New insights into landslide processes around volcanic islands from Remotely Operated Vehicle (ROV) observations offshore Montserrat

Abstract

Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition, and emplacement are hindered by the challenges of investigating deposits directly. Here we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment core and geophysical data. These complementary approaches enable a more comprehensive view of large-scale mass-wasting processes around island-arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5-14 ka (Deposit 1; 1.7 km 3 ) and formed a radially spreading hummocky deposit that is morphologically similar to many subaerial debris-avalanche deposits. Hummocks comprise angular lava and hydrothermally altered fragments, implying a deep-seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km 3 ) occurred at 130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris-avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris.

Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)

“…Multiple marine expeditions have been dedicated to studying the seafloor bathymetry and shallow crust around the island, allowing identification of volcaniclastic and debris avalanche deposits (Deplus et al, 2001;Boudon et al, 2007;Le Friant et al, 2008;Trofimovs et al, 2010Trofimovs et al, , 2013Lebas et al, 2011;Watt et al, 2012aWatt et al, , 2012bCassidy et al, 2013Cassidy et al, , 2014aCassidy et al, , 2014bCassidy et al, , 2015Le Friant et al, 2015;Watt et al, 2015;Coussens et al, 2016). Before the IODP 340 Expedition, coring offshore Montserrat was restricted to shallow piston cores or vibrocores characterizing volcanic eruptions and mass wasting events of Soufrière Hills and South-Soufrière Hills extending back to ca.…”

“…Before the IODP 340 Expedition, coring offshore Montserrat was restricted to shallow piston cores or vibrocores characterizing volcanic eruptions and mass wasting events of Soufrière Hills and South-Soufrière Hills extending back to ca. 110 ka (Le Friant et al, 2008;Trofimovs et al, 2013;Cassidy et al, 2013Cassidy et al, , 2014aCassidy et al, , 2014bWatt et al, 2015).…”

A decade of volcanic construction and destruction at the summit of NW Rota‐1 seamount: 2004–2014

“…Once a height and slope threshold of instability is reached, these edifices begin to disaggregate in the form of small‐scale creep, episodic landsliding, and major sector collapse. Our knowledge of seamount evolution has been largely based on ocean‐island volcanoes with subaerial portions and unstable flanks, such as Hawaii, the Canary Islands, and Tristan da Cunha [ Holcomb and Searle , ; Moore et al ., ; Krastel et al ., ; Mitchell et al ., ; Mitchell , ; Caratori Tontini et al ., ] and more recently on arc‐related volcanic islands such as Montserrat and other sites in the Lesser Antilles [e.g., Coussens et al ., ; Le Friant et al ., ; Watt et al ., ]. The repetitive growth and collapse of smaller, completely submarine arc volcanoes is more poorly studied.…”

“…They proposed a similar explanation, relating the more lithologically diverse flanks of arc volcanoes to their greater susceptibility to mass wasting. However, blocks from large sector collapse events have been observed below sea level at Montserrat as well, the largest of which is composed of volcanic breccia [ Watt et al ., ]. Therefore, large‐scale sector collapse is indeed possible at arc volcanoes, most likely in cases where volcanic breccias are abundant and well consolidated.…”

“Volcanism in the Chugoku region: a review for the first UNESCO field school on Geoenvironmental disaster reduction”

“…Several mass wasting deposits have been identified in the submarine successions of the Shimane Peninsula (see previous paragraph). Comparison with recent examples is difficult due to limited exposure of modern deposits although some recent documentations exist (Watt et al 2015). It remains open whether some of the events forming these deposits were accompanied by the Fig.…”