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In hot Iron bearing rocks or magmas the iron minerals they contain align themselves parallel to the Earth's magnetic field. When they cool below about 500 degrees Celsius the the iron minerals 'freeze' in this magnetic orientation and are no longer able to adapt to any changes in the Earth's field (which occasionally reverses polarity and wanders). This frozen magnetic alignment is a fossil of the ancient magnetic field and can be 'read' to find out what the Earth's magnetic field has been doing in the past.

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15y ago

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