Formation and Composition
A cay is formed when ocean currents transport loose sediment across the surface of a reef to a depositional node, where the current slows or converges with another current, releasing its sediment load. Gradually, layers of deposited sediment build up on the reef surface (Hopley 1981, Gorlay 1998). Such nodes occur in windward or leeward areas of reef surfaces and sometimes occur around an emergent outcrop of old reef or beach rock.
The island resulting from sediment accumulation is made up almost entirely of biogenic sediment – the skeletal remains of plants and animals – from the surrounding reef ecosystems (Hopley 1982). If the accumulated sediments are predominantly sand, then the island is called a cay; if they are predominantly gravel, the island is called a motu.
Cay sediments are largely composed of calcium carbonate (CaCO3), primarily of aragonite, calcite, and high magnesium calcite. They are produced by myriad plants (e.g., coralline algae, species of the green algae Halimeda) and animals (e.g., coral, molluscs, foraminifera). Small amounts of silicate sediment are also contributed by sponges and other creatures (Chave 1964, Folk and Robles 1964, Scoffin 1987, Yamano 2000). Over time, soil and vegetation may develop on a cay surface, assisted by the deposition of sea bird guano.
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