DEVELOPMENT OF THE GEOMAGNETIC POLARITY REVERSAL TIMESCALE

John W. Hillhouse

Photo of Paleomagnetic Lab at the USGS Menlo Park, CA.

During 1959-1966, the scientists of the Rock Magnetics Laboratory in Menlo Park were involved in a race to prove that reverse-polarity magnetization in rocks was the result of global reversals of the geomagnetic field. To prove their case, the scientists had to show that rocks of the same age throughout the world had the same magnetic polarity. They collected samples from Alaska to Antarctica and from Hawaii to New Mexico. In the laboratory, they measured the magnetic polarity of rock samples with a spinner magnetometer and determined the ages of samples with an argon mass spectrometer. As more and more polarity-age pairs were gathered at this laboratory and elsewhere, a consistent pattern began to emerge, demonstrating that indeed the earth’s magnetic field had reversed polarity many times in the past. Within a few years, the Geomagnetic Polarity Reversal Timescale was published. When it was recognized that the polarity timescale was matched by magnetic patterns on the sea floor, the concepts of sea-floor spreading and mobile continents became widely accepted. This discovery about earth magnetism became the foundation for modern plate-tectonic models that explain mountain building, the formation of ocean basins, and the distribution of continents.

In 1995, recognition of the fundamental scientific contributions made by Allan Cox, Richard Doell, and Brent Dalrymple from 1959 to 1966 led to listing of the Rock Magnetics Laboratory on the National Register of Historic Places. Accordingly, the essence of the original laboratory and many of the historic research materials are preserved in an exhibit. A documentary video, "Secrets in Stone," was also commissioned to record this history.

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