托福阅读材料之Taking a Volcano’s Temperature
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!
译文：
大多数时候，火山看起来和其他的山体没有两样：固体，平静，被积雪覆盖-露营的不错选择。
除非，当然，它突然喷发，熔浆覆盖你的营地。如果我们能够预测火山什么时候会喷发，是不是更好呢?科学家们最近在此领域的研究取得了进展。
让我们一起来看看火山的内部结构吧!中心是一个熔岩的“液态区”，叫岩浆。岩浆温度很高，并且处于一定的压力之下。当雨水渗入山缝后，岩浆将其蒸发为水蒸汽，又让它排放出来。岩浆也同样通过这些山缝散发一些热气。
这是一座火山的正常状态。当气体和蒸汽可以从火山的裂缝排放时，它可能并不会喷发。然而，有时顶层的岩浆会冷却，并在外部形成一个坚硬的壳。这时，雨水聚集在岩缝中，不能蒸发。这些雨水阻止了岩浆热气的排放。在山缝中的科学仪器记录到了温度的陡然下降，和火山气体的减少。
这是即将喷发的一个预兆。冷却的岩浆壳让内部的液体压力增大。就像把你的拇指放在被摇过的苏打水瓶口上，使其密封，增加内部压力。岩浆上部的密封壳被冲开时，就像你的拇指从苏打水瓶上拿开的时候一样，在这种情况下，你就不会想要在火山上露营了。
