The Japanese archipelago is thought to lie above the edges of five tectonic plates: the Pacific Plate, North American Plate (or Okhotsk Plate, according to some geologists), Philippine Sea Plate, Amur Plate and Nankai Micro Plate. These huge slabs move about, collide and slide under and over each other, making Japan more prone to earthquakes and volcanic eruptions than almost any other country.
Mount Fuji is located just about in the middle of the archipelago, almost directly above where three of the plates meet. It has erupted violently many times in its history.
The Fuji we see today developed on top of two older volcanoes. About 10,000 years ago, one of these older mountains, Ko Fuji Kazan (“Old Fuji Volcano,” elevation about 3,000 meters), began throwing out huge quantities of lava in all directions. Over the next several thousand years, Ko Fuji Kazan was eventually swallowed up, along with an even older volcano to the northeast, Ko Mitake Kazan (“Small Mitake Volcano”). Out of this chaos came the basic shape of the mountain that now exists. Other later eruptions gave the finishing touches to the beautiful cone seen today.
The most recent period of frequent violent activity lasted about 300 years, during the 9th, 10th and 11th centuries A.D. The Jogan eruption of 864 on the northwestern slope was the biggest in recorded history. It discharged massive amounts of lava that quickly transformed the base of the mountain on the north side. The large forested highland called Aokigahara and numerous lakes are the result we see today.

View of Aokigahara, a dense, mature forest spreading across a lava plateau northwest of the mountain. The plateau has an area of about 30 km². (Photo credit: The Mainichi Newspapers Co.)

There was no major activity from the 12th to the mid-15th century, when the volcano awoke again. Then in 1707, the Hoei eruption opened up three craters on the southeastern slope and caused tremendous damage to nearby villages and farmland. It also sent huge clouds of volcanic ash that rained down on the big city of Edo (present-day Tokyo), more than 100 kilometers to the east.
Fuji has been quiet for almost 300 years now. But for a volcano with a life span of hundreds of thousands of years, three centuries are almost no time at all. It would be perfectly normal for the mountain to erupt again at any time. After all, it is only sleeping.