Investigations of the orientation and(or) intensity of the Earth's magnetic field in the past, as recorded in geologic materials. The magnetic poles wander about the Earth's axis of rotation, and the paleomagnetic pole position at the time of cooling of a volcanic rock is "frozen" by magnetic minerals. An empirical calibration of this "secular variation" over time allows eruption ages to be constrained and isolated outcrops to be correlated with one another.

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