Mount Fuji, Japan’s highest and most famous volcano, dominates the landscape southwest of Tokyo. Determining which tectonic plate it sits on is complex because the stratovolcano is situated at the unique meeting point where the boundaries of three major tectonic plates converge. This geological intersection is responsible for the mountain’s existence and its continued volcanic activity.
The Three Tectonic Plates Converging at Fuji
Mount Fuji is a product of the intense geological activity caused by the convergence of the Philippine Sea Plate, the Eurasian Plate, and the North American Plate. These three segments of the Earth’s lithosphere come together beneath the central part of Honshu island. In this region, the Eurasian Plate is often called the Amurian Plate, and the North American Plate is called the Okhotsk Plate. The Philippine Sea Plate, an oceanic plate lying to the south, is actively moving northward. This plate drives the subduction process that generates the magma fueling the volcano. The Pacific Plate also contributes to the overall tectonic stress by subducting further east along the Japan Trench.
Defining the Triple Junction
The meeting point of the Philippine Sea, Eurasian (Amurian), and North American (Okhotsk) plates is known as the Fuji Triple Junction. A triple junction is a point where the boundaries of three tectonic plates intersect, representing a site of concentrated crustal stress and deformation. Mount Fuji is situated directly atop this geological feature.
This positioning subjects the mountain to pressure and friction from three different directions. The Philippine Sea Plate collides with and slides underneath the continental plates, causing the crust to buckle and fracture. This intense convergence creates structural weaknesses in the overlying crust, providing a direct pathway for magma to rise to the surface.
The volcanic structure of Mount Fuji is built on the highly fractured crust of the Izu volcanic arc, which is being driven into Honshu. This collision zone generates the heat and pathways necessary for sustained volcanic activity. The stresses from the triple junction have created a magma reservoir beneath the mountain, estimated to sit at a depth of around 20 kilometers.
Subduction and the Formation of Mount Fuji
The geological mechanism responsible for the formation of Mount Fuji is subduction, where one tectonic plate slides beneath another. In this region, the denser oceanic plates—the Philippine Sea Plate and the Pacific Plate—are diving beneath the lighter continental plates. The Philippine Sea Plate subducts beneath the Eurasian (Amurian) and North American (Okhotsk) plates along two distinct boundaries: the Sagami Trough and the Suruga Trough.
As the oceanic plates sink deeper into the mantle, they carry water trapped within the rock structure. The increasing temperature and pressure cause this water to be released from the subducting slab. This water then rises into the overlying mantle wedge, the layer of rock directly above the descending plate.
The introduction of water significantly lowers the melting point of the mantle rock, a process known as flux melting. This causes the solid rock to partially melt, generating buoyant plumes of magma. This magma rises through the fractured crust created by the triple junction’s stress, eventually erupting to form the massive, symmetrical stratovolcano. The volcano is an island arc volcano, built up over hundreds of thousands of years through alternating eruptions of lava and ash.
Japan’s Location on the Ring of Fire
Mount Fuji’s tectonic environment places it within the Pacific Ring of Fire, a belt that circles the Pacific Ocean. This zone is defined by a continuous series of oceanic trenches, volcanic arcs, and plate movements. Approximately 75% of the world’s active and dormant volcanoes and 90% of the world’s earthquakes occur along this path.
Japan’s position along the Ring of Fire is a direct consequence of the continuous subduction of multiple oceanic plates beneath the continental crust. The island nation is a chain of volcanic arcs formed by the rising magma generated by this process. This high concentration of plate boundaries and subduction zones accounts for Japan’s frequent, powerful earthquakes and its abundance of volcanoes.