The relationship between degassing and ground deformation at Soufriere Hills Volcano, Montserrat I.M. Watson a, * , C. Oppenheimer a , B. Voight c , P.W. Francis b , A. Clarke c , J. Stix d , A. Miller e , D.M. Pyle f , M.R. Burton a,b , S.R. Young g , G. Norton g , S. Loughlin g , B. Darroux h , MVO Staff h a Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK b Department of Earth Sciences, Open University, Milton Keynes, Bucks, UK c Department of Geosciences, Penn State University, PA, USA d De ´partment de Ge ´ologie, Universite ´ of Montre ´al, Montre ´al, Canada e Geowalks, 24 Argyle Place, Edinburgh, UK f Department of Earth Sciences, University of Cambridge, Cambridge, UK g British Geological Survey, West Mains Road, Edinburgh, UK h Montserrat Volcano Observatory, Mongo Hill, Montserrat, WI, USA Received 18 November 1999 Abstract We examine the correlations between SO 2 emission rate, seismicity and ground deformation in the month prior to the 25 June 1997 dome collapse of the Soufriere Hills Volcano, Montserrat. During this period, the volcano exhibited a pattern of cyclic inflation and deflation with an 8–14 h period. We find that SO 2 emission rates, measured by COSPEC, correlate with the amplitude of these tilt cycles, and that higher rates of SO 2 emission were associated with stronger ground deformation and enhanced hybrid seismicity. Within tilt cycles, degassing peaks coincide with maximum deformation gradients. Increases in the amount of gas in the magma conduit feeding the dome, probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude, hybrid seismicity and SO 2 emission rate.  2000 Elsevier Science B.V. All rights reserved. Keywords: degassing–deformation correlation; cyclic activity; COSPEC; Soufriere Hills Volcano 1. Introduction Soufriere Hills Volcano (SHV), Montserrat, began erupting on 18 July 1995 with phreatic explosions, following seismic crises in 1992 and 1994 (Young et al., 1997). A new andesitic lava dome was first observed on 15 November 1995. Subsequent develop- ment of the eruption included periods of rapid dome growth, explosive events, and gravitational dome and fountain collapse generating pyroclastic density currents (Young et al., 1998a). At the time of writing (July 1999), there has been no dome growth since March 1998, though SO 2 emission rates remain at elevated levels of around 2–3 kg s -1 . Surveillance at SHV has been carried out by the Montserrat Volcano Observatory (MVO), comprised of local and visiting scientists, and using a combination of seismology, ground deformation, petrology and geochemistry, Journal of Volcanology and Geothermal Research 98 (2000) 117–126 0377-0273/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0377-0273(99)00187-0 www.elsevier.nl/locate/jvolgeores * Corresponding author.