Archaeomagnetic dating example
If we were able to use a compass deep under southern Africa, we would see that in this unusual patch north actually points south.This patch is the main culprit creating the South Atlantic Anomaly.For example, clay used to make pottery contains small amounts of magnetic minerals, such as magnetite.When the clay is heated to make a pot, its magnetic minerals lose any magnetism they may have held.Each floor is a small magnetic observatory, with its compass frozen in time immediately after burning.With our colleagues, we’ve focused our sampling on Iron Age village sites that dot the Limpopo River Valley, bordered today by Zimbabwe to the north, Botswana to the west and South Africa to the south. What we found reveals a period in the past, near A. 1300, when the field in that area was decreasing as rapidly as it is today.In numerical simulations, unusual patches similar to the one beneath southern Africa appear immediately prior to geomagnetic reversals.The poles have reversed frequently over the history of the planet, but the last reversal is in the distant past, some 780,000 years ago.
And because more radiation would reach Earth’s surface under very low field strengths during a global reversal, it also might affect rates of cancer.
This collapse is centered in a huge expanse of the Southern Hemisphere, extending from Zimbabwe to Chile, known as the South Atlantic Anomaly.
The magnetic field strength is so weak there that it’s a hazard for satellites that orbit above the region – the field no longer protects them from radiation which interferes with satellite electronics.
These Iron Age people lived in huts built of clay, and stored their grain in hardened clay bins.
As the first agriculturists of the Iron Age of southern Africa, they relied heavily on rainfall.
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Just as in the case of the firing and cooling of a pot, the clay in these structures recorded Earth’s magnetic field as they cooled.