The Volcano's Pulse
In central Kyushu, the Aso caldera exhales a steady breath of volcanic gases. The most prominent of these is sulfur dioxide (SO2), released from the active Nakadake crater at a rate that can fluctuate between 200 and 1,000 tons per day, sometimes more. This constant degassing shows the active magma system deep beneath the surface. As magma rises, the reduction in pressure allows dissolved gases to escape, which then travel to the surface through fissures and vents.
The caldera itself is one of the world's largest, measuring 25 kilometers from north to south and 18 kilometers east to west. It was formed by four immense pyroclastic flow eruptions between 270,000 and 90,000 years ago. The final and largest of these events, Aso-4, was a super-eruption that ejected an estimated 384 cubic kilometers of rock and ash, blanketing much of the Japanese islands.
Today, activity is focused on the central cone group, specifically the Nakadake crater. This crater often contains a hyperacidic lake called Yudamari. During calm periods, this lake of hot water sits at around 60°C, its striking green color caused by dissolved iron ions. The lake is fed not by rainfall, but by superheated fluids and gases from the underlying hydrothermal system. The state of Yudamari—its temperature, water level, and even its existence—indicates the volcano's condition. Disappearance of the lake often precedes more intense eruptive activity.
Forecasting by Gas
Scientists from the Japan Meteorological Agency (JMA) and other institutions constantly monitor the gases released from Nakadake to forecast phreatic eruptions—violent steam-driven explosions. These eruptions occur when hot magma interacts with ground or surface water, flashing it to steam and causing a sudden, violent expansion. Changes in the composition of volcanic gases provide clues that such an event may be imminent.
One measurement is the ratio of sulfur dioxide (SO2) to hydrogen sulfide (H2S). Instruments like scanning differential optical absorption spectrometers (DOAS) are used to measure the absorption of ultraviolet light by these gases in the volcanic plume, allowing for continuous, remote monitoring. Under normal conditions, the high-temperature magmatic gas is rich in SO2. If the proportion of H2S increases relative to SO2, it can indicate that the magmatic gases are passing through a cooler, shallower hydrothermal system. This interaction can scrub the more water-soluble SO2 from the gas mixture, altering the ratio and signaling an increased risk of a steam explosion.
This data, combined with seismic monitoring and ground deformation measurements, allows the JMA to issue volcanic alert levels for Mount Aso. These levels, ranging from 1 (normal) to 5 (evacuate), determine the size of the exclusion zone around the crater, often restricting access for public safety.