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.

【生詞注釋】
switch v.轉換
magnetic field 磁場
flip vt.快速翻動
tip n. 頂端
solid adj. 固體的, 堅固的
molten adj. 熔化的
partway adv. 到某種程度
interior n. 內部
close up n. 靠近
bubble v. 冒泡, 沸騰

【參考譯文】

在先前的節目中我們討論了每隔20萬年，地球的南北極都要互換位置，不僅是地點的移動，而是整個地球的磁場快速翻轉，導致我們現在稱作“北極”的地方原來在地球的南端，正好與原來磁場相反。這種現象為什么會發生呢？盡管地球的外殼看起來很牢固，實際上它處于融化的狀態中，那意味著地殼處于液體和固體間的中間形態。這種溶化物質的大部分組成部分是鐵和鎳。這些都是極易導電的物質。它們也是在地球內部流動的液體金屬，從而形成了整個地球的磁場。盡管從近處看，這些金屬就像熱湯那樣冒泡，你也可以說從整體上來看，他們的運動遵循著一種十分穩定的模式。那也是地球磁場十分穩定的原因。