Mount Fuji stands as Japan’s most recognizable and culturally significant peak, often depicted as a symbol of tranquility. Yet, this iconic mountain is a volcano, leading to confusion over its current geological status. The question of whether Mount Fuji is active, dormant, or extinct directly relates to the potential threat it poses to the densely populated areas of Japan. To understand its nature, it is necessary to first define the specific terms volcanologists use for classification.
Defining Active, Dormant, and Extinct
Volcanologists use a spectrum of terms to categorize a volcano’s potential for future eruption. An active volcano is one that has erupted within the Holocene epoch (the last 11,700 years) or exhibits clear signs of an imminent eruption, such as strong seismic activity or gas emissions. These volcanoes possess a youthful magmatic system and are expected to erupt again.
The term dormant is applied to an active volcano that is not currently erupting but has the potential to do so. A dormant volcano has usually erupted within historical memory and maintains an intact magmatic plumbing system. It often shows low-level signs of activity, such as slight ground deformation or minor seismic tremors, confirming that it is merely resting.
In contrast, an extinct volcano is one that scientists do not expect to erupt ever again because it has not shown signs of activity for tens of thousands of years. The magma source for an extinct volcano is believed to have been cut off or completely solidified. Mount Fuji cannot be classified as extinct due to its history of recent, powerful eruptions.
Mount Fuji’s Eruptive History
Mount Fuji’s history of volcanism over the past several millennia confirms its long-term potential for activity. Records indicate at least ten reliable eruptions have occurred since 781 CE. The most recent and best-documented event was the Hōei eruption, which began on December 16, 1707.
This major event was likely triggered by a powerful tectonic shift, specifically the M8.6 Hōei earthquake that occurred 49 days earlier off the coast of Japan. The earthquake caused a change in the stress field beneath the mountain, which pressurized the magma chambers. The Hōei eruption lasted until February 24, 1708, and was characterized by a massive explosive phase without any accompanying lava flow.
The eruption reached a magnitude of VEI 5 (Volcanic Explosivity Index) and ejected approximately 800 million cubic meters of ash. Wind carried the volcanic ash to Japan’s capital, Edo (now Tokyo), nearly 100 kilometers away, where it blanketed the city in a thick layer. The scale and recency of this event, which occurred just over 300 years ago, establishes a clear historical precedent for future activity.
The Scientific Basis for Its Dormant Classification
Mount Fuji is classified as dormant, a status supported by ongoing seismic and geodetic monitoring data. The continuous presence of a pressurized magma chamber beneath the mountain is the most compelling evidence. Scientists monitor the volcano using a network of instruments to detect subterranean movements and stress changes.
Evidence of its viability emerged following the 2011 Great East Japan Earthquake (Tōhoku earthquake), a powerful M9.0 event. This massive tremor, and a subsequent M5.9 earthquake near the volcano, caused measurable changes within Fuji’s magmatic system. Researchers detected a significant increase in the rate of deep low-frequency earthquakes (LFEs) beneath the mountain, which are tremors associated with the movement of magma or volcanic fluids.
These seismic changes indicated that the large earthquakes had reactivated the volcano’s plumbing system by disturbing crustal stress levels. Although the volcano did not erupt, the data confirmed that the deep magma chamber is still viable and responsive to external forces. This constant internal pressure, even without surface activity like gas venting or steam plumes, means the volcano is merely resting. Therefore, the scientific consensus is that a future eruption is a certainty, solidifying its classification as dormant.