شرکت در آزمون آنلاین زبان انگلیسی 1 - زبان انگلیسی 1 | آزمون شماره 12976

Earth’s magnetic poles do not stay in one place. Today, the magnetic pole in the north is in a different place from where it was 20 years ago. In fact, not only does the position of the magnetic poles change, but also the Earth’s magnetic field sometimes reverses direction. For example, 700 thousand years ago, a compass needle that now points north would point south. (During the past 20 million years, Earth’s magnetic field has reversed direction more than 70 times.) The magnetism of ancient rocks contains a record of these magnetic field changes. When some types of molten rock cool, magnetic domains of iron in the rock line up with the Earth’s magnetic field. After the rock cools, the orientation of these domains is frozen into position. Consequently, these old rocks preserve the orientation of the Earth’s magnetic field as it was long ago.

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