THE ASAHI SHIMBUN

An area known as the "Chiba section," a rock strata that shows signs of geomagnetic reversal (Tabito Fukutomi)

For the first time, Japan has carved out a spot for itself among the hundred or so geologic ages recognized by scientific organizations.

The International Union of Geological Sciences (IUGS) on Jan. 17 chose “Chibanian,” or Chiba age, for the period between about 129,000 and 774,000 years ago.

The name stems from the discovery of the geological strata defining the boundary on the international geologic time scale in the city of Ichihara, Chiba Prefecture.

A team of researchers from the National Institute of Polar Research and Ibaraki University discovered the rock strata in Ichihara that clearly had signs of the geomagnetic reversal that occurred about 770,000 years ago. While such geomagnetic reversals have occurred a number of times, the strata in Ichihara was from the last such reversal.

“We have finally been able to include a name from Japan in Earth’s history,” said Makoto Okada, a geology professor at Ibaraki University, who led the research group that discovered the strata.

Similar strata were also found in Italy, but a number of assessments by the IUGS determined that the Chiba strata more clearly showed signs of the geomagnetic reversal.

In the final assessment, a majority of IUGS directors favored awarding the age name to Chiba.

The Chibanian age coincides with the period in which Homo sapiens first appeared on Earth.

Earth’s geological history of 4.6 billion years is classified into 100 or so ages, which are based on major phenomenon, such as climate change and the extinction of certain living things. Until now, most of the ages have been taken from European place names because those areas were the sites of the specific rock strata for that age.

For example, the Jurassic Period between 150 million and 200 million years ago, when dinosaurs still roamed the Earth, was so named because of the Jura mountains, which now serves as the national border between France and Switzerland.

Likewise, the Cambrian Period between 490 million and 540 million years ago was named after a region in Wales.

The area with the boundary that most clearly designates a rock strata is selected as the Global Boundary Stratotype Section and Point (GSSP) for the age.

(This article was written by Roku Goda and Maiko Kobayashi.)