雅思阅读备考素材：Amazon River

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　　Amazon River shaped by see-saw tilt of continent

　　SOUTH AMERICA'S AMAZON RIVER Basin was a vast wetland for millions of years. That was until tectonic processes deep in the Earth transformed it into the world's largest drainage system, new research suggests.

　　Geoscientists from the University of Sydney and the California Institute of Technology believe a process called 'continental tilting', which began about 30 million years ago and continued for about 16 million years, tipped South America like a see-saw and drained its wetlands.

　　The event, funnelled water over 6400 km east to the Atlantic Ocean in what the team describes as the world's largest water slide. "What we're describing involves the whole continent," lead author Grace Shephard told Australian Geographic. "This tilting led to the formation of the Amazon River as we know it today."

　　Sinking slabs

　　The team used computer simulation to demonstrate the continent's movement across a subduction zone, where slabs of the Earth's crust sink into the softened rocks of the mantle, causing it to tilt. With time, as South America moved further west of these sinking slabs, the northeast subsided by up to 400 m, while the west was uplifted.

　　"We had a hunch that the ultimate forces leading to this fundamental shift in continental topography had something to do with the westward motion of South America over dense, sinking mantle rocks while the Atlantic Ocean opened up," explains Grace.

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