Reconstructing the initial shape of volcanic islands to quantify long-term coastal erosion
Authors: Bossis Rémi , Regard Vincent , Carretier Sébastien 
Published: March 2021

&lt;p&gt;The global solid flux from continent to ocean is usually reduced to the input of sediments from rivers, and is estimated at approximately 20 Gt/year. Another input of sediments to ocean is coastal erosion, but this flux is difficult to estimate on a global scale and it is often neglected, perhaps wrongly according to regional studies [1,2]. Most studies attempting to quantify coastal erosion have focused on the coasts of developed countries and are limited to the timescale of decades or less [3]. The difficulty in quantifying long-term coastal erosion is that there are still many uncertainties about the factors controlling coastal erosion on this time scale, and it would be necessary to know the initial geometry of coastlines to calculate an eroded volume.&lt;/p&gt;&lt;p&gt;Volcanic islands, as geomorphological objects, seem to be very good objects of study to remedy these limitations. Indeed, many young volcanic islands are made of only one central edifice with a strong radial symmetry despite its degradation by erosion [4,5]. By knowing the age of an island and by comparing reconstructed shape with current shape, we can calculate a total eroded volume and an integrated average coastal erosion rate on the age of the island. Moreover, due to their geographical, petrological and tectonic diversity, volcanic islands allow to compare the influence of different factors on long-term coastal erosion, such as climate, wave direction and height, rock resistance or vertical movements. Thus, we will be able to prioritize them to propose coastal erosion laws that would applicable to all rocky coasts.&lt;/p&gt;&lt;p&gt;Here we built on previous works that have used aerial geospatial databases to reconstruct the initial shape of these islands [6,7] but we improve this approach by using offshore topographic data to determine the maximum and initial extension of their coasts. From both onshore and offshore topographies, we determine a long-term mean coastal erosion rate and we quantify precisely its uncertainty. Using the example of Corvo Island, in the Azores archipelago, we show how our approach allows us to obtain first estimates of long-term coastal erosion rate around this island.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;[1] Landemaine V. (2016). Ph.D. thesis, University of Rouen.&lt;/p&gt;&lt;p&gt;[2] Rachold V., Grigoriev M.N., Are F.E., Solomon S., Reimnitz E., Kassens H., Antonow M. (2000). International Journal of Earth Sciences, 89(3), 450-460.&lt;/p&gt;&lt;p&gt;[3] Pr&amp;#233;maillon M. (2018). Ph.D. thesis, University of Toulouse.&lt;/p&gt;&lt;p&gt;[4] Kar&amp;#225;tson D., Favalli M., Tarquini S., Fornaciai A., W&amp;#246;rner G. (2010). Journal of Volcanology and Geothermal Research, 193, 171-181.&lt;/p&gt;&lt;p&gt;[5] Favalli M., Kar&amp;#225;tson D., Yepes J., NannipierI L. (2014). Geomorphology, 221, 139-149.&lt;/p&gt;&lt;p&gt;[6] Lahitte P., Samper A., Quidelleur X. (2012). Geomorphology, 136, 148-164.&lt;/p&gt;&lt;p&gt;[7] Kar&amp;#225;tson D., Yepes J., Favalli M., Rodr&amp;#237;guez-Peces M.J., Fornaciai A. (2016). Geomorphology, 253, 123-134.&lt;/p&gt;