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Rainwater and ash leachate analysis as proxies for plume chemistry at Soufrière Hills volcano, Montserrat

By
M. Edmonds
M. Edmonds
Montserrat Volcano Observatory, Mongo Hill, Montserrat, West Indies (email: marie@mvo.ms)
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C. Oppenheimer
C. Oppenheimer
Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK.
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D. M. Pyle
D. M. Pyle
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
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R. A. Herd
R. A. Herd
Montserrat Volcano Observatory, Mongo Hill, Montserrat, West Indies (email: marie@mvo.ms)
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Published:
January 01, 2003

Abstract

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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Contents

Geological Society, London, Special Publications

Volcanic Degassing

C. Oppenheimer
C. Oppenheimer
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D. M. Pyle
D. M. Pyle
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J. Barclay
J. Barclay
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Geological Society of London
Volume
213
ISBN electronic:
9781862394612
Publication date:
January 01, 2003

GeoRef

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