| abstract
| - In the early 20th century geologists first noticed that some volcanic rocks were magnetized in a direction opposite to what was expected. The first examination of the timing of magnetic reversals was done by Motonori Matuyama in the 1920s, who observed that there were rocks in Japan whose magnetic fields were reversed and those were all of early Pleistocene age or older. At the time he published his proposal suggesting that the magnetic field had been reversed, the magnetic field itself was poorly understood so there was little interest in the possibility that it had reversed. Three decades later, theories existed of the cause of the magnetic field and some of these included the possibility of field reversal. 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. 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. Because of the complex routes of cruises, associating navigational data with magnetometer readings was difficult. But when data was plotted on a map, it became apparent that there were remarkably regular and continuous magnetic stripes across the ocean floors. 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. Canadian L. W. Morley independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journals Nature and Journal of Geophysical Research, and not published until 1967 in the literary magazine Saturday Review. Further information: Morley-Vine-Matthews hypothesis 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. The same magnetic anomalies were found over most of the world's oceans, and allowed estimation of the timing of the creation of most of the oceanic crust. Through analysis of palaeomagnetic data, we now know that the field has reversed its orientation tens of thousands of times since its formation very early on in earth history. With the increasingly accurate Global Polarity Timescale (GPTS) it has become apparent that the rate at which reversals occur has varied considerably throughout the past. During some periods of geologic time (e.g. Cretaceous Long Normal), the Earth's magnetic field is observed to maintain a single orientation for tens of millions of years. 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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![[RDF Data]](/fct/images/sw-rdf-blue.png)