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)