万物简史 第310期:地下的烈火(18)（在线收听）

 　　So all that can be said is that at some slightly indeterminate point as we head toward the center of Earth we leave the asthenosphere and plunge into pure mantle. 因此，我们只能说，在前往地球中心的过程中，我们会在某个不大确定的地点离开软流圈，进入纯粹的地幔。

　　Considering that it accounts for 82 percent of the Earth's volume and 65 percent of its mass, 地幔占到地球体积的82%，质量的65%，
　　the mantle doesn't attract a great deal of attention, 而之所以没有引起足够的重视，
　　largely because the things that interest Earth scientists and general readers alike 很大程度上是因为地球上的科学家和普通读者感兴趣的东西，
　　happen either deeper down (as with magnetism) or nearer the surface (as with earthquakes). 不是在地下深处（比如磁力），就是接近地面（比如地震）。
　　We know that to a depth of about a hundred miles the mantle consists predominantly of a type of rock known as peridotite, 我们知道，到了大约150公里的深处，地幔主要是由一种名叫橄榄岩的岩石组成，
　　but what fills the space beyond is uncertain. 但以下的2650公里是什么就不清楚。
　　According to a Nature report, it seems not to be peridotite. 《自然》杂志的一篇报道说，似乎不是橄榄岩。
　　More than this we do not know. 更多的情况我们就不知道了。
　　Beneath the mantle are the two cores—a solid inner core and a liquid outer one. 地幔下面是两个地核，一个坚硬的内核，一个液态的外核。
　　Needless to say, our understanding of the nature of these cores is indirect, but scientists can make some reasonable assumptions. 不用说，我们对两个地核的性质的了解是间接的，但科学家们可以作一些合理的假设。
　　They know that the pressures at the center of the Earth are sufficiently high, 他们知道，地球中央的压力很大，
　　something over three million times those found at the surface—to turn any rock there solid. 大约是地面上最大压力的300多万倍——足以使那里的岩石变得坚硬。
　　They also know from Earth's history (among other clues) that the inner core is very good at retaining its heat. 他们还（在许多别的线索之中）从地球史中得知，内核很善于保存自己的热量。
　　Although it is little more than a guess, it is thought that in over four billion years the temperature at the core has fallen by no more than 200 degrees Fahrenheit. 尽管只不过是个猜测，据认为地核的温度在过去的40多亿年间下降了不到110摄氏度。
　　No one knows exactly how hot the Earth's core is, 谁也不知道地核的温度到底有多高，
　　but estimates range from something over 7,000 degrees Fahrenheit to 13,000 degrees Fahrenheit—about as hot as the surface of the Sun. 但估计是在4000-7000摄氏度——大致相当于太阳表面的温度。
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