A volcano that grows taller in the rain? Mount Fuji has been hiding a geological secret

Japan’s iconic peak has been secretly doing something surprising, and scientists have just figured out why.

When the ground around a volcano starts to bulge, it sets off alarm bells. Known as ground uplift, such movement is among the most closely watched warning signs in volcanology. That usually means magma is pushing up from below and an eruption might be coming. So when sensors around Mount Fuji started recording ground lifting by as much as two centimeters during storms, you would expect scientists to sit up and take notice.

They did. But what they found was not what anyone expected.


Rain, not magma, is moving the ground
Shuo Zheng, a hydrological geodesist at Hong Kong Polytechnic University, and his team spent years poring over GPS data collected from a dense network of monitoring stations around the mountain from 2017 to 2023. Uplift sensors at two stations within 10 km of the summit repeatedly recorded unmistakable signals of uplift of 1 to 2 centimeters, well above the millimeter-level noise normally recorded by these sensors.

But when the team compared those readings with local rainfall records, the pattern was as clear as day. The ground lifted almost immediately with the heavy rain and dropped back down within a day or two of the rain stopping. This was rain lifting, not volcanic activity, reported Eos, the science news outlet for the American Geophysical Union.

The strange geology under Mount Fuji
This happens because of something quirky about Fuji's internal structure. Each of the underground layers of lava in the volcano is capped with what geologists call clinkers, thin layers of shattered rock that form when the outer surface of a lava flow cools rapidly while the inside is still flowing. These clinker layers are like natural sponges and absorb and retain groundwater.

These clinker layers fill up fast when a big rainstorm dumps water into the ground near the summit. Water is almost incompressible. Once those pores are full and there is no room for the water to spread sideways, the ground above has only one place to go, up.

At greater distances, 25 to 40 kilometers from the peak, the opposite effect was actually observed. In many parts of the world, the ground will sink. This is the typical response during heavy rainfall. But near the summit, the clinker layers prevent that sinking from dominating, creating the strange uplift the team measured.

Why it matters for volcano monitoring in the US and around the world
This is not a quirky Japan story. It has serious implications for how scientists track volcanoes everywhere, including the dozens of active ones in the United States.

Ground deformation is the primary way to track magma movement and eruption forecasts, according to the USGS Hawaiian Volcano Observatory. GPS networks instantly detect swelling and subsidence, and those measurements are used to make decisions about hazard alerts.

The problem, as Fuji now reveals, is that not all uplift is volcanic. Kosuke Heki, a geophysicist at Hokkaido University and a member of Zheng's team, says rain-induced uplift stops when the rain stops. Magma operates on a wholly different time scale: it keeps pushing for weeks or months, building steadily, ignoring what the weather is doing.

The most useful thing to take from this research, it turns out, is that timing difference. A short burst of uplift in a storm that is gone within 48 hours is almost certainly water. Uplift that keeps growing after the skies clear, that is the signal worth losing sleep over.

A strange case, or more common than we think?
Zheng’s team wanted to know whether the same rainfall effect was occurring at other Japanese volcanoes, particularly on an island chain south of Tokyo. It didn't. They theorize the clinker layers at those volcanoes are low enough to allow water to drain away before pressure can build.

Whether the same geology exists beneath volcanoes elsewhere in the world, the Cascades in the Pacific North-west, or Hawaii, for example, is an open question. The unusual combination of preserved clinker layers and relatively sealed near-surface structure at Mount Fuji may be unusual, or may be more widespread than anyone has yet looked for.

Either way, the study, published in the journal Geology, adds an important nuance to how scientists read the ground. False alarms in volcano monitoring cost real money, disrupt communities, erode public trust, and strain the economy. The missed warnings carry far greater consequences, and knowing that sometimes a rainstorm is just a rainstorm, no matter how much the ground is moving, is exactly the kind of knowledge that allows monitoring to be more precise.