quarta-feira, 6 de fevereiro de 2008

Massive volcano beneath Antarctic ice

Agence France-Presse
Monday, 21 January 2008

A powerful volcano erupted under the icesheet of Antarctica around 2,000 years ago and it might still be active today, a finding which raises questions about ice loss from the white continent.

The explosive event – rated "severe" to "cataclysmic" on an international scale of volcanic force – punched a massive breach in the icesheet and spat out a plume some 12 kilometres into the sky, said British scientists behind the find.

Occasional volcanism
Most of Antarctica is seismically stable. But its western part lies on a rift in Earth's crust that gives rise to occasional volcanism and geothermal heat, occurring on the Antarctic coastal margins.
This is the first evidence for an eruption under the ice sheet itself – a slab of frozen water, hundreds of metres thick in places, that holds most of the world's stock of fresh water.
Reporting in the journal Nature Geoscience this week, the investigators from the British Antarctic Survey (BAS), In Cambridge, England, describe the finding as "unique."
It extends the range of known volcanism in Antarctica by some 500 km and raises the question whether this or other sub-glacial volcanoes may have melted so much ice that global sea levels were affected, they said.
The volcano, located in the Hudson Mountains, blew around 207 BC, give or take 240 years, according to their paper.
Anomalous radar readings
Evidence for this comes from a British-American airborne geophysical survey completed between 2004 and 2005. This used radar to delve deep under the ice sheet to map the terrain beneath. The team spotted anomalous radar reflections over 23,000 square kilometres - an area bigger than Wales.
They interpret this signal as being a thick layer of ash, rock and glass, formed from fused silica, that the volcano spewed out in its fury.
The amount of material – 0.31 cubic kilometres – indicates an eruption of between three and four on a yardstick called the Volcanic Explosive Index (VEI).
By comparison, the eruption of Mount St. Helens in 1980, which was greater, rates a VEI of five, and that of Mount Pinatubo in 1991 is a VEI of six.
Melting ice
"We believe this was the biggest eruption in Antarctica during the last 10,000 years," said lead author Hugh Corr. "It blew a substantial hole in the icesheet and generated a plume of ash and gas that rose around 12 km into the air."
The eruption occurred close to the massive Pine Island Glacier, an area where movement of glacial ice towards the sea has been accelerating alarmingly in recent decades.
"It may be possible that heat from the volcano has caused some of that acceleration," said co-author David Vaughan, who stresses though that global warming is by far still the most likely culprit.
Volcanic heat "cannot explain the more widespread thinning of West Antarctic glaciers that together are contributing nearly 0.2mm (0.008 of an inch) per year to sea-level rise," he adds.
"This wider change most probably has its origin in warming ocean waters."

source: http://www.cosmosmagazine.com

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