英语听力文摘507 山脉不再上升
山脉为什么停止上升了
If you live up in the mountains, understanding the continually shifting landscape can mean survival.
Mountains are continually shifting? Because of the erosion? Not just erosion.
Erosion wears mountains down, but active mountain ranges are also continuously rising. And here’s the interesting thing: despite this, once mountains reach a certain height, they achieve a state of equilibrium. So they rise but do not get any taller? Weird.
You see, mountain ranges form in places where tectonic plates meet–those are the thick sheets of rigid rock that cover the earth’s surface. When one tectonic plate slides beneath the other, the parts of it that are scraped off pile up in buckling folds to form mountains.
Of course, we’re talking geological time here, which is measured in tens of thousands of years. So one question geologists are investigating is which forces counteract the shifting and colliding of the tectonic plates and enable still- active mountain ranges to level off at a certain height.
And that's where erosion comes yet. Yes, but erosion is caused by many factors, like weather and rivers carving into the mountains. Also, as mountains get higher, they get steeper, and the greater the slope, the more sediment ends up sliding down into the oceans instead of accumulating on the mountains themselves.
So the big question is what role each of these factors plays in maintaining the height of mountain ranges. And that’s where the debate comes in– and it’s a tough one.
如果你居住在山上，了解持续不断移动的风景就意味着生存。山脉是不断移动的吗?因为侵蚀?不仅仅是因为侵蚀。
侵蚀磨损山体使之高度下降。但是活跃的山脉也同样不断在上升。这有个有趣的现象：尽管山脉会不断上升，但上升到一定高度后，它们就达到一种平衡状态。所以它们上升但高度不会继续增加?真不可思议。
你看。山脉是在地壳板块交接的地方形成的，地壳板块是覆盖地球的坚硬岩石厚片。当某地壳板块俯冲到另一板块下面时，被摩擦掉的那部分岩石就以褶皱的形式堆积起来形成山脉。
当然，我们现在谈论的是地史时期，是以数万年测量的。因此地质学家一直研究的一个问题就是哪些力量中和了板块的移动和碰撞，并使得仍活跃的山脉在某一特定高度呈平衡状态。
那就是侵蚀出现的原因了。是的，但是侵蚀是由多种原因引起的，比如天气和侵蚀山脉的河流。同样，山脉变高时，他们也变陡峭了，坡度越大，就会有越多的沉积物滑向海洋，而不是堆积在山脉上。
所以最大的问题就是这些因素在保持山脉高度时各自扮演着什么角色。而且那就是争论的源头——是个难题。
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