Cay
Overview
 
A cay also spelled caye or key, is a small, low-elevation, sandy island formed on the surface of coral reefs. Cays occur in tropical environments throughout the Pacific
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

, Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 and Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

s (including in the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

 and on the Great Barrier Reef
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...

 and Belize Barrier Reef
Belize Barrier Reef
The Belize Barrier Reef is a series of coral reefs straddling the coast of Belize, roughly offshore in the north and in the south within the country limits...

), where they provide habitable and agricultural land for hundreds of thousands of people. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.
A cay is formed when ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

s transport loose sediment across the surface of a reef to a depositional
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...

 node, where the current slows or converges with another current, releasing its sediment load.
Unanswered Questions
Encyclopedia
A cay also spelled caye or key, is a small, low-elevation, sandy island formed on the surface of coral reefs. Cays occur in tropical environments throughout the Pacific
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

, Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 and Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

s (including in the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

 and on the Great Barrier Reef
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...

 and Belize Barrier Reef
Belize Barrier Reef
The Belize Barrier Reef is a series of coral reefs straddling the coast of Belize, roughly offshore in the north and in the south within the country limits...

), where they provide habitable and agricultural land for hundreds of thousands of people. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.

Formation and composition

A cay is formed when ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

s transport loose sediment across the surface of a reef to a depositional
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...

 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
Windward and leeward
Windward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...

 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
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, then the island is called a cay; if they are predominantly gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

, the island is called a motu
Motu
Motu may refer to:*Motu language, a language of Papua New Guinea*Motu proprio, a type of Papal document*MOTU, also known as "Mark of the Unicorn", a maker of professional audio hardware and software...

.

Cay sediments are largely composed of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 (CaCO3), primarily of aragonite, calcite, and high magnesium calcite. They are produced by myriad plants (e.g., coralline algae
Coralline algae
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls...

, species of the green algae Halimeda
Halimeda
Halimeda is a genus of green macroalgae. The algal body is composed of calcified green segments. Calcium carbonate is deposited in its tissues, making it inedible to most herbivores....

) and animals (e.g., coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

, molluscs
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

, foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

). Small amounts of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

 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
Guano
Guano is the excrement of seabirds, cave dwelling bats, and seals. Guano manure is an effective fertilizer due to its high levels of phosphorus and nitrogen and also its lack of odor. It was an important source of nitrates for gunpowder...

.

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 which bind, erode and mix sediments) living in surrounding reef ecosystems (Harney and Fletcher 2003, Hart and Kench 2007).

Significant changes in cays and their surrounding ecosystems can result from natural phenomena such as severe El Niño Southern Oscillation (ENSO) cycles. Also, tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

s can help build or destroy these islands (Scoffin 1993, Woodroffe 2003).

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 (Kench and Cowell 2003, Hart 2003). There is also debate around whether these islands are relict features which effectively stopped expanding two thousand years ago during the late Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 or, as recent research suggests, they are currently still growing with significant new additions of reef sediments (Woodroffe et al. 2007).

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 fragile.

Examples

Examples of cays include:
  • Heron Island, a coral cay on the southern Great Barrier Reef
  • Warraber Island in central Torres Strait
    Torres Strait
    The Torres Strait is a body of water which lies between Australia and the Melanesian island of New Guinea. It is approximately wide at its narrowest extent. To the south is Cape York Peninsula, the northernmost continental extremity of the Australian state of Queensland...

     (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 (Woodroffe et al. 2000).
  • The Florida Keys
    Florida Keys
    The Florida Keys are a coral archipelago in southeast United States. They begin at the southeastern tip of the Florida peninsula, about south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry...

     are composed primarily of exposed ancient coral reefs and oolite
    Oolite
    Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...

     beds formed behind reefs. A few of the Florida Keys, such as Sand Key
    Sand Key Light
    Sand Key Light is located six nautical miles southwest of Key West, Florida, between Sand Key Channel and Rock Key Channel, two of the channels into Key West. It is located on a reef that is intermittently covered by sand. At times the key has been substantial enough to have trees, and in 1900 nine...

    , are "cays" as defined above.
  • Tobacco Caye
    Tobacco Caye
    Tobacco Caye is a tiny island in Belize, about 10 miles east of Dangriga. It has a permanent population of about 20, and sees a regular stream of travelers. The island was ravaged by Hurricane Mitch in 1998, but has since been rebuilt....

    , Dangriga, Belize
  • Prickly Pear Cays
    Prickly Pear Cays
    The Prickly Pear Cays, sometimes spelled Prickley Pear Cays, are a small pair of uninhabited islands about six miles from Road Bay, Anguilla, in the Leeward Islands of the Caribbean. They are divided by a narrow boat channel between Prickly Pear East and Prickly Pear West...

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