Artificial Volcanoes Created in New York Lab
Bijal P. Trivedi
National Geographic Today
|October 8, 2003|
In a windowless laboratory at the American Museum of Natural History in New York City, scientists are simulating the conditions that caused two of recent history's most explosive volcanic eruptions.
James Webster, a geochemist and curator of mineral deposits at the museum, and his colleague Charles Mandeville, a volcanologist, are recreating the chemistry of the magma chambers of the volcanoesthe massive underground cauldrons of liquid rock that fuel an eruption.
The research bears directly on volcano hazard predictions in a world where an estimated 500 million people live near an active volcano.
The nature of the magmaparticularly the quantity and identity of gases in itmay help determine whether future eruptions resemble the epic explosions like Mount Vesuvius and Mount St. Helens, or the more passive spillovers like the fountains of fire and rivers of lava at Mount Kilauea on the island of Hawaii.
In the museum lab today, the magma of choice is from Mount Vesuvius, in Italy. In A.D. 79 the eruption there destroyed Pompeii and Herculaneum, killed more 3,000 people, and lofted a cubic mile of ash in a 20-mile-high (32-kilometer-high) plume.
Statesman and orator Pliny the Younger witnessed the eruption and described it in a letter to the historian Tacitusthe first detailed such account. The eruption launched a "pine tree"-shaped cloud, Pliny wrote. Now volcanologists identify such massive explosions as "plinian."
Plinian eruptions are so violent that they can toss refrigerator-size rocks miles into the atmosphere.
With or Without Gas
The difference between plinian and passive eruptions depends largely on the underlying magma, according to Webster. "Volcanoes with similar flavors of magma tend to produce similar eruptions," he said.
Generally, volcanoes with high quantities of water and carbon dioxide in the magma tend to be more explosivelike a bottle of champagne suddenly uncorked. Volcanoes with less gassy magma, and/or with a plumbing system that lets them "burp off the gas," erupt more passively, said Webster.
Magmas rich in silica can produce even more cataclysmic explosions because the magma is more viscous, making it difficult for trapped gases to escape, Mandeville explained.
"Gas drives eruptions," said Jacob Lowenstern, a volcanologist at the U.S. Geological Survey in Menlo Park, California. "Without gas, the magma would just ooze out on the surface."
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.
National Geographic Today, 7 p.m. ET/8 p.m. PT in the United States, is a daily news journal available only on the National Geographic Channel. Click here to learn more about it.
Got a high-speed connection? Watch National Geographic Today in streaming video.
|© 1996-2008 National Geographic Society. All rights reserved.|