Abstract
Abstract Volcanic archipelagos are a source of numerous on- and offshore geohazards, including explosive eruptions and potentially tsunamigenic large-scale flank collapses. Fogo Island in the southern Cape Verdes is one of the most active volcanoes in the world, making it both prone to collapse (as evidenced by the c. 73 ka Monte Amarelo volcanic flank collapse), and a source of widely distributed tephra and volcanic material. The offshore distribution of the Monte Amarelo debris avalanche deposits and the surrounding volcaniclastic apron were previously mapped using only medium-resolution bathymetric data. Here, using recently acquired, higher-resolution acoustic data, we revisit Fogo's flank collapse and find evidence suggesting that the deposition of hummocky volcanic debris originating from the failed eastern flank most likely triggered the contemporaneous, multi-phase failure of pre-existing seafloor sediments. Additionally, we identify, for the first time, multiple mass-transport deposits in the southern part of the volcaniclastic apron of Fogo and Santiago based on the presence of acoustically chaotic deposits in parametric echo sounder data and volcaniclastic turbiditic sands in recovered cores. These preliminary findings indicate a long and complex history of instability on the southern slopes of Fogo and suggest that Fogo may have experienced multiple flank collapses.
Highlights
ObjectivesA detailed analysis of the distribution of the landslide deposits and failure mechanism are critical for constraining the hazard linked to the collapse of a volcanic flank
Based on the difference in their acoustic character, we distinguish between two main types of deposits related to the Monte Amarelo volcanic flank collapse: hummocky, debris avalanche deposits proximal to Fogo (MTD-A), and finer-grained, acoustically transparent landslide deposits (MTD-B) at greater distances from Fogo that were previously not recognized (Fig. 3)
We revisited the Monte Amarelo volcanic flank collapse of Fogo Island and found that the deposition of the debris avalanche material may have triggered subsequent failures of pre-existing seafloor sediments in two phases. This is similar to what has been observed in the Lesser Antilles (Le Friant et al 2015; Brunet et al 2016), at La Réunion (Lebas et al 2018) and at Ritter Island (Karstens et al 2019; Watt et al 2019)
Summary
A detailed analysis of the distribution of the landslide deposits and failure mechanism are critical for constraining the hazard linked to the collapse of a volcanic flank. This characterization is crucial for tsunami hazard modelling, given that the mode, volume and run-out of a collapse will have a profound effect on the resulting tsunami waves (Grilli et al 1997; Abadie et al 2012; Watt et al 2012a). The offshore distribution of the Monte Amarelo flank-collapse debris (i.e. debris avalanche deposits) and the surrounding volcaniclastic apron were previously mapped using only medium-resolution (100 m grid cell size) multibeam bathymetric data (Masson et al 2008). We aim to (1) map out the lateral extent and characterize the acoustic nature of the Monte Amarelo debris avalanche deposits; (2) identify possible additional landslides on the slopes south of the islands of Fogo and Santiago; and (3) consider the related preconditioning and triggering processes
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