Ever wondered how the Maldivian islands were formed? This island chain made up of 19 atolls which hosts up to 1200 islands, owes its formation to a sequence of several different processes. Understanding the formation of the Maldives is vital to know the vulnerability of these dynamic islands against impacts of climate change and develop and implement appropriate and effective mitigation strategies. Despite the importance of understanding the formation of Maldives for its protection, the history of formation, processes involved in, and the factors influencing formation of Maldives are uncertain and a source of speculation over the years (Perry et al, 2013), hence an important research gap to be more thoroughly studied.
The origin and formation of atolls, including the reefs (Faru) and the lagoons (Vilu) was first defined by Darwin (1842) and is commonly used to explain the formation of Maldives. Darwin’s subsidence model suggests that the atolls formed as a result of volcanic edifice being fully buried following subsidence of volcanoes, and growth of corals around the subsiding volcanoes that evolve from fringing reefs to barrier reefs (Droxler & Jorry, 2021). This theory was developed based on observations from Darwin’s expedition on the HMS Beagle. Although widely expected and still used to this day, the Darwin theory of atoll formation was primarily based on observations from the pacific islands and studies from just one true atoll, the Cocos Atoll (Droxler & Jorry, 2021). Therefore, generalisation to the different atoll systems can form uncertainties, since different systems have very distinct features (Liang et al., 2022). Moreover, vital information that needs to be taken into account to completely understand and explain atoll formation, such as ice-sheet induced sea-level fluctuations was not included in Darwin’s subsidence theory as basic concepts of glaciation and sea-level fluctuations were not known at the time (Droxler & Jorry, 2021).
Since then, there have been several well-proven alternative and complementary theories, one being the Karst model. Karst model states that the modern atolls were formed during the late Quaternary when the Pliocene flat-topped banks drowned and were reflooded after a long period of exposure that led to erosion and dissolution of the centre of the bank through Karstification. This favoured coral growth along the raised margins, hence enhancing the difference between reef margins and lagoon seafloors. The Karst model takes into account, in addition to subsidence as in Darwin’s theory, the late Quaternary high-amplitude sea-level fluctuations. Moreover, it also relates Karst topography formed during glacial periods where the sea-level falls to the formation of modern atolls that consists of a central lagoon enclosed by a reef rim (Droxler et al., 2021).
Islands form when these small lagoonal patch reefs on the rims of the larger atolls formed, depending on the size and position, get infilled with sediments, becoming flat-topped banks, of which on top the islands form (Droxler & Jorry, 2021). This process is mainly a result of accumulation of reef rubble, along with sediment accumulation through wave action (Naylor et al., 2015). There are various factors that influence island formation sequence and timing in the Maldives. Studies show that there is a significant difference in the timing of island formation in the Maldives depending on whether the island is located on the peripheral rim of the atoll or on the lagoon reef platforms. Islands that form on reef platforms inside larger lagoons of the atoll periphery such as the islands studied in Maalhosmadulu atoll formed approximately 4500 years ago under rising sea level and across sediment filled lagoons, whereas islands on the peripheral reef rim in Huvadhoo atoll was identified to being formed in the past 2500 years during a period of sea level oscillation (East et al., 2018; Liang et al., 2022; Kench et al., 2005).
The Maldivian atoll system is also distinct in that they have a high number of islands that have dynamic beaches. Dynamic beaches are also observed in a few other atoll systems such as Marshall islands, Kiribati, Tuvalu, and French Polynesia (Aslam & Kench, 2017). Despite long-term morphological stability, they undergo island mobility through erosional and accretional processes to adjust to changes in environmental boundary conditions such as sediment availability, waves, and sea level (Aslam & Kench, 2017). Moreover, seasonal monsoon current shifts, along with monsoonal swells can also influence the movement of sands around the perimeter of the islands (David & Schlurmann, 2020; Naylor et al., 2015). This natural process to maintain the islands’ size and dynamically respond to ocean-climate pressures can be interrupted due to anthropogenic activities such as building hard coastal management structures and infrastructure as an effort to prevent erosion (Aslam & Kench, 2017; David & Schlurmann, 2020). However this effort can have the negative effect and impact sediment availability for accretion and hence lead to more erosion (David & Schlurmann, 2020). Therefore, adequate research and assessment is required on the specific island to understand its dynamics prior to taking anthropogenic actions such as hard-structure for coastal management and prioritise nature-based and natural dynamics to improve resilience and prevent interference to natural processes that maintains the islands (David & Schlurmann, 2020).
In conclusion, studies conducted to understand formation of atolls, particularly complex systems such as the Maldivian atoll system, is very scarce, highlighting the need for atoll specific studies. Multiple theories have been proposed to explain atoll formation, from Darwin’s subsidence theory to the more recent Karst model that builds that takes into account the modern understanding of sea-level fluctuations. Island formation on the lagoon flats on the rims of these atolls through sediment accumulation can be influenced by a range of conditions, from currents and weather to position of the island. Further research is also essential to better understand the highly dynamic nature of the Maldivian islands and how to best protect and manage these island dynamics, especially given the growing human-induced climate-ocean pressures on these vulnerable systems.
References
Aslam, M. and Kench, P.S., 2017. Reef island dynamics and mechanisms of change in Huvadhoo Atoll, Republic of Maldives, Indian Ocean. Anthropocene, 18, pp.57-68. https://doi.org/10.1016/j.ancene.2017.05.003
David, C.G. and Schlurmann, T., 2020. Hydrodynamic drivers and morphological responses on small coral islands—the thoondu spit on fuvahmulah, the Maldives. Frontiers in Marine Science, 7, p.538675. https://doi.org/10.3389/fmars.2020.538675
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(1), pp.537-573. https://doi.org/10.1146/annurev-marine-122414-034137
Liang, C.Y., Kench, P.S., Ford, M.R. and East, H.K., 2022. Lagoonal reef island formation in Huvadhoo atoll, Maldives, highlights marked temporal variations in island building across the archipelago. Geomorphology, 414, p.108395. https://doi.org/10.1016/j.geomorph.2022.108395
Naylor, A.K., 2015. Island morphology, reef resources, and development paths in the Maldives. Progress in Physical Geography, 39(6), pp.728-749. https://doi.org/10.1177/0309133315598269
Perry, C.T., Kench, P.S., Smithers, S.G., Yamano, H., O’Leary, M. and Gulliver, P., 2013. Time scales and modes of reef lagoon infilling in the Maldives and controls on the onset of reef island formation. Geology, 41(10), pp.1111-1114. https://doi.org/10.1130/G34690.1