H2O Emission Rate of Volcanic Plume During the 2000-2002 Miyakejima Volcanic Activity
Abstract
After the onset of the 2000 eruption, Miyakejima has effused a large amount of volcanic gases from its summit crater. We have observed the volcanic plume and estimated H2O emission rate from the summit crater, using infrared thermal images of the plume and meteorological data, such as air-temperature, humidity and atmospheric pressure (Matsushima and Nishi, 2001). The infrared thermal images provide the plume upward velocity, cross section and mean temperature at arbitrary heights. The plume is modeled as the mixture of volcanic gas and entrained atmosphere. Mass and energy conservation equations are derived from the plume model. Substituting the observed values and the physical constants into the equations, we calculate the emission rate of H2O discharging from the vent. The mean values and standard deviations of plume temperature, cross section and upward velocity are evaluated for the continuous data recorded for 300 sec on 19 September 2000. From the data fluctuations and sensitivity tests, the error of H2O emission rate is estimated to be about 30 %. The H2O emission rate showed the highest value of 1,000 kton/day in September 2000, and then it decreased gradually to 100 kton/day with large fluctuations as of June 2002. The variation is similar with that of SO2 emission rate (Kazahaya et al. 2001) except for the high value in September 2000. The average H2O/SO2 wt ratio is about 10, which corresponds to the melt gas content analyzed from the glass inclusion (Saito et al., 2001). The rapid decrease of H2O emission rate from September to October suggests that the ground water had progressively evaporated by the heat of uprising volcanic gas. The constant H2O/SO2 wt ratio after October 2000 means that the ground water is dried up or expelled from the conduit and the plume is mainly composed of volcanic gases. This interpretation is confirmed by mathematical simulation. The simulation accounts for multiphase mass and heat transport within a porous medium, and the supply of superheated vapor from degassing magma. Considering the fitting between the simulated H2O emission rate and the observation, practical hydrothermal conditions within the conduit and surrounding rocks are investigated.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V11C0512M
- Keywords:
-
- 8419 Eruption monitoring (7280);
- 8424 Hydrothermal systems (8135)