The Earth’s magnetic field comes from the movement of molten iron in the core. The field’s strength and structure are constantly changing. But paleomagnetists (scientists who study the history of the Earth’s magnetic field) thought the changes were usually small and slow, fluctuating by about 16 percent over the course of a century.
But a new study of ancient copper mines in southern Israel found that the strength of the magnetic field could double and then fall back down in less than 20 years.
“The magnetic field reached an intensity that was much higher than anyone had ever thought before, two and a half times the present field,” said graduate student Ron Shaar of the Hebrew University of Jerusalem, lead author of the new study. “And you can have dramatic changes in the intensity of the field in periods of less than decades.” Shaar presented his results in a poster here at the American Geophysical Union meeting Dec. 14, and in a paper to appear in Earth and Planetary Science Letters.
To measure the strength of the magnetic field, Shaar and colleagues turned to piles of waste metal left near an ancient Egyptian copper mine.
When melted iron cools rapidly, it freezes with a signature of the Earth’s magnetic field at that instant. Paleomagnetists have traditionally studied the glass-like rocks thrown from volcanoes to build a picture of how the magnetic field has changed over time. Their measurements, plus theoretical models, showed that the magnetic field’s strength peaked around 3,000 years ago in the middle Egypt’s Iron Age.