H2O Emission Rate of Volcanic Plume During the 2000-2001 Miyakejima Volcanic Activity

Immediately after the onset of the eruption of last year, Miyakejima has effused a large amount of volcanic gases from its summit crater. In order to obtain the basic data about the volcano-hydrothermal system, we observed the volcanic plume and estimated H2O emission rate from the summit crater. A temporally observation was made from a patrol ship on September 19, 2000 and then continuous observation has been conducted at Ako district of Muyakejima since October 25, 2000. Infrared thermal image and video image of the plume were recorded in digital tape at 2 seconds intervals in minimum. At the same station, air-temperature, humidity and atmospheric pressure were recorded at 30 minutes intervals. By pursuing the light and shade patterns of the infrared thermal images or video images, we measured the rising velocity of the plume. From the temperature distribution of the plume, which was obtained by the infrared thermal image, we measured the width and mean temperature of the plume at an arbitrary height. When the clear image of the plume was taken, we conducted analysis. From the observation data: plume rising velocity, plume width, plume mean temperature and meteorological conditions, H2O emission rate of the plume was estimated using the conservation law of mass and energy (Fukui, 1995). The result was the underestimation because the mean temperature was evaluated at the plume margin. The average H2O emission rate from October 2000 to February 2001 was 400ktons/day, which was one order smaller than that observed on September 19, 2000 (2000kton/day). The average sulfur dioxide emission rate at the same period was 40ktons/day (Kazahaya et al, 2001). Because the glass inclusion analysis indicated that the melt in the magma had 1.4wt% H2O and 0.2wt% SO2 content (Saito et al, 2001), the H2O degassing rate from the intruded magma was estimated to be 280kton/day. The H2O emission rate decreased to the value near the degassing rate after October 2000, suggesting that the ground water surrounding the intruded magma had been exhausted by the progress of the volcanic activity and the plume was mainly constituted by the H2O degassed from the intruded magma.