Mount Fuji
Geology

Mount Fuji is located at a triple junction trench where the Amurian Plate, Okhotsk Plate, and Philippine Sea Plate meet. These three plates form the western part of Japan, the eastern part of Japan, and the Izu Peninsula respectively. The Pacific Plate is being subducted beneath these plates, resulting in volcanic activity. Mount Fuji is also located near three island arcs: the Southwestern Japan Arc, the Northeastern Japan Arc, and the Izu-Bonin-Mariana Arc. The Fuji triple junction is only 400 kilometres (250 mi) from the Boso Triple Junction.

Fuji's main crater is 780 m (2,560 ft) in diameter and 240 m (790 ft) deep. The bottom of the crater is 100–130 m (330–430 ft) in diameter. Slope angles from the crater to a distance of 1.5–2 km (0.93–1.24 mi) are 31°–35°, the angle of repose for dry gravel. Beyond this distance, slope angles are about 27°, which is caused by an increase in scoria. Mid-flank slope angles decrease from 23° to less than 10° in the piedmont.

Scientists have identified four distinct phases of volcanic activity in the formation of Mount Fuji. The first phase, called Sen-komitake, is composed of an andesite core recently discovered deep within the mountain. Sen-komitake was followed by the " Komitake Fuji", a basalt layer believed to be formed several hundred thousand years ago. Approximately 100,000 years ago, "Old Fuji" was formed over the top of Komitake Fuji. The modern, "New Fuji" is believed to have formed over the top of Old Fuji around 10,000 years ago.

Pre-Komitake started erupting in the Middle Pleistocene in an area seven km (4 + 1 ⁄ 2 mi) north of Mount Fuji. After a relatively short pause, eruptions began again which formed Komitake Volcano in the same location. These eruptions ended 100,000 years ago. Ashitake Volcano was active from 400,000 to 100,000 years ago and is located 20 km (12 mi) southeast of Mount Fuji. Mount Fuji started erupting 100,000 years ago, with Ko-Fuji (old-Fuji) forming 100,000 to 17,000 years ago, but which is now almost completely buried. A large landslide on the southwest flank occurred about 18,000 years ago. Shin-Fuji (new-Fuji) eruptions in the form of lava, lapilli and volcanic ash, have occurred between 17,000 and 8,000 years ago, between 7,000 and 3,500 years ago, and between 4,000 and 2,000 years ago. Flank eruptions, mostly in the form of parasitic cinder cones, ceased in 1707. The largest cone, Omuro-Yama, is one of more than 100 cones aligned NW-SE and NE-SW through the summit. Mt. Fuji also has more than 70 lava tunnels and extensive lava tree molds. Two large landslides are at the head of the Yoshida-Osawa and Osawa-Kuzure valleys.

As of December 2002, the volcano is classified as active with a low risk of eruption. The last recorded eruption was the Hōei eruption which started on December 16, 1707 (Hōei 4, 23rd day of the 11th month), and ended about January 1, 1708 (Hōei 4, 9th day of the 12th month). The eruption formed a new crater and a second peak, named Mount Hōei, halfway down its southeastern side. Fuji spewed cinders and ash which fell like rain in Izu, Kai, Sagami, and Musashi. Since then, there have been no signs of an eruption. However, on the evening of March 15, 2011, there was a magnitude 6.2 earthquake at shallow depth a few kilometres from Mount Fuji on its southern side.
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