岩浆在地幔上升过快会引起火山爆发

If some volcanoes operate on geologic¹ timescales, Costa Rica's Irazú had something of a short fuse（火爆脾气）. In a new study in the journal Nature, scientists suggest that the 1960s eruption² of Costa Rica's largest stratovolcano（成层火山） was triggered by magma rising from the mantle³ over a few short months, rather than thousands of years or more, as many scientists have thought. The study is the latest to suggest that deep, hot magma can set off an eruption fairly quickly, potentially providing an extra tool for detecting an oncoming volcanic⁴ disaster. "If we had had seismic⁵ instruments in the area at the time we could have seen these deep magmas coming," said the study's lead author, Philipp Ruprecht, a volcanologist at Columbia University's Lamont-Doherty Earth Observatory⁶. "We could have had an early warning of months, instead of days or weeks."

 

Towering more than 10,000 feet and covering almost 200 square miles, Irazú erupts about every 20 years or less, with varying degrees of damage. When it awakened⁷ in 1963, it erupted for two years, killing⁸ at least 20 people and burying hundreds of homes in mud and ash. Its last eruption, in 1994, did little damage.

 

Irazú sits on the Pacific Ring of Fire, where oceanic crust is slowly sinking beneath the continents, producing some of earth's most spectacular fireworks. Conventional wisdom holds that the mantle magma feeding those eruptions⁹ rises and lingers for long periods of time in a mixing chamber¹⁰ several miles below the volcano. But ash from Irazú's prolonged explosion is the latest to suggest that some magma may travel directly from the upper mantle, covering more than 20 miles in a few months.

 

"There has to be a conduit from the mantle to the magma chamber," said study co-author Terry Plank¹¹, a geochemist at Lamont-Doherty. "We like to call it the highway from hell."

 

Their evidence comes from crystals of the mineral olivine separated from the ashes of Irazú's 1963-1965 eruption, collected on a 2010 expedition to the volcano. As magma rising from the mantle cools, it forms crystals that preserve the conditions in which they formed. Unexpectedly, Irazú's crystals revealed spikes¹² of nickel, a trace element found in the mantle. The spikes told the researchers that some of Irazú's erupted magma was so fresh the nickel had not had a chance to diffuse¹³.

 

"The study provides one more piece of evidence that it's possible to get magma from the mantle to the surface in very short order," said John Pallister, who heads the U.S. Geological Survey (USGS) Volcano Disaster Assistance Program in Vancouver, Wash. "It tells us there's a potentially shorter time span we need to worry about."

 

Deep, fast-rising magma has been linked to other big events. In 1991, Mount Pinatubo in the Philippines spewed so much gas and ash into the atmosphere that it cooled Earth's climate. In the weeks before the eruption, seismographs recorded hundreds of deep earthquakes that USGS geologist¹⁴ Randall White later attributed to magma rising from the mantle-crust boundary. In 2010, a chain of eruptions at Iceland's Eyjafjallajökull volcano that caused widespread flight cancellations also indicated that some magma was coming from down deep. Small earthquakes set off by the eruptions suggested that the magma in Eyjafjallajökull's last two explosions originated 12 miles and 15 miles below the surface, according to a 2012 study by University of Cambridge researcher Jon Tarasewicz in Geophysical Research Letters.

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