by Lucas Joel Thursday, February 16, 2017
Beneath a Bolivian volcano called Cerro Uturuncu sits one of Earth’s largest-known magma reservoirs, the Altiplano-Puna Magma Body (APMB), which may have a volume as large as 500,000 cubic kilometers. Dissolved in the APMB magma, scientists report in a new study in Earth and Planetary Science Letters, is enough water to fill Lake Superior or Lake Huron — two of the largest lakes in the world.
Water levels in magma bodies can help geologists predict how explosive future volcanic eruptions might be, says Terry Plank, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, who was not involved in the new study. During an eruption, water in magma vaporizes, and “like opening a seltzer bottle, the gas coming out is what drives the eruption,” she says. “The more water there is, the more potentially explosive it is.”
But measuring exact water levels in magma is difficult. In “rocks that have just erupted, all the water just goes ‘Pssshhh!' and it’s gone,” Plank says. Additionally, the APMB is between 15 and 35 kilometers below the surface, which rules out directly measuring its water content.
To study Cerro Uturuncu, researchers sampled rocks from multiple eruptions along the slopes of the volcano. In the lab, petrologist Mickael Laumonier of the Universities of Orléans in France and Bayreuth in Germany and his colleagues then infused the samples with water as they heated and pressurized the rocks to conditions seen in the actual magma body: up to 1,400 degrees Celsius and up to 30,000 times standard atmospheric pressure. By measuring electrical conductivity in their makeshift APMB magmas during the experiments, the team could tell how much water dissolved in the rock; the more conductive the sample, the more water was dissolved. They found that the APMB ought to be “between 8 and 10 percent weight of water,” Laumonier says, which is about twice the amount proposed in previous estimates for similar magmas. “It was so common to find 4 to 5 weight-percent of water … that we didn’t expect to find 8 to 10.”
What is unclear from the new work, Plank says, is whether the magma might include other substances that, like water, can also increase the magma’s conductivity. “They didn’t really consider other conductive sources,” she says, noting that there could be fluid carbon dioxide, which is very conductive.
If water levels in the APMB are as high as the new study reports, however, it’s possible that eruptions from volcanoes that tap the magma body, like Cerro Uturuncu — one of the fastest-growing volcanoes in the world — could be particularly explosive. Cerro Uturuncu last erupted about 271,000 years ago, and volcanologists do not know when the next eruption will be. But the landscape around the volcano “is inflating, so it looks like new magma is arriving every day,” Plank says.
On the other hand, if the volcano were to erupt, Plank notes that the water vapor in the magma may not escape quickly enough to increase the eruption’s explosiveness. “It’s how fast you open the cap of the seltzer bottle that’s important,” she says. “You could open the cap slowly” and the gas would escape more slowly, causing a relatively gentle eruption. Regardless of exactly how much water the APMB contains, though, it represents a significant hazard, Plank says. “It’s huge, it’s molten and, in the past, it has had enormous eruptions.”
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