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 真正的世界末日其实在2012年，如果这一天真的到来的话，我们该做点什么呢？ |
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磁极翻转的依据是什么？ -- 惊涛拍岸 - (0 Byte) 2008-7-25 周五, 02:40 (496 reads) |
Big
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加入时间: 2008/06/04
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作者：Big 在 海归商务 发贴, 来自【海归网】 http://www.haiguinet.com
[Ref. Wikipedia]
Magnetic field reversals
Main article: geomagnetic reversal
based upon the study of lava flows of basalt throughout the world, it has been proposed that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. The last such event, called the Brunhes-Matuyama reversal, is theorized to have occurred some 780,000 years ago.
There is no clear theory as to how the geomagnetic reversals might have occurred. 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 magnetic "North" pole pointing either North or South. External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when magnetic "North" reappears in the opposite direction this is a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion.
Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day at some time in Earth's history, which significantly challenges the popular understanding of how the Earth's magnetic field works. [4]
This has been found to be consistent, by measuring magnetism across ocean ridges. The molten lava (typically basalt or tholeiite) is extruded from volcanoes at well over the Curie temperature and then cools to adopt whatever magnetic field was present at the time. As time goes on more lava flows and bands of opposite magnetic fields are made present.
Using a magnetic detector (a variant of a compass), scientists have measured the historical direction of the Earth's magnetic field, by studying sequences of relatively iron-rich lava flows. Typically such layers have been found to record the direction of Earth's magnetic field when they cool (see paleomagnetism). They have found that the poles have shifted a number of times throughout the past.
作者：Big 在 海归商务 发贴, 来自【海归网】 http://www.haiguinet.com
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