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Geomagnetic polarity dating

In 1963 Frederick Vine and Drummond Matthews provided a simple explanation, by combining the seafloor spreading theory of Harry Hess with the known time scale of reversals: if new sea floor acquired the present magnetic field, spreading from a central ridge would produce magnetic stripes parallel to the ridge. Starting in 1966, Lamont-Doherty Geological Observatory scientists found the magnetic profiles across the Pacific-Antarctic Ridge were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridges.

Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces resulting from very large impact events.This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7-15 years (see: solar cycle).However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.Originally, however, the past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor. Morley, Frederick John Vine and Drummond Hoyle Matthews made the connection to seafloor spreading in the Morley-Vine-Matthews hypothesis which soon led to the development of the theory of plate tectonics.Given that the sea floor spreads at a relatively constant rate, this results in broadly evident substrate "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer along the sea floor.At first it seemed that reversals happen every one million years, but during the 1960s it became apparent that the time between reversals is erratic.During the 1950s and 1960s research ships gathered information about variations in the Earth's magnetic field.Most paleomagnetic research in the late 1950s was examining the wandering of the poles and continental drift.Although it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks contained traces of the Earth's magnetic field at the time the rock cooled.Other events seem to have occurred very rapidly, with two reversals in a span of 50 thousand years.The last reversal was the Brunhes-Matuyama reversal approximately 780 thousand years ago.

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