Journal of Conference Abstracts

Generation and Emplacement Mechanisms of Gravity-Driven Pyroclastic Flows from the 1996 Lava Dome of Soufriere Hills Volcano, Montserrat

Eliza S. Calder¹ (Eliza.Calder@Bristol.ac.uk), Paul D. Cole² (paul.cole@luton.ac.uk), R. Steve J. Sparks¹ (Steve.Sparks@Bristol.ac.uk) & Simon R. Young³ (SRY@wpo.nerc.ac.uk)

¹ Department of Geology, University of Bristol, Bristol, UK.

² Department of Geology, University of Luton, Bedfordshire, UK.

³ British Geological Survey, Keyworth, Nottingham, UK.

The 1996 lava dome of Soufriere Hills volcano first generated small pyroclastic flows in April. Since then continued growth of the dome has lead to the generation of numerous and successively larger flows in the Tar river valley. Many of these reached the sea 3km from crater and have now built up a substantial delta of deposits on the coast. Video footage of some of the flows was obtained as they entered the sea, preliminary analyses of these tapes give flow front velocities in the order of 70-40km/h. Up until the 17th September all flows were a product solely of gravitational collapse. The juvenile andesitic material is nearly totally non-vesicular. The upper fan deposits, directly beneath the dome, consist of a large number of overlapping lobes of coarse blocky deposits with well defined levee and channel structures. The distribution and character of the larger flow deposits is consistent with deposition from a two component flow. The dense basal avalanche giving rise to coarse block-rich material in the valley bottom and an upper turbulent ash cloud surge depositing a thinner layer of finer ash-rich material over a broader less topographically constrained area. Larger flows are consistently generated after sustained periods of smaller, 'decapping' rockfalls. Autobrecciation of hot, pre-stressed blocks and volume of collapsed material are important factors governing run-out distance of the flows. These types of deposits are thought to accumulate from a concentrated suspension of cohesionless fragments where dispersive pressure plays an important role in the suspension of clasts. The overriding ash clouds are however, dilute and turbulent and are considered to have formed by mixing with air in turbulent boundary layers across the upper surface of the basal avalanche. The different properties of the two components allows flow separation and in places detachment to occur as was the case when flows entered the sea.

The Great Glen Fault Transects the Whole Lithospheric Plate: Petrological and Isotopic Evidence

J. C. Canning¹ (J.C.Canning@bham.ac.uk), M. A. Morrison¹ (M.A.Morrison@bham.ac.uk), P. Henney² (P.J.Henney@british-geological-survey.ac.uk), P. W. C. van Calsteren³ (P.V.Calsteren@open.ac.uk), J. W. Gaskarth¹ (J.W.Gaskarth@bham.ac.uk), P. Holden⁴ (pholden@rupture.ucsc.edu) & A. Swarbrick¹

¹ School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham, UK.

² British Geological Survey, Keyworth, Nottingham, UK.

³ Department of Earth Science, The Open University, Milton Keynes, UK.

⁴ Department of Earth Science, The University of California, Santa Cruz, CA, USA.

Two suites of lamprophyres (mica- and hornblende-phyric varieties) have been used to constrain the composition and extent of sub-continental lithospheric mantle (SCLM) domains in Late Caledonian Northern Britain. Only primitive samples with compositions typical of primary magmas, unaffected by fractionation and/or contamination were used. Two distinct compositional groups, representing a Northern Highlands mantle domain and a Southern one, can be resolved. These are separated by a sharp boundary co-incident with the surface expression of the Great Glen Fault (GGF). This is apparent for the data for both lamprophyres suites. The Northern Highlands domain is characterised by low (sum)Nd (-6.4 to -12.8) while the Southern domain displays higher (sum)Nd (+3.9 to -5.3), with no overlap between them. It is clear from this that at the end of the Caledonian orogeny the GGF was a major discontinuity within the SCLM.

Samples derived from the two mantle domains tightly bracket the GGF with a surface separation of less than 20km. Mineralogical and chemical constraints require that these magmas have risen from a depth of at least 60km. The GGF cannot be listric and must be effectively vertical. Any imbrication or splaying of the fault zone that could have caused physical mixing of the source regions must be similarly restricted to a zone much less than 20km wide.