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Why Australia was left high and dry

点击量:   时间:2017-06-25 08:02:03

By Jeff Hecht THE puzzle of how Australia managed to stay dry at a time when the sea covered nearly half the surfaces of other continents 70 million years ago may have been solved. At that time, says geophysicist Michael Gurnis of the California Institute of Technology in Pasadena, Australia was 250 metres higher than it is today. To find out why, Gurnis developed a computer model of the movement of tectonic plates and the heat flow in the underlying mantle. His starting point was a slab of heavy ocean crust that was dipping down beneath eastern Australia about 130 million years ago. The model showed that when the subduction stopped, Australia moved over the cold sinking slab, which would initially pull it down. That could explain why over half the continent was flooded 120 million years ago. Then Australia bent back upwards, and reached its highest point about the time global sea levels peaked (Science, vol 279, p 1499). Since then, Australia has moved north and sunk to its present level. Gurnis says the model predicts that the slab which kept the continent dry is now in the lower mantle, just below a mid-ocean ridge south of Australia—”the oddest place on the seafloor”, he says. This region has a curious ocean-floor composition and some parts that Gurnis thinks lie above the slab are unusually deep. Gurnis says that material rising to form new seafloor may have included “wisps” from the subducted slab,