Schematic illustration of Earth's magnetic field.
Credits: Peter Reid, The University of Edinburgh
A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.
About 770,000 years ago, Earth’s magnetic fields reversed, swapping magnetic north and south for the last known time. That ushered in a new geological age which scientists have now named the Chibanian.
The Chibanian age is named after the Japanese prefecture Chiba where a cliff wall was found with an exposed layer of marine deposits and mineral debris about 770,000 years old.
When geologists studied the minerals inside, they found evidence of the last known shifting of Earth’s magnetic fields. The planet’s outer core generates its magnetic field, a kind of shield that protects Earth from solar wind.
As molten rock cools, iron-bearing minerals form. They align themselves with the magnetic field, then solidify, acting as a kind of snapshot of Earth’s magnetic field at the time cooling occurred.
The minerals in Chiba allowed geologists to date the last known switch of magnetic fields to about 774,000 years ago. They named the reversal event the Brunhes-Matuyama reversal in honor of the French geophysicist Bernard Brunhes (1867-1910) and the Japanese geophysicist Motonori Matuyama (1884-1958).
Matuyama was the first to provide systematic evidence that the Earth's magnetic field had been reversed in the early Pleistocene and to suggest that long periods existed in the past in which the polarity was reversed.
Antoine Joseph Bernard Brunhes was a pioneer in paleomagnetism. His 1906 discovery of geomagnetic reversal has since been verified. The current period of normal polarity, called Brunhes Chron, is named for him.