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Friday, November 18, 2011

Ghost alps of Antarctica secret revealed

For more than half of a century, geologists have wrangled over the origins of astonishing range of mountains found beneath ice up to three kilometers (two miles) thick in East Antarctica.

It came to be known as Gamburtsev mountains, which was named after a Soviet geophysicist who detected them in 1958 during the first International Polar Year exploration. The mountains are 1,200 km (750 miles) long, with jagged peaks up to 2,700 meters (8,900 ft.) high intersected by deep troughs and valleys.

How this chain came into being is one of the many mysteries of the great white continent.

The Gamburtsevs are located at high elevation and on a continent that geologically is ancient and long free of the tectonic upheaval that throws up mountains.

Yet their sharp edges, similar to Europe's Alps, clearly attest to a youthful range barely touched by the erosive forces of wind, snow and water.

An international team of scientists, reporting in the journal Nature on Wednesday, say they have the answer to the riddle.

The key, they believe, lies in a network of lakes and rifts in the bedrock, remarkably mirroring features found in the parched African tropics half a world away.

"The East Antarctic rift system resembles one of the geological wonders of the world, the East African rift system," said head investigator Fausto Ferraccioli of the British Antarctic Survey (BAS).

"It provides the missing piece of the puzzle that helps explain the Gamburtsev Subglacial Mountains. The rift system was also found to contain the largest subglacial lakes in Antarctica."

The rift network was found, owing it to the second exploration of the Gamburtsevs, carried out in the last International Polar Year, which despite its name ran from 2007-2009 in order to cover the seasons at both poles.

The seven-nation project mapped the subglacial topography in central East Antarctica using two Twin Otter aircraft fitted with penetrating radar and sensors to map changes in the Earth's gravitational and magnetic field.

What they propose is a narrative starting a billion years ago.

Several mini-continents collided together to form a super-continent called Gondwana, creating a mountain range at the point of impact.

Eventually, the uplifted rock collapsed under its own weight and over eons it eroded away, leaving an underlying crustal "root."

There next followed two periods of rifting, some 250 million years ago and again about 100 million years ago, in which Gondwana pulled part in tectonic agony.

This created a 3,000 km (2,000 mile) fracture in the planet's crust that extends from Eat Antarctica across the ocean to India.

The residual "root," combined with the rifting, helped force up the land that is now East Antarctica.

In doing so, this developed an extensive rift-valley system, whose flanks were incised by rivers, and then by glaciers, as Earth moved from warmth into deepening chill.

Some 34 million years ago, the magnificent mountains became smothered by the East Antarctic ice sheet, an area the size of Canada. Like Sleeping Beauty, they retained their eerie youthfulness.

Piecing together the story of the Gamburtsevs as a tale of mountain rejuvenation was an exercise in humility, said Carol Finn of the US Geological Survey (USGS).

"We are accustomed to thinking that mountain-building relates to a single tectonic event rather than sequences of events," she said.

"The lesson we learned about multiple events forming the Gamburtsevs may inform studies of the history of other mountain belts."


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