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[[File:PamalicanAfterLiftOff.jpg|thumb|[[Pamalican]] island with surrounding reef, [[Sulu Sea]], [[Philippines]]]]
[[File:Reef.jpg|thumb|A reef surrounding an [[islet]]]]
[[File:Vanatinai, Louisiade Archipelago.jpg|thumb|Reefs off [[Vanatinai|Vanatinai Island]] in the [[Louisiade Archipelago]]]]▼
A '''reef''' is a ridge or [[shoal]] of rock, [[coral]] or similar relatively stable material, lying beneath the surface of a natural body of water.<ref name="NatGeo" /> Many reefs result from natural, [[abiotic component|abiotic]] (non-living) processes such as [[deposition (geology)|deposition]] of sand{{citation needed|reason=for reef formation by deposition of sand|date=February 2021}} or [[wave erosion]] planing down rock outcrops. However, reefs such as the [[coral reef]]s of tropical waters are formed by [[biotic component|biotic]] (living) processes, dominated by corals and [[coralline algae]]. [[Artificial reef]]s such as shipwrecks and other man-made underwater structures may occur intentionally or as the result of an accident, and are sometimes designed to increase the physical complexity of featureless sand bottoms to attract a more diverse range of organisms. Reefs are often quite near to the surface, but not all definitions require this.<ref name="NatGeo" />
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=== 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.]]
▲[[File:Vanatinai, Louisiade Archipelago.jpg|thumb|Reefs off [[Vanatinai|Vanatinai Island]] in the [[Louisiade Archipelago]]]]
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|ore deposits]].
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