雅思阅读：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.

　　The creation of the Amazon River had previously been attributed to the growth of the Andes Mountains damming the westward movement of water from the wetland and creating a smaller sloped effect. Grace says the team’s discovery is important because it shows the way in which the interaction between the Earth’s crust and the mantle underneath can fundamentally reshape the topography and ecosystems of continents.

　　"Wobble board" effects

　　While demonstrated most spectacularly in the Amazon Basin, continental tilting is known to have occurred elsewhere, including in our own backyard.

　　"The idea of sinking slabs is shown to have occurred in North America and there are applications to Australia," says Grace, where eastern parts of the country are believed to "have swung up and down like a giant wobble board." Grace says it is possible that similar tilting has created and destroyed other large river systems over time, such as a Triassic-era (from 200 to 250 million years ago) movement perhaps responsible for forming Sydney’s bedrock, the Hawkesbury sandstone.

　　Geophysicist Louis Moresi from Melbourne’s Monash University, who was not involved in the research, says the team’s work is fascinating and demonstrates the way in which continental tilting can influence river systems over millions of years.

　　"An additional 400 m of topography might not seem much in contrast with the height of the Andes, but it is a very broad-scale tilting of the continent which drainage and erosion cannot resist," says Louis.