Draft:Reef geomorphology

Global Views of Coral Reef Geomorphology

From Research Vessels

"The circularly-formed Coral Islands in the Pacific occasionally afford excellent land-locked harbours, with a sufficient entrance, and would be well adapted to any nice astronomical observations which might require to be carried on in undisturbed tranquillity. While these are quietly proceeding, and the chronometers rating, a very interesting inquiry might be instituted respecting the formation of these coral reefs .... A modern and very plausible theory has been put forward, that these wonderful formations, instead of ascending from the bottom of the sea, have been raised from the summits of extinct volcanoes ..."

From Orbiting Satellites

Pale green when viewed through the water of the Persian Gulf, coral reefs fringe the shoreline and islands of the United Arab Emirates in this Landsat satellite image. Credit: Jesse Allen, NASA, United States Geological Survey.

"Pale green when viewed through the water of the Persian Gulf, coral reefs fringe the shoreline and islands of the United Arab Emirates in this Landsat satellite image. The reefs are more extensive than the islands they surround in many cases. The natural-color image was made from data collected by the Landsat 7 satellite on July 3 and July 10, 2002."

"The reefs are an oasis of biological diversity, and they are among the most extensive reefs in the Gulf. Starting in January 2005, a division of the World Wildlife Federation in Abu Dhabi used Landsat images such as this one to create the first map of coral reefs off Abu Dhabi in the United Arab Emirates and Qatar. The map will help resource managers assess the quality and quantity of marine resources in the region and to make decisions that protect them."

From Google Earth

Until very recently, a synoptic view of reef geomorphology showing the impact of varying ecological processes has not been possible, and previous attempts to summarize information have had to rely on parochial descriptions of what was accessible or visible at the time (e.g., Hopley, 1982; Guilcher, 1988). Following this approach, vast areas of modern reefs have been inadequately characterized and, with some exceptions, quantitative differences and the similarities between different regions have gone undocumented. This deficient situation is about to change. The recent advent of widely available orthorectified satellite imagery (Google Earth) and rapid development and cost reduction in geographic information systems, multi-beam sonar, LiDAR, and other remote-sensing technologies are making large-scale morphometric quantification of reefs feasible (Naseer and Hatcher, 2001; Andrefouet et al., 2001; Storlazzi et al., 2003; Naseer and Hatcher, 2004). As such, we are on the cusp of being able to provide a more holistic view of modern coral-reef form and a development that will provide timely insight into the resilience and resistance of reefs to human-induced global environmental change. In this entry, satellite imagery from Google Earth is used along with local descriptions of reefs and attempt to summarize what is presently known about the geomorphy of coral reefs, outline their formative processes, and assess their development.

Geomorphic Reef Zonation

The coral reefs of the Atlantic, the Caribbean and the Indo-Pacific do not differ fundamentally in their structural forms, their habitats and the interaction of their species, even though the organisms occupying specific ecological roles vary greatly between oceans and even between individual reefs (Goreau et al. 1979, p. 135). Despite the similarity between the geomorphology of coral reefs noted by the Goreaus and others (Wells, 1957; Stoddart, 1969), much of the emphasis in characterizing coral-reef zonation has been ecological using either statistically defined coral assemblages or more simply, the morphology of the corals themselves (Wells, 1957; Rosen, 1975; Geister, 1977; Pichon, 1978; Chappell, 1980; Done, 1982; Done, 1983). Most of these studies have found that shallow coral assemblages show a distinct zonation as wave energy, and hydrodynamic disturbance varies with depth and margin exposure. For example, Done (1982) found that shallow communities changed with a decreasing cross-shelf wave exposure in the central Great Barrier Reef. Individual reefs also showed the same pattern, with shallow-wave-adapted communities being replaced along leeward margins by upward shifts in the positions of deeper communities. Growth form of corals also show similar zonation patterns in response to depth and exposure-related variation in wave energy (Pichon, 1978; Chappell, 1980; Madin and Connely, 2006). Such zonation patterns have led to a clear distinction between rough-water reefs exposed to trade-wind-, swell-, and storm-generated wave fields and calm-water reefs protected from them (Geister, 1977, 1980).

A comparison of zonation schemes, however, shows that the coral assemblages in each zone can be highly variable on both inter-reefal and regional scale, and there is a significant overlap between the members of each assemblage zone (e.g., Done, 1982). Despite this “coral-scale” variability, the geomorphology of these reefs is very similar. Indeed, equally similar reef structures are produced in areas with order-of-magnitude differences in species abundances, and even where reefs have depauperate species numbers, their basic geomorphology is identical (e.g., Glynn et al., 1996). This consistency implies that gradients in wave exposure not only control the main environmental tolerances of shallow coral assemblages but also the form of the structures they build (Graus et al., 1977; Graus and Macintyre, 1989). It therefore follows that reefs might be better classified according to their geomorphic form (Wells, 1957).

Standard Zonation

Image shows geomorphic donation of rough-water reefs. Credit: Blanchon.{{free media}}

Geomorphic zonation of rough-water reefs. Both swell- and trade-wind-dominated reefs develop five standard reef zones: lagoon, reef flat, reef front, reef slope, and fore reef (not shown). These zones are delineated by simple slope breaks: the sand slope between lagoon and reef flat, the reef crest between the reef flat and reef front, the shelf break between the reef front and reef slope, and the break between the subvertical lower slope and the talus cones of the fore-reef sediment. These standard zones are divided into several widely occurring subzones, which vary with respect to margin orientation, area, and region.

A comparison of well-ordered, rough-water reefs shows a remarkable degree of conformity between the size and character of their major morphological features. All of them consist of five first-order or “standard” zones: a calm reef lagoon, with varying number of patch reefs; a largely barren, shallow reef flat strewn with detritus; a narrow, gently sloping reef front exposed to waves and swells; a steeper reef slope bearing corals to as deep as 100 m; and a less steeply inclined fore-reef apron, where coral and reef detritus accumulate (Figure on the right). The boundaries between these standard zones are delineated by simple slope breaks. The break between the lagoon and the reef flat is either a sand slope or a small reef scarp. The break between the level reef flat and the gently sloping reef front occurs at the reef crest. Similarly, the shelf break separates the gently sloping reef front from the steeper reef slope, and the break between the reef slope and fore reef is the transition into slopes that reflects the angle of sediment repose. Each of these standard and easily defined reef zones can in turn be subdivided into commonly recurring second order zones or subzones, which may not be present on all reefs and, if they are present, may vary between margin, area, and region. As a consequence, their boundaries can be more difficult to define.

Reef Lagoons

Patches, Knolls and Pinnacles, Cellular Networks

Reef Flats

Algal Rims, Coralgal Flat, Sediment Flat, Island and Storm deposits

Reef Fronts

Spur and Groove, Mid-shelf Scarp, Sand Terrace

Reef Slopes

Chute and Buttress, Upper Reef Slope, Lower Reef Slope

Fore Reefs

Talus aprons

Geomorphic Reef Types

Platform reefs

Theoretical platform reef is diagrammed. Credit: Normann Z.

Platform reefs, variously called bank or table reefs, can form on the continental shelf, as well as in the open ocean, in fact anywhere where the seabed rises close enough to the surface of the ocean to enable the growth of zooxanthemic, reef-forming corals. Platform reefs are found in the southern Great Barrier Reef, the Swain and Capricorn Group on the continental shelf, about 100–200 km from the coast. Some platform reefs of the northern Mascarenes are several thousand kilometres from the mainland. Unlike fringing and barrier reefs which extend only seaward, platform reefs grow in all directions. They are variable in size, ranging from a few hundred metres to many kilometres across. Their usual shape is oval to elongated. Parts of these reefs can reach the surface and form sandbanks and small islands around which may form fringing reefs. A lagoon may form In the middle of a platform reef. Platform reefs can be found within atolls. There they are called patch reefs and may reach only a few dozen metres in diameter. Where platform reefs form on an elongated structure, e. g. an old, eroded barrier reef, they can form a linear arrangement. This is the case, for example, on the east coast of the Red Sea near Jeddah. In old platform reefs, the inner part can be so heavily eroded that it forms a pseudo-atoll. These can be distinguished from real atolls only by detailed investigation, possibly including core drilling. Some platform reefs of the Laccadives are U-shaped, due to wind and water flow.

Fringing Reefs

A theoretical fringing reef is diagrammed. Credit: Normann Z.

The fringing reef is at Eilat at the southern tip of Israel. Credit: Mark A. Wilson.

The Vivari Channel in Albania links Lake Butrint with the Straits of Corfu. Credit: Roquai.

A fringing reef, also called a shore reef, is directly attached to a shore, or borders it with an intervening narrow, shallow channel or lagoon. It is the most common reef type. Fringing reefs follow coastlines and can extend for many kilometres. They are usually less than 100 metres wide, but some hundreds of metres wide. Fringing reefs are initially formed on the shore at the low water level and expand seawards as they grow in size. The final width depends on where the sea bed begins to drop steeply. The surface of the fringe reef generally remains at the same height: just below the waterline. In older fringing reefs, whose outer regions pushed far out into the sea, the inner part is deepened by erosion and eventually forms a lagoon. Fringing reef lagoons can become over 100 metres wide and several metres deep. Like the fringing reef itself, they run parallel to the coast. The fringing reefs of the Red Sea are "some of the best developed in the world" and occur along all its shores except off sandy bays.

Def.

1. a "physical confine of a river or slough, consisting of a bed and banks",
2. a "natural or man-made deeper course through a reef, bar, bay, or any shallow body of water",
3. a "navigable part of a river", or
4. a "narrow body of water between two land masses" is called a channel.

Barrier Reefs

Satellite image shows part of the Great Barrier Reef adjacent to the Queensland coastal areas of Airlie Beach and Mackay. Credit: NASA, by MISR.{{free media}}

The approximately 20,000-year-old Great Barrier Reef offers an example of how coral reefs formed on continental shelves. Sea level was then 120 m (390 ft) lower than in the 21st century. 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 60 m (200 ft) lower than at present, and many hills of the coastal plains had become continental islands. As sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming cays and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years. The age of living reef structure is estimated to be between 6,000 and 8,000 years. 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, 300–1,000 m (980–3,280 ft) from shore, stretching for 2,000 km (1,200 mi).

The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching longer than 2,600 kilometers over an area of approximately 344,400 square kilometers. The reef is located in the Coral Sea, off the coast of Queensland, Australia. Identifying individual reefs or the precise boundaries of the reef becomes arbitrary. It is composed of billions of tiny organisms, known as coral polyps. These grow and die as part of the greater ocean ecosystem they are part of. Attempts to identify a definitive boundary between the reef and the sea arbitrarily separate each from the other it depends on.

Atoll Reefs

Formation of an atoll is a theory of Charles Darwin. Credit: NOAA.

North and South Malosmadulu Atols are in the Maldives. Credit: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team.{{free media}}

Atolls or atoll reefs are a more or less circular or continuous barrier reef that extends all the way around a lagoon without a central island. They are usually formed from fringing reefs around volcanic islands. Over time, the island erodes away and sinks below sea level. Atolls may also be formed by the sinking of the seabed or rising of the sea level. A ring of reefs results, which enclose a lagoon. Atolls are numerous in the South Pacific, where they usually occur in mid-ocean, for example, in the Caroline Islands, the Cook Islands, French Polynesia, the Marshall Islands and Micronesia. Atolls are found in the Indian Ocean, for example, in the Maldives, the Chagos Islands, the Seychelles and around Cocos Island. The entire Maldives consist of 26 atolls.

This animation on the left shows the dynamic process of how a coral atoll forms. Corals (represented in tan and purple) begin to settle and grow around an oceanic island forming a fringing reef. It can take as long as 10,000 years for a fringing reef to form. Over the next 100,000 years, if conditions are favorable, the reef will continue to expand. As the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef. When the island completely subsides beneath the water leaving a ring of growing coral with an open lagoon in its center, it is called an atoll. The process of atoll formation may take as long as 30,000,000 years to occur.

North and South Malosmadulu Atols on the right are in the Maldives, an island republic in the northern Indian Ocean, southwest of India. Maldives is made up of a chain of 1192 small coral islands that are grouped into clusters of atolls. It has a total area of 298 sq km and a population of about 330,000. The capital and largest city is Male, with a population of about 80,000. Arguably the lowest-lying country in the world, the average elevation is 1 m above sea level. Waves triggered by the great tsunami of December 2004 spilled over sea walls to flood Male with sand-clouded water and then swept out just as suddenly. Residents fear this was a foreboding of disasters to come from sea-level rise due to global warming. The simulated natural color ASTER image was acquired 22 December 2002, covers an area of 50.8 x 90.4 km, and is centered near 5.3 degrees north latitude, 73.9 degrees west longitude.

Raa Atoll is located at 5°38'N 72°55'E.

The first Maldivians did not leave any archaeological artifacts. Their buildings were probably built of wood, palm fronds and other perishable materials, which would have quickly decayed in the salt and wind of the tropical climate. Moreover, chiefs or headmen did not reside in elaborate stone palaces, nor did their religion require the construction of large temples or compounds.

Comparative studies of Maldivian oral, linguistic and cultural traditions and customs confirm that the first settlers were people from the southern shores of the neighboring Indian subcontinent, including the Giraavaru people mentioned in ancient legends and local folklore about the establishment of the capital and kingly rule in Malé.

A strong underlying layer of Dravidian population and culture survives in Maldivian society, with a clear Tamil-Malayalam substratum in the language, which also appears in place names, kinship terms, poetry, dance, and religious beliefs. Malabari seafaring culture led to the settlement of the Islands by Malayali seafarers.

The earliest written history of the Maldives was marked by the arrival of Sinhalese people in Sri Lanka and the Maldives (Mahiladvipika) circa 543 to 483 BC, as reported in the Mahavansa. Their settlement marks a significant change in demographics and the development of the Indo-Aryan language Dhivehi language.

Submerged Reefs

Landsat image is of Roncador Reef. Credit: NASA.{{free media}}

Satphoto shows Jarvis Island; note the submerged reef beyond the eastern end. Credit: NASA.{{free media}}

Map shows the central Pacific Ocean including Jarvis and neighboring islands. Credit: United States Central Intelligence Agency's World Factbook.

Roncador Reef is a reef in the Solomon Islands, south of Ontong Java Atoll and north of Santa Isabel Island that is a submerged reef forming a small geographic group together with Nukumanu and Ontong Java, the two nearest atolls located further north.

This reef was first sighted by Europeans at the time of the Spanish expedition of Álvaro de Mendaña and Pedro Sarmiento de Gamboa on 1 February 1658, charted as Bajos de la Candelaria (shoals of Our Lady of Candlemas in Spanish).

Its sighting was also reported by Spanish naval officer Francisco Mourelle de la Rúa on 22 January 1781 on board of frigate La Princesa, charted as Peregrino Roncador (Snoring Pilgrim in Spanish).

Jarvis Island, formerly known as Bunker Island, or Bunker's Shoal is located at 0°22'S 160°01'W in the South Pacific Ocean.

It is an unincorporated, unorganized territory of the United States, administered by the United States Fish and Wildlife Service of the United States Department of the Interior as part of the National Wildlife Refuge system.

The indigenous people who inhabit the islands of Polynesia are termed Polynesians, and share many similar traits including language family, culture, and beliefs.

Physical and biological controls on Reef Geomorphology

Reef Zonation

PAR limitation on coral growth forms with depth
Biomechanical limitation on coral and algal growth forms and therefore zonation
Wave exposure (Trades, extratropical swell) around reefs and with depth
Wave-set up (wave pumping) and its impact on reef flats and lagoons
Tides and their modulation of waves and currents on reef flats and lagoons
Storms and their effects on reef-front and reef-flat zonation
Tsunamis.

Reef Types

Wind-wave, swell, storm directions predict reef types

Geological controls on Reef Geomorphology

Reef Zonation

relative sea-level changes (coseismic, eustatic)

Reef Types

Darwin Point relative sea-level changes

See also

• Reefkeeping (8 kB) (25 January 2020)
• Earth science (5 kB) (28 May 2019)
• Equatorials (147 kB) (17 August 2019‎)
• Geochemistry (40 kB) (1 February 2020)
• Geomorphology (71 kB) (31 January 2020)
• Green radiation astronomy (106 kB) (5 July 2019‎)
• Hydromorphology (56 kB) (10 July 2019)
• Mediterranean continental shelves (12 kB) (1 December 2019)
• North east American continental shelves (80 kB) (10 August 2019‎)
• North Sea continental shelves (104 kB) (14 September 2019)
• North west African continental shelves (8 kB) (26 April 2019)
• Oceanography (25 kB) (8 August 2019)
• Oxidanes (5 kB) (28 January 2019)
• South east African continental shelves (13 kB) (14 September 2019)
• South east Asian continental shelves (6 kB) (23 May 2019‎)
• South west African continental shelves (3 kB) (31 July 2019‎)
• Volcanoes (100 kB) (8 December 2019)
• West European continental shelves (32 kB) (28 April 2019‎)