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Magnetic ReversalThe Earth's Magnetic Field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of ~250,000 years. It is believed that this last occurred some 780,000 years ago, referred to as the Brunhes-Matayama reversal. Past field reversals are recorded in the "frozen" magnetic domains of solidified lava that has welled up along spreading ocean floor ridges; since the sea floor spreads at a fairly constant rate, this results in broad "stripes" of sea floor from which the past magnetic field direction can be read. At least once in Earth's history, the magnetic field held a constant direction for as long as 30 million years (see: Cretaceous long normal). The mechanism responsible for geomagnetic reversals is not well understood. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the North pole pointing either up or down. When the North reappears in the opposite direction, we would interpret this as a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion. Magnetic reversals are very useful to paleontologists as they serve as a basis for dating fossils. This is generally used in combination with Radiometric dating. Radiometric dating generally provides an absolute date with a wide margin of error, while magnetic reversals can provide a much smaller margin of error when comparing two locations, but no absolute date. At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the field to disappear, albeit temporarily, by about 4000 AD.1 Other sources have put the date of field collapse as early as 3000 AD. The deterioration began roughly 150 years ago and has accelerated in the past several years. So far the strength of the earth's field has decreased by 10 to 15 percent. However, one should note that no one knows if field decay will continue in the future. Also, since a magnetic field reversal has never been observed by humans, and the mechanism of field generation is not well understood, it is difficult to say what the characteristics of the magnetic field might be leading up to such a reversal.
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