chibanian chiba prefecture japan yoro river earth magnetic field reversal polarity kazusa
chibanian chiba magnetic field polarity reversal japan geology

Newly Named Chibanian Age Demarcates Earth’s Last Magnetic Flip

The time period, which spans 770,000 to 126,000 years ago, started with a reversal of the planet’s magnetic field.

kerry grens
Kerry Grens

Kerry served as The Scientist’s news director until 2021. Before joining The Scientist in 2013, she was a stringer for Reuters Health, the senior health and science reporter at...

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ABOVE: The Yoro River in Chiba Prefecture, Japan, has mineral deposits that document a reversal in the Earth’s magnetic field, which marked the start of the Chibanian.

The International Union of Geological Sciences has designated the time in Earth’s history from 770,000 to 126,000 years ago as the Chibanian, notable for being the most recent reversal of the planet’s magnetic poles, The Japan Times reported January 17. It’s named for the Chiba Prefecture in Japan, where a deposition of minerals and marine fossils reveals the flip in polarity that occurred at the start of the Chibanian.

According to The Washington Post, iron within minerals of the deposition aligned with Earth’s magnetic field at the time the rocks cooled from a molten form, logging the field’s change in polarity. “This sedimentary sequence, called the Kazusa Group, has a total thickness of 3 kilometers with an anomalously high deposition rate reaching 2 meters per thousand years on average,” Makoto Okada, a professor of paleomagnetic studies at Ibaraki University in Mito, Japan, tells Eos. “[T]his sequence provides us reliable geomagnetic polarity signals and abundant marine microfossils.”

The Chibanian designates the first geological age named for Japan, The Japan Times reports. “As a Japanese geologist, I am happy [the International Union of Geological Sciences] made a good decision,” Hiroshi Kitazato, a professor at Tokyo University of Marine Science and Technology and an executive member of the International Union of Geological Sciences (IUGS) who participated in the discussions for naming the Chibanian, tells Eos. “The Chibanian section is certainly the most well preserved paleomagnetic reversal transition from Reversal (Matuyama) to Normal (Brunhes).”

The Japan Times notes that when geologists had requested to register the name Chibanian with IUGS in 2017, another group accused them of fabricating data. This delayed the decision by the IUGS to choose Chibanian, The Mainichi reported in 2018, but, ultimately, the allegations were determined to be unfounded. 

Kerry Grens is a senior editor and the news director of The Scientist. Email her at