The recent history of the Soufrière Hills volcano, Montserrat, Lesser Antilles volcanic arc, is deduced using data obtained from a submarine core collected in 2002. The core contains concentrations of ash and several tephra layers, which are identified by the abundance of glass shards, dense and poorly vesiculated particles, and scoria. The tephra layers have been dated using micropalaeontology and stable isotope stratigraphy. Tephra layers in a marine sediment core off the coast of Montserrat record the volcanic history of South Soufrière Hills–Soufrière Hills volcano back to 250 ka. Eight layers are composed of dense juvenile ash related to dome eruptions, five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land. Six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions. A marker sequence of basalt tephra layers is dated at 124–147 ka and is correlated with construction of the South Soufrière Hills basaltic stratocone. Pelagic sediments between the main tephra layers have low abundances of volcanogenic components (<15%) and suggest long periods ( c . 10 4 years) of dormancy or low activity.

Mass flows on volcanic islands generated by volcanic lava dome collapse and by larger‐volume flank collapse can be highly dangerous locally and may generate tsunamis that threaten a wider area. It is therefore important to understand their frequency, emplacement dynamics, and relationship to volcanic eruption cycles. The best record of mass flow on volcanic islands may be found offshore, where most material is deposited and where intervening hemipelagic sediment aids dating. Here we analyze what is arguably the most comprehensive sediment core data set collected offshore from a volcanic island. The cores are located southeast of Montserrat, on which the Soufriere Hills volcano has been erupting since 1995. The cores provide a record of mass flow events during the last 110 thousand years. Older mass flow deposits differ significantly from those generated by the repeated lava dome collapses observed since 1995. The oldest mass flow deposit originated through collapse of the basaltic South Soufriere Hills at 103–110 ka, some 20–30 ka after eruptions formed this volcanic center. A ~1.8 km3 blocky debris avalanche deposit that extends from a chute in the island shelf records a particularly deep‐seated failure. It likely formed from a collapse of almost equal amounts of volcanic edifice and coeval carbonate shelf, emplacing a mixed bioclastic‐andesitic turbidite in a complex series of stages. This study illustrates how volcanic island growth and collapse involved extensive, large‐volume submarine mass flows with highly variable composition. Runout turbidites indicate that mass flows are emplaced either in multiple stages or as single events.

Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and masswasting events spanning 250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high-resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density-currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption-fed and mass-wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of 3000Geochemistry, Geophysics, Geosystems PUBLICATIONS submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.

No abstract