Coastal erosion rates of lava deltas around oceanic islands

Rates of coastal erosion are needed for planning purposes and to improve understanding of how the shelves of islands develop, ultimately becoming submerged banks. Near-shore submarine platforms created by erosion of lava deltas of known age provide an opportunity to quantify erosion rates, and to investigate how those rates vary between different types and ages of lava flows, as well as how they vary with wave climate. We have compiled data on deltas formed during historical and Holocene eruptions (age ≤ 6 ka), from both 'a'ā and pāhoehoe lava flows, and from diverse localities (Azores and Hawaiian islands and Ascension Island). Near-shore platforms were interpreted from multibeam sonar, bathymetric Light Detection And Ranging (LiDAR) and historical sounding data. From them, we estimated time-averaged shoreline retreat rates from the seaward distances between the modern coastlines and the submarine platform edges. The 35 interpreted platforms have rugged dipping surfaces left behind by erosion with an average gradient of 5° (range 2°–10°), eroded distance of 436 m (78–1119 m) and depth of platform edge of 25 m (5–56 m). To evaluate the latter depths, we computed seabed orbital speeds due to surface waves at the platform edges. Depths of platform edges correlate only modestly with those speeds, suggesting that platform surfaces have adjusted to wave conditions with a wide range of particle sizes and hence thresholds of motion. The largest retreat distances (≥800 m) are associated with platforms of the central Azores Islands, which are exposed to higher waves. However, retreat rates are highly varied (0.08–12.5 m/yr) and weakly correlated with measures of wave power, suggesting that waves are only a secondary influence. Given that lack of correlation, we suspect that the main factor contributing to variability arises from varied cliff resistance to erosion. Blocks between joints in lava flows, and hence resistance to erosion, vary greatly in size, from the thicker and less densely jointed 'a'ā flows to the thinner and shell-like layered lavas in pāhoehoe flows.