Magnetic Sampling and Analysis

determine the location of the magnetic pole. By assuming that the magnetic pole position is always near the rotational pole position for the Earth the geologist can then draw conclusions about the location of the rock when it was lithified, and thus the location of the continents through time. While such paleomagnetic evidence is compelling as a proof for the plate tectonic theory, the best evidence for the movement of the plates came with the study of the ocean floors.

Rocks taken for magnetic testing must be sampled very precisely by measuring the samples orientation with respect to true north. Geologists use two methods, alternating field demagnetization and thermal demagnetization, to demagnetize overlaid magnetic patterns in the rock. Once the sample is cleaned up its magnetic properties, intensity, direction, and inclination, are measured by either a spin magnetometer or a cryogenic magnetometer. The results allow geologists to determine the relative location of the continents at the time that the rock was lithified.

References:
Seyfert, Carl and Sirkin, Leslie; Earth History and Plate Tectonics: an Introduction to Historical Geology; 1973, Harper & Row Publishers.

Prothero, Donald R.; Interpreting the Stratigraphic Record; 1990, W.H. Freeman & Company.

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