On a previous show we discussed how, every 200,000 years or so, the north and south poles switch places. Not that the places themselves move—rather, the entire magnetic field of the earth flips around, resulting in the pole we now call “north” being on the southern tip of the planet, and vice versa. Why does this happen? Although it seems pretty solid, the outer core of our planet is in a molten state. That means it’s partway between what most folks would call liquid and solid. Much of this molten interior is iron and nickel. These are elements that conduct electricity very well. It’s the fluid metals that move about inside earth that generate a magnetic field around our entire planet. Although from close up these metals are bubbling like hot soup, you could also say that in general their motions follow a pretty steady pattern. That’s why the magnetic field on earth is pretty stable.

【生词注释】
switch v.转换
magnetic field 磁场
flip vt.快速翻动
tip n. 顶端
solid adj. 固体的, 坚固的
molten adj. 熔化的
partway adv. 到某种程度
interior n. 内部
close up n. 靠近
bubble v. 冒泡, 沸腾

【参考译文】

在先前的节目中我们讨论了每隔20万年，地球的南北极都要互换位置，不仅是地点的移动，而是整个地球的磁场快速翻转，导致我们现在称作“北极”的地方原来在地球的南端，正好与原来磁场相反。这种现象为什么会发生呢？尽管地球的外壳看起来很牢固，实际上它处于融化的状态中，那意味着地壳处于液体和固体间的中间形态。这种溶化物质的大部分组成部分是铁和镍。这些都是极易导电的物质。它们也是在地球内部流动的液体金属，从而形成了整个地球的磁场。尽管从近处看，这些金属就像热汤那样冒泡，你也可以说从整体上来看，他们的运动遵循着一种十分稳定的模式。那也是地球磁场十分稳定的原因。