Soufriere Hills
Soufriere Hills volcano, Montserrat, lies within the Lesser Antilles subduction zone and is thought to have been active for 300 Ka (Harford 2002). It has been erupting since 1995 and it’s activity is characterized by periods of dome growth, followed by long periods (1-18 months) where extrusive activity pauses (residually active period). Pyroclastic flows and vulcanian explosions punctuate both these residual and effusive periods.
Prior to the current eruption increased seismicity was detected beneath the volcano from 1992, followed by a seismic swarm in 1994 10-15 km deep (Aspinall 1998). Eruption commenced on the 18th of July 1995, beginning with phase one of dome extrusion. Phase one growth is characterized by irregular extrusion with smaller, more frequent pyroclastic flows (Herd 2005). The first phase ceased in March 1998, and residual activity was recorded till November 1999, where pyroclastic flows were generated as the dome cooled and became unstable.
Effusive eruption 2010 from: http://dearmissmermaid.blogspot.com/2010/02/pics-of-soufriere-hills-volcano-on.html
5-8th November 1999 a seismic swarm was detected and was followed by phreato-magmatic explosions, marking the beginning of the second phase of dome growth (Edmonds 2002). Phase two is characterized by continuous, faster extrusion (1-4 m3/s) (Herd 2005), less frequent but larger pyroclastic flows that allowed the dome to attain larger volumes (Herd 2005). Dome growth continued till 2003, where the end was marked by the largest dome collapse ever recorded. 210 million m3 collapsed in 18 hours, the majority in just over 2 hours. Shockwaves from one of the vulcanian eruptions following the dome collapse were thought to have caused structural damage to buildings 3km away.
Phase 3 of dome growth began in 2005 and continued till 2007. Extrusion rates were higher still (5.6 m3/s) with even fewer collapses than in phase two (Ryan 2010). Following another period of residual activity from 2007- 2008 with several pyroclastic flows, dome growth continued in 2009 but shifted west, forming a new dome on the side of the older one.
Prior to the eruption of 1995 seismic swarms in 1897, 1933, and 1966 were detected (Perret 1939, Shepherd 1971, and Robertson 2000) and are thought to have been events where magma was rising beneath the volcano.
Ash eruption 2002 from: http://www.rsiphotos.com/gallery_detail.php?PhotoID=119
Gas emissions
Some work has already been conducted on the gasses emitted from Soufriere Hills, but mainly on HCl and SO2. Work on CO2 emissions has only just begun (Edmonds 2010). Edmonds 2002 showed that HCl degassing occurred mainly at shallow levels during magma ascent as they were strongly related to effusion rates and agreed well with pertrological predictions. Furthermore, using HCl concentrations in glass, closed and open degassing systems could be distinguished. Samples from 1997 had higher HCl contents than in 1999 and 2000 samples. HCl strongly partitions into the fluid phase, in an open system this fluid is lost resulting in lower HCl contents in glass. So the higher concentrations of the 1997 samples indicate that the fluids could not escape (closed system), allowing equilibration of HCl content in the fluid and glass, resulting in higher HCl concentrations in the glass. The closed system degassing may explain the explosive activity of 1997, thus potentially HCl contents of glass could be a useful tool in prediction of explosive activity.
SO2 and CO2 fluxes have revealed the presence of an underlying body of degassing mafic magma. Edmonds 2010 used petrological constraints on andesitic magma composition and showed via models that the volume of SO2 emitted requires a magma volume 14-27 times the erupted volume, and CO2 fluxes require 3-6 times the erupted volume of magma. It was also noted that the gasses emitted require a source more reduced than the andesite, which is oxidised (NNO+1 according to Fe-Ti oxides). So the gasses cannot solely be derived from the erupting andesite. Gas emissions fit well with models where the majority of gas is derived from a mafic magma that is thought to degas on contact with the cooler andesite (Edmonds 2010). This also solves the oxidation inconsistency as the mafic magma provides the reduced source. The gases produced at the contact between the two magmas are thought to be able to percolate through some form of a permeable bubble network in the conduit. Variations in permeability of the conduit and dome have been noted by varying SO2 emissions (Edmonds 2003).
Figure from Edmonds 2010 "The modeled results of mixing between a gas phase derived from quenching and vesiculation of mafic magma of basaltic andesite composition at 150 MPa with gases derived from the decompressional degassing of rhyolite."
There is abundant petrological evidence for the presence of an underlying mafic body. Enclaves of the mafic magma can be found and are usually highly vesiculated, suggesting the mafic magma underwent rapid cooling and degassing on contact with the cooler (850°C) andesitic magma (Edmonds 2001). Furthermore, abundant disequilibrium textures can be observed such as sieve textures, reverse and oscillatory zoning (Edmonds 2001,2010). Oscillatory zoning suggests multiple mafic injections.
In order to explain gaseous fluxes and compositions and petrological observations a model where mafic magma injection into a lower chamber (~12km) that periodically interacts with a shallower (~6km) chamber containing rhyolite has been suggested (Edmonds 2010). This model, rather conveniently, also fits well with models constructed based on inflation and deflation patterns (Mattioli 2003).
CLICK HERE FOR TABLE OF SOUFRIERE HILLS GAS COMPOSITION
References
Allen A G, Baxter P J, Ottley C J. Gas and particle from Soufriere Hills Volcano, Montserrat, West Indies: characterization and health hazard assessment. Bull volcanol. 2000; 62:8-19
Aspinall W P, Miller A D, Lynch L L, Latchman J L, Stewart R C, White R A, Power J A. Soufriere Hills eruption, Montserrat, 1995–1997: volcanic earthquake locations and fault plane solutions. Geophys. Res. Lett., 1998;25: 3397–3400.
Harford C L, Pringle M S, Sparks R S J, Young S R. The volcanic evolution of Montserrat using 40Ar/39Ar geochronology. In: Druitt, T.H. & Kokelaar, B.P. (eds) The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs 2002; 21: 93–113.
Herd R A, Edmonds M, Bass V A. Catastrophic lava dome failure at Soufrière Hills Volcano, Montserrat, 12–13 July 2003. J Volcanol Geotherm Res 2005;148:234–252
Edmonds M, Pyle D M, Oppenheimer C M. A model for degassing at Soufriere Hills Volcano, Montserrat, West Indies based on geochemical data. Earth Planet Sci Lett 2001;186:159–173
Edmonds M, Pyle D, Oppenheimer C. HCl emissions at Soufrie`re Hills Volcano, Montserrat, West Indies, during a sec- ond phase of dome building: November 1999 to October 2000, B. Volcanol., 2002;64: 21–30.
Edmonds M, Oppenheimer C, Pyle D M, Herd R A, Thompson G. SO2 emissions from Soufrire Hills Volcano and their relationship to conduit permeability, hydrothermal interaction and degassing regime. J Volcanol Geotherm Res 2003;124:1-2: 23- 43. DOI:10.1016/S0377-0273(03)00041-6
Edmonds M, Aiuppa A, Humphreys M, Moretti R, Giudice G, Martin R S, Herd R A, Christopher T. Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano, Geochem Geophys Geosyst. 2010 doi:10.1029/2009GC002781,
Mattioli G S, Herd R A. Correlation of cyclic surface deformation recorded by GPS geodesy with surface magma flux at Soufrière Hills Volcano, Montserrat, Seismol. Res. Lett. 2003;74:2:230.
Perret F. The volcanic-seismic crisis at Montserrat 1933-1937. Carnegie Institution of Washington, Washington, DC, 1939;512:75.
Robertson R E A, Aspinall W P, Herd R A, Norton G E, Sparks R S J, Young S R. The 1995-98 eruption of the Soufrière Hills volcano, Montserrat, W.I. Philos. Trans. R. Soc. London, 2000;358:1619–1638.
Ryan G A, Loughlin S C, James M R, Jones L D, Calder E S, Christopher T, Strutt M H, Wadge G. Growth of the lava dome and extrusion rates at Soufriere Hills Volcano, Montserrat West Indies: 2005–2008, Geophys. Res. Lett., 2010;37, L00E08, doi:10.1029/ 2009GL041477.
Shepherd J B, Tomblin J F, Woo D A. Volcano-seismic crisis in Montserrat, West Indies, 1966-67. Bull. Volcanol., 1971;35:143–163.
Young S R, Sparks R S, Aspinall W P, Lunch L L, Miller D D, Robertson E A, Shepherd J B. Overview of the eruption of Soufriere Hills Volcano, Montserrat, 18 July 1995 to December 1997. J. Geophys. Res. Let. 1998;25:18:3389-3392
