Last month, the International Union of Geological Sciences formally adopted the name “Chibanian Age” for the period between 770,000 and 126,000 years ago, Kyodo News reported at the time.
The beginning of the period is defined by the most recent reversal of Earth’s magnetic field, called the Brunhes-Matuyama reversal. The flip took about 22,000 years from start to finish, according to a 2019 paper in Science Advances. Signs of the Brunhes-Matuyama reversal, which was named for pioneers in the research of Earth’s magnetic field, can be found around the world. But a cliff wall in Japan’s Chiba prefecture holds some of the clearest and most extensive evidence for the major geological event.
The Chiba cliffs, located near the city of Ichihara southeast of Tokyo, are home to a sedimentary deposit called the Kazusa Group, which is nearly two miles of rock layers made of compressed silt and clay that were once under the ocean. Volcanic ash in the layers indicates its age is about 770,000 years old. As Erin Blakemore writes for the Washington Post, when the rock was molten, minerals with iron suspended in the molten soup pointed in the direction of Earth’s magnetic field.
When the rock solidified, it trapped the iron-containing minerals in whatever direction they were pointing at the time. Scientists today can look at the captured iron like a time capsule that indicates the history of Earth’s magnetic field. The magnetic field protects Earth’s surface from the radiation of outer space, and as it shifts, that protection becomes weaker.
https://www.smithsonianmag.com/smart-news/chibanian-age-earths-newly-named-geological-period-180974224/