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 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 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 toward 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 suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate 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.
Key Points
Heightened volcanic activity on Montserrat at 120–190, 760–810, and 900–930 ka
Large landslides coincide with rapid sea level rise at island arc volcanoes
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 mass‐wasting 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 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.
Key Points
Biostratigraphy, isotope geochemistry, and clast componentry of IODP Site U1396
Deposits are correlated across sites to the south and south west of Montserrat
Results highlight the spatial heterogeneity of deposits around volcanic islands
Volcanism on Montserrat (Lesser Antilles arc) has migrated southwards since the formation of the Silver Hills ~2.5Ma, and has formed three successively active volcanic centres. The Centre Hills ...volcano was the focus of volcanism from ~1–0.4Ma, before activity commenced at the currently active Soufrière Hills volcano. The history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes. Here, we describe the pyroclastic stratigraphy of subaerial exposures around central Montserrat; identifying 11 thick (>1m) pumiceous units derived from sustained explosive eruptions of Centre Hills from ~0.8–0.4Ma. Over 10 other, less well- exposed pumiceous units have also been identified. The pumice-rich units are interbedded with andesite lava breccias derived from effusive, dome-forming eruptions of Centre Hills. The stratigraphy indicates that large (up to magnitude 5) explosive eruptions occurred throughout the history of Centre Hills, alongside effusive activity. This behaviour at Centre Hills contrasts with Soufrière Hills, where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano, from ~175–130ka. Subsequent eruptions at Soufriere Hills have been dominated by andesitic effusive eruptions. The bulk composition, petrography and mineral chemistry of volcanic rocks from Centre Hills and Soufrière Hills are similar throughout the history of both volcanoes, except for occasional, transient departures to different magma compositions, which mark shifts in vent location or dominant eruption style. For example, the final recorded eruption of Centre Hills, before the initiation of activity at Soufrière Hills, was more silicic than any other identified eruption on Montserrat; and the basaltic South Soufrière Hills episode marked the transition to the current stage of predominantly effusive Soufrière Hills activity. The compositional stability observed throughout the history of Centre Hills and Soufrière Hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma, but may reflect local changes in long-term magma storage conditions that characterise individual episodes (on 105year timescales) of volcanism on Montserrat.
•Presentation of a comprehensive subaerial stratigraphy of Montserrat spanning 1Ma.•Eruptive products from both active centres are compositionally similar.•Activity at the older centre tended towards more sustained explosive eruptions.•Variation may be due to differences in magma ascent, temperature or storage.
Montserrat is a small island arc volcano in the Caribbean island arc. The island comprises three main volcanic centres: Silver Hills, active between 2.5-1 Ma; Centre Hills, active between ~1 to 0.5 ...Ma; and the Soufrière Hills-South Soufrière Hills volcanic complex, active from ~0.3 Ma. Here an extensive (> 1 Ma) and detailed stratigraphic record is compiled for Montserrat using both the subaerial and submarine (in the form of three International Ocean Drilling Program cores) records. This combined record gives valuable insight into the evolution of volcanic and mass-wasting processes at Montserrat, and may be useful for future hazard mitigation. The stratigraphic record shows that eruptive styles, volcanic intensity and mass-wasting processes have varied through time. Dome-style eruptions have dominated the past 1 Ma of volcanic activity at Montserrat. At the older edifice of Centre Hills, regular large-magnitude explosive eruptions (represented by >1m thick pumiceous sequences onshore) also occurred, but such explosive eruptions are rare at the younger Soufrière Hills-South Soufrière Hills volcanic complex. Periods of heightened volcanic activity occurred between 1.1-0.9 Ma, 0.3 Ma, and 0.2-0.1 Ma. Another period of increased volcanism may have also occurred at ~0.5±0.2 Ma. These coincide with periods of increased, mass-wasting, identified at 1.1-0.9 Ma, 0.6-0.5 Ma, 0.3 Ma, and 0.2-0.1 Ma and suggest that increased volcanic activity may facilitate mass-wasting processes. This may be due to increased deposition of material on the island flanks, or increased seismic activity that can trigger collapses. The ages of the largest landslide deposits (volumes >0.3 km3) observed offshore of Montserrat also coincide with periods of faster sea-level rises. Analysing the global database, large landslides coincide with rapid sea-level rise at other island arc volcanoes, but not at ocean islands. The reason for this difference in behavior is unclear, but maybe associated with differences in island composition and size, or tectonic regimes.
Montserrat is a small island arc volcano in the Caribbean island arc. The island comprises three main volcanic centres: Silver Hills, active between 2.5-1 Ma; Centre Hills, active between ~1 to 0.5 ...Ma; and the Soufrière Hills-South Soufrière Hills volcanic complex, active from ~0.3 Ma. Here an extensive (> 1 Ma) and detailed stratigraphic record is compiled for Montserrat using both the subaerial and submarine (in the form of three International Ocean Drilling Program cores) records. This combined record gives valuable insight into the evolution of volcanic and mass-wasting processes at Montserrat, and may be useful for future hazard mitigation. The stratigraphic record shows that eruptive styles, volcanic intensity and mass-wasting processes have varied through time. Dome-style eruptions have dominated the past 1 Ma of volcanic activity at Montserrat. At the older edifice of Centre Hills, regular large-magnitude explosive eruptions (represented by >1m thick pumiceous sequences onshore) also occurred, but such explosive eruptions are rare at the younger Soufrière Hills-South Soufrière Hills volcanic complex. Periods of heightened volcanic activity occurred between 1.1-0.9 Ma, 0.3 Ma, and 0.2-0.1 Ma. Another period of increased volcanism may have also occurred at ~0.5±0.2 Ma. These coincide with periods of increased, mass-wasting, identified at 1.1-0.9 Ma, 0.6-0.5 Ma, 0.3 Ma, and 0.2-0.1 Ma and suggest that increased volcanic activity may facilitate mass-wasting processes. This may be due to increased deposition of material on the island flanks, or increased seismic activity that can trigger collapses. The ages of the largest landslide deposits (volumes >0.3 km3) observed offshore of Montserrat also coincide with periods of faster sea-level rises. Analysing the global database, large landslides coincide with rapid sea-level rise at other island arc volcanoes, but not at ocean islands. The reason for this difference in behavior is unclear, but maybe associated with differences in island composition and size, or tectonic regimes.
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