ORIGINAL RESEARCH published: 13 December 2018 doi: 10.3389/feart.2018.00223 Complex and Cascading Triggering of Submarine Landslides and Turbidity Currents at Volcanic Islands Revealed From Integration of High-Resolution Onshore and Offshore Surveys Michael A. Clare 1*, Tim Le Bas 1, David M. Price 1,2, James E. Hunt 1, David Sear 3, Matthieu J. B. Cartigny 4, Age Vellinga 2, William Symons 2, Christopher Firth 5 and Shane Cronin 6 1 National Oceanography Centre, University of Southampton Waterfront Campus, Southampton, United Kingdom, 2 National Oceanography Centre, School of Ocean and Earth Science, University of Southampton, Southampton, United Kingdom, 3 Department of Geography & Environment, University of Southampton, Southampton, United Kingdom, 4 Department of Geography, Durham University, Durham, United Kingdom, 5 Department of Earth and Planetary Sciences, Macquarie Edited by: University, Sydney, NSW, Australia, 6 School of Environment, University of Auckland, Auckland, New Zealand Ivar Midtkandal, University of Oslo, Norway Reviewed by: Submerged flanks of volcanic islands are prone to hazards including submarine Gijs Allard Henstra, landslides that may trigger damaging tsunamis and sediment-laden seafloor flows (called University of Bergen, Norway “turbidity currents”). These hazards can break seafloor infrastructure which is critical for Miquel Poyatos Moré, University of Oslo, Norway global communications and energy transmission. Small Island Developing States are *Correspondence: particularly vulnerable to these hazards due to their remote and isolated nature, small size, Michael A. Clare high population densities, and weak economies. Despite their vulnerability, few detailed [email protected] offshore surveys exist for such islands, resulting in a geohazard “blindspot,” particularly in Specialty section: the South Pacific. Understanding how these hazards are triggered is important; however, This article was submitted to pin-pointing specific triggers is challenging as most studies have been unable to link Sedimentology, Stratigraphy and Diagenesis, continuously between onshore and offshore environments, and focus primarily on large- a section of the journal scale eruptions with sudden production of massive volumes of sediment. We address Frontiers in Earth Science these issues by integrating the first detailed (2 × 2 m) bathymetry data acquired from Received: 19 September 2018 Tanna Island, Vanuatu with a combination of terrestrial remote sensing data, onshore Accepted: 21 November 2018 Published: 13 December 2018 and offshore sediment sampling, and documented historical events. Mount Yasur on Citation: Tanna has experienced low-magnitude Strombolian activity for at least the last 600 years. Clare MA, Le Bas T, Price DM, We find clear evidence for submarine landslides and turbidity currents, yet none of the Hunt JE, Sear D, Cartigny MJB, Vellinga A, Symons W, Firth C and identified triggers are related to major volcanic eruptions, in contrast to conclusions Cronin S (2018) Complex and from several previous studies. Instead we find that cascades of non-volcanic events Cascading Triggering of Submarine (including outburst floods with discharges of >1,000 m3/s, and tropical cyclones), that Landslides and Turbidity Currents at Volcanic Islands Revealed From may be separated by decades, are more important for preconditioning and triggering of Integration of High-Resolution landslides and turbidity currents in oversupplied sedimentary regimes such as at Tanna. Onshore and Offshore Surveys. Front. Earth Sci. 6:223. We conclude with a general model for how submarine landslides and turbidity currents doi: 10.3389/feart.2018.00223 are triggered at volcanic and other heavily eroding mountainous islands. Our model Frontiers in Earth Science | www.frontiersin.org 1 December 2018 | Volume 6 | Article 223 Clare et al. Hazard-Triggering Offshore From Volcanic Islands highlights the often-ignored importance of outburst floods, non-linear responses to land-use and climatic changes, and the complex interactions between a range of coastal and tectonic processes that may overshadow volcanic regimes. Keywords: cascading hazards, turbidity current, submarine landslide, tropical cyclone, outburst flood, volcanic island, crescentic bedforms, Strombolian volcano INTRODUCTION 2002). In addition to evidence of past slope failures, deep- water seafloor surveys (>25m cell size) of active volcanoes have Active volcanic islands can create a variety of subaerial hazards revealed that previously undocumented, crescentic bedforms including explosive eruptions that disrupt air transport (e.g., are common from shallow to deep-water modern marine Gudmundsson et al., 2012), emission of gases harmful to health volcaniclastic systems (Wright et al., 2006; Hoffmann et al., (e.g., Horwell and Baxter, 2006), fast-moving pyroclastic flows 2008; Silver et al., 2009; Gardner, 2010; Leat et al., 2013; Pope and lahars (e.g., Cronin et al., 1997; Calder et al., 1999), and ash et al., 2018). These bedforms may be diagnostic of turbidity falls that destroy agriculture and pollute water supplies (e.g., currents triggered by major, episodic volcanic events, including: Wilson et al., 2012). Given their high relief in the surrounding (i) sector or flank collapse; (ii) powerful Vulcanian eruptions; deep ocean (up to 7 km above the surrounding seafloor), the or (iii) sustained Plinian eruptions that can produce high- subaerial extents of volcanic islands are typically dwarfed by flux, sustained pyroclastic density currents (Pope et al., 2018). their submerged flanks (Watt et al., 2014). These submarine These past studies focussed on large and powerful scenarios, slopes are often affected by dynamic sediment transport processes but what should we expect if and when volcanoes erupt that can also pose a major hazard (Watt et al., 2014). Subsea at lower rates or have long-term low but steady outputs? 3 flank collapses can be prodigious in scale (>100s of km ) and These cases are arguably most common, with large explosive trigger damaging tsunamis (Moore et al., 1989; Keating and volcanoes undergoing centuries or millennia of quiescence McGuire, 2000; Carey et al., 2001; Tappin et al., 2001; Coussens between events and many less-explosive volcanoes having regular et al., 2016). Strategically important seafloor infrastructure, small eruptions (e.g., throughout Vanuatu; Tonga, Solomon such as the subsea telecommunications cable network that Islands, Papua New Guinea and New Zealand). Only one transmits more than 95% of all digital data traffic worldwide, existing bathymetric study is known for a regularly erupting low- is vulnerable to submarine landslides or powerful sediment explosivity volcano (Stromboli: Aeolian Archipelago; Casalbore avalanches (called “turbidity currents”) that occur offshore from et al., 2010, 2014), which also reveals similar crescentic bedforms. volcanic islands (Carter et al., 2014; Pope et al., 2017). Small Thus, it is plausible that crescentic bedforms offshore from Island Developing States are disproportionately vulnerable to volcanic islands may signify sudden catastrophic collapses that both subaerial and submarine hazards; largely due to their originate after long-term preconditioning and by a range of remote and isolated nature, small size, high population densities multiple possible triggering mechanisms—directly related to at or near sea-level and weak economies (Briguglio, 1995; volcanism or otherwise. With smaller individual eruptions Pelling and Uitto, 2001; Cronin et al., 2004; Terry and Goff, known at such sites, crescentic bedforms this bedforms thus may 2012; Hodgson et al., 2018). Understanding the threats posed not constitute evidence for major volcanic events. Crescentic to seafloor cables is particularly important for these islands, bedforms offshore from some volcanic islands have been as telecommunication links underpin many critical areas for tentatively linked to turbidity currents triggered by non-volcanic development, including access to regional markets, overseas bank processes, with documented examples including ephemeral transactions and booking for tourism (ICPC, 2016). Despite sediment-laden river floods that plunge directly into the sea their vulnerability, remarkably few detailed offshore surveys (Babonneau et al., 2013; Quartau et al., 2018). Morphologically- exist for Small Island Developing States in the South Pacific; similar crescentic bedforms have also been described in many largely due to geographic and economic constraints (CClare other subaqueous, non-volcanic settings worldwide and have et al., 2018). Therefore, the South Pacific has been identified as been related to a wide range of triggers, including: subaqueous a “hazard blind-spot” with respect to submarine landslides and delta collapses (Clare et al., 2016; Hughes Clarke, 2016); dense associated tsunamis (Terry and Goff, 2012; Goff and Terry, 2016). river-water plunging (Casalbore et al., 2011, 2017); settling of Furthermore, the link between onshore and offshore sediment sediment from river plumes (Hizzett et al., 2018); wave and transport processes, and hence, identification of the triggers storm resuspension (Xu et al., 2004; Normandeau et al., 2016); for offshore hazards, is often unclear as integrated subaerial and glacial outburst floods (Duller et al., 2008). Thus, there is and submarine surveys are limited to relatively few volcanic a large degree of ambiguity in linking bedform morphology at islands worldwide (e.g., Casalbore et al., 2010; Babonneau et al., volcanic islands