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Formation of Mt. Fuji
Mt. Fuji is a basaltic stratovolcano born from the base of Mt. Komitake about 100,000 years ago. Its current beautiful cone shape was formed over two generations of volcanic activity turning the old Mt. Fuji into the current Mt. Fuji. Recent research also suggests there may have been a volcanic predecessor to Mt. Komitake also. In 1707, during the Edo period, an explosive eruption created the Hoei crater and volcanic ash formed a vast volcanic plane to the eastern side of the mountain. There have been no further eruptions since.
Location and Shape
As a typical stratovolcano, Mt. Fuji has gradual slopes and a wide skirt spreading in all directions. The skyline rapidly fills with Mt. Fuji as one approaches the summit accentuating the beauty of everything from the vast skirting regions to the summit area. The major source of Mt. Fuji’s volcanic activity is said to be subduction of the Pacific Place beneath the Philippine Plate, as is the activity of other volcanoes in the area. Three tectonic plates meet in the vicinity of Japan, and Mt. Fuji is located at a point where the Nankai Trough is kinked northwards as the Izu Peninsula collides with Honshu.
Characteristic Geology
Mt. Fuji has a stratovolcanic structure resulting from sequences over vivacious volcanic activity which created multiple layers of lava and volcanic projectiles. This volcanic ejecta is made of basaltic rock, setting it apart from the more common andesite volcanoes of Japan.
Precious Geological Phenomena
Basaltic lava has a low viscosity and thus flows more freely, spreading out over a wide area, giving rise to various special land formations such as lava caves and lava trees.
Lava Caves and Lava Trees
Lava caves can be formed when lava flows begin to harden. The slower to harden core may continue to flow and hollow out the area inside, else, gas ejections can create an empty region within the hardened exterior. One can see the stages of cooling and hardening from the initial nearly 1000℃ hot lava flow.
Lava trees are created when lava flows around trees, which the burn leaving hollowed out regions in the lava according to their shape. These can tell us how the forest looks at the time of eruption, as well as the direction the lava flowed.
Osawa Valley
Mt. Fuji had more or less than same shape as today and was a young, new volcano as of about 10 thousand years ago. However, rain, wind and snow have been slowly causing erosion and creating valleys along its surface. The largest of these is the Osawa Valley, situated directly below the summit on the west side of the mountain, which began to be formed about 1000 years ago.
The east side of Mt. Fuji has been frequently covered in ash due to eruptions over the years, yet very little has landed on the west side. It is thought that this is reason for this large valley.

Mt. Fuji Formation History
Birth of Mt. Komitake (200 to 100 thousand years ago)
Old Mt. Fuji eruption (100 to 10 thousand years ago)
New Mt. Fuji eruption (less than 10 thousand years ago)

Formation of Mt. Fuji

New Mt. Fuji Volcanic Activity History
11000 to 8000 years ago Large-scale eruption from the crater at the summit
8000 to 4500 years ago Intermittent eruptions of volcanic ash largely from the summit crater
4500 to 3000 years ago Large lava eruptions and small-scale ash eruptions from the summit crater and neighboring volcanoes.
3000 to 2000 years ago Frequent explosive eruptions mostly from the summit crater.
2000 years ago to today Series of lava or volcanic ash eruptions from the group of volcanoes. Large scale eruptions have been occurring at Mt. Fuji roughly once every 500 years. The 864 and 1707 eruptions were two particularly large eruptions among those in recorded history.
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