Types
Since Darwin's identification of the three classical reef formations – the fringing reef around a
volcanic island becoming a barrier reef and then an atoll [30] – scientists have identified further reef
types. While some sources find only three,[31][32] Thomas and Goudie list four "principal large-scale
coral reef types" – the fringing reef, barrier reef, atoll and table reef [33] – while Spalding et al. list five
"main types" – the fringing reef, barrier reef, atoll, "bank or platform reef" and patch reef. [34]
Fringing reef
Fringing reef
Fringing reef at Eilat at the southern tip of Israel
Main article: Fringing reef
A fringing reef, also called a shore reef, [35] is directly attached to a shore,[36] or borders it with an
intervening narrow, shallow channel or lagoon.[37] It is the most common reef type.[37] Fringing reefs
follow coastlines and can extend for many kilometres.[38] They are usually less than 100 metres wide,
but some are hundreds of metres wide.[39] Fringing reefs are initially formed on the shore at the low
water level and expand seawards as they grow in size. The final width depends on where the sea
bed begins to drop steeply. The surface of the fringe reef generally remains at the same height: just
below the waterline. In older fringing reefs, whose outer regions pushed far out into the sea, the
inner part is deepened by erosion and eventually forms a lagoon.[40] Fringing reef lagoons can
become over 100 metres wide and several metres deep. Like the fringing reef itself, they run parallel
to the coast. The fringing reefs of the Red Sea are "some of the best developed in the world" and
occur along all its shores except off sandy bays.[41]
Barrier reef
Barrier reef
Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon.[37] They
resemble the later stages of a fringing reef with its lagoon but differ from the latter mainly in size and
origin. Their lagoons can be several kilometres wide and 30 to 70 metres deep. Above all, the
offshore outer reef edge formed in open water rather than next to a shoreline. Like an atoll, it is
thought that these reefs are formed either as the seabed lowered or sea level rose. Formation takes
considerably longer than for a fringing reef, thus barrier reefs are much rarer.
The best known and largest example of a barrier reef is the Australian Great Barrier Reef.[37][42] Other
major examples are the Belize Barrier Reef and the New Caledonian Barrier Reef.[42] Barrier reefs
are also found on the coasts of Providencia,[42] Mayotte, the Gambier Islands, on the southeast coast
of Kalimantan, on parts of the coast of Sulawesi, southeastern New Guinea and the south coast of
the Louisiade Archipelago.
Platform reef
Platform reef
Platform reefs, variously called bank or table reefs, can form on the continental shelf, as well as in
the open ocean, in fact anywhere where the seabed rises close enough to the surface of the ocean
to enable the growth of zooxanthemic, reef-forming corals.[43] Platform reefs are found in the southern
Great Barrier Reef, the Swain[44] and Capricorn Group[45] on the continental shelf, about 100–200 km
from the coast. Some platform reefs of the northern Mascarenes are several thousand kilometres
from the mainland. Unlike fringing and barrier reefs which extend only seaward, platform reefs grow
in all directions.[43] They are variable in size, ranging from a few hundred metres to many kilometres
across. Their usual shape is oval to elongated. Parts of these reefs can reach the surface and form
sandbanks and small islands around which may form fringing reefs. A lagoon may form In the middle
of a platform reef.
Platform reefs can be found within atolls. There they are called patch reefs and may reach only a
few dozen metres in diameter. Where platform reefs form on an elongated structure, e. g. an old,
eroded barrier reef, they can form a linear arrangement. This is the case, for example, on the east
coast of the Red Sea near Jeddah. In old platform reefs, the inner part can be so heavily eroded that
it forms a pseudo-atoll.[43] These can be distinguished from real atolls only by detailed investigation,
possibly including core drilling. Some platform reefs of the Laccadives are U-shaped, due to wind
and water flow.
