Yesterday was the birthday of Motonori Matuyama, a Japanese geophysicist born in 1884 who discovered that Earth's magnetic poles have flip-flopped throughout history. Matuyama studied traces of Earth's ancient magnetic field frozen into basalt, a volcanic rock. In 1929 he reported that the field had changed over relatively short time periods. Moreover, he discovered it had completely reversed during a period from 2.4 million to 700,000 years ago, which is now called the "Matuyama reversed epoch." Later research showed that the magnetic field has reversed more than 20 times during the past 5 million years, seemingly at random.
[Source: Emily McMurray, Ed., Notable Twentieth Century Scientists (Gale Research Inc., ITP, 1995).]