Editing Coral reef (section)

== Formation ==
{{Further|Atoll|Fringing reef|The Structure and Distribution of Coral Reefs}}

Most coral reefs were formed after the [[Last Glacial Period]] when melting ice caused [[sea level]] to rise and flood [[continental shelf|continental shelves]]. Most coral reefs are less than 10,000 years old. As communities established themselves, the reefs grew upwards, pacing rising [[sea level]]s. Reefs that rose too slowly could become drowned, without sufficient light.<ref>{{cite web |url=http://www.eoearth.org/article/Coral_reef#Types_of_Coral_Reefs |title=Coral reef |last=Kleypas |first=Joanie |date=2010 |website=The Encyclopedia of Earth |archive-url=https://web.archive.org/web/20100815052312/http://www.eoearth.org/article/Coral_reef |archive-date=August 15, 2010 |url-status=dead |access-date=April 4, 2011}}</ref> Coral reefs are also found in the deep sea away from [[continental shelf|continental shelves]], around [[Island#Oceanic islands|oceanic islands]] and [[atoll]]s. The majority of these islands are [[volcano|volcanic]] in origin. Others have [[tectonics|tectonic]] origins where [[plate tectonics|plate movements]] lifted the deep ocean floor.

In ''[[The Structure and Distribution of Coral Reefs]]'',<ref name="The structure and distribution of coral reefs">{{cite book|last=Darwin|first=Charles R.|title=The Structure and Distribution of Coral Reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836.|year=1842|publisher=Smith Elder and Co.|location=London |url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F271&pageseq=1}} [https://archive.org/details/b29331213 Via Internet Archive]</ref> [[Charles Darwin]] set out his theory of the formation of atoll reefs, an idea he conceived during the [[second voyage of HMS Beagle|voyage of the ''Beagle'']]. He theorized that [[tectonic uplift|uplift]] and [[subsidence]] of Earth's [[oceanic crust|crust]] under the oceans formed the atolls.<ref name=cr>{{Cite web|url=http://darwin-online.org.uk/EditorialIntroductions/Chancellor_CoralReefs.html |title=Introduction to ''Coral reefs'' |author=Chancellor, Gordon |year=2008 |publisher=Darwin Online |access-date=January 20, 2009}}</ref> Darwin set out a sequence of three stages in atoll formation. A [[fringing reef]] forms around an extinct [[volcanic island]] as the island and ocean floor subside. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef.

<gallery widths="120" heights="80">

File:Atoll forming-volcano.png|Darwin's theory starts with a [[volcanic island]] which becomes extinct
File:Atoll forming-Fringing reef.png|As the island and ocean floor subside, coral growth builds a [[fringing reef]], often including a shallow lagoon between the land and the main reef.
File:Atoll forming-Barrier reef.png|As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper [[lagoon]] inside.
File:Atoll forming-Atoll.png|Ultimately, the island sinks below the sea, and the barrier reef becomes an [[atoll]] enclosing an open lagoon.
</gallery>

Darwin predicted that underneath each [[lagoon]] would be a [[bedrock]] base, the remains of the original volcano.<ref>{{Cite web|date=2017-03-11|title=4 Main Theories of Coral Reefs and Atolls/Oceans/Geography|url=https://www.geographynotes.com/oceans/4-main-theories-of-coral-reefs-and-atolls-oceans-geography/2704|access-date=2020-08-01|website=Geography Notes|language=en-US}}</ref> Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the [[tropics]] where the water is agitated, but can only live within a limited depth range, starting just below low [[tide]]. Where the level of the underlying earth allows, the corals grow around the coast to form fringing reefs, and can eventually grow to become a barrier reef.

[[File:Coral atoll formation animation.gif|thumb|right|A fringing reef can take ten thousand years to form, and an atoll can take up to 30 million years.<ref>[http://oceanservice.noaa.gov/education/kits/corals/media/supp_coral04a.html Animation of coral atoll formation] {{Webarchive|url=https://web.archive.org/web/20120714035333/http://oceanservice.noaa.gov/education/kits/corals/media/supp_coral04a.html |date=July 14, 2012 }} [[National Oceanic and Atmospheric Administration|NOAA]] Ocean Education Service. Retrieved January 9, 2010.</ref>]]

Where the bottom is rising, fringing reefs can grow around the coast, but coral raised above sea level dies. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of older, dead coral, forming a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls do not usually form complete circles but are broken in places by storms. Like [[sea level rise]], a rapidly subsiding bottom can overwhelm coral growth, killing the coral and the reef, due to what is called ''coral drowning''.<ref name="Webster Coral subsidence">{{cite journal|last=Webster|first=Jody M. |author2=Braga, Juan Carlos |author3=Clague, David A. |author4=Gallup, Christina |author5=Hein, James R. |author6=Potts, Donald C. |author7=Renema, Willem |author8=Riding, Robert |author9=Riker-Coleman, Kristin |author10=Silver, Eli |author11=Wallace, Laura M. |title=Coral reef evolution on rapidly subsiding margins|journal=Global and Planetary Change|date=1 March 2009 |volume=66 |issue=1–2|pages=129–148|doi=10.1016/j.gloplacha.2008.07.010 |bibcode=2009GPC....66..129W}}</ref> Corals that rely on [[zooxanthellae]] can die when the water becomes too deep for their [[symbionts]] to adequately [[photosynthesis|photosynthesize]], due to decreased light exposure.<ref name="Webster coral drowning">{{cite journal|last=Webster|first=Jody M. |author2=Clague, David A. |author3=Riker-Coleman, Kristin |author4=Gallup, Christina |author5=Braga, Juan C. |author6=Potts, Donald |author7=Moore, James G. |author8=Winterer, Edward L. |author9=Paull, Charles K. |title=Drowning of the −150 m reef off Hawaii: A casualty of global meltwater pulse 1A?|journal=Geology|date=1 January 2004|volume=32|issue=3|page=249 |doi=10.1130/G20170.1 |bibcode=2004Geo....32..249W}}</ref>

The two main variables determining the [[geomorphology]], or shape, of coral reefs are the nature of the [[substrate (biology)|substrate]] on which they rest, and the history of the change in sea level relative to that substrate.

The approximately 20,000-year-old [[Great Barrier Reef]] offers an example of how coral reefs formed on continental shelves. Sea level was then {{convert|120|m|ft|abbr=on}} lower than in the 21st century.<ref>{{cite report |year=2006 |title=Reef Facts for Tour Guides: A "big picture" view of the Great Barrier Reef |url=http://www.gbrmpa.gov.au/__data/assets/pdf_file/0017/12437/Reef-Facts-01.pdf |access-date=June 18, 2007 |publisher=Great Barrier Reef Marine Park Authority |url-status=dead |archive-url=https://web.archive.org/web/20070620013057/http://www.gbrmpa.gov.au/__data/assets/pdf_file/0017/12437/Reef-Facts-01.pdf |archive-date=June 20, 2007}}</ref><ref name="AIMSage">{{cite web |last=Tobin |first=Barry |title=How the Great Barrier Reef was formed |publisher=Australian Institute of Marine Science |orig-year=1998 |year=2003 |url=http://www.aims.gov.au/pages/research/project-net/reefs/apnet-reefs00.html |access-date=November 22, 2006 |url-status=dead |archive-date=October 5, 2006 |archive-url=https://web.archive.org/web/20061005122324/http://www.aims.gov.au/pages/research/project-net/reefs/apnet-reefs00.html}}</ref> As sea level rose, the water and the corals encroached on what had been hills of the Australian coastal plain. By 13,000 years ago, sea level had risen to {{convert|60|m|ft|abbr=on}} lower than at present, and many hills of the coastal plains had become [[Island#Continental islands|continental islands]]. As sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming [[cay]]s and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years.<ref name="AIMSage"/> The age of living reef structure is estimated to be between 6,000 and 8,000 years.<ref name="CRCage">{{cite web|author=CRC Reef Research Centre Ltd |title=What is the Great Barrier Reef? |url=http://www.reef.crc.org.au/discover/coralreefs/coralgbr.html|access-date=May 28, 2006|archive-url= https://web.archive.org/web/20060822015653/http://www.reef.crc.org.au/discover/coralreefs/coralgbr.html |archive-date=August 22, 2006 |url-status=dead}}</ref> Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, Darwin's principles apply. Development stopped at the barrier reef stage, since Australia is not about to submerge. It formed the world's largest barrier reef, {{convert|300|–|1,000|m|ft|abbr=on}} from shore, stretching for {{convert|2,000|km|mi|abbr=on}}.<ref>[http://www.stanford.edu/group/microdocs/typesofreefs.html Four Types of Coral Reef] {{Webarchive|url=https://web.archive.org/web/20121024014723/http://www.stanford.edu/group/microdocs/typesofreefs.html |date=24 October 2012 }} Microdocs, Stanford Education. Retrieved January 10, 2010.</ref>

Healthy tropical coral reefs grow horizontally from {{convert|1|to|3|cm|in|abbr=on}} per year, and grow vertically anywhere from {{convert|1|to|25|cm|in|abbr=on}} per year; however, they grow only at depths shallower than {{convert|150|m|ft|abbr=on}} because of their need for sunlight, and cannot grow above sea level.<ref>{{cite encyclopedia|author=MSN Encarta |year=2006 |title=Great Barrier Reef |url=http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html|access-date=December 11, 2006 |archive-url=https://web.archive.org/web/20091028020755/http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html |archive-date=October 28, 2009 |url-status=dead}}</ref>

===Material===
As the name implies, coral reefs are made up of coral skeletons from mostly intact coral colonies. As other chemical elements present in corals become incorporated into the calcium carbonate deposits, [[aragonite]] is formed. However, shell fragments and the remains of [[coralline algae]] such as the green-segmented [[genus]] ''[[Halimeda]]'' can add to the reef's ability to withstand damage from storms and other threats. Such mixtures are visible in structures such as [[Eniwetok Atoll]].<ref name=murph>{{cite book |title=Coral Reefs: Cities Under The Seas |last=Murphy |first=Richard C. |year=2002 |isbn=978-0-87850-138-0 |publisher=The Darwin Press |url={{google books |plainurl=y |id=WyYVAQAAIAAJ}}}}</ref>{{page needed|date=November 2022}}

=== In the geologic past ===
[[File:Ancient coral reefs.jpg|thumb|upright=1.2|right|Ancient coral reefs]]
The times of maximum reef development were in the [[Middle Cambrian]] (513–501 [[Annum|Ma]]), [[Devonian]] (416–359 Ma) and [[Carboniferous]] (359–299 Ma), owing to [[extinction|extinct]] order [[Rugosa]] corals, and [[Late Cretaceous]] (100–66 Ma) and [[Neogene]] (23 Ma–present), owing to [[Order (biology)|order]] [[Scleractinia]] corals.<ref>{{Citation |last=Hallock |first=Pamela |title=Reefs and Reef Limestones in Earth History |date=1997 |work=Life and Death of Coral Reefs |pages=13–42 |place=Boston, MA |publisher=Springer US |doi=10.1007/978-1-4615-5995-5_2 |doi-broken-date=2 November 2024 |isbn=978-0-412-03541-8}}</ref>

Not all reefs in the past were formed by corals: those in the [[Early Cambrian]] (542–513 Ma) resulted from calcareous [[algae]] and [[Archaeocyatha|archaeocyathids]] (small animals with conical shape, probably related to [[sponges]]) and in the [[Late Cretaceous]] (100–66 Ma), when reefs formed by a group of bivalves called [[rudists]] existed; one of the valves formed the main conical structure and the other, much smaller valve acted as a cap.<ref name="Johnson_2002" />

Measurements of the oxygen isotopic composition of the aragonitic skeleton of coral reefs, such as ''[[Porites]]'', can indicate changes in [[sea surface temperature]] and sea surface salinity conditions during the growth of the coral. This technique is often used by climate scientists to infer a region's [[paleoclimate]].<ref name="Cobb">{{cite journal |last1=Cobb |first1=K. |last2=Charles |first2=Christopher D. |last3=Cheng |first3=Hai |last4=Edwards |first4=R. Lawrence |year=2003 |title=El Nino/Southern Oscillation and tropical Pacific climate during the past millennium |url=http://eas8001.eas.gatech.edu/papers/Cobb_Nature_2003.pdf |url-status=dead |journal=Nature |volume=424 |issue=6946 |pages=271–276 |bibcode=2003Natur.424..271C |doi=10.1038/nature01779 |pmid=12867972 |archive-url=https://web.archive.org/web/20120111134948/http://eas8001.eas.gatech.edu/papers/Cobb_Nature_2003.pdf |archive-date=January 11, 2012 |s2cid=6088699}}</ref>