When there are signs of ground movement around a live volcano, scientists typically pay close attention, as such deformation often indicates magma flow beneath the surface. However, a discovery at Japan's most famous peak, Mount Fuji, is changing how geologists interpret subtle ground deformation around volcanoes.
Rainfall-Induced Swelling
Sensors placed around Mount Fuji have revealed that the ground expands upward by as much as two centimeters during periods of heavy rainfall. This temporary swelling is unrelated to molten rock or volcanic threats, but instead results from a highly unusual structural layout hidden beneath the mountain slope.
The findings emerged from careful analysis by a research team at the Hong Kong Polytechnic University. They examined daily observations accumulated over six years from a network of observation stations surrounding the volcano. By comparing precise measurements with weather data, the researchers discovered that the volcano rose briefly after heavy rainfall and returned to its original height once precipitation ended.
Role of Porous Rock Layers
According to researchers, the phenomenon is linked to Mount Fuji's structure. The volcano was formed by layers of solidified lava flows, each covered by porous rock called clinkers. Clinkers are loose rocks formed by the rapid cooling of lava flow edges.
This mechanism is documented in a study published in the journal Geology titled Heavy rains inflate Mount Fuji, central Japan. The authors explain that clinker horizons act like natural underground aquifers, collecting and storing rainwater. When heavy rain falls on the upper slopes, water fills pore spaces in these shallow aquifers.
Since water is not easily compressible, this accumulation creates outward pressure when sideways flow is blocked. This pressure lifts the ground surface, detected by sensitive equipment. Surprisingly, stations 25–40 kilometers from the summit recorded slight subsidence during heavy rains.
Implications for Volcano Monitoring
The findings provide volcanologists a practical way to distinguish between rainstorms and potentially dangerous underground activity. Rain-driven uplift is short-lived, typically lasting only a day or two, while magma-driven deformation persists or evolves over weeks, months, or years.
By recognizing that temporary inflation is a hydrological response to weather, researchers can avoid false alarms and improve accuracy in assessing volcanic risks.
