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How Burned Iron Age Villages Recorded the Earth’s Magnetic Field

Researchers found evidence of a time of "rapid change" in Earth's magnetic field from the 13th to 16th centuries.
July 28, 2015, 3:55pm
Modern grain bins, similar to those found during the Iron Age. Image: John Tarduno/University of Rochester

A team of American and South African researchers just published a paper in Nature Communications outlining how they've discovered a record of the Earth's magnetic fields literally scorched into the ground. By measuring magnetic samples from ritualistic fires in Iron Age communities, they found evidence of a time of "rapid change" in Earth's magnetic field from the 13th to 16th centuries.

Paleomagnetism is a lot like paleontology, except instead of the study of plants and animals, it's the study of the Earth's magnetic fields. Rather than dig for fossils, researchers have to be a bit craftier to find data on Earth magnetism beyond the past 160 years that we've been observing the phenomenon.

The Earth's magnetic field has a weak spot, where the Van Allen belt, a collection of charged particles from the Sun are held in place by magnetism, dips close to the Earth's surface. The spot, called the South Atlantic Anomaly, spans from South America to southern Africa, and may be the starting spot for the reversal of Earth's poles, which happens every 800,000 years or so. This phenomenon that takes place over literally thousands of years, but we've been collecting data on the magnetic fields since the 1840s. Extending the record back further took some looking.

One record was uncovered thanks to a long-standing ritual practiced the along near the borders of present-day South Africa, Botswana and Zimbabwe, during the region's "Iron Age," from the 11th through 16th centuries. For at least 2,000 years, agricultural populations in southern Africa lived in semi-permanent villages, with buildings made of clay, mud, and grass for holding their livestock and harvests of millet, beans and peas, Rory Cottrell, one of the study's co-authors told me in an email.

Magnetic field strength in the South Atlantic Anomaly. Image: Michael Osadciw/University of Rochester

"In times of sustained drought, poor growing seasons, and animals dying unexpectedly, the villages were burned as part of a ritualistic cleansing," Cottrell said. As the buildings burned, the firing temperatures reached at least 1,000 degrees Celsius. That amount of heat is above the Curie temperature for magnetic materials like magnetite, where noticeable changes occur to a material's magnetic properties, creating a record of magnetic strength and direction at the time of burning.

"This is a temperature where the magnetic characteristics of a particular material change from being unable to retain a permanent magnetic field above this temperature, and recording or 'freezing' the magnetic field as the material cools through and below the Curie temperature," Cottrell said. "We sampled burnt features from localities in southern Africa to look at what magnetic fields were recorded for different time periods throughout the Iron Age of southern Africa."

According to the 160 years of records collected by magnetic field observatories, the Earth's magnetic field has been weakening and changing direction, which is normal and could indicate that we're in the early stages of a pole reversal, which could take 15,000 years to complete.

Looking back even further, through the burned remains of villages, researchers found that the rate of directional change today is small potatoes compared to what was going on in the South Atlantic Anomaly from the 13th through 16th centuries. According to their observations, from 1225 to 1550, the rate of change was twice what it is today.

Weakening magnetic fields can strengthen without a pole reversal, so at this point researchers are only speculating on what the magnetic field will do next. But the more data they have to base those speculations on, the more accurate they will be. For phenomena that take hundreds of thousands of years to complete, every record helps.