Abstract
Submerged flanks of volcanic islands are prone to hazards including submarine landslides that may trigger damaging tsunamis and sediment-laden seafloor flows (called ‘turbidity currents’). These hazards can break seafloor infrastructure which is critical for global communications and energy transmission. Small Island Developing States are particularly vulnerable to these hazards due to their remote and isolated nature, small size, high population densities and weak economies. Despite their vulnerability, few detailed offshore surveys exist for such islands, resulting in a geohazard ‘blindspot’, particularly in the South Pacific. Understanding how these hazards are triggered is important; however, pin-pointing specific triggers is challenging as most studies have been unable to link continuously between onshore and offshore environments, and focus primarily on large-scale eruptions with sudden production of massive volumes of sediment. Here we focus on an area with a high volcanic sediment supply, which is also similar to locations (volcanic or not) where human-induced vegetation change over-supplies sediments to coastal margins. We address these issues by integrating the first detailed (2 m x 2 m) bathymetry data acquired from Tanna Island, Vanuatu with a combination of terrestrial remote sensing data, onshore and offshore sediment sampling, and documented historical events. Mount Yasur on Tanna has experienced low-magnitude Strombolian activity for at least the last 600 years. We find clear evidence for submarine landslides and turbidity currents, yet none of the identified triggers are related to major volcanic eruptions, in contrast to conclusions from several previous studies. Instead we find that cascades of non-volcanic events (including outburst floods with discharges of >1000 m3/s, and tropical cyclones), that may be separated by decades, are more important for preconditioning and triggering of landslides and turbidity currents in oversupplied sedimentary regimes such as at Tanna. We conclude with a general model for how submarine landslides and turbidity currents are triggered at volcanic and other heavily eroding mountainous islands. Our model 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.
Highlights
Active volcanic islands can create a variety of subaerial hazards including explosive eruptions that disrupt air transport (e.g., Gudmundsson et al, 2012), emission of gases harmful to health (e.g., Horwell and Baxter, 2006), fast-moving pyroclastic flows and lahars (e.g., Cronin et al, 1997; Calder et al, 1999), and ash falls that destroy agriculture and pollute water supplies (e.g., Wilson et al, 2012)
We ask whether submarine landslides and crescentic bedforms found offshore from volcanic islands are always directly linked to major eruptive volcanic activity or flank collapses? We identify possible volcanic and non-volcanic triggers for submarine landslides and turbidity currents offshore Tanna Island based on documented historical events, and through integration of onshore and offshore analysis
On the submarine flanks of Stromboli volcano, Casalbore et al (2010) only observed crescentic bedforms on slopes of
Summary
Active volcanic islands can create a variety of subaerial hazards including explosive eruptions that disrupt air transport (e.g., Gudmundsson et al, 2012), emission of gases harmful to health (e.g., Horwell and Baxter, 2006), fast-moving pyroclastic flows and lahars (e.g., Cronin et al, 1997; Calder et al, 1999), and ash falls that destroy agriculture and pollute water supplies (e.g., Wilson et al, 2012) Given their high relief in the surrounding deep ocean (up to 7 km above the surrounding seafloor), the subaerial extents of volcanic islands are typically dwarfed by their submerged flanks (Watt et al, 2014). The link between onshore and offshore sediment transport processes, and identification of the triggers for offshore hazards, is often unclear as integrated subaerial and submarine surveys are limited to relatively few volcanic islands worldwide (e.g., Casalbore et al, 2010; Babonneau et al, 2013)
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