雅思阅读:Amazon River shaped by see-saw tilt of continent
SOUTH AMERICAS AMAZON RIVER Basin was a vast wetland for millions of years. That was until tectonic processes deep in the Earth transformed it into the worlds 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 worlds largest water slide. What were 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 continents movement across a subduction zone, where slabs of the Earths 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 teams discovery is important because it shows the way in which the interaction between the Earths 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 movement perhaps responsible for forming Sydneys bedrock, the Hawkesbury sandstone.
Geophysicist Louis Moresi from Melbournes Monash University, who was not involved in the research, says the teams 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.
“五步法”攻克四六级听力选择题
2013英语四级听力答题技巧:说明文解析
英语四级听力:职业身份题相关信息词
四六级考试在即 四六级考试大神成功经验助考
英语听力过关场景词必备
英语四级考前20天名师问答——听力篇
2013听力提高法:能力培养+题型结合+模拟
英语四级听力:如何应对复合式听写
英语级听力正确答案有哪些特征?
英语四级听力解题技巧:预读
找不到感觉如何应对?
2013英语四级短文听力材料篇例题解析
英语四六级听力提分策略
英语四六级听力冲刺:复合式听写如何提分
英语听力常用人名、地名、国家名
英语四级听力考试得分7类关键词
考前一个月突破四级听力“四遍式”方法
英语四级听力技巧:谨防四大误区
四六级考试难度持续提升 名师支招如何应对
英语四级听力答题应试技巧
冲刺浓缩7大技巧
英语听力提高法:有意识训练法和无意识训练法
英语四级听力考试临考心态如何调整?
英语四级听力细节辨认题
英语听力考试题型及要点解析
英语四级听力考试复习的三副良药
英语四级听力真题训练六步法
英语四级听力中的十大类标志词
英语四六级听力有效提分三大要点
英语四级听力:逻辑分析及思维能力需提升
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |