Wednesday, December 05, 2007

Gosse Bluff - A mysterious crater

While dinosaurs forage in the green heart of Australia, a huge fireball plunges into the plain, shattering the landscape with a force hundreds of thousands of times greater than that of the nuclear bomb that destroyed the Japanese city of Hiroshima in 1945. Slowly, a huge mushroom cloud of dust and debris rises from the scene, blotting out the sun and darkening the skies of the Southern Hemisphere for months.
Such was the dramatic birth of Gosse Bluff, a massive, rock-rimmed crater gouged out by the impact of a comet 130 million years ago. The comet - a ball of frozen carbon dioxide, ice and dust one-third of a mile (660m) across became a flaming furnace as it hurled to Earth. It penetrated only about half a mile (800m) into the ground, but blew up some 150sq miles (400sq km) of the surrounding land, sending shock waves across the globe and flinging up gigantic rings of earth and stone like ripples in a pond.
Massive thumbprint
The original crater was about 12 miles (120km) in diameter. Today's crater, only 2.5 miles (4km) across, is just its central core. Aeons of erosion have worn away the innumerable tons of debris that once covered it. The bluff, the crater's double-walled rim of hard sandstone crags, now rises to 590ft (180m) above the plain. This sandstone was pushed up by the explosion - layers of of similar rock have been identified over 1¼ miles (2km) beneath the surface, giving some idea of the massive force involved.
Pictured by space satellite, Gosse Bluff looks like a massive thumbprint on the otherwise flat and featureless Missionary Plain 100 miles (160km) west of Alice Springs. The sandstone ring stands out as one of the most impressive impact scars in a landscape littered with meteor craters.
A 19th-century explorer named Edmund Gosse is credited with its discovery in 1873. Long before this, however, the crater was well known to the Aborigines and the area (now a registered sacred site) is rich in campsite remains, hunting hides and rock shelters decorated with the red hand stencils of long-gone inhabitants.
Earnest Giles, who explored the area in 1875, described it in detail. without the benefit of an aerial view, he was unable to appreciate the symmetry of the crater, and did not realise its significance. Unimpressed, he wrote: 'A few cypress pines are rooted in the rocky, shelving sides of the range, which is not of such elevation as it appeared at a distance. The highest points are not more than from 700 to 800 feet.'
Until recently, the origins of Gosse Bluff were a mystery, and rival theories abounded. One held that gases below the surface may have forced their way up, creating a powerful eruption of soil and water known as a 'mud volcano'. Another theory was that a meteorite could have been the cause, and that the lack of any remaining fragments was because of millions of years of weathering. But scientific research in recent times points to a different origin.
As with most similar sites, Gosse Bluff has a pattern of geological fractures radiating out from the centre. As they weather, the rocks fall apart along the fracture lines in cone-shaped patterns called 'shatter cones'. By studying their formations, scientists have verified that the crater is an impact crater, and that the colliding object had a high speed but a relatively low density - suggesting a comet's composition rather than the rock of a meteorite.
Not far from Gosse Bluff are the Henbury Craters, made by 12 fragments of a meteorite that split as it hurled in from space 4700 years ago. The Aboriginal name, sun walk fire devil rock', suggests that someone saw it happen.

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