英語(yǔ)聽(tīng)力文摘507 山脈不再上升

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山脈為什么停止上升了

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，了解持續(xù)不斷移動(dòng)的風(fēng)景就意味著生存。山脈是不斷移動(dòng)的嗎?因?yàn)榍治g?不僅僅是因?yàn)榍治g。

侵蝕磨損山體使之高度下降。但是活躍的山脈也同樣不斷在上升。這有個(gè)有趣的現(xiàn)象：盡管山脈會(huì)不斷上升，但上升到一定高度后，它們就達(dá)到一種平衡狀態(tài)。所以它們上升但高度不會(huì)繼續(xù)增加?真不可思議。

你看。山脈是在地殼板塊交接的地方形成的，地殼板塊是覆蓋地球的堅(jiān)硬巖石厚片。當(dāng)某地殼板塊俯沖到另一板塊下面時(shí)，被摩擦掉的那部分巖石就以褶皺的形式堆積起來(lái)形成山脈。

當(dāng)然，我們現(xiàn)在談?wù)摰氖堑厥窌r(shí)期，是以數(shù)萬(wàn)年測(cè)量的。因此地質(zhì)學(xué)家一直研究的一個(gè)問(wèn)題就是哪些力量中和了板塊的移動(dòng)和碰撞，并使得仍活躍的山脈在某一特定高度呈平衡狀態(tài)。

那就是侵蝕出現(xiàn)的原因了。是的，但是侵蝕是由多種原因引起的，比如天氣和侵蝕山脈的河流。同樣，山脈變高時(shí)，他們也變陡峭了，坡度越大，就會(huì)有越多的沉積物滑向海洋，而不是堆積在山脈上。

所以最大的問(wèn)題就是這些因素在保持山脈高度時(shí)各自扮演著什么角色。而且那就是爭(zhēng)論的源頭——是個(gè)難題。