Coral islands

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It is recommended to read this article together with the article Coral reefs.


Coral island development

Fig. 1. Maldives archipelago, a chain of 26 atolls in the Indian Ocean southwest of Sri Lanka and India. In the period 2000-2017, the archipelago has gained 37.50 km2 of land area by natural accretion[1]. Photo credit Fabio Di Lupo, Flickr Creative Commons.

Charles Darwin assumed that coral islands (reef islands) formed on geological timescales through coral colonization of slowly sinking volcanoes in mid-ocean basins. Observations a century later confirmed that the majority of coral islands indeed have a sinking volcanic base. However, an important adjustment to Darwin's theory for the recent evolution of coral islands includes the effect of the strongly fluctuating sea level during the Quaternary, with an important role of karst dissolution processes during sea level low-stand periods[2][3].

Most coral islands are situated in the tropical zones of the Pacific and Indian oceans, e.g. Kiribati, Maldives (Fig. 1), Marshall Islands, Tokelau and Tuvalu. Coral island development results from the interplay of constructive and destructive processes, all of which are important in reef construction. Constructive processes include carbonate production by reef building corals (highly variable, up to about 4 kg CaCO3 m-2year-1) and secondary framework builders (e.g., crustose coralline algae) and benthic organisms (foraminifera, bryozoans, calcareous algae, and mollusks) and precipitation of cements that bind and stabilize sediments. Destructive processes include bioerosion and physical processes, whereby waves mechanically (e.g. abrasion of the reef edge) break the skeletal structure of carbonate material[4]. Bioerosion results from the action of grazing organisms (e.g. scraping by parrotfish (Fig. 2) and urchins), mechanical boring (macroborers, including sponges, bivalves, worms and microborers, including algae, fungi, cyanobacteria and foraminifera), and chemical dissolution, mainly by microborers[5].

Fig. 2. Bumphead parrotfish. Parrotfish contribute to bioerosion by rasping algae from coral and other rocky substrates with their teeth tightly packed on the external surface of their jaw bones. For example, parrotfish grazing accounted for the production of 85% of island sand in the Maldives because of their high biomass and feeding intensity[6]. Photo credit Jenny Huang. Flickr creative commons licence.

Vertical reef building dominates when the reef has to catch up with sea level rise and lateral expansion when sea level is stabilizing or falling. During vertical reef growth, carbonate sediment is retained in the reef framework. However, once reefs attain sea level, excess carbonate is shed from the reef system. In the past millennium, the average rate of vertical reef growth was below 5 mm/year, while before 6000 years ago, when sea level rise was several times faster, the vertical reef growth rate was several times larger too[7][8]. In contrast, lagoon infill rates are increasing and are currently in the range of 0.5-4 mm/year. Reef island accretion largely results from the accumulation of detrital sediment (coral sand/gravel, shell fragments) derived from the reef flat by waves and currents. Wave interaction with coral reef platforms is recognized as a major geomorphic process in the accretion of reef islands[4][9], see Coral reefs#Sediment transport.

The present coral reef islands developed when the rate of sea level rise was slowing, about 5-6 thousand years ago. The sea level in the central Pacific reached a highstand of about 1–2 m above present level a few thousand years ago. Detrital sand accumulations, bound and stabilized by precipitation of calcium carbonate, then reached a few meters above present sea level[10][11]. Reef islands still evolve as a result of erosion and accretion processes causing shoreline displacement, changes in island shape and migration across the reef platform without necessarily a reduction in total island area[12].

Coral reef island protection

Fig. 3. Example of a typical reef shape.

Coral reefs provide important protection for coasts and coastal populations against the destructive forces of the sea under storm conditions. Coral reefs are particularly effective wave attenuators. A meta-analysis by Ferrario et al. (2014[13]) found an average wave-height reduction of 84% (76–89%) across whole reefs. Reef crests accounted for the strongest attenuation, with an average wave-height reduction of 64%; reef flats provided additional attenuation (Fig. 3). Both reef crests and reef flats dissipate disproportionately more wave energy as the incident wave energy increases. In cases where reefs have been degraded, restoration can be advantageous compared to conventional coastal protection and may provide both coastal-protection and ecological benefits[13]. Recovery of the reef crest is most effective. However, there is not yet much experience with large-scale reef restoration, see Coral reefs#Coral reef restoration and creation. To be effective, recovery techniques require first eliminating the original causes of reef degradation.

Climate change impact on coral islands

Coral islands are exposed to sea level rise and other effects of climate change, including increasing seawater temperature, ocean acidification and possible changes in cyclone climate. Their low elevation increases susceptibility to flooding, while their morphological response also depends strongly on the continued supply and redistribution of reef-derived sediment[5]. Evidence from field observations, physical and numerical modelling suggest that the rate of sea level rise is a crucial factor, along with sediment availability. Observations show that coral islands have increased in size during the past decades in spite of an increase in the rate of sea level rise; this holds in particular for the larger sand-gravel islands[14][15][1]. Observations and models suggest that this can be explained by the influx of sediment eroded from the surrounding reefs by storm waves[16]. Under higher sea levels, reef erosion by energetic waves can increase, thus providing sediments for island accretion[14][17]. The observed morphological resilience of many reef islands implies that sea-level rise does not necessarily lead to rapid island disappearance but adaptation measures will likely be needed. Future island adjustment depends on continued supply of reef-derived sediment. Climate-driven changes in reef-community composition, carbonate production and bioerosion can therefore alter both the amount and type of sediment available for island building. The long-term fate of many coral islands therefore remains uncertain, particularly if the rate of sea-level rise continues to increase[14][9].


Related articles

Coral reefs


References

  1. ↑ 1.0 1.1 Holdaway, A., Ford, M. and Owen, S. 2021. Global-scale changes in the area of atoll islands during the 21st century. Owen Anthropocene 33, 100282
  2. ↑ Droxler, A.W. and Jorry, S.J. 2021. The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory. Annual Review of Marine Science 13: 537-573
  3. ↑ Liu, J., Webster, J. M., Salles, T., Wang, S., Ma, Y., Xu, W., Li, G. and Yan, W. 2022. The formation of atolls: New insights from numerical simulations. Journal of Geophysical Research: Earth Surface 127, e2022JF006812
  4. ↑ 4.0 4.1 Kench, P.S. and Owen, S.D. 2022. Coral Systems. Ch. 8.22 of Treatise on Geomorphology (ed. Shroder, J.F.), Elsevier
  5. ↑ 5.0 5.1 Browne, N.K., Cuttler, M.V.W., Moon, K., Morgan, K.M., Ross, C.L., Castro-Sanguino, C., Kennedy, E.V., Harris, D.L., Barnes, P., Bauman, A.G., Beetham, E.P., Bonesso, J., Bozec, Y., Cornwall, C.E., Dee, S., Decarlo, T.M., D’olivo, J.P., Doropoulos, C., Evans, R.D., Eyre, Bradley D., Gatenby, P., Gonzalez, M., Hamylton, S., Hansen, J.E., Lowe, R.J., Mallela, J., O’Leary, M.J., Roff, George, Saunders, B.J. and Zweifler, A. 2021. Predicting responses of geological carbonate reef systems to climate change: A conceptual model and review. Oceanography and Marine Biology, pp. 229–370
  6. ↑ Morgan, K.M. and Kench, P.S. 2016. Parrotfish erosion underpins reef growth, sand talus development and island building in the Maldives. Sedimentary Geology 341: 50–57
  7. ↑ Montaggioni, L.F. 2005. History of Indo-Pacific coral reef systems since the last glaciation: Development patterns and controlling factors. Earth-Science Reviews 71: 1-75
  8. ↑ Gischler, E. and Hudson, J.H. 2019. Holocene tropical reef accretion and lagoon sedimentation: A quantitative approach to the influence of sea-level rise, climate and subsidence (Belize, Maldives, French Polynesia). Depositional Rec. 5: 515–539
  9. ↑ 9.0 9.1 East, H.K., Perry, C.T., Kench, P.S., Liang, Y. and Gulliver, P. 2018. Coral reef island initiation and development under higher than present sea levels. Geophys. Res. Lett. 45: 11265–11274
  10. ↑ Dickinson, W. R. 2003. Impact of mid-Holocene hydro-isostatic highstand in regional sea level on habitability of islands in Pacific Oceania. J. Coastal Res. 19: 489–502
  11. ↑ Woodroffe, C.D. and Webster, J.M. 2014. Coral reefs and sea-level change. Marine Geology 352: 248–267
  12. ↑ Kench, P.S. 2025. Effects of Environmental and Climatic Changes on Coral Reef Islands. Annu. Rev. Mar. Sci. 17: 301–324
  13. ↑ 13.0 13.1 Ferrario, F., Beck, M.W., Storlazzi, C.D., Micheli, F., Shepard, C.C and Airoldi, L. 2014. The effectiveness of coral reefs for coastal hazard risk reduction and adaptation. Nature Communications 5: 3794 , DOI: 10.1038/ncomms4794
  14. ↑ 14.0 14.1 14.2 Kench, P.S., Ford, M.R. and Owen, S.D. 2018. Patterns of island change and persistence offer alternate adaptation pathways for atoll nations. Nature Communications 9: 605
  15. ↑ Duvat, V. K. E. 2018. A global assessment of atoll island planform changes over the past decades. Wiley Interdisc. Rev. Clim. Change 10, e557
  16. ↑ Tuck, M.E, Ford, M.R., Kench, P.S. and Masselink, G. 2021. Sediment supply dampens the erosive effects of sea‑level rise on reef islands. Nature Scientific Reports 11: 5523
  17. ↑ Masselink, G., Beetham, E. and Kench, P. 2020. Coral reef islands can accrete vertically in response to sea level rise. Sci. Adv. 6 : eaay3656


The main author of this article is Job Dronkers
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Citation: Job Dronkers (2026): Coral islands. Available from http://www.coastalwiki.org/wiki/Coral_islands [accessed on 12-09-2026]
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