SAN FRANCISCO — Slag left over from Iron Age copper smelting shows the Earth’s magnetic field was stronger and more variable than scientists ever imagined.
“This is a very challenging result,” said geomagnetist Luis Silva of the University of Leeds, who was not involved in the new work. “It’s completely outside of anything we thought could be happening in the core.”
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.
“We don’t have volcanic glass in Israel, but we do have slag,” Shaar said. When the ancient Egyptians (in what is now Israel) melted ore to produce copper, they created a lot of leftover molten rock that they threw immediately on a waste heap. The rock cooled quickly, preserving a signature of the magnetic field.
“It’s like a small scale lava flow,” Shaar said.
To see what the magnetic field was doing 3,000 years ago, Shaar and his colleagues collected slag samples from the ancient copper mines of Timna in southern Israel. They found remnants of wheat, dates, grapes and human hair, too, which allowed them to use carbon dating to figure out how long ago the slag layers were laid down. Combined with slag from a previous study of the Khirbat en-Nahas mines to the northeast in Jordan, their samples spanned almost two centuries, from 3,050 to 2,870 years ago.
Back in the lab, the team melted and re-froze some of the slag in the presence of a known magnetic field, to make sure they could trust the rock to faithfully trap the field strength. Then they measured the field strength in the raw slag.
They found that the magnetic field abruptly spiked twice during the 180 years they studied, once around 2,990 years ago and once around 2,900 years ago. Both times, the field jumped up in strength and then fell by at least 40 percent in the space of about 20 years.
“These geomagnetic spikes are very different from what we see now or have seen before,” Shaar said.
“He sees the field changing 5 to 10 times faster than anything else we have seen so far,” said geomagnetist Cathy Constable of the Scripps Institute of Oceanography in San Diego, who makes global maps of the changing magnetic field but was not involved in the new work.
Constable notes that the spikes seem to happen only in the part of the Middle East that Shaar studied, not everywhere on Earth. That suggests that the spike could be caused by a small piece of especially magnetic molten iron moving through the Earth’s core right under Israel.
Shaar and his colleagues plan to visit Roman mines in Cyprus to see if similar spikes happened there.
Image: Flickr/Chadica
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