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.[...]
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
Link via reddit | Photo: unrelated piece of copper slag by the University of California at Irvine