Flank collapse and large‐scale landsliding in the Cape Verde Islands, off West Africa

Abstract

Large‐scale landslides occur on the flanks of many volcanic oceanic islands worldwide. None have taken place in historical time, but their geohazard potential, especially their ability to generate tsunamis, is large. The Cape Verde Islands are a group of 10 large and several smaller volcanic islands off the coast of West Africa between 15 and 17°N. A single flank landslide has previously been described from the island of Fogo, but systematic analysis of the Cape Verde group has until now been lacking. This paper describes and interprets a multibeam bathymetry data set covering the slopes of the western Cape Verde Islands, including those of the islands with the most recent volcanic activity, Fogo in the southwest, and Santo Antao in the northwest. All of the larger islands show evidence of large flank landslides, although only Fogo and the southwest part of Santo Antao have failed in the last 400 ka. Tope de Coroa, the volcano at the southwest end of Santo Antao, has been inactive for the past 170 ka and is judged to have a low landslide potential unless volcanic activity resumes. In contrast, there would seem to be a high probability of a future east directed landslide on Fogo, from the area of the highly active Pico do Fogo volcano, although it is impossible to predict a timescale for such an event. A tsunami generated by such a landslide could have a catastrophic effect on the adjacent island of Santiago and possibly even farther afield on the West African coast.

Multi-storey calcrete profiles developed during the initial stages of the configuration of the Ebro Basin's exorrheic fluvial network

“…According to the methods that these authors applied to the Canary Islands, in the case of Cape Verde, a young island such as Fogo should lack platform as it really occurs (Masson et al, 2008) whereas Boa Vista, with a shelf between 6 and 14 km wide, would have an age be tween 9 and 20 Ma, an idea in agreement with the radiometric age data.…”

“…An alternative interpretation is that the depression had formed as a result of a slide, a common event that occurred in other islands of Cape Verde, in Fogo, Santo Antao, Sao Nicolau or Sao Vicente (Elsworth and Day et al, 1999;Le Bas et al, 2007;Masson et al, 2008;Ancochea et al, 2010). The mass-wasting event could account for the absence of trachytes and phonolites or other volcanic rock reliefs in the western sector of the island (Fig.…”

“…Serralheiro et al (1974) and Macedo et al (1995) identified these outcrops as the "Complexo eruptivo interno antigo", a unit also existent in some other islands of the archipelago (i.e. in Santiago, Matos Alves et al, 1979) that represented an ancient basement for all the younger vol canic units.…”

Investigating volcanic hazard in Cape Verde Islands through geophysical monitoring: network description and first results

“…2). In addition to onshore evidence for the lateral collapse (Day et al, 1999), Masson et al (2008) reported results of a multibeam bathymetric survey to the east of Fogo, which were interpreted as the avalanche debris of the lateral collapse. Foeken et al (2009) Day et al (1999) and Fonseca et al (2003).…”

“…The Planalto Norte plateau was built by the infilling with young volcanic rocks of two escarpments opened to the southwest, which Tarff and Day (2013) attribute to two successive lateral collapses. Masson et al (2008) used multibeam and acoustic backscatter images to map areas of blocky bathymetry offshore of the island, which they interpreted as two different submarine debris avalanche deposits originating from Topo de Coroa. Airfall pumice deposits of phonolitic composition are found almost everywhere on the island, but are especially thick (up to 8 m) on the Planalto Norte.…”