Editing Reef (section)

==Classification==
Reefs may be classified in terms of their origin, geographical location, depth, and [[topography]]. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.

=== Biotic ===
{{see also|Coral reef#Formation|Sponge reef#Structure of sponge reefs|Bivalve reef}}
[[File:Great-Barrier-Reef-2018-Luka-Peternel.jpg|thumb|Part of [[Great Barrier Reef]]]]
A variety of biotic reef types exists, including [[oyster reef]]s and [[sponge reef]]s, but the most massive and widely distributed are tropical [[coral reef]]s.<ref name="NatGeo" /> 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]].

Oyster larvae prefer to settle on adult oysters and thereby develop layers building upwards. These eventually form a fairly massive hard stony calcium carbonate structure on which other reef organisms like [[sponges]] and [[seaweeds]] can grow, and provide a habitat for mobile benthic organisms.<ref name="NatGeo" />

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 reef]]s, [[barrier reef]]s, and [[atoll]]s. A fringing reef is a reef that is attached to an island. Whereas, a barrier reef forms a calcareous barrier around an island, resulting in a [[lagoon]] between the shore and the reef. Conversely, an atoll is a ring reef with no land present.

The reef front, facing the ocean, is a high energy locale. Whereas, the internal lagoon will be at a lower energy with fine grained sediments.

==== Mounds ====
Both mounds and reefs are considered to be varieties of organosedimentary buildups, which are sedimentary features, built by the interaction of organisms and their environment. These interactions have a synoptic relief and whose biotic composition differs from that found on and beneath the surrounding [[sea floor]]. However, reefs are held up by a macroscopic skeletal framework, as what is seen on coral reefs. [[Corals]] and calcareous algae grow on top of one another, forming a three-dimensional framework that is modified in various ways by other organisms and inorganic processes.<ref name=":0">{{Cite web |title=Reading: Shorelines {{!}} Geology |url=https://courses.lumenlearning.com/geo/chapter/reading-shorelines/ |access-date=2024-04-20 |website=courses.lumenlearning.com}}</ref>

Conversely, mounds lack a macroscopic skeletal framework. Instead, they are built by microorganisms or by organisms that also lack a skeletal framework. A microbial mound might be built exclusively or primarily by [[cyanobacteria]]. Examples of [[biostrome]]s formed by cyanobacteria occur in the [[Great Salt Lake]] in [[Utah]], United States, and in [[Shark Bay]] on the coast of [[Western Australia]].<ref name=":0" /><ref>{{Cite journal |last=Wood |first=Rachel |date=2001-12-15 |title=Are reefs and mud mounds really so different? |url=https://www.sciencedirect.com/science/article/pii/S0037073801001464 |journal=Sedimentary Geology |series=Carbonate Mounds: sedimentation, organismal response, and diagenesis |volume=145 |issue=3 |pages=161–171 |doi=10.1016/S0037-0738(01)00146-4 |bibcode=2001SedG..145..161W |issn=0037-0738|url-access=subscription }}</ref>

Cyanobacteria do not have skeletons, and individual organisms are microscopic. However, they 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. [[Stromatolite]]s, for instance, are microbial mounds with a laminated internal structure. Whereas, [[bryozoan]]s and [[crinoid]]s, common contributors to marine sediments during the [[Mississippian (geology)|Mississippian period]], produce a different kind of mound. Although bryozoans are small and crinoid skeletons disintegrate, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.<ref name=":0" /><ref>{{Cite journal |last=crossref |title=Chooser |url=https://chooser.crossref.org/ |access-date=2024-04-20 |website=chooser.crossref.org |language=en |doi=10.2307/3514838|jstor=3514838 |url-access=subscription }}</ref>

The [[Proterozoic]] [[Belt Supergroup]] contains evidence of possible [[microbial mat]] and dome structures similar to stromatolite and chicken reef complexes.{{clarify|what are chicken reef complexes?|date=November 2024}}<ref name=":0" /><ref>{{Cite journal |last=Schieber |first=Jürgen |date=1998 |title=Possible indicators of microbial mat deposits in shales and sandstones: examples from the Mid-Proterozoic Belt Supergroup, Montana, U.S.A. |url=https://sepm04.sitehost.iu.edu/PDF/JS-J24-microbial_mat_challenge |journal=Sedimentary Geology |volume=120 |issue=1 |pages=105–124|doi=10.1016/S0037-0738(98)00029-3 |bibcode=1998SedG..120..105S |url-access=subscription }}</ref>

=== Geologic ===
<!-- target for redirect [[Rocky reef]] -->
Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief. They can be found in depth ranges from [[intertidal]] to deep water and provide a substrate for a large range of sessile benthic organisms, and shelter for a large range of mobile organisms.<ref name="NOAA fisheries" /> They are often located in sub-tropical, temperate, and sub-polar latitudes.
{{expand section|relative abundance of rocky vs biotic reef and global distribution, ecological importance|date=February 2021}}

==== Structures ====
[[File:Fossil Reef Windley Key 1.jpg|thumb|Fossil [[brain coral]] (''Diploria'') at the [[Windley Key Fossil Reef Geological State Park]].  [[Quarter (United States coin)|US Quarter]] near top for scale.]]

Ancient reefs buried within [[stratigraphy|stratigraphic]] sections are of considerable interest to [[geologist]]s because they provide paleo-environmental information about the location in [[history of Earth|Earth's history]]. In addition, reef structures within a sequence of [[sedimentary rock]]s provide a discontinuity which may serve as a trap or conduit for [[fossil fuel]]s or mineralizing fluids to form [[petroleum]] or [[ore deposit]]s.<ref>{{Cite journal |last1=Gorokhovich |first1=Yuri |last2=Learning |first2=Lumen |title=Coastal Geology: Shorelines |url=https://pressbooks.cuny.edu/gorokhovich/chapter/coastal-geology-shorelines/ |language=en}}</ref>

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 [[rudists|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.<ref>{{Cite web |title=Archaeocyathans |url=https://ucmp.berkeley.edu/porifera/archaeo.html#:~:text=The%20first%20archaeocyaths%20appear%20roughly,creation%20of%20the%20first%20reefs. |access-date=2024-04-20 |website=ucmp.berkeley.edu}}</ref> 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). They became increasingly important reef builders throughout the [[Mesozoic]] Era.<ref>{{Cite journal |last1=Pruss |first1=Sara B. |last2=Bottjer |first2=David J. |date=2005-09-01 |title=The reorganization of reef communities following the end-Permian mass extinction |url=https://www.sciencedirect.com/science/article/pii/S163106830500045X |journal=Comptes Rendus Palevol |volume=4 |issue=6 |pages=553–568 |doi=10.1016/j.crpv.2005.04.003 |bibcode=2005CRPal...4..553P |issn=1631-0683|url-access=subscription }}</ref> 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]].<ref>{{Cite web |date=2021-06-30 |title=Rugose Coral |url=https://www.colorado.edu/cumuseum/2021/06/30/rugose-coral |access-date=2024-04-20 |website=Museum of Natural History |language=en}}</ref> However, there are some unusual examples of well-preserved aragonitic rugose corals in the [[Lopingian|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).<ref name=":0" />

=== Artificial ===
{{main|Artificial reef}}
An artificial reef is a human-created underwater structure, typically built to promote [[Marine biology#Reefs|marine life]] in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of [[trawling]] nets,<ref name="Gray 2018" /> or improve [[surfing]].<ref name="Bournemouth Echo" />

Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through the [[Rigs-to-Reefs]] program), [[Sinking ships for wreck diving sites|scuttling ships]], or by deploying [[rubble]] or [[construction debris]]. Other artificial reefs are purpose built (e.g. the [[The Reef Ball Foundation|reef balls]]) from [[Polyvinyl chloride|PVC]] or concrete. Shipwrecks become artificial reefs on the seafloor. Regardless of construction method, artificial reefs generally provide stable hard surfaces where [[algae]] and invertebrates such as [[barnacle]]s, corals, and [[oyster]]s attach; the accumulation of attached marine life in turn provides intricate structure and food for [[biocoenosis|assemblages]] of fish.