Mount Fuji - Geology

Geology

Mount Fuji is located at the triple junction where the Amurian Plate, the Okhotsk Plate, and the Philippine Sea Plate meet. Those plates form the western part of Japan, the eastern part of Japan, and the Izu Peninsula respectively.

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.

The volcano is currently 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) during the Edo period. The eruption formed a new crater and a second peak (named Hōei-zan after the Hoei era) halfway down its 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. In 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. But according to the Japanese Meteorological Service there was no sign of any eruption. In September 2012, mathematical models created by the National Research Institute for Earth Science and Disaster Prevention suggest that the pressure in Mount Fuji's magma chamber could be at 1.6 megapascals higher than it was in 1707. However, since there is no known way to directly measure the pressure of a volcano's magma chamber, such research is only speculative.