Richard R. Doell
1969
<div class="hlFld-Abstract"><div class="NLM_paragraph">Direct<span> </span>measurements<span> </span>of the direction and strength of the earth's<span> </span>magnetic field<span> </span>have provided a knowledge of the field's form and behavior during the last few hundreds of years. For older times, however, it has been necessary to<span> </span>measure<span> </span>the<span> </span>magnetism<span> </span>of certain rocks to learn what the geomagnetic field was like. For example, when a lava flow<span> </span>solidifies<span> </span>(at<span> </span>temperatures<span> </span>near 1000°C) and cools through the<span> </span>Curie point<span> </span>of the<span> </span>magnetic minerals<span> </span>contained in it (around 500°C) it acquires a remanent<span> </span>magnetism<span> </span>that is (1) very weak, (2) very stablel, (3) paralle to the direction of the ambient geomagnetic field, and (4) proportional in intensity to the ambient field. Separating, by various analytical means, this<span> </span>magnetization<span> </span>from other ``unwanted''<span> </span>magnetizations<span> </span>has allowed paleomagnetists to study the historical and prehistorical behavior of the earth's field. It has been learned, for example, that the strength of the field was almost twice its present value 2000 years ago and that it has often completely reversed its polarity. Paleo‐magnetists have also confirmed that most<span> </span>oceans<span> </span>are,<span> </span>geologically<span> </span>speaking, relatively new features, and that the<span> </span>continents<span> </span>have markedly changed their positions over the surface of the earth.</div></div>
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10.1063/1.1657800
en
AIP
History of the geomagnetic field
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