Cosmic rays provide penetrating insight into volcanic activity

Volcanic activity is driven by magma movement within a volcano’s “plumbing system.” These subsurface processes often cause volcanic inflation due to pressurized gas and magma buildup, and deflation as pressure is released during and after eruptions. While satellite-based remote sensing can monitor ground surface deformation, only about half of the deformations precede eruptions, so deformation monitoring via satellite alone may not be sufficient when it comes to predicting eruptions.

László Oláh and Hiroyuki Tanaka combined muography, a geophysical imaging technique that uses subatomic cosmic ray muons, with satellite-based data to monitor the internal volcanic magma dynamics of Sakurajima, one of Japan’s most active volcanoes. The integrated approach helped identify causes of eruptions and improved mid-term hazard assessment.

“During quiet periods, magma accumulated beneath the crater, forming a dense plug that increased pressure within the volcano and caused its surface to rise,” said Oláh. “During eruptive phases, this plug was destroyed and replaced by fresher, lower-density magma, which allowed volcanic gases to escape, reducing pressure and causing the volcano’s surface to subside.”

The research represents the first time muography was used to monitor magma density changes between two adjacent active Sakurajima craters. The results showed that as one crater became active, its magma conduit density increased, while the adjacent crater’s conduit showed a corresponding decrease — suggesting a dynamic, shared plumbing system.

“Currently, muography provides monthly density measurements, making it a valuable tool for mid-term monitoring of volcanic unrest,” said Oláh. “By combining muon data with surface deformation and gas emission observations, scientists can quantify a volcanic unrest index, a metric that can feed into probabilistic hazard models to improve eruption forecasting and risk mitigation strategies.”

Source: “Towards joint muography and ground deformation monitoring for volcanic unrest assessment,” by László Oláh and Hiroyuki K. M. Tanaka, Journal of Applied Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0275038 .

This paper is part of the Muography: Discoveries, Innovations and Applications Collection, learn more here .