Geologic ages are part of the geological time scale, a dating system that matches layers of rock to time.

Ages are tied to geologic events such as the development of Greenland’s ice sheet (the Piacenzian age) or the appearance of the trilobite species Glyptagnostus reticulatus (the Paibian age).

The Chibanian age is named after the Japanese prefecture Chiba, home to the city of Ichihara. A cliff wall was found there 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 Bernard Brunhes, a pioneer in paleomagnetism, and Motonori Matuyama, who first suggested Earth’s magnetic field undergoes periodic reversals.

Most sites that define measures of geologic time are in Europe, so the Japanese site is an outlier in the geological world.

“In that section in Chiba, you have one of the best records of the reversal interval of anywhere in the world,” geologist Stanley Finney told Tim Hornyak of Eos, which publishes Earth and space science information. “It’s a significant record of past Earth history that helps us see what may happen now.”

Earth’s magnetic poles are on the move again, and a flip could hurt humans and animals and wreak havoc on technology. But it is unclear how long a reversal could take. The Brunhes-Matuyama reversal is thought to have taken about 22,000 years — if repeated, perhaps that would be long enough for humanity to come up with a plan.