Gravitational, erosional and depositional processes on volcanic ocean islands: Insights from the submarine morphology of Madeira Archipelago

Complex and Cascading Triggering of Submarine Landslides and Turbidity Currents at Volcanic Islands Revealed From Integration of High-Resolution Onshore and Offshore Surveys

“…This is in line with observations by Micallef and Mountjoy (2011) who identified a minimum slope threshold (5 • ) for the formation of linear gullies, arguing that a critical bed shear stress can only be attained on such steep slopes. Quartau et al (2018) only found linear gullies on the volcanic islands of the Madeira archipelago at slopes of >15 • . On the submarine flanks of Stromboli volcano, Casalbore et al (2010) only observed crescentic bedforms on slopes of <5 • .…”

“…Third, sediment delivered by a river flood rapidly accumulates at the river mouth and periodically becomes unstable, thus triggering a delayed slope failure that initiates a turbidity current (Clare et al, 2016;Hughes Clarke, 2016). Submarine channels with crescentic bedforms occur offshore from river outflows on volcanic islands in La Reunion and the Madeira Archipelago, and have been tentatively linked to ephemeral periods of flash flooding that may trigger hyperpycnal flow (Babonneau et al, 2013;Quartau et al, 2018). This may indicate that river floods are a potential trigger; however, Pope et al (2018) explicitly ruled out river floods as a potential explanation for the formation of crescentic bedforms on volcanic islands of the Kermadec Arc, on the basis of a small hydrologic system that prevents TABLE 1 | Review of possible triggers for turbidity currents and submarine landslides at volcanic islands, with specific reference to documented events at Tanna Island (parenthesized numbers are cross-references to Figure 11).…”

“…With smaller individual eruptions known at such sites, crescentic bedforms this bedforms thus may not constitute evidence for major volcanic events. Crescentic bedforms offshore from some volcanic islands have been tentatively linked to turbidity currents triggered by non-volcanic processes, with documented examples including ephemeral sediment-laden river floods that plunge directly into the sea (Babonneau et al, 2013;Quartau et al, 2018). Morphologicallysimilar crescentic bedforms have also been described in many other subaqueous, non-volcanic settings worldwide and have been related to a wide range of triggers, including: subaqueous delta collapses (Clare et al, 2016;Hughes Clarke, 2016); dense river-water plunging (Casalbore et al, 2011(Casalbore et al, , 2017; settling of sediment from river plumes (Hizzett et al, 2018); wave and storm resuspension (Xu et al, 2004;Normandeau et al, 2016); and glacial outburst floods (Duller et al, 2008).…”

“…Thus, there is a large degree of ambiguity in linking bedform morphology at volcanic islands with triggering mechanisms. One constraint to our understanding has been the challenge of acquiring detailed multibeam data in water depths of <100 m; hence, few studies have acquired data that extends shallow enough to link the subaerial volcano to offshore and of appropriate resolution to image bedforms (Casalbore et al, 2010;Quartau et al, 2018).…”

Neogene marine sediments and biota encapsulated between lava flows on Santa Maria Island (Azores, north‐east Atlantic): An interplay between sedimentary, erosional and volcanic processes

“…As rates of deposition increase, less sediment remains in the water column, resulting in a decrease in the deposition rate farther seaward over the bottomset portion of the clinoforms (Pirmez et al , 1998; Walsh et al , 2004). On narrow shelves in very energetic environments, a large portion of such sediments will ultimately be transported beyond the shelf edge, covering the submarine slopes of the island edifice and feed submarine canyon networks that drain into the deep sea (Ávila et al , 2008; Quartau et al , 2018; Santos et al , 2019); possibly, this process was exacerbated during glacial sea‐level lowstands, when shelves were at their narrowest extension.…”