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托福听力材料:海底扩张

2016-07-25 11:41:54来源：网络

　　Mid-ocean ridges and seafloor spreading can also influence sea levels. As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense. This increases the volume of the ocean basin and decreases the sea level. For instance, a mid-ocean ridge system in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea—contributed to shallower oceans and higher sea levels in the Paleozoic era. Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years.

　　Seafloor spreading disproves an early part of the theory ofcontinental drift. Supporters of continental drift originallytheorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity.

　　Keeping Earth in Shape

　　Seafloor spreading is just one part of plate tectonics. Subduction is another. Subduction happens where tectonic plates crash into each other instead of spreading apart. At subduction zones, the edge of the denser plate subducts, or slides, beneath the less-dense one. The denser lithospheric material then melts back into the Earth's mantle.

　　Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant.

　　海底扩张(Sea-FloorSpreading)到20世纪50年代，地理学家们才能用先进的技术测绘出海底世界。测绘结果显示：海底有座相当高耸的海洋“山脊”，形成了一道水下“山脉”，绵延约83683.6千米，穿过世界上所有的海洋，海洋底部的“山脊”也叫断裂谷，断裂谷里不断地冒出岩浆，岩浆冷却后，在大洋底部造成了一条条蜿蜒起伏的新生海底山脉，这个过程就叫海底扩张，而这些新生的海底山脉则称为海岭。由于断裂谷里添了新岩石，断裂谷两边的岩石就逐渐远离了洋脊中央。所以，距离“山脉”越远的岩石就越古老。所以科学家们都说“古老的海洋，年轻的洋底。”当海岭和新的海底平原形成后，断裂谷的岩浆还会继续喷出，它们起着“传送带”的作用，把一条条新海岭从地壳岩层中推送出来，同时又把它们慢慢地从地壳岩层中推落下去，重新熔化到地幔中去，达到新生和消长的平衡。

　　以上就是新东方在线托福网为你带来的托福听力材料:海底扩张，更多精彩敬请关注新东方在线托福网。

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