For the scientists the initial challenge is to recover samples of the magma, which provide essential clues to interpret eruptions. Webster collected rock samples at Mount Vesuvius last year.
Mandeville focused on the volcanic fallout around Mount Mazama in Crater Lake, Oregon. About 7,600 years ago, Mount Mazama erupted explosively, ejecting 50 cubic kilometers (12 cubic miles) of debris over an area of one million square kilometers (400,000 square miles).
Back in the lab, Mandeville slices the samplesin this case, glassy pieces of rockin search of so-called "melt inclusions," microscopic pockets of magma sealed in volcanic rock.
Inclusions are time capsules that reveal the chemical signature of the magma before the eruption. Within the sample slices, an electron microprobe detects silica and other components, and infrared spectroscopy measures water and carbon dioxide.
The process, Mandeville said, "is like looking at a champagne bottle before you open the cork, when all the gases are still dissolved."
"These melt inclusions are the only direct evidence of the type of gas powering ancient and modern eruptions," Lowenstern said. "If you want to model how eruptions occur, you need to know how the gas is distributed."
Taking what Mandeville has learned about the chemistry of the inclusions, Webster brews ancient magmas. He packs a tiny gold capsule with pulverized rock from Mount Vesuvius and other gases determined to be present in the original magma.
The capsule then goes into an internally heated pressure vessel that the scientists affectionately call the "bomb." The bomb simulates heat and pressure of magma within the Earth. At Mount Vesuvius, the temperature in the magma chamber just prior to eruption reached 1,000° to 1,100° Celsius (1800° to 2,000° Fahrenheit); the pressure, 22,000 pounds per square inch, the equivalent of six kilometers (four miles) underground.
Predicting the Bang
By manipulating temperatures and pressures inside the bomb, Webster and Mandeville can gauge how the magma may behave in an eruption.
Around the world, scientists closely monitor active volcanoes with satellites and ground instruments. "Monitoring can reveal how much gas is emitted," Lowenstern said, "but that doesn't give you any clues where this gas came from"the magma or other sources.
For example, magma chambers often heat nearby subterranean water supplies, generating steam, which pours from the volcano and nearby regions. But a satellite cannot distinguish between steam from hydrothermal sources and steam emerging directly from the magma.
"Webster and Mandeville's work tries to address how much gas is in the magma, and how this affects the explosive potential," said Charles Bacon, a USGS volcanologist in Menlo Park, California.
The scientists hope that their findings will correlate with the gas emissions that satellites and ground instruments detect on the slopes of active volcanoes. Thus volcanologists may improve their predictionsnot only when an eruption will occur but how big the bang will be.
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