Volcanic Island Sector Collapse: Reconstruction of volcanic activity and implications for subsequent mass movements from marine records drilled with MeBo70 offshore Montserrat (Lesser Antilles)

Volcanic island sector collapses produce some of the volumetrically largest mass movements on Earth. They may trigger devastating tsunamis that pose hazards to coastal communities and endanger seafloor installations. However, very little is currently known about the interplay between volcanic activity and subsequent mass wasting (volume, source location, and trans­port dis­tance) as well as their specific em­place­ment pro­cesses (tim­ing, kin­emat­ics, and dy­nam­ics). Moreover, these are key information to de­vel­op­ a re­li­able tsunami haz­ard as­sess­ment for sec­tor col­lapses.

The volcanic island of Montserrat in the Lesser Antilles is an ideal target to study the timing, frequency, and kinematics of sec­tor col­lapses as well as subsequent mass wasting. In 2019, Meteor expedition M154 investigated the major landslide complex – Deposit 2, located in the southeast offshore sector of Montserrat and provided an outstanding geophysical (M154-1) and sedimentological dataset. Here, the second leg, M154-2, focused on sediment sampling. Within and in the vicinity of Deposit 2, drill cores were taken with the MeBo70 drill rig from up to 63 mbsf. Ad­di­tion­ally, 21 sup­ple­ment­ing grav­ity cores were taken in the vi­cin­ity of Me­Bo70 drill sites and along systematic transects across the slid masses. Sedimentological, geophysical, geotechnical as well as geochemical analyses of these sediment cores enable a unique opportunity to gain new insights into timing of mass wasting events and complement information on the volcanic island evolution.

Based on these sediment cores, this pro­ject aims at con­trib­ut­ing to the gen­eral com­pre­hen­sion of vol­canic is­land sec­tor col­lapses, par­tic­u­larly the in­ter­re­la­tion­ship of vol­canic pro­cesses and as­so­ci­ated mass move­ments by establishing an event chronostratigraphy for the marine sediment records off Montserrat volcanic island.

Samples from four MeBo70 drill sites at the undisturbed slope, the central and distal part of Deposit 2, and south of Montserrat were analyzed for their componentry and composition. The sediments predominantly comprise mud-rich facies interbedded with fine to coarse-grained, better-sorted sands. The sandy intervals sometimes show multiple units defined by normally-graded beds or sharp color changes with variable proportions of volcanic and biogenic clasts. In a small number, coarse volcanic sands to volcaniclastic gravels were encountered. Tuffaceous deposits are less frequent. Petrographic analyses of selected samples by polarized light microscopy enable the investigation of clast inventories to differentiate between sediment units. Geochemical fingerprinting of major elements of volcanic glasses by electron microprobe elucidates this differentiation. The geochemical analyses further show a mainly basaltic to rhyolitic volcanism in the range of Arc Tholeiitic and Calc-alkaline series. The analyzed samples represent different stages of volcanic island evolution with periods of increased volcanic activity and eruptions, flank collapses, submarine mass wasting events, and periods of relative inactivity. Moreover, trace element analyses by laser ablation inductively coupled plasma-mass spectrometry of selected potential primary volcanic layers enable the possibility to better distinguish between single eruptions and also to narrow down their source area(s) as well as that of the sedimentary material.