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'Killer' Volcanoes Not Guilty

13 August 2010 12:18 pm
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Wikipedia

Fatal or flop? Eruptions of flood basalts like this one that formed 15 million years ago in Washington State can wipe out half the life on Earth—or do nothing at all.

Approximately 260 million years ago, a volcanic province known as the Emeishan Traps burst forth in what is now Southwest China. In the geologic blink of an eye—half a million years—500,000 cubic kilometers of lava poured into the ocean and threw billions of tons of toxic sulfur dioxide into the air. More than half of the marine species on Earth disappeared.

For years researchers have blamed quick, massive eruptions like this one for numerous large-scale extinctions. That includes the largest one 250 million years ago that exterminated more than three-fourths of the species on Earth, an event so catastrophic it now carries the name the "Great Dying." There's just one problem: One of the biggest eruptions on record—an event that spewed 1.5 million cubic kilometers of lava over what is today southern Brazil, Paraguay, Uruguay, and southwestern Africa—didn't cause much damage at all. As far as scientists can tell, the 130-million to 140-million-year-old blast killed off just a few species in the Atlantic Ocean.

One reason could be duration. Some geologists who have analyzed the resulting Paraná flood basalts—mountains of lava rock stretching over more than a million square kilometers—have concluded that the eruption lasted for 11 million years, making it too gradual to cause a mass extinction. But others have found that the Paraná eruption was relatively quick, lasting less than a million years.

To set the record straight, geologists led by David Thiede and Paulo Vasconcelos of the University of Queensland in Australia redated some of the rocks analyzed by these teams using a method called laser incremental heating. The technique involves heating the rocks at multiple temperatures and analyzing the gas that is released at each step. The method is considered accurate because it is less susceptible to the ravages of time and weathering than are other approaches. In a study published in the August issue of Geology, Thiede and Vasconcelos report that the eruption that formed the Paraná basalts occurred about 135 million years ago and lasted less than 1.2 million years, enough time to provoke a mass extinction.

Paul Renne, a geochronologist at the University of California, Berkeley, calls the results a "nail in the coffin" for the idea of an 11-million-year eruption. But that leaves advocates of the volcano-extinction link with a nagging question: Why was Paraná so mild?

Geologist Paul Wignall of the University of Leeds in the United Kingdom thinks it's what's above volcanoes that makes all the difference. The lava from the Siberian Traps eruption, which preceded the Great Dying, had to ascend through a bed of coal. The hot magma could have released enormous quantities of climate-altering carbon and sulfur when it rose. Paraná, on the other hand, erupted under a desert. There wasn't a lot of dangerous gas to release in that sediment, says Wignall.

Vincent Courtillot, a geophysicist at the University of Paris Diderot, has a different theory. He thinks the number of volcanic pulses—individual episodes of volcanism during the million-year eruption period—and how closely they occur together could determine how lethal an eruption is. Spaced out, they wouldn't be disastrous. But if several occurred very quickly, it could generate what Courtillot calls a "runaway effect." The climate wouldn't have time to recover between pulses, and life would suffer.

Thiede says he believes both ideas are possible.

This article has been corrected to read "geologists David Thiede and Paulo Vasconcelos." It previously read "a team of geologists led by David Thiede."

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