Sedimentology of deposits from the pyroclastic density current of 26 December 1997 at Soufrière Hills Volcano, Montserrat

Abstract

abstractAt 03:01 (local time) on 26 December 1997, major sector collapse followed by collapse of the andesitic lava dome occurred at Soufrière Hills Volcano, Montserrat. The collapse of the dome involved explosive disintegration and formation of a highly energetic pyroclastic density current (PDC), which was dispersed principally to the SW and devastated an area of 10 km2. The deposits of the PDC are divisible into valley-confined and unconfined facies. The latter is characterized by two bipartite units (Units I and II), both of which are composed of a fines-poor layer (layer 1) typically overlain by a finer-grained, fines-rich layer (layer 2). The sequence is interpreted as recording strongly pulsatory (unsteady) flow and is capped by Unit III, an accretionary lapilli-rich fallout layer. There are pronounced variations of lithofacies, thickness, grain size and sedimentary structures related to local topography. The PDC was highly erosive: it sculpted isolated mounds of deposit and heavily scoured the pre-existing substrate. Lithofacies are granulometrically distinct, with median diameter (Mdø) increasing as sorting coefficient (Ãø) decreases. Lithofacies characteristics depend strongly on azimuth over a 70° sector, with major lateral (cross-flow) changes at similar radial distances from the dome. The deposits are similar to those produced in the blast eruptions of Mont Pelée in 1902 and Mount St Helens in 1980. We infer that particle size sorting occurred during explosive expansion of the collapsing lava dome, such that the resulting PDC was initially stratified in both grain size and density. The marked lateral and vertical variations in grain size of the deposits indicate efficient further development of density stratification and grain-size sorting during transport, due to air entrainment and sedimentation.