A cay (/ˈkiː/ or /ˈkeɪ/), also spelled caye or key, is a small,
low-elevation, sandy island on the surface of a coral reef. Cays occur
in tropical environments throughout the Pacific, Atlantic and Indian
Oceans (including in the
Caribbean and on the
Great Barrier Reef
Great Barrier Reef and
Belize Barrier Reef).
1 Formation and composition
2 Development and stability
4 See also
Formation and composition
Cay sand under an optical microscope
A cay forms 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. Such nodes occur in windward or leeward areas of reef
where surfaces 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. If the
accumulated sediments are predominantly sand, then the island is
called a cay; if they are predominantly gravel, the island is called a
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. Over time, soil and vegetation may
develop on a cay surface, assisted by the deposition of sea bird
Development and stability
A range of physical, biological and chemical influences determines the
ongoing development or erosion of cay environments. These influences
include: the extent of reef surface sand accumulations, changes in
ocean waves, currents, tides, sea levels and weather conditions, the
shape of the underlying reef, the types and abundance of carbonate
producing biota and other organisms such as binders, bioeroders and
bioturbators (creatures that bind, erode, and mix sediments) living in
surrounding reef ecosystems.
Significant changes in cays and their surrounding ecosystems can
result from natural phenomena such as severe El Niño Southern
Oscillation (ENSO) cycles. Also, tropical cyclones can help build or
destroy these islands.
There is much debate and concern over the future stability of cays in
the face of growing human populations and pressures on reef
ecosystems, and predicted climate changes and sea level rise.
There is also debate around whether these islands are relict features
that effectively stopped expanding two thousand years ago during the
Holocene or, as recent research suggests, they are still growing,
with significant new additions of reef sediments.
Understanding the potential for change in the sediment sources and
supply of cay beaches with environmental change is an important key to
predicting their present and future stability. Despite, or perhaps
because of all the debate around the future of cays, there is
consensus that these island environments are very complex and somewhat
Warraber Island, Torres Strait
Examples of cays include:
Florida Keys are composed primarily of exposed ancient coral reefs
and oolite beds formed behind reefs. A few of the Florida Keys, such
Sand Key, are "cays" as defined above.
Heron Island, Australia, a coral cay on the southern Great Barrier
Prickly Pear Cays, Anguilla
Rama Cay, Nicaragua
Tobacco Caye, Dangriga, Belize
Warraber Island in central
Torres Strait (10°12' S, 142°49' E),
Australia, a small ‘vegetated sand cay’ according to the
classification schemes of McLean and Stoddart (1978) and Hopley
(1982). Approximately 750 by 1500 m wide, this island is situated
on the leeward surface of a large 11 km2 emergent reef platform.
This cay and the surrounding reef flat are
Holocene in origin, having
formed over an antecedent Pleistocene platform.
Elbow Cays, Bahamas
Great Goat Island, Jamaica
^ Hopley, D. (1981). "Sediment movement around a coral cay, Great
Barrier Reef, Australia". Pacific Geology. 15: 17–36.
^ Gourlay, M.R. (1988) "
Coral cays: products of wave action and
geological processes in a biogenic environment" pp. 497–502. In
Choat, J.H. et al. (eds.) Proceedings of the 6th International Coral
Reef Symposium: Vol. 2: Contributed Papers. Townsville, Australia.
^ a b Hopley, D. (1982) The Geomorphology of the Great Barrier Reef
– Quaternary Development of
Coral Reefs. Wiley-Interscience
Publication, John Wiley and Sons Ltd., New York, ISBN 0471045624.
^ Chave, K. (1964) "Skeletal Durability and Preservation". In: J.
Imbrie and N. Newell (Eds.), Approaches to Palaeoecology. John Wiley
and Sons Inc., Sydney.
^ Folk, R.; Robles, P. (1964). "Carbonate sands of Isla Perez, Alacran
Reef Complex, Yucatan". Journal of Geology. 72 (3): 255–292.
doi:10.1086/626986. JSTOR 30075161.
^ Scoffin, T.P. (1987) Introduction to Carbonate Sediments and Rocks.
Blackwell, Glasgow, ISBN 0216917891.
^ Yamano, H., Miyajima, T. and Koike, I. (2000). "Importance of
foraminifera for the formation and maintenance of a coral sand cay:
Green Island, Australia".
Coral Reefs. 19: 51.
doi:10.1007/s003380050226. CS1 maint: Uses authors parameter
^ Harney, J.N.; Fletcher, C.H. (2003). "A budget of carbonate
framework and sediment production, Kailua Bay, Oahu, Hawaii" (PDF).
Journal of Sedimentary Research. 73 (6): 856–868.
^ Hart, D.E.; Kench, P.S. (2006). "Carbonate production of an emergent
reef platform, Warraber Island, Torres Strait, Australia" (PDF). Coral
Reefs. 26: 53. doi:10.1007/s00338-006-0168-8. hdl:10092/312.
^ Scoffin, T.P. (1993). "The geological effects of hurricanes on coral
reefs and the interpretation of storm deposits".
Coral Reefs. 12
(3–4): 203–221. doi:10.1007/BF00334480.
^ Woodroffe, C.D. (2003) Coasts: Form, Process and Evolution.
Cambridge University Press, Cambridge, ISBN 0521011833.
^ Kench, P.S.; Cowell, P. (2002). "Erosion of low-lying reef islands".
TIEMPO. 46: 6–12. Archived from the original on 2013-05-10.
^ Hart, D.E. (2003) "The importance of Sea-Level in an Inter-Tidal
Reef Platform System, Warraber Island, Torres Strait". Proceedings of
the 22nd Biennial New Zealand Geographical Society Conference,
Auckland, 2003. pp 77–81.
^ Woodroffe, C.D., Samosorn, B., Hua, Q. and Hart, D. E. (2007).
"Incremental accretion of a sandy reef island over the past 3000 years
indicated by component-specific radiocarbon dating". Geophysical
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^ McLean, R.; Stoddart, D. (1978). "Reef island sediments of the
northern Great Barrier Reef". Philosophical Transactions of the Royal
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(1378): 101. doi:10.1098/rsta.1978.0093. JSTOR 75221.
^ Woodroffe, C.D., Kennedy, D.M., Hopley, D., Rasmussen, C.E. and
Smithers, S.G. (2000). "
Holocene reef growth in Torres Strait". Marine
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doi:10.1016/S0025-3227(00)00094-3. CS1 maint: Uses authors
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