How can we tell the Earth's magnetic poles reverse polarity with rocks?

How can we tell the Earth's magnetic poles reverse polarity with rocks?

• A. When rocks are molten, they magnetize, and when they solidify, they retain the same magnetic polarity
• B. Rocks reveal polarity only during meteor impacts
• C. Rocks never indicate polarity
• D. Rocks don't record polarity

Answer: (A)

Rocks preserve the Earth's magnetic field at the moment they solidify. As molten rock cools, magnetic minerals in the melt, like magnetite, line up with the direction of the Earth's current magnetic field. When the rock hardens, that alignment is locked in, so the rock records the polarity at that time. Over long periods, the field has reversed many times, so rocks of different ages show alternating normal and reversed polarity. By studying many rocks, scientists can map when these reversals occurred. This is why the mechanism described—magnetization during cooling and retention once solid—is the correct way to tell past polarity from rocks. The other ideas don’t fit because rocks do record polarity, not only during meteor impacts, and they do not fail to indicate polarity.

Rocks preserve the Earth's magnetic field at the moment they solidify. As molten rock cools, magnetic minerals in the melt, like magnetite, line up with the direction of the Earth's current magnetic field. When the rock hardens, that alignment is locked in, so the rock records the polarity at that time. Over long periods, the field has reversed many times, so rocks of different ages show alternating normal and reversed polarity. By studying many rocks, scientists can map when these reversals occurred. This is why the mechanism described—magnetization during cooling and retention once solid—is the correct way to tell past polarity from rocks. The other ideas don’t fit because rocks do record polarity, not only during meteor impacts, and they do not fail to indicate polarity.