Mount Fuji, an iconic and culturally significant landmark, stands majestically in Japan. This towering peak, revered for its symmetrical cone and natural beauty, is a testament to powerful geological forces. Its familiar shape is not the result of a single event, but rather a complex history of volcanic activity spanning hundreds of thousands of years.
Mount Fuji’s Tectonic Foundation
The formation of Mount Fuji is directly linked to its unique position within a volatile geological region. It sits at a triple junction, where three major tectonic plates converge: the Amurian Plate (often considered part of the Eurasian Plate), the Okhotsk Plate (sometimes referred to as part of the North American Plate), and the Philippine Sea Plate. This specific arrangement creates a complex environment conducive to intense volcanic activity. The Philippine Sea Plate is actively subducting, or sliding, beneath both the Amurian and Okhotsk Plates. This process generates immense heat and pressure, causing the mantle rock to melt and form magma that then rises to the surface, fueling the eruptions that built Mount Fuji.
The Predecessor Volcanoes
Mount Fuji’s current form is built upon the remnants of older volcanic structures that paved the way for its growth. The earliest known phase, called Sen-komitake, represents an ancient andesite core discovered deep within the mountain. Following this, the Komitake Volcano emerged, forming a basalt layer several hundred thousand years ago. The peak of Komitake, which ceased erupting around 100,000 years ago, still protrudes from Mount Fuji’s northern slope. Building upon Komitake, the Ko-Fuji, or “Old Fuji,” began to form approximately 100,000 years ago. This earlier volcano developed through alternating layers of lava and ash, creating the bulk of the present-day mountain’s underlying structure, with its ejected materials largely covering the Komitake structure over millennia. These predecessor volcanoes provided the foundational mass and geological setting upon which the current Mount Fuji would eventually rise.
The Building of Modern Fuji
The majestic cone we see today, known as Shin-Fuji or “New Fuji,” began its formation approximately 10,000 to 11,000 years ago. This phase involved significant eruptive activity, characterized by massive basaltic lava flows and the accumulation of pyroclastic materials like ash and lapilli. These eruptions gradually filled in the slopes of the older Ko-Fuji, contributing to the mountain’s nearly perfect tapered form. Major eruptive phases between 17,000 and 8,000 years ago, and again between 7,000 and 3,500 years ago, significantly contributed to its growth. Further activity occurred between 4,000 and 2,000 years ago, shaping the mountain into its current impressive size. The last major eruption, known as the Hōei eruption, occurred from 1707 to 1708, forming a new crater on its southeastern side.
A Stratovolcano’s Construction
Mount Fuji is classified as a stratovolcano, also known as a composite volcano, whose conical shape and steep slopes result directly from how these volcanoes are built. They are constructed by alternating layers of viscous lava flows and explosive pyroclastic materials, such as ash, cinders, and volcanic bombs. The lava flows from stratovolcanoes are typically thick and sticky, preventing them from spreading far and instead building up steep flanks around the vent. Interspersed with these lava flows are layers of fragmented rock and ash ejected during powerful, explosive eruptions. This process of sequential layering, where new material is deposited on top of older eruptions, gradually builds the volcano higher and wider, with the repeated accumulation of these diverse materials creating the stable, symmetrical, and iconic conical profile that defines Mount Fuji and distinguishes it from other types of volcanoes.