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Ancient eruption tapped the Earth's depths.

Lofting its mushroom cloud of ash high into the atmosphere, the catastrophic blast at Mount Pinatubo in 1991 may seem the stereotypical volcanic eruption. But the largest lava outpourings in Earth's history actually resemble floods more than nuclear explosions. Research on the remains of one such lava deluge in western India pinpoints for the first time the origin of the rock that erupts in these so-called flood basalts.

Asish R. Basu of the University of Rochester (N.Y.) and colleagues made this discovery while studying the Deccan Traps, stair-like layers of basalt formed during a series of eruptions 65 million years ago. An estimated 1.5 million cubic kilometers of lava spewed over several million years. If spread evenly, that amount of rock would envelop the entire world in a layer more than 3 meters thick.

Geoscientists believe the Deccan Traps and other flood basalts form when narrow plumes of hot rock rise through the mantle and burn through the crust. Most researchers think the plumes come from the base of the mantle near the top of Earth's core, but others suggest the plumes start much closer to the surface.

Basu and his co-workers traced the origin of the Deccan Trap rocks by measuring the ratio of two helium isotopes preserved within bubbles in the basalt. These rocks contain an abundance of light helium, a geochemical signature of the deep mantle, the researchers report in the Aug. 13 Science.

Geochemists have seen similar helium ratios in young lavas from Hawaii and other sites where deep mantle plumes reach the surface. But the known flood basalts on Earth are all geologically old, and researchers had assumed such rocks could not preserve helium for tens of millions of years.

Basu and his co-workers found the gas by looking at basaltic rocks that formed at the very start of the Deccan event, when magma from the plume first started rising through the crust. Unlike the bulk of the Deccan basalts, these rocks north of the main lava flows did not erupt at the surface, but cooled underground and trapped the helium instead of releasing it into the atmosphere.

According to Basu, these findings "open up a new avenue of research where people will be able to look at older rocks and look for helium."

The bulk of the Deccan eruptions occurred at roughly the same time as a mass extinction that wiped out the last remaining dinosaurs and a large fraction of other life forms at the end of the Cretaceous period. To better bracket the timing of the eruptions, the researchers dated basalt formations from the beginning and end of the Deccan sequence by measuring a particular argon isotope created by the decay of radioactive potassium.

The dating shows the early eruptions began 68.5 million years ago, long before most of the outpourings. The last eruptions ended about 65 million years ago, plus or minus 110,000 years. Within the errors of the dating technique, that age coincides with the crash of a major meteorite that left a 180-kilometer-wide crater in the Yucatan Peninsula.

While speculations about the cause of the extinctions have centered on the meteorite, some researchers think the timing of the Deccan eruptions suggests they too played a role. The eruptions would have emitted vast amounts of heat-trapping carbon dioxide and poisonous sulfur gases, says Robert A. Duncan of Oregon State University in Corvallis. "The coincidence of two rare events, namely flood basalt activity and the meteorite impact, are probably the explanation for the mass extinctions," he says.
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Title Annotation:research on volcanic rocks
Author:Monastersky, Richard
Publication:Science News
Date:Aug 14, 1993
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