20 million years ago, the place where Mt. Fuji is located was still under the sea.
After long time of tectonic movements, the once seabed has itself raised above sea level. Some hundred thousand years before now, the initial volcanic movement had created the Komitake Volcano and Ashitaka Volcano.
Eighty thousand years after the birth of the two volcanoes, the old Fuji Volcano which was just south of the Komitake Volcano erupted. Massive Volcanic rocks burst out from the volcano and raised the height of the volcano up to 3000 metres.
At the same time, the rocks and ashes eventually covered the whole Komitake Volcano. It maintained quiet for quite a long time, till 10000 years before now. The same crater burst out again. This time, magma was the main component. The result is what we now call as New Fuji Volcano, or Mt. Fuji. As the result of countless eruption and creation of lateral volcanoes, the old Fuji Volcano has been totally covered up by magma and ashes.
Mt. Fuji continued her eruption even to the recorded history. Two of the main eruptions were Zyogan Eruption of Mount Fuji from AD 864 to AD 866, and Hoei Eruption of Mount Fuji from Dec 1707 to Jan 1708. After the Hoei eruption, Mt. Fuji has her volcanic movement stopped.