Mount Fuji: A Stratovolcano Built in Overlapping Phases
Mount Fuji is a stratovolcano, or a composite volcano formed by the accumulation of lava flows, ash, and pyroclastic materials deposited over 2.6 million years of eruptions. The mountain's unusual steep, symmetrical cone profile is caused by relatively high-viscosity lava, which forms thick sequences near the eruptive vent rather than spreading across the landscape. This "grey volcano" designation reflects its proclivity for explosive eruptions that generate volcanic ash clouds rather than flowing lava.
The geological formation happened in several separate periods. The first phase, known as Sen komitake, included an andesite core discovered deep within the mountain in 2004. This stratum forms the northernmost slope and precedes any observable structures. Komitake Fuji, a basalt layer that originated hundreds of thousands of years ago, is now mostly buried beneath younger materials. Old Fuji ("Ko-Fuji") formed 100,000 to 17,000 years ago, resulting in a large mountain that now serves as the mountain's deeper structural basis. Finally, New Fuji ("Shin-Fuji") began to form as the highest layer around 10,000 years ago, creating the summit region and making the nearly flawless tapering cone evident today.
Mount Fuji's crater is approximately 780 meters (2,560 feet) in diameter and 240 meters (790 ft) deep, with slopes ranging from 31 to 35 degrees—exactly the angle of repose for dry gravel. The mountain's base is 40-50 kilometres in diameter and contains numerous parasitic cinder cones (over 100 aligned cones) and over 70 lava tunnels. The top consists of eight separate peaks that encircle the central crater.
Recent volcanic history demonstrates inconsistent activity. Lava, lapilli, and ash were created during eruptions that occurred between 17,000 and 8,000 years ago. Subsequent eruptions occurred between 7,000 and 3,500 years ago, then again between 4,000 and 2,000 years ago. Flank eruptions, which generally formed parasitic cones, ended in 1707. The destructive Hoei eruption (1707-1708) created a new crater and secondary peak (Mount Hōei) midway down the southeastern face, sending cinders and ash to far areas. Since then, no eruptions have been reported, though a magnitude 6.2 earthquake occurred in March 2011 a few km from Mount Fuji's southern flank, momentarily drawing scientific attention.
