Carbon dioxide degassing by advective flow from Usu volcano, Japan

Magmatic carbon dioxide (CO2) degassing has been documented before the 31 M arch 2000 eruption of Usu volcano, Hokkaido, Japan. Six months before the e ruption, an increase in CO2 flux was detected on the summit caldera, from 1 20 (September 1998) to 340 metric tons per day (September 1999), followed b y a sudden decrease to 39 metric tons per day in June 2000, 3 months after the eruption. The change in CO2 flux and seismic observations suggests that before the eruption, advective processes controlled gas migration toward t he surface. The decrease in flux after the eruption at the summit caldera c ould be due to a rapid release of CO2 during the eruption from ascending da citic dikes spreading away from the magma chamber beneath the caldera.