Public message 3283 SCIENCE Area 19:51 Wednesday 4-Sep-91 To: MIKE DAVEY Re: EARTH'S M

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Public message 3283 SCIENCE Area 19:51 Wednesday 4-Sep-91 From: HENRY SHAW To: MIKE DAVEY Re: EARTH'S MAGNETIC FIELD 1 MB> I understand that 'magnetic mapping' (for lack of the real term) MB> has produced evidence that the magnetic 'north pole' has shifted MB> many times in the earths past leaving some forms of rocks with MB> magnetic 'map' pointing to where the magnetic pole was at the MB> time the rock was formed. MB> Just out of curiousity, do you happen to know how many times this MB> shift is evident and the average time interval between them, and MB> when the last one was? Many times. Dozens. But I don't know an exact number. The last major reorganization of the magnetic field was a reversal at about 700,000 years ago. There does not appear to be any regularity to the pattern of reversals. That is, they are not periodic. Recently, people have postulated that they are a "chaotic" (in the strict sense) phenomenon. Our knowledge of the history of the earth's magnetic field, naturally enough, gets poorer as one goes back in time. This is due to several things: (1) older rocks are relatively rarer than younger rocks; (2) older rocks are more likely to have undergone more complex histories (e.g. undergone multiple heating events, which act to "reset" the remanent magnetism); (3) absolute errors (not percentage errors) on the age of samples increases with the age of the sample, making it difficult to do the correlations required for high resolution magnetostratigraphy. The basis for paleomagnetic studies is the fact that many igneous rocks contain minerals that are naturally magnetic (magnetite, or "lodestone" is an obvious example of a common rock forming mineral like this, but there are others). These minerals are only magnetic below a certain temperature known as the "Curie point". Above this temperature, the minerals have no net magnetism. When a rock cools below this temperature, the minerals acquire a magnetic moment (i.e. become like little compass needles) aligned with the earth's field. This direction is "locked in" over a relatively small temperature range, and persists even if the the relative orientation of the rock and the earth's field change at some later time (either by movement of the field, or by movement of the rock). [The following stuff is taken from a message I wrote to Pat Goltz in this echo several years ago.] The magnetic field of the earth is not a static thing. It undergoes fluctuations on timescales from milliseconds to hundreds of millions of years. The high frequency fluctuations are due to external (to the earth) causes, mainly variations in the solar wind. The longer-term fluctuations are due to variations in the motion of the fluid outer core of the earth at a depth of ~2900km. (For a review of the subject, see: Courtillot, V. and LeMouel, J.-L., 1988, Geomagnetic Time Variations, Annual Review of Earth and Planetary Science, v16, pp389-476.) continued next message.... --- TBBS v2.1/NM * Origin: Diablo Valley PCUG-BBS, Walnut Creek, CA 510/943-6238 (1:161/55)


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