Initial shape reconstruction of a volcanic island as a tool for quantifying long-term coastal erosion: the case of Corvo Island (Azores)

Abstract

Long-term coastal erosion is not yet well studied given that it is difficult to quantify. The quantification of long-term coastal erosion requires reconstruction of the coast's initial geometry and the determination of where and when the erosion started. Volcanic islands fulfill these two conditions: their initial shape is roughly conical and the age of the lavas that generated this geometry is easily measured. We have developed a method to reconstruct the initial shape of simple volcanic edifices from aerial and submarine topographic data. The reconstructed initial shape and associated uncertainties allow us to spatially quantify the coastal erosion since the building of the island. This method is applied to Corvo Island in the Azores archipelago. We calculated that, due to coastal erosion, the island has lost a volume of 6.5 ± 2.7 km3 and roughly 80 % of its surface area since it first came into being. Taking the large uncertainty in the age of the topmost lava flows (0.43 ± 0.34 Myr) into account, we have estimated that Corvo Island has lost an average of 5000 to 100 000 m3 yr-1 of its volume due to coastal erosion. Lastly, we show a strong correlation between long-term coastal erosion and the spatial distribution of the waves. Specifically, we highlight a stronger control on erosion by smaller and more frequent waves than by storm waves. The next step will be to apply this method to other volcanic islands in order to (i) streamline and improve the method and (ii) verify the correlations observed in the present study.