Rainwater and ash leachate analysis as proxies for plume chemistry at Soufrière Hills volcano, Montserrat
M. Edmonds, C. Oppenheimer, D. M. Pyle, R. A. Herd, 2003. "Rainwater and ash leachate analysis as proxies for plume chemistry at Soufrière Hills volcano, Montserrat", Volcanic Degassing, C. Oppenheimer, D. M. Pyle, J. Barclay
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Chloride and sulphate concentrations in rainwater and water-soluble leachates from volcanic ash samples track the compositions of gas emissions at the Soufriere Hills Volcano, Montserrat, from 1996 to 2001. There are both systematic spatial and temporal variations in the chloride/sulphate ratio (expressed as the equivalent HCI/SO2 mass ratio) in rainwater and ash leachates. Temporal variations reflect changes in eruption rate and eruptive style. Mass ratios of HCl/SO2 in ash leachates correspond closely with those obtained by open-path Fourier transform infrared (OP-FTIR) spectroscopy, and reflect changes in volatile emissions throughout the eruption. Both leachate and OP-FTIR spectroscopic analyses show mass ratios of HCl/SO2 > 1 during dome growth, and HCl/SO2 < 1 during non-eruptive periods.
The HCl/SO2 mass ratios in rainwater samples from 1996 and 1997 show temporal variations that correlate with changes in extrusion rate. The HCl/SO2 ratios in plume-affected rainwater and ash leachates from June and July 2001 correlate positively with increasing rock-fall energy, and with increasing eruption rate prior to a dome collapse event. The HCl/SO2 mass ratios in water-soluble ash leachates and rainwater samples collected at the same time and from the same sites, are linearly correlated, with rainwater HCl/SO2 ratios systematically two to three times higher than ash leachate ratios. Spatial patterns of rainwater pH, and HCl/SO2 in rainwater and ash leachates are principally influenced by the proximity of the sampling sites to the active dome, and to the typical pattern of dispersion of the plume by tropospheric winds. These results demonstrate that rainwater chemistry and ash leachate analysis provides a useful indicator of volcanic activity, and represents a valuable supplement to volcano surveillance efforts.
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Humans have long marvelled at (and feared) the odorous and colourful manifestations of volcanic emissions, and, in some cases, have harnessed them for their economic value. The degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions. Moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment, and climate. Even more fundamental are the relationships between the history of planetary outgassing, differentiation of the Earth’s interior, chemistry of the atmosphere and hydrosphere, and the origin and evolution of life. This book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma, the feedbacks between degassing and magma dynamics, and the composition, flux, and environmental, atmospheric and climatic impacts of volcanic gas emissions.