Magnetic Sampling and Analysis

When a geologist measures a rocks magnetism in the lab, all the magnetization the rock has acquired since it was first formed is still present. This is called the rocks natural remanent magnetization (NRM) and frequently the original (or primary) magnetization - the one the geologist is interested in - is overprinted by a younger, but weaker, magnetization picked up by the Earth's current magnetic field. The geologist must "clean up" this overprinted magnetization so that only the primary magnetization remains. This "cleaning up" is accomplished by demagnetizing the weaker magnetic fields.

Demagnetization can be accomplished by one of two methods. With many samples both methods are used.

Alternating Field (AF) Demagnetization: A sample is placed in a strong alternating field. As the magnetic minerals in the sample respond to the rapid changes of the field direction the weaker components of magnetization are removed. As the field strength increases more of the magnetization in the rock sample is removed. Finally, after the weaker fields have been cleaned up, only the primary field remains.

Thermal demagnetization: In this case the rock sample is heated to higher temperatures. The rock is cooled in a place shielded from the Earth's magnetic field and then measured. The overprinted magnetization disappears at the lower temperatures as the rock is heated. The primary component will remain unless the rock is heated above the Curie point of the magnetic mineral. Different magnetic minerals have different Curie points, so this method can be useful to remove the magnetization of minerals with lower Curie points.

The process of demagnetization is a complicated, long, and tedious procedure. Sometimes the samples are never completely demagnetized or the primary magnetization is accidentally removed. Demagnetization problems often lead to bad results and the geologist makes erroneous conclusions based on the bad data.

In the end, the careful sampling, cleaning up, and measurement of the rock samples will show the intensity, direction, and inclination of the Earth's magnetic field when the rocks was lithified. The magnetic inclination can be used to determine the magnetic latitude at which the rock formed. A high inclination indicates that the rock lithified near either the magnetic north or south pole. A low inclination indicates the rock lithified near the magnetic equator. The direction of the magnetic field is used to determine the location of the magnetic pole. Several samples collected from a wide geographic area can be used to determine

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