時差N小時:地球磁場的變化(下)

By calculating how old the rocks are where we findthese ancient compasses, we can figure out howlong ago the magnetized parts were locked in. Thattells us which direction was magnetic north tens ofthousands of years ago. Surprisingly, it’s not alwaysthe same as north is now. Occasionally the poleschange places. What is the reversal period like? So,did the north pole drift slowly southward, so that atsome point magnetic north was in, say Mexico city? No. The rocks tell us that, during polereversals, there is a several thousand year period during which the earth’s magnetic field iseither gone or very much weakened. At these times north and south become arbitrary terms.Eventually the magnetic field re-establishes itself, and it seems to be a matter of chancewhich side comes out north and which south.

通過計算在發(fā)現(xiàn)古老指南針地點的巖石年齡，我們能計算出被磁化的部分已經(jīng)被封存了多久。那會告訴我們數(shù)萬年前那個方向是北極。令人驚奇的是，北極并不總是在一個方向。有時候南北極會調(diào)換。磁極調(diào)換期間會是什么樣子?北極會緩慢地變?yōu)槟蠘O，所以在某個階段，磁極可能在墨西哥城?不是的。巖石告訴我們，在磁極變換過程中，有幾千年地球的磁場要么是消失不見了，要么是被大大削弱。在這期間，南北極變得十分隨意。最終磁極重新恢復(fù)，那端變?yōu)楸睒O，那端成為南極都是偶然的。

calculate vt.計算，核算

magnetized (magnetize的過去分詞)意思相當(dāng)于形容詞，指“磁化了的”

reversal n. 翻轉(zhuǎn)

weakened (weake的過去分詞)意思相當(dāng)于形容詞，指“削弱了的”

arbitrary adj. 任意的

re-establish v.恢復(fù)