托福閱讀材料之Taking a Volcano’s Temperature

　　火山噴發(fā)(volcanic eruption)是一種奇特的地質(zhì)現(xiàn)象，是地殼運動的一種表現(xiàn)形式，也是地球內(nèi)部熱能在地表的一種最強烈的顯示。是巖漿等噴出物在短時間內(nèi)從火山口向地表的釋放。由于巖漿中含大量揮發(fā)分，加之上覆巖層的圍壓，使這些揮發(fā)分溶解在巖漿中無法溢出，當(dāng)巖漿上升靠近地表時，壓力減小，揮發(fā)分急劇被釋放出來，于是形成火山噴發(fā)。

　　Most of the time, volcanoes look like other mountains: solid, serene, capped with snow–a nice place to go camping.

　　Unless, of course, it happens to explode and pour molten lava all over your campsite! Wouldn’t it be nice if we could predict when a volcano was about to erupt? Scientists have recently taken a big step closer toward doing just this.

　　Let’s look inside a volcano. At the center is a reservoir of molten rock, known as magma. This magma is very hot, and under a lot of pressure. When rainwater seeps into cracks in the mountain, the magma turns it into steam and forces it out again. The magma also vents away hot gasses through these same cracks.

　　This is a volcano’s normal state. When gasses and steam vent from a volcano’s cracks, it’s probably not about to erupt. Sometimes, however, the top layer of magma can cool, and form a hard crust over the rest of the molten rock. When this happens, rainwater is able to collect in the cracks without evaporating, and this water captures a lot of gasses that would ordinarily vent from the magma. Scientific instruments inside the volcano’s cracks record a sudden drop in temperature, and much less volcanic gas.

　　This is a warning sign for an imminent eruption. That cool magma seal allows pressure to build up inside the liquid magma, much like your thumb over the top of a shaken soda bottle will seal in the pressure. When that magma seal gives way it’s like taking your thumb off the top of the bottle, and you don’t want to be camping on the volcano when that happens!

　　譯文：

　　大多數(shù)時候，火山看起來和其他的山體沒有兩樣：固體，平靜，被積雪覆蓋-露營的不錯選擇。

　　除非，當(dāng)然，它突然噴發(fā)，熔漿覆蓋你的營地。如果我們能夠預(yù)測火山什么時候會噴發(fā)，是不是更好呢?科學(xué)家們最近在此領(lǐng)域的研究取得了進展。

　　讓我們一起來看看火山的內(nèi)部結(jié)構(gòu)吧!中心是一個熔巖的“液態(tài)區(qū)”，叫巖漿。巖漿溫度很高，并且處于一定的壓力之下。當(dāng)雨水滲入山縫后，巖漿將其蒸發(fā)為水蒸汽，又讓它排放出來。巖漿也同樣通過這些山縫散發(fā)一些熱氣。

　　這是一座火山的正常狀態(tài)。當(dāng)氣體和蒸汽可以從火山的裂縫排放時，它可能并不會噴發(fā)。然而，有時頂層的巖漿會冷卻，并在外部形成一個堅硬的殼。這時，雨水聚集在巖縫中，不能蒸發(fā)。這些雨水阻止了巖漿熱氣的排放。在山縫中的科學(xué)儀器記錄到了溫度的陡然下降，和火山氣體的減少。

　　這是即將噴發(fā)的一個預(yù)兆。冷卻的巖漿殼讓內(nèi)部的液體壓力增大。就像把你的拇指放在被搖過的蘇打水瓶口上，使其密封，增加內(nèi)部壓力。巖漿上部的密封殼被沖開時，就像你的拇指從蘇打水瓶上拿開的時候一樣，在這種情況下，你就不會想要在火山上露營了。