New Insights into the Emplacement Dynamics of Volcanic Island Landslides

Volcanic islands form the highest topographic structures on Earth and are the sites of some of the planet's largest landslides. These landslides can rapidly mobilize hundreds of cubic kilometers of rock and sediment, and potentially generate destructive tsunamis on ocean-basin scales. The main unknown for tsunami hazard assessment is the way in which these landslides are emplaced. Understanding of landslide dynamics relies on interpretation of deposits from past events: it is necessary to understand where material within the deposit originated and the temporal sequence of the deposit's formation. The degree of fragmentation in a volcanic landslide is controlled by its relative proportions of dense lavas and weak pyroclastic rocks; fragmentation is generally reduced during subaqueous relative to subaerial transport. In the submarine environment, the seafloor-sediment substrate commonly fails during emplacement of a volcanic landslide. However, in many cases, this sediment failure remains almost in situ as a deformed package rather than disaggregating to form a debris flow. Because seafloor sediment makes up a large proportion of many landslide deposits around volcanic islands, the magnitude of the primary volcanic failure cannot be readily assessed without a clear understanding of deposit constituents. Both the dimensions of the volcanic failure and the way in which it fails are of key importance for tsunami generation. Turbidite deposits suggest that some volcanic landslides occur in multiple retrogressive stages. This significantly reduces potential tsunami magnitude relative to models that assume emplacement of the landslide in a single stage.