Systematic Slowing of Initially Rapid Retreat of New Coasts Formed by Historical Eruptions in Volcanic Islands

Due to their exposure to waves, volcanic island coasts typically retreat with cliff collapses and other erosional processes. Understanding how retreat rates vary over time and in response to environmental and other factors could be useful for geohazard assessment, coastal management and landform reconstruction. Historical eruptions can create new coasts with volcanic materials that are friable. The retreat of such coastlines can be fast and more easily observed than for many older rocky coasts. Here we assemble coastline retreat distances and rates of 12 coasts formed by historical eruptions from literature sources and remote-sensing data. In the cases with observations at many time steps, post-eruptive coastline retreat was initially rapid and declined with time. We adapt an empirical equation found earlier to represent the coastline retreat of a Surtseyan cone, finding that it represents the systematic variation in retreat distances with time well where coastal evolution is known in more than 5 time steps. The slowing is interpreted to arise from (a) increasing wave attenuation with abrasion platform widening, (b) exposure of progressively more resistant materials at cliffs, and (c) from increasingly taller cliffs, which lead to increasingly large volumes of debris from cliff collapses, temporarily protecting cliff bases. Coastline retreat rates also follow inverse power-law relationships with varied time intervals of measurement; hence, they are affected by erosion episodicity. Comparisons with wave height and precipitation surprisingly reveal no strong co-variation with the retreat rates. We hypothesize that varied lithology, fracture density and other factors dominate retreat rates of young volcanic coastlines.