
The researchers made the discovery while reconstructing the history of a massive eruption 4.5 million years ago in a field of volcanoes called Heise, in what's now eastern Idaho. The Heise supervolcano spewed enough ash and molten rock to fill Lake Ontario. In most supervolcanoes, magma simmers in underground chambers for hundreds of thousands of years before erupting. But in Heise, the team has determined, several small reservoirs of magma pooled together in less than 10,000 years to spur the eruption.
Supervolcanoes are capable of outbursts thousands of times more powerful than typical volcanic eruptions, causing regional devastation and dramatic changes in global climate. “We now have a better idea of what the magma reservoirs under these supervolcanoes look like just prior to eruption,” says geologist and lead author Jörn-Frederik Wotzlaw of the University of Geneva.
Heise was fueled by an abnormally hot spot deep in Earth’s mantle that has sprouted a string of North American volcanoes over the last 16.5 million years, including the supervolcano beneath Yellowstone National Park that last erupted 640,000 years ago. The new finding could help researchers predict how quickly a new supervolcano could emerge and erupt in the region, Wotzlaw says.
Wotzlaw and colleagues studied tiny translucent crystals embedded in rocks blasted from the Heise eruption. Scientists know that over thousands of years, the tiny crystals grew inside the volcano’s subterranean magma and absorbed oxygen from the surrounding molten rock. The oxygen serves as a fingerprint of where the crystals formed, Wotzlaw says, because all crystals within a magma reservoir should have a single ratio of forms of oxygen called isotopes.
Looking at these ratios, the researchers found that the crystals in their sample didn’t all arise in a single magma chamber. The oldest part of each crystal, its core, was born in one of four magma reservoirs. The younger outer rims of the crystals, however, grew inside one of two distinct magma reservoirs. Because previous studies had demonstrated that the Heise supervolcano contained only one magma reservoir when it erupted, the team realized that the crystals must have grown in different magma batches that merged before eruption.
While in these reservoirs, the crystals absorbed not only oxygen but uranium, which slowly decays into lead. By comparing each crystal layer’s number of uranium and lead atoms, the team determined how long ago it formed. The researchers estimated that the four smaller magma chambers formed, merged and erupted in only 1,000 to 10,000 years.
Wotzlaw proposes that the assembly of the magma reservoirs set off the eruption without an external trigger such as an earthquake. In January, a team reported that the buoyancy of magma pushing upward against Earth’s denser crust might be enough by itself to prompt an eruption. Because the force from a huge magma chamber would be stronger than from each smaller individual reservoir, Wotzlaw says that when the reservoirs teamed up, they could set off the eruption.
Heise’s magma is spent and the volcano will not erupt again. But the Yellowstone supervolcano contains several large magma reservoirs. Still, geochemist Kenneth Sims of the University of Wyoming in Laramie warns that people need to be careful in comparing the two volcanoes. The magma under Yellowstone is older and less runny than the molten rock that fueled the Heise eruption and therefore less likely to combine or initiate a major eruption, Sims says. “The possibility of another eruption certainly exists, but right now there isn’t that kind of evidence.”
But a new supervolcano will one day probably form northeast of Yellowstone, Wotzlaw says. The North American tectonic plate has been slowly and steadily drifting southwest over the region’s hot spot, spawning volcanoes. If a new volcano follows Heise’s lead, he adds, it could quickly evolve into an eruptible state.
“If the process only takes a few thousand years,” he says, “then it is relevant on a human scale; maybe not for you and me. But if a volcano erupts in 2,000 years, that may not look good for the people living at that time.”
Editor's Note: This story was updated July 31, 2014, to correct the name of the supervolcano being studied. A previous version of the article had used a colloquial but inaccurate name, the Kilgore Tuff supervolcano.
J.-F. Wotzlaw et al. Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes. Geology. Published online July 21, 2014. doi: 10.1130/G35979.1.
W. Malfait et al. Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers. Nature Geoscience. Published online January 5, 2014. doi: 10.1038/ngeo2042.
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