Kyodo News
Mount Fuji has long been thought to contain two old volcanoes inside its cone shape, but the results of a three-year drilling project that ended in February suggest the existence of yet another volcano deep within it, researchers said Saturday.
"What we are learning about how Mount Fuji was formed is quite different from the conventional theory," said Setsuya Nakada, a professor at the University of Tokyo's Earthquake Research Institute who led the research.
Previously, researchers believed the 3,776-meter mountain took its shape through three major periods of volcanic activity that started several hundred thousand years ago.
The volcano created in the first major eruptions is called Komitake, which is believed to have been around 2,500 meters high.
The second major eruptions occurred about 100,000 years ago, which eventually led to the formation of the volcano known as Old Fuji. Komitake is contained within Old Fuji.
The last major eruptions took place around 10,000 years ago. As a result, New Fuji, or the current Mount Fuji, was formed over Old Fuji.
But researchers at the institute said that when they drilled into the depths of the mountain, they found andesite, a type of rock that had never been discovered at the mountain.
Komitake and Old Fuji are composed of basalt, which is completely different in nature from andesite.
The newly discovered layer of andesite is located beneath Komitake, which suggests that a different volcanic activity took place prior to the formation of Komitake, they said.
The researchers have named their discovery Sen-Komitake, or Pre-Komitake.