Japan’s Oldest Rock Found in Tsuwano

March 30, 2019

Hiroshima- A group of researchers have found a rock in Tsuwano, Shimane Prefecture, that dates back some 2.5 billion years, making it the oldest ever discovered in Japan.

"The rock is an important material that will help us uncover how the Japanese archipelago was formed," said Yasutaka Hayasaka, associate professor at Hiroshima University, who led the group.

Members of the group discovered fragments of the granitic gneiss rock in the western Japan town in 2017, measuring its age from its proportion of components such as uranium and lead.

Then they found that the granitic gneiss was included in two rock masses 2 and 1.5 kilometers long, respectively, that were trapped in a Permian stratum formed 250 to 300 million years ago due to fault activity before Japan was detached from the Asian continent.

The rock is believed to have been part of a very old continent crust called North China Craton and reached an area which is now Tsuwano after the portion of the craton containing the rock moved some 500 kilometers due to a fault slip, the researchers also said.

The previously known oldest rock in the country was one some 1.85 billion years old that was found in another Shimane town of Okinoshima, located on Dogo, the biggest island in the Oki group. Jiji Press