Intensity of main dipole field for last 800,000 years

P. David in 1904 and B. Brunhes detected reversed magnetic fields in lava flows from the Massif Central mountains in France. In 1926, P. L. Mercanton (Terrestrial Magnetism, vol 31, P.187) observed reversed magnetization in both old lava flows and baked clays, and went on to suggest that such polarity reversals might be used to test the continental drift hypothesis. Also, in 1929, M. Matuyama published a paper describing polarity reversals in Japan, Manchuria and Tyosen dating from the Quaternary era.

Since then, advanced dating techniques have revealed over 60 reversals during the past 20 million years, and the last one was about 800,000 years ago.

The above diagram shows the intensity of the main field since the time of the last reversal about 780,000 years ago. Notice that it is very irregular and frequently dips to levels that are a quarter of its present strength at the far left. The individual dips do not correspond to reversals. It seems that only the major dips to levels of 0.5 to 1.0 on the scale, are suitable for making the transition. Prior to 800,000 years ago, the Earth's field was reverse to its present polarity.

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