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 250,000 years ago.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.