时差N小时 地球磁场的变化(上)（在线收听）

You’re a polar explorer and you’ve just reached the north pole. The huskies are yapping, the cameras are ready and you’re just about to unfurl the flag when, all of a sudden, the north pole becomes the south pole. Imagine your chagrin. Okay, so it doesn’t really happen that fast, but it’s true that in the past the earth’s magnetic poles have switched places. In fact, it seems to happen roughly every 250,000 years or so. How do we know this? When lava is in its molten state, the atoms in it can move around fairly easily. At this time some atoms, like iron, will align themselves with the earth’s magnetic field in a recognizable way. Then, as the lava solidifies into rock, these little compasses get frozen in position. It’s like writing your name in wet cement.

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你是名极地探险者，你刚刚到达北极。爱斯基摩犬在狂吠，相机已经准备好了，你正要展开旗帜，突然，北极变成了南极。想象下你的懊恼。事实上，磁场变换并没有那么快。但是在过去地球的磁场确实发生了变换，事实上，大概每25万年左右，磁场就要发生改变。我们是如何知道的？当熔岩处于融化状态时，其中的原子能轻易地移动。这时候，有些原子比如铁原子，以一种可识别的方式按照地球磁场的方向排列，当熔岩凝固成岩石时，这些小的指南针会在原地冷凝。这就像将你的名字写在湿的水泥上。