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| A reef surrounding
an islet. |
Reefs
A reef is a bar of rock, sand, coral or similar
material, lying beneath the surface of water. Many reefs
result from natural, abiotic processes—deposition of
sand, wave erosion planing down rock outcrops, etc.—but
the best known reefs are the coral reefs of tropical
waters developed through biotic processes dominated by
corals and coralline algae.
Artificial reefs (e.g. shipwrecks) sometimes have a role
in enhancing the physical complexity of featureless sand
bottoms, in order to attract a diverse assemblage of
organisms, especially algae and fish.
Earth's largest reef system is the Great Barrier Reef in
Australia, at a length of over 2,300 kilometres (1,400
miles). |
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| Reefs off Vanatinai
in the Louisiade Archipelago. |
Biotic reef
There is a variety of biotic reef types, including
oyster reefs and sponge reefs, but the most massive and
widely distributed are tropical coral reefs. Although
corals are major contributors to the framework and bulk
material comprising a coral reef; the organisms most
responsible for reef growth against the constant assault
from ocean waves are calcareous algae, especially,
although not entirely, coralline algae.
These biotic reef types take on additional names
depending upon how the reef lies in relation to the
land, if any. Reef types include fringing reefs, barrier
reefs, and atolls. A fringing reef is a reef that is
attached to an island. A barrier reef forms a calcareous
barrier around an island resulting in a lagoon between
the shore and the reef. An atoll is a ring reef with no
land present. The reef front (ocean side) is a high
energy locale whereas the internal lagoon will be at a
lower energy with fine grained sediments. |
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| Coral reef at Nusa
Lembongan, Bali, Indonesia. |
Geologic reef
Geologic reef structures
Ancient reefs buried within stratigraphic sections are
of considerable interest to geologists because they
provide paleo-environmental information about the
location in Earth's history. In addition, reef
structures within a sequence of sedimentary rocks
provide a discontinuity which may serve as a trap or
conduit for fossil fuels or mineralizing fluids to form
petroleum or ore deposits.
Corals, including some major extinct groups Rugosa and
Tabulata, have been important reef builders through much
of the Phanerozoic since the Ordovician Period. However,
other organism groups, such as calcifying algae,
especially members of the red algae Rhodophyta, and
molluscs (especially the rudist bivalves during the
Cretaceous Period) have created massive structures at
various times. During the Cambrian Period, the conical
or tubular skeletons of Archaeocyatha, an extinct group
of uncertain affinities (possibly sponges), built reefs.
Other groups, such as the Bryozoa have been important
interstitial organisms, living between the framework
builders. The corals which build reefs today, the
Scleractinia, arose after the Permian–Triassic
extinction event that wiped out the earlier rugose
corals (as well as many other groups), and became
increasingly important reef builders throughout the
Mesozoic Era. They may have arisen from a rugose coral
ancestor. Rugose corals built their skeletons of calcite
and have a different symmetry from that of the
scleractinian corals, whose skeletons are aragonite.
However, there are some unusual examples of
well-preserved aragonitic rugose corals in the Late
Permian. In addition, calcite has been reported in the
initial post-larval calcification in a few scleractinian
corals. Nevertheless, scleractinian corals (which arose
in the middle Triassic) may have arisen from a
non-calcifying ancestor independent of the rugosan
corals (which disappeared in the late Permian). |
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Biotic mound structures
One useful definition distinguishes reefs from mounds as
follows: Both are considered to be varieties of
organosedimentary buildups – sedimentary features, built
by the interaction of organisms and their environment,
that have synoptic relief and whose biotic composition
differs from that found on and beneath the surrounding
sea floor. Reefs are held up by a macroscopic skeletal
framework. Coral reefs are an example of this kind.
Corals and calcareous algae grow on top of one another
and form a three-dimensional framework that is modified
in various ways by other organisms and inorganic
processes. By contrast, mounds lack a macroscopic
skeletal framework. Mounds are built by microorganisms
or by organisms that don't grow a skeletal framework. A
microbial mound might be built exclusively or primarily
by cyanobacteria. Examples of biostromes formed by
cyanobacteria occur in the Great Salt Lake in Utah, and
in Shark Bay on the coast of Western Australia.
Cyanobacteria do not have skeletons, and individuals are
microscopic. Cyanobacteria can encourage the
precipitation or accumulation of calcium carbonate to
produce distinct sediment bodies in composition that
have relief on the seafloor. Cyanobacterial mounds were
most abundant before the evolution of shelly macroscopic
organisms, but they still exist today; stromatolites are
microbial mounds with a laminated internal structure.
Bryozoans and crinoids, common contributors to marine
sediments during the Mississippian, for instance,
produced a very different kind of mound. Bryozoans are
small and the skeletons of crinoids disintegrate.
However, bryozoan and crinoid meadows can persist over
time and produce compositionally distinct bodies of
sediment with depositional relief.
The Proterozoic Belt Supergroup contains evidence of
possible microbial mat and dome structures similar to
stromatolite reef complexes. |
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 Kiddle: Reefs
Wikipedia: Reefs |
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