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Science Frontiers ONLINE No. 34: Jul-Aug 1984 |
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Behind magnetic flip-flops
The earth's magnetic field frequently reverses its polarity. Such flips can of-ten be correlated with climate changes, global ice volumes, sea-floor spreading rates, and deposition of black shales, tektite falls, biological extinctions, etc. The frustrating thing is the lack of clear-cut cause and effect; that is, how these phenomena are linked physically to the geomagnetic field. Part of the problem is that we can only guess at how the geomagnetic field is generated.
Let us assume that the earth's magnetic field is created by dynamo action in the planet's fluid core. P. Olson finds analytically that the core dynamo may reverse sign due to fluctuations in core turbulence caused by two competing energy sources: heat loss at the mantle-core boundary and progressive growth of the inner core. In concept, the heat lost at the core-mantle boundary might be linked to climate changes and sea-floor spreading.
Taking a different tack, D. Gubbins has investigated the possibility that field reversals are triggered by ice ages and meteorite impacts (tektite falls). The physical mechanism here would be the increase in pressure upon the core, which affects the rate of freezing in the outer core, and thus the power available to the core dynamo. Gubbins found that these externally caused pressure changes were too small to explain the polarity changes. However, the parameters involved are not well-known, and external triggers cannot yet be written off. Summarizing, very little progress has been made in explaining how the earth's field is generated and how polarity changes are linked to other geophysical parameters.
(Jacobs, J.A.; "What Triggers Reversals of the Earth's Magnetic Field?" Nature, 309:115, 1984.)
Reference. Anomalous reversals of the geomagnetic field appear in section EZP2 in our Catalog: Inner Earth. To order this book, visit: here.