Submarine platform development by erosion of a Surtseyan cone at Capelinhos, Faial Island, Azores

Abstract

Erosion of volcanic islands ultimately creates shallow banks and guyots, but the ways in which erosion proceeds to create them over time and how the coastline retreat rate relates to wave conditions, rock mass strength and other factors are unclear. The Capelinhos volcano was formed in 1957/58 during a Surtseyan and partly effusive eruption that added an ~2.5 km2 tephra and lava promontory to the western end of Faial Island (Azores, central North Atlantic). Subsequent coastal and submarine erosion has reduced the subaerial area of the promontory and created a submarine platform. This study uses historical information, photos and marine geophysical data collected around the promontory to characterize how the submarine platform developed following the eruption. Historical coastline positions are supplemented with coastlines interpreted from 2004 and 2014 Google Earth images in order to work out the progression of coastline retreat rate and retreat distance for lava‐ and tephra‐dominated cliffs. Data from swath mapping sonars are used to characterize the submarine geometry of the resulting platform (position of the platform edge, gradient and morphology of the platform surface). Photographs collected during SCUBA and ROV dives on the submarine platform reveal a rugged surface now covered with boulders. The results show that coastal retreat rates decreased rapidly with time after the eruption and approximately follow an inverse power‐law relationship with coastal retreat distance. We develop a finite‐difference model for wave attenuation over dipping surfaces to predict how increasing wave attenuation contributed to this trend. The model is verified by reproducing the wave height variation over dipping rock platforms in the UK (platform gradient 1.2° to 1.8°) and Ireland (1.8°). Applying the model to the dipping platform around Capelinhos, using a diversity of cliff resistance predicted from known lithologies, we are able to predict erosion rate trends for some sectors of the edifice. We also explore wider implications of these results, such as how erosion creates shallow banks and guyots in reef‐less mid‐oceanic archipelagos like the Azores. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

The Medial Offshore Record of Explosive Volcanism Along the Central to Eastern Aegean Volcanic Arc: 2. Tephra Ages and Volumes, Eruption Magnitudes and Marine Sedimentation Rate Variations

“…Subaerial and submarine erosion rates can also be greatly enhanced when explosive volcanic eruptions emplace large volumes of easily erodible tephra. For example, Zhao et al (2019) documented very high erosion rates immediately after emplacement of the Capelinhos cone (Faial, Azores) that declined in inverse power-law fashion through later times. However, none of the cores in the central Aegean region shows a significant increase in sedimentation rate after the largest eruptions on Santorini, nor is such feature observed in the eastern cores after the Kos, Nisyros and Yali eruptions (Figure 9).…”

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

“…The Azores Archipelago is frequently exposed to local wind‐waves and high‐energy swells generated by distant Atlantic storms (Carvalho, 2003; Rusu & Guedes Soares, 2012; Zhao et al ., 2019). Previous analysis of model outputs of the European Centre for Medium‐Range Weather Forecasts (ECMWF) showed that prevailing waves approach Santa Maria Island mainly from the north‐west (35%) and west (22%), with average significant wave heights of 2 m (Ricchi et al ., 2020).…”

“…Previous analysis of model outputs of the European Centre for Medium‐Range Weather Forecasts (ECMWF) showed that prevailing waves approach Santa Maria Island mainly from the north‐west (35%) and west (22%), with average significant wave heights of 2 m (Ricchi et al ., 2020). Waves are higher in winter than in summer (Zhao et al ., 2019). The North Atlantic Oscillation and Eastern Atlantic atmospheric circulation brings large storms (wave heights >16 m and peak periods >18 s) which affect the Azores Archipelago once every seven years on average (Andrade et al ., 2008; Quartau et al ., 2012; Zhao et al ., 2019; Ricchi et al ., 2020).…”

A model for wave abrasion on underwater bedrock, with an application to rapidly downwearing tephra cones adjacent to Surtsey Island in Iceland

“…Extending temporal and spatial scales, we will be able to envisage morphologies like erosional continental shelves and volcanic insular shelves. The wave-base concept is of importance for modelling for continental shelves (Sunamura, 1978b;Trenhaile, 2001) and island shelves (Quartau et al, 2010(Quartau et al, , 2018Zao et al, 2019), and also for exploring the origin of submarine erosion surfaces (Rovere et al, 2011;Thom et al, 2010;Wilson & Luyendyk, 2006). Although multiple estimates of the wave base have been reported from many locations (Sunamura, 1992, table 6.2), almost all of them have not been based on sound field evidence.…”