Earth’s Magnetic Reversals

When I took my earth science students on our annual field trip to Lake Michigan, we did a “magnetism and history” exercise showing Earth’s magnetic reversals. We would go to the large sand dune known as “Old Baldy” (no, not named after me), cover a bar magnet with a white handkerchief, and stick it into the sand. The handkerchief would come out completely covered with little black needles of magnetite which is a magnetic iron oxide.

There was a nearby pool where the water was calm and where you could see the magnetic pieces with a pair of binoculars. They pretty much lined up in the same direction, and I showed the students that they were lined up with a compass needle. We would then dig down several feet into the sand to a layer of mud where the magnetite pieces pointed every which way. I would ask my students to tell me why that would be. Most of them said that the Earth must have lost its magnetic field at the time when that mud was deposited. I then told them if we dug down deeper we would find the magnetite pieces lined up again, but in the opposite direction. The point was obvious–the magnetic field of the Earth had flipped.

In reality, there are several other possible causes for the scrambled magnetite pieces, but the magnetic reversal did happen. In the mid-ocean ridge of the Atlantic, lava flows have trapped the magnetite. The process is called “thermoremanent magnetization.” Because the lava flows happened multiple times, they show a picture of the history of the Earth’s magnetic reversals. These lava flows have recorded over a dozen “flips” in the magnetic field of Earth.

Archaeologist Dr. Erez Ben-Yosef of Tel Aviv University has led studies of the history of magnetic changes on the Earth. Ancient pottery has tiny magnetic minerals that were aligned with the Earth’s magnetic field when the pottery was fired in the kiln. The stronger the magnetic field, the greater the alignment of the magnetic particles. From the late eighth to the second century B.C., ceramic jars bearing the impressions of royal stamps were manufactured in and around Jerusalem. Scientists have studied handles from 67 pieces of pottery, and they have found that the magnetic field of the Earth has gone through periodic rises and falls throughout history. The mapping of the magnetic fields gives a record of history, with small changes producing a pattern that improves dating techniques by a huge factor.

What causes the Earth’s magnetic field is still not completely understood, and the magnetic reversals are even more of a mystery. By measuring the magnetic particles in the pottery, scientists can date the pottery very accurately. This dating method can answer many questions about ancient kings and kingdoms including the major figures in the history of Israel.

For more on this go to https://www.haaretz.com/archaeology/1.771427
–John N. Clayton © 2018