The Japanese island of Honshu is the meeting place of four tectonic
plates: the Pacific Plate, the Okhotsk Plate, the Amur Plate and the Philippine
Plate. The Okhotsk Plate was once part of the North American Plate, but is now an
independent plate. Similarly, there is debate as to whether the Amur Plate is
independent or whether it is still part of the Eurasian Plate. The Amur and
Okinawa Plates are subducting the Philippine Plate. The Philippine and Okhotsk
Plates are subducting the Pacific Plate.
Through time, these plates have given rise to the landforms of
Japan. One of these is the composite volcano Mt. Fuji.
Mt. Fuji is located about twenty-three miles from the plate
convergence zone. Composite volcanoes are common at subduction zones, as
evidenced by the volcanoes of the Ring of Fire.
When an ocean plate is subducted by a continental plate,
magma can rise as dewatering occurs. Dewatering is the release of water from hydrated
minerals. Dewatering happens at specific temperatures and pressures for
different minerals.
The released water lowers the melting point of the mantle
rock. The mantle rock partially melts and becomes less dense than the
surrounding rock. As it rises, it pools temporarily in the lower region of the continental
lithosphere. The magma then rises through the crust and gathers crustal silica
rock. Then it pools in the magma chamber of a volcano. Pressure and gases build
up until the volcano erupts, spewing lava and pyroclastic flows over the landscape
with explosive force. This is how Mt. Fuji and the surrounding area came to be
volcanic.
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