Abstract 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.

中文翻译：

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大洋岛屿周围熔岩三角洲的海岸侵蚀率

摘要 为了规划目的和提高对岛屿架如何发展的理解，最终成为淹没的银行，需要海岸侵蚀率。由已知年龄的熔岩三角洲侵蚀形成的近岸海底平台提供了量化侵蚀率的机会，并研究这些速率如何在不同类型和年龄的熔岩流之间变化，以及它们如何随波浪气候变化。我们收集了历史和全新世喷发（年龄≤ 6 ka）、来自'a'ā 和pāhoehoe 熔岩流以及来自不同地点（亚速尔群岛和夏威夷群岛以及阿森松岛）的三角洲的数据。近岸平台通过多波束声纳、测深光探测和测距 (LiDAR) 和历史探测数据进行解释。从他们，我们估计了现代海岸线和海底平台边缘之间的向海距离的时间平均海岸线后退率。解释的35个平台具有平均坡度5°（范围2°~10°）、侵蚀距离436 m（78~1119 m）、平台边缘深度25 m（5~ 56 m)。为了评估后者的深度，我们计算了由于平台边缘的表面波引起的海床轨道速度。平台边缘的深度与这些速度仅适度相关，这表明平台表面已经适应了具有广泛颗粒尺寸和运动阈值的波浪条件。最大的撤退距离（≥800 m）与亚速尔群岛中部的平台有关，这些平台暴露于更高的波浪中。然而，撤退率差异很大（0.08-12. 5 m/yr) 并且与波浪功率的测量值相关性较弱，这表明波浪只是次要影响。鉴于缺乏相关性，我们怀疑导致变化的主要因素来自不同的悬崖对侵蚀的抵抗力。熔岩流中接头之间的块，因此抵抗侵蚀，大小差异很大，从较厚和较不密集的'a'ā流到pāhoehoe流中更薄和壳状的层状熔岩。