What is Fossa Magna and what does it have to do with jade and the islands of Japan?

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Ancient jadeite showing skillful techniques

Fossa Magna Museum

This wonderful geology museum is located in the city of Itoigawa. It is named after the Great Crack in the middle of Japan, the terminus of the Japan Median Tectonic Line (map below). The Fossa Magna (Latin for great rift) refers to the place where Pacific/Oceanic and Continental plates meet. You can learn all about it at this museum. Itoigawa is the home of Japanese jade, and this is its museum.

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Itoigawa City, Home of Jade

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Itoigawa is a charming small city on the coast of the Japan Sea, in the prefecture of Niigata. Itoigawa is on the Japan Median Tectonic Line, and its location is marked with a red arrow. The Fossa Magna zone is shown in yellow. Itoigawa is known as Japan’s source of jade, the mineral/gem jadeite. Jade has a long history in Japan, and it has been worked by early Jomon people. Jadeite is called hisui. It is usually written in kana characters, ヒスイ or  ひすい , so perhaps this term is of Jomon origin. Jadeite was named the National Stone of Japan in 2016.

Nunakawa-hime, Lady of Jade

Nunakawahime statueIn Koshi-no-kuni, also known as Esshū province, now known as Hokuriku, a folk heroine of Itoigawa is Nunakawahime. Legend says she was beautiful and wise, and she knew how to make magic using jade. Her fame was so great that the famous Ōkuninushi came from Izumo in the west to win her hand. And he did. Their son became Takeminakata, the kami revered at Suwa Taisha in nearby Nagano. This statue of Nunakawahime holding a large jade stands on the north side of the Itoigawa station.

Birth of the Oldest Jade in the World

How was jade created? The process began 500 million years ago. Jadeite crystallizes from a hydrothermal liquid in the subduction zone of two tectonic plates. In a zone about 30 to 50 km below the surface, elements (Na, Al, Si, and H2O) formed jadeite at temperatures of 250 to 650 C. Jadeite was created during the period 100 to 500 million years ago.

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Itoigawa lies on the border of two plates. 250 million years ago, the Oceanic plate (right) pushed below the Continental plate (left). Jadeite (colored turquoise) that had been created deep below rose to the surface.

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Jadeite comes in various colors depending on the elements present in it. The colors range from white, through various shades of green, lavender, and blue, to black.IMG_2816

History of Jade in Japan

first jade jewelry

6000 years ago, jade jewelry in the form of small pendants were made. They were of gemstone quality and may be the oldest human-worked jade in the world.

5000 years ago, there were jade workshops making larger pendants in Itoigawa, as found at the Chojogahara and Teraji archaeological sites in Itoigawa.

3000 years ago, magatama pendants were made. 

first magatamaMagatama means curved bead. They were generally comma-shaped, hence their name; they were also in the shapes of animals and insects.

8th century, a Buddhist statue, containing jade, is made for Todaiji. Subsequently jade mysteriously disappears from the scene. It was believed for a long time that there was no more jade in Japan.

In 1938, the tanka poet Gyofu Soma suggested that jade might be found in Itoigawa because of the Nunakawahime legend. And jade was rediscovered in Japan.

Fossa Magna and The Birth of the Japanese Islands

There is a stunning video at the museum which explains the role of the Fossa Magna in forming the islands of Japan. The Fossa Magna was once under sea. 20 million years ago, the land was still connected to Asia. Then it broke apart from Asia and the Sea of Japan was created. The land split into two parts: the southern part of the land rotated one way and the northern part rotated the other way, creating an opening, like a door. Fossa Magna was in the sea in this gap. Then the eruption of subterranean volcanoes filled the Fossa Magna sea with ash, sand, and mud. And connected the two parts of land. Thus was the main island of the archipelago created. Volcanoes on land formed 2.6 million years ago in the Fossa Magna zone, like Mt. Fuji on the main island of Honshu. The dark area in the middle is the Kanto Mountains, and Mt. Fuji is on the left edge.

volcanoes of fossa

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