• Kyodo


Japanese researchers said Monday they have found the country’s oldest known rock, dating back 2.5 billion years, in Shimane Prefecture — around 500 million years earlier than the previous oldest discovery.

A team from Hiroshima University collected fragments of the rock, which is located near a forest road in the town of Tsuwano. The granite-gneiss metamorphic rock, formed by intense heat, has a striped appearance.

The team estimated the age of the rock by analyzing the isotopic ratios of uranium and lead it contains.

Many rocks of the same age exist in North Korea and northeastern China, and analysis of the latest specimen is likely to provide insight into the formation of the Japanese archipelago, which was originally connected to the Asian continent.

“We will study the chemical composition of the rock and clarify from which part of the continent it came,” said Yasutaka Hayasaka, an associate professor at Hiroshima University.

The previous oldest rock in Japan, which consists of 2 billion-year-old gneiss, was found in Hichiso, Gifu Prefecture, in 1970.

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