The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: A long-term (>1 Ma) record offshore Montserrat, Lesser Antilles

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings

Shear-derived mixing in dense granular flows

In flume experiments, granular avalanches run onto loose substrates develop 3-D architectures that record shear-derived mixing between the flow and the substrate. Spherical silica beads 0.250 mm diameter are run onto stratified substrates of various topographies composed of identical but colored materials. A method of setting and serial sectioning experimental granular deposits is presented. Experiments investigating the interactions between these granular charges and substrates reveal that centimeter-scale vortical reworking features are produced by the highly unsteady flows, with localized erosion depth of the same order as the flow thickness. The structure of the reworking features indicates predominantly bed-normal and streamwise particle motions and is interpreted as most likely to reflect velocity-shear instability growth, similar to Kelvin-Helmholtz instabilities formed in Newtonian fluids. This constitutes the first observation of such features formed within granular fluids by motions within the vertical plane. The scale of the features and degree of mixing generated by them has implications for the reliability of stratigraphic interpretations of geophysical granular flow deposits, such as those formed by debris avalanches and pyroclastic density currents. Sheared "flame structures" commonly observed at the bases of geophysical flow deposits might be explained in some cases by reworking by unsteady currents, or by rapid vertical migration of the active shear zone in long-lived steady currents. Copyright © 2011, SEPM (Society for Sedimentary Geology)