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 volcanoes—the 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 magma—particularly the quantity and identity of gases in it—may 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 Tacitus—the 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 explosive—like 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."
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