Mount Fuji, with its iconic conical shape, stands as a prominent natural landmark in Japan. Its geological nature often prompts curiosity. Understanding its volcanic classification provides insight into its formation and potential behavior.
Mount Fuji’s Volcanic Classification
Mount Fuji is classified as a stratovolcano, also commonly known as a composite volcano. This designation indicates a volcano built up from many layers, or strata, of hardened lava, tephra (rock fragments), pumice, and volcanic ash. Stratovolcanoes are characterized by their typically steep slopes and a summit crater. Their construction involves alternating periods of both effusive lava flows and explosive eruptions.
The term “composite” reflects the varied materials that compose these volcanoes, deposited over successive eruptions. These layers solidify, contributing to the volcano’s structure.
Understanding Stratovolcanoes
Stratovolcanoes are typically tall, conical formations with slopes that become steeper near the summit. They are built from alternating layers of viscous lava flows and pyroclastic material, such as ash, cinders, and volcanic bombs. The viscous nature of the lava, often rich in silica, prevents it from flowing far, instead causing it to pile up and contribute to the volcano’s steep profile.
This type of volcano commonly forms at subduction zones, where one tectonic plate slides beneath another. The process generates magma that rises to the surface, forming volcanic arcs. Eruptions from stratovolcanoes are often intermittent and can be explosive due to the buildup of pressure from trapped gases within the highly viscous magma. This pressure release results in forceful expulsion of ash, gases, and rock fragments. The materials expelled typically include andesite, dacite, and rhyolite, reflecting the silica-rich composition of the magma. These volcanoes develop over thousands of years through repeated volcanic activity.
Mount Fuji’s History and Structure
Mount Fuji’s base is approximately 125 kilometers (78 miles) in circumference. Its summit crater spans about 500 meters (1,600 feet) in diameter and descends to a depth of roughly 250 meters (820 feet).
Geologically, Mount Fuji is situated in the “Ring of Fire,” a region known for high seismic and volcanic activity. It lies at a triple junction where the Eurasian, North American, and Philippine Sea plates meet, with the Pacific Plate subducting beneath them.
Mount Fuji’s formation occurred in several phases, beginning with the Komitake volcano around 700,000 years ago, followed by Old Fuji, and then the modern New Fuji which began forming about 10,000 years ago.
The last major eruption of Mount Fuji was the Hoei eruption, which occurred from December 1707 to January 1708. This explosive eruption produced significant ashfall, reaching Edo (present-day Tokyo) 100 kilometers away. The Hoei eruption was triggered by a magnitude 8.6 earthquake that struck 49 days prior, releasing pressure from magma chambers. Although currently dormant, Mount Fuji is still considered an active volcano, typical of stratovolcanoes with long periods of inactivity.