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21-547: Bligh Reef , sometimes known as Bligh Island Reef , is a reef off the coast of Bligh Island in Prince William Sound , Alaska . This was the location of the 1989 Exxon Valdez oil spill . After the incident, 33 US Code § 2733 mandated the operation of an automated navigation light to prevent future collisions with the reef. Despite these efforts the tug Pathfinder ran aground on Bligh Reef on Dec 24, 2009, rupturing its tanks and spilling diesel fuel. Bligh Reef

42-445: 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. Both mounds and reefs are considered to be varieties of organosedimentary buildups, which are sedimentary features, built by

63-429: 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. 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. Whereas,

84-676: A large portion of the structure's volume. This density of skeletal material allows the structure to withstand sea currents and stand freely. Reef knolls can be divided into bioherms and biostromes. A bioherm is a landform of organic sedimentary rock enclosed or surrounded by rock of different origin. A biostrome is a distinctly bedded or broadly lenticular sedimentary rock landform. Krumbein additionally used these terms to distinguish different shapes of stromatolites : "Distinctly bedded, widely extensive, blanketlike build-ups are biostromes. Nodular, biscuit-like, dome-shaped or columnar stromatolites are also referred to as bioherms". Examples on

105-649: 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 biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah , United States, and in Shark Bay on the coast of Western Australia . Cyanobacteria do not have skeletons, and individual organisms are microscopic. However, they can encourage

126-580: A temperate rocky intertidal reef. A variety of biotic reef types exists, 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 . Oyster larvae prefer to settle on adult oysters and thereby develop layers building upwards. These eventually form

147-578: Is also where Alaska Steamship Company 's Olympia ran aground in 1910. Bligh Reef serves as a fishing ground for halibut and a harvesting area for shrimp . The nearest town is Tatitlek , which lies seven miles (eleven kilometers) to the northeast. The reef was named after William Bligh , of future HMS Bounty fame, who served as Master aboard ship during James Cook 's third world voyage. 60°50′26″N 146°52′48″W  /  60.84056°N 146.88000°W  / 60.84056; -146.88000 Reef Earth's largest coral reef system

168-650: Is the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles). The word "reef" traces its origins back to the Old Norse word rif, meaning "rib" or "reef". Rif comes from the Proto-Germanic term ribją meaning "rib". Reefs may be classified in terms of their origin, geographical location, depth, and topography . For example a tropical coral fringing reef, or

189-522: The Derbyshire / Staffordshire border include Thorpe Cloud and Bunster Hill in southern Dovedale , and also Chrome Hill and Parkhouse Hill at the northern end. These structures are often most clearly seen where the surrounding rocks are much softer and so can be preferentially eroded . All the Derbyshire examples quoted lie at the edge of the limestone areas; Chrome and Parkhouse lie at

210-546: The 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. The Proterozoic Belt Supergroup contains evidence of possible microbial mat and dome structures similar to stromatolite and chicken reef complexes. Rocky reefs are underwater outcrops of rock projecting above

231-476: The Rigs-to-Reefs program), scuttling ships , or by deploying rubble or construction debris . Other artificial reefs are purpose built (e.g. the reef balls ) from 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 barnacles , corals, and oysters attach;

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252-519: The accumulation of attached marine life in turn provides intricate structure and food for assemblages of fish. Biostrome A reef knoll is a landform that comprises an immense pile of calcareous material that had previously accumulated on an ancient sea floor. Reef knolls are geological remnants of reefs and other organic concentrations of calcareous organisms. Reef knolls are often fossil-rich, with prehistoric corals , sponges , calcareous algae , and other reef-builders contributing to

273-481: 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. They are often located in sub-tropical, temperate, and sub-polar latitudes. Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about

294-540: The divide between limestone and the much softer shale . Examples in the Yorkshire Dales lie on the downthrow side (north) of the Mid Craven Fault . There is one set located around Thorpe (Skelterton, Butter Haw, Stebden, Elbolton, Thorpe Kail, Myra Bank and Hartlington Kail); one set located around Malham (Burns Hill, Cawden, and Wedber); and a set around Settle (High Hill and Scaleber). It

315-608: 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. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of

336-486: 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. Conversely, mounds lack

357-674: 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

378-436: 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 , for instance, are microbial mounds with a laminated internal structure. Whereas, bryozoans and crinoids , common contributors to marine sediments during

399-598: 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

420-421: The rugosan corals (which disappeared in the late Permian). An artificial reef is a human-created underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, or improve surfing . Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through

441-471: 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

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