Mount Fuji, a nearly symmetrical cone, is revered as a sacred symbol of Japan. This iconic peak is a towering stratovolcano classified by scientists as active yet dormant. Its history is marked by periods of intense volcanic activity, shaping the landscape and impacting the region. Understanding the last time this volcano erupted provides context for assessing its present status and potential for future activity.
The Great Hoei Eruption (1707)
The most recent confirmed eruption of Mount Fuji began on December 16, 1707, known as the Great Hoei Eruption. This explosive period continued intermittently for over two months, ceasing on February 24, 1708. The eruption was a powerful Plinian event, rated 5 on the Volcanic Explosivity Index (VEI), and was preceded by a massive magnitude 8.7 earthquake a few weeks earlier.
The eruption dramatically reshaped the southeastern flank of the mountain, creating three new vents known as the Hoei craters. This event was characterized by an enormous expulsion of ash and scoria, estimated at 800 million cubic meters, without any lava flow reaching the surface.
Westerly winds carried this massive cloud of ash over a vast distance, causing the worst ash-fall disaster in Japan’s recorded history. Edo, the former name for modern Tokyo, which lies almost 100 kilometers away, was blanketed with several centimeters of ash. Agricultural land closer to the volcano was devastated, leading to widespread crop failure and subsequent famine.
The immediate disaster was followed by secondary issues, as the ash clogged rivers and led to destructive mudflows and flooding during the following summer’s rainy season. The widespread economic and environmental damage cemented the Hoei Eruption as the last major event in Mount Fuji’s history. Since 1708, the mountain has remained in a state of relative quiet, leading to its current classification as dormant.
Mt. Fuji’s Current Status and Monitoring
Mount Fuji is designated as an active volcano, meaning it has erupted within the last 10,000 years and still shows signs of internal activity. Since 1708, a complex network of instruments continuously monitors its internal state for any sign of unrest. The Japanese Meteorological Agency (JMA) employs advanced observation techniques to track subtle changes beneath the surface.
Seismic Monitoring
Seismic monitoring uses a dense network of seismometers to detect earthquakes and volcanic tremors. Scientists watch for deep low-frequency (DLF) earthquakes, which are associated with the movement of magma or volcanic fluids deep within the crust. An increase in DLF events was observed in 2000 and 2001, though it did not lead to an eruption.
Ground Deformation Tracking
Ground deformation tracking is a primary method, involving Global Positioning System (GPS) stations and tiltmeters placed around the volcano. These instruments measure minute changes in the mountain’s shape, such as swelling or inflation, which indicates magma accumulating in a shallow reservoir. A period of inflation was detected between 2008 and 2010, suggesting a movement of deep underground fluids, but this activity eventually subsided.
Gas Emissions Analysis
Monitoring also includes the continuous analysis of volcanic gas emissions, such as sulfur dioxide and carbon dioxide, from fumaroles and vents. A sustained increase in the output of these gases would signal that new magma is rising and degassing closer to the surface. The combination of these monitoring systems provides a robust, real-time assessment of the volcano’s present condition.
Assessing the Likelihood of Future Activity
Scientific consensus holds that an eventual re-eruption of Mount Fuji is a certainty, given its geological history and active status. The primary concern is the potential trigger mechanism, which historical data suggests is often linked to major tectonic earthquakes. The 1707 Hoei Eruption was preceded by the Great Hoei earthquake, which scientists believe compressed the magma chamber and forced magma toward the surface.
The massive 2011 Tohoku earthquake and the subsequent East Shizuoka earthquake caused calculated stress changes in the area surrounding Mount Fuji’s magma system. While these events did not lead to an eruption, they demonstrated that tectonic shifts can influence the volcano’s stability. Experts calculate the probability of a future eruption based on these tectonic and magmatic interactions.
The government and local authorities have developed detailed preparedness plans using the 1707 event as the baseline scenario. Hazard maps and evacuation routes have been established for the surrounding areas. The most significant impact is projected to be volcanic ashfall, which could deposit several centimeters of ash over the greater Tokyo metropolitan area, paralyzing transportation and infrastructure within hours.
Preparedness includes simulations showing that a layer of just three centimeters of ash is enough to halt road traffic and trains. These proactive measures, including action plans for ash removal and supply stockpiling, reflect the long-term recognition of the calculated risk. Monitoring and planning are crucial for mitigating the unavoidable impact of the next eruption.