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doi:10.2204/iodp.proc.340.204.2016

Introduction

It is important to understand the significant hazards posed by volcanic islands such as Montserrat, which include various types of volcanic eruption and flank collapses that can trigger tsunamis. The best record of these past events may be found offshore in marine sequences because most of the eruption or landslide material ends up offshore (Le Friant et al., 2010). Terrestrial outcrops are particularly hard to interpret because of erosion or burial of deposits. Marine sediments are also more easily dated as a result of intervening layers of hemipelagic mud (Trofimovs et al., 2012). Integrated Ocean Drilling Program (IODP) Expedition 340 in 2012 provided some of the few long cores from offshore oceanic volcanic islands (>1 Ma) and the first that penetrated through major submarine landslide deposits (Le Friant et al., 2012, 2015). During the Expedition a total of nine sites were drilled offshore Montserrat, Martinique, and Dominica in the Lesser Antilles (Expedition 340 Scientists, 2012) (Fig. F1).

Montserrat is an outstanding natural laboratory for understanding the growth and decay of a volcanic island. It comprises four volcanic centers: the Soufrière Hills, South Soufrière Hills, Centre Hills, and Silver Hills centers. The Soufrière Hills volcano has erupted episodically since 1995, with the last major eruption in 2010. These subaerial eruptions have been studied in unusual detail (Wadge et al., 2014; Druitt and Kokelaar, 2002). The offshore record has also been studied in unprecedented detail. Previous offshore work includes arguably the most complete set of shallow (<10 meters below seafloor; mbsf) sediment cores available for a volcanic island (Trofimovs et al., 2006, 2008, 2010, 2013; Le Friant et al., 2008; Cassidy et al., 2013, 2014a, 2014b), the first repeat bathymetric surveys of offshore pyroclastic flow deposits emplaced during recent eruptions (Le Friant et al., 2004, 2009, 2010; Trofimovs et al., 2006, 2008), and detailed 2-D and 3-D seismic reflection surveys (Deplus et al., 2001; Lebas et al., 2011; Watt et al., 2012a, 2012b; Crutchley et al., 2013; Karstens et al., 2013; Le Friant et al., 2004, 2015). In particular, this previous work identified a series of large (1–20 km³) offshore landslide deposits with much larger volumes than any of the 1995–recent eruptions. These landslides are termed Deposit 1 to Deposit 8 (Deplus et al., 2001; Le Friant et al., 2004; Lebas et al., 2011; Watt et al., 2012a, 2012b).

IODP Expedition 340 provides important information on the growth and decay of volcanic islands in the Lesser Antilles. Three sites (U1394, U1395, and U1396) were successfully drilled offshore Montserrat; a fourth site (U1393) recovered little material and is not included here. Site U1394 is located ~10 km southeast of Montserrat, Site U1395 is located ~25 km southeast of Montserrat, and Site U1396 is located ~35 km southwest of Montserrat (Fig. F1). Sites U1394 and U1395 are situated in the Boulliante-Montserrat Graben, which is a major route for turbidites originating from southeast of Montserrat. Site U1396 is located on a topographic high to capture a record of fallout from eruptions that sent material southwest of the island. Identifying and dating the deposits will enable a better understanding of how volcanic island arcs evolve on long timescales (>1 My). Results are summarized as two oversized-format figures shown in “Appendix A” and “Appendix B.” Here we outline the methods used to obtain these figures and the key results that they show.

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