Mount Fuji, a symbol of Japan’s geography and culture, is the nation’s highest peak, rising to 3,776 meters southwest of Tokyo. Despite its tranquil, snow-capped appearance and centuries of silence, the mountain is not extinct. It is officially classified as an active volcano, a status requiring constant scientific attention from Japanese authorities. The current lack of an eruption is a period of dormancy, not a sign that its geological activity has ended.
The Geological Classification of Mount Fuji
The term “active volcano” uses a technical definition that often differs from the common perception of a constantly erupting mountain. Under modern vulcanology standards, a volcano is classified as active if it has erupted at least once within the last 10,000 years. Mount Fuji’s last event occurred just over 300 years ago, placing it firmly within this geological window. This classification distinguishes it from a dormant volcano, which has been quiet for a longer period, and an extinct volcano, which scientists believe will never erupt again.
Japanese governmental agencies, including the Japan Meteorological Agency (JMA), categorize Fuji as a Class A active volcano. This designation highlights its potential for a large-scale eruption that could affect densely populated areas. The volcano sits at a triple junction where the Eurasian, North American, and Philippine Sea tectonic plates meet. This geological setting drives its ongoing activity, ensuring the magma system deep beneath the mountain remains dynamic and pressurized.
Historical Eruption Record
Mount Fuji’s history includes at least 10 reliable documented eruptions since the year 781 CE, demonstrating a pattern of repeated activity. The most recent and significant event was the Hōei eruption, which began on December 16, 1707, following a major earthquake 49 days prior. This explosive event was a Plinian-style eruption, characterized by a massive, sustained column of gas and ash reaching high into the stratosphere. The eruption lasted intermittently for 16 days, ending in early 1708, and ejected an estimated 800 million cubic meters of ash and pumice.
The Hōei eruption did not produce lava flows, but the voluminous ashfall caused severe disruption across a wide area. Ash was carried by prevailing winds as far as Edo (the former name for Tokyo), over 100 kilometers away, where it accumulated to a depth of several centimeters. This long period of inactivity since 1707, now spanning over 300 years, is unusual based on Fuji’s historical record. This extended dormancy suggests the potential for pressure to build over time, necessitating rigorous monitoring efforts today.
Current Monitoring and Indicators of Activity
Scientists utilize a sophisticated network of instruments to track the mountain’s internal state, providing continuous data on possible signs of magma movement. This monitoring system, called the Fundamental Volcano Observation Network (V-net), includes more than 50 instruments placed around the volcano. Seismic monitoring is a primary tool, using highly sensitive seismometers to detect deep low-frequency (DLF) earthquakes 15 to 20 kilometers below the summit. These tremors suggest the movement of magma or volcanic fluids within the crust.
Ground deformation is measured using Global Navigation Satellite System (GNSS) receivers and tiltmeters to detect subtle changes in the mountain’s shape. Swelling or deflation of the volcano’s flanks indicates the buildup or release of pressure in the shallow magma chamber. The National Research Institute for Earth Science and Disaster Resilience (NIED) monitors these changes, noting periods of heightened seismic activity, such as swarms detected in 2000-2001 and after the 2011 Tohoku earthquake. Analyzing gas emissions, particularly sulfur dioxide and carbon dioxide, provides another physical indicator, as increasing levels of these gases can signal magma rising closer to the surface.
Potential Impact of a Future Eruption
A future eruption would pose a significant hazard, primarily due to the widespread distribution of volcanic ash over the Greater Tokyo Area. Prevailing winds typically carry ash eastward, meaning the world’s largest metropolitan area lies directly in the path of potential fallout. A repeat of the 1707 event could deposit several centimeters of ash on central Tokyo, with up to 30 centimeters accumulating in closer western districts. Even a small layer of ash can paralyze transportation networks, including roads, railways, and air travel, due to reduced visibility and mechanical failures in engines.
Heavier ashfall poses a risk of structural collapse for wooden buildings, particularly when combined with rain or snow. Ash can also contaminate freshwater reservoirs and disrupt power lines, leading to widespread utility outages affecting millions of people. Japanese government expert panels have developed detailed action plans for these scenarios, advising residents to stay indoors when ashfall is under 30 centimeters. The primary concern is not lava flow, which is typically confined to the mountain’s immediate slopes, but the cascading infrastructural failure caused by widespread ash blanketing a dense urban environment.