Mount Fuji is Japan’s highest and most striking mountain. It is also still considered to be an active volcano by geologists, although it is not expected to erupt in the near future. In addition to being a memorable and striking vista and a potential source of danger, however, Mt. Fuji is also extremely important to geology because it is an example of a “triple junction” where three different tectonic plates meet.
According to the modern theory of plate tectonics, Earth’s surface consists of extremely large rock formations called plates, which drift across the surface of the globe at the rate of an inch or two per year and regularly bump and jostle one another on the long fault zones where two plates meet. When one plate passes under another, the fault line where they meet is often the site of frequent earthquakes as one plate hangs up on the other for a brief period, then suddenly gives way and moves as much as several feet. In addition to earthquakes, however, fault zones can also be the site of volcanic eruptions.
Volcanoes erupt when the shifting plates create a rupture in the crust through which liquid magma escapes from the Earth’s molten interior. The magma gradually cools and hardens into solid rock, creating conical mountains like Mount Fuji. Mount Fuji’s last eruption was in the early 1700s, when escaping magma formed a massive ash field running down the east side of the mountain. Geologists do not believe Mount Fuji is truly dormant (inactive) now, since it still lies along the meeting zone of the plates which caused the last eruption. However, there are currently no definitive signs that another eruption is imminent.
Although volcanoes are common along certain types of faults where plates are colliding or moving away from each other, Mount Fuji is especially unusual and important to geology because it sits at the meeting point not of two plates, but three. This is referred to as a “triple junction.” Below Mount Fuji converge the Amurian plate (part of Eurasia) from the west, the Okhotsk plate (originally part of North America but running underneath the Pacific) from the east, and the Filipino plate from the south.
In addition, geologists now realize that Mount Fuji is an unusual form of basalt-based composite volcano – a volcano which has accumulated its present form through several successive massive eruptions. Mount Fuji’s first life, known as its Komitake phase, began about 700,000 years ago, when the volcano was still emitting andesite rather than basalt. The northern side of Mount Fuji still contains rock from the Komitake volcano. Next, about 80,000 years ago, a new series of eruptions began known as “Old Fuji” or “Older Fuji,” with the new volcano forming along the south side of the remains from Komitake. The present-day Mount Fuji is in the middle of a third phase, “Younger Fuji,” which began 11,000 years ago.
The last Younger Fuji eruption occurred in the 1700s. Most scientists believe another eruption is still very possible, but probably not imminent. Some geologists have noted that pressure might be climbing within the mountain’s magma chamber again, however, indicating that the volcano might be building toward the next major eruption.