Journal of Conference Abstracts

Ground Deformation Monitoring at Mount Etna: the Transition from IR-EDM to GPS

Jane L. Moss (jmoss@chelt.ac.uk) & W. J. McGuire (w.mcguire@ucl.ac.uk)

Centre for Volcanic Research, Cheltenham and Gloucester College of Higher Education, Cheltenham GL50 4AZ, UK.

Mt. Etna is the largest and most active volcano in Europe. The near constant extrusive and intrusive activity is monitored through the analysis of ground deformation. This occurs in relation to sub-surface stresses associated with (i) the intrusion of new magma within the edifice, (ii) gravitational loading of the summit, (iii) surface creep. A network of survey stations has been established to specifically examine the active 'Southern rift-zone', which lies SE of the central craters. This dynamically deforming area is characterised by a complex pattern of surface fissures and cracks. Temporal changes in the co-ordinates of the survey stations are modelled to understand the causative stress regime operating below.

Here we examine the transition from the IR-EDM to the GPS monitoring method over a two year period. Four established networks encompassing the summit area and the tectonic faults dissecting the lower eastern flank, were combined to form one comprehensive network of three-dimensional absolute co-ordinates. Difficulties encountered during the transition include processing the GPS data to a fixed absolute co-ordinate in a stable area, so that the measurements could be repeated in subsequent years, and secondly in combining the use of IR-EDM with the GPS method for networks with sky-view obstacles.

The 1995 and 1996 occupations of the networks using the GPS method confirmed the similarity of the IR-EDM and the GPS baseline lengths. The final co-ordinates were achieved through network adjustment using a fixed absolute co-ordinate, these were linked to the local Italian geodetic network, and determined relative to the WGS84 reference system. Calculated semi-major and semi-minor error ellipses have an average of 10mm and 7.5mm respectively and an estimated height error of 15mm.

Petrology of the Soufriere Hills Volcano, Montserrat, West Indies

M. D. Murphy¹ (m.d.murphy@bristol.ac.uk), J. Barclay¹, R. S. J. Sparks¹, M. R. Carroll¹ & R. Macdonald²

¹ Geology Department, Bristol University, Bristol, UK.

² Environmental Science Division, Lancaster University, Lancaster, UK.

The Montserrat magma is a crystal-rich (~40%) andesite (57­59% SiO₂ ), with phenocrysts of plagioclase (28­30%), amphibole (3­7%), orthopyroxene (3­5%), titanomagnetite (1.5­2%), quartz (<1%), augite microphenocrysts (<1%) and accessory apatite and ilmenite. The groundmass consists of plagioclase, orthopyroxene, augite, pigeonite, titanomagnetite and quartz in a residual high silica rhyolite glass (77­80% SiO₂). The crystal chemistry is complex. The predominant plagioclase population is relatively sodic (An_48­55) but strong reverse zoning up to about An₈₀ and fine dusty sieve-textures are very common. Plagioclase microphenocrysts (80­250 µm) are mostly relatively calcic (An_65­75) whereas smaller plagioclase microlites range from about An_50­70, becoming more calcic over the timescale of the recent eruption. Mafic inclusions (51­55% SiO₂), ranging from <1 mm to about 40 cm, form an important component of the magma. The inclusions are mostly elliptical although angular inclusions also occur. The inclusions contain phenocrysts of plagioclase in a quench-textured diktytaxitic matrix of plagioclase, amphibole, orthopyroxene, augite, pigeonite and titanomagnetite. The mineral chemistry of the inclusions is distinct reflecting crystallisation from a more mafic melt. The inclusions contain xenocrysts of plagioclase, amphibole and orthopyroxene, derived from the andesite. The quench textures and occurrence of andesitic xenocrysts imply that incorporation of the inclusions in a molten state. The older (16,000­24,000 years) volcaniclastics of the Soufriere Hills volcano are very similar in mineral assemblage and chemistry to the recent magma and also contain ubiquitous mafic inclusions. Geothermometry suggests that the andesite magma crystallised prior to 22,000 years at temperatures between about 800­875°C to a crystal mush which has subsequently been periodically intruded by hotter (about 1050°C) mafic magma. Influx of fresh mafic magma is believed to reheat and remobilise the resident magma resulting in eruption.