Atoll Island Formation

The probability of significant transport of large coral clasts in the Spratly Islands is unlikely in the absence of a major cyclone or series of major cyclones passing through the area. Waves greater than the annual average significant wave height (~1.0m) would be required to pick up and move this type of rock, necessitating an extreme weather event. Even in the case of extreme weather events, the single greatest motu growth from a storm as captured in literature is a 3.9m overnight, as described in Baines et al. (1976). However, once coral begins to accumulate in the center of a reef flat, this will facilitate subsequent natural trapping of sand, allowing the feature to grow further on its own (Ortiz and Ashton, 2019). Additionally, while motu tend to naturally shift left and right across the reef flat by meters per year, there were no examples of motu vertically accreting higher than 3-5m above sea level, and the average height is 1-2m above sea level (Woodroffe, 2008). The lateral growth (expanding shoreline) that does occur naturally tends to be concentrated at “points of weakness” morphologically—spits, embayments, or other irregularities in a pre-existing motu that capture sediment as it travels up or down the coast of the islet, or where an inlet/channel separates two pre-existing islets and then fills in. Finally, in the case of motu destruction after a storm, a study of the reformation of a destroyed feature showed a gradual regrowth over 30 years, not weeks or months (Ford and Kench, 2016). Following a significant destruction of islands by large storms in the Marshall Islands (a roughly analogous environment to the Spratlys), the estimated amount of time it would take for the islands to reform to their original area is 96 years (the fastest) to 843 years (the slowest) (Ford and Kench, 2016)

在没有大型气旋或一系列大型气旋经过该地区的情况下，南沙群岛出现大型珊瑚碎屑大量搬运的可能性不大。需要大于年平均有效波高（约1.0米）的海浪才能掀起并移动这种类型的岩石，这就需要极端天气事件。即使在极端天气事件的情况下，文献中记载的风暴导致的最大的礁岛一夜增长幅度为3.9米，如贝恩斯等人（1976年）所述。然而，一旦珊瑚开始在礁坪中心堆积，这将有利于后续沙子的自然截留，使该地貌能够自行进一步生长（奥尔蒂斯和阿什顿，2019年）。此外，虽然礁岛往往每年会在礁坪上左右自然移动数米，但没有礁岛在海平面以上垂直堆积高于3 - 5米的例子，平均高度是在海平面以上1 - 2米（伍德罗夫，2008年）。确实会自然发生的横向生长（海岸线扩张）在形态上往往集中在“薄弱点”——岬角、海湾，或者现有礁岛的其他不规则之处，这些地方会在沉积物沿着小岛海岸上下移动时截留沉积物，或者在一个入口/水道将两个现有小岛分开然后又被填满的地方。最后，在风暴过后礁岛遭到破坏的情况下，一项对被破坏地貌重新形成的研究表明，是在30年中逐渐重新生长，而不是几周或几个月（福特和肯奇，2016年）。在马绍尔群岛的岛屿被大型风暴严重破坏之后（与南沙群岛环境大致相似），这些岛屿恢复到原来面积所需的估计时间为96年（最快）到843年（最慢）（福特和肯奇，2016年）