- 1 Global Views of Coral Reef Geomorphology
- 2 Geomorphic Reef Zonation
- 3 Geomorphic Reef Types
- 4 Physical and biological controls on Reef Geomorphology
- 5 Geological controls on Reef Geomorphology
- 6 References
- 7 External links
Global Views of Coral Reef Geomorphology
From Research Vessels
From Orbiting Satellites
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).
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 1). 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.
Patches, Knolls and Pinnacles, Cellular Networks
Algal Rims, Coralgal Flat, Sediment Flat, Island and Storm deposits
Spur and Groove, Mid-shelf Scarp, Sand Terrace
Chute and Buttress, Upper Reef Slope, Lower Reef Slope
Geomorphic Reef Types
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.
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.
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).
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.
Physical and biological controls on Reef Geomorphology
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
Wind-wave, swell, storm directions predict reef types
Geological controls on Reef Geomorphology
relative sea-level changes (coseismic, eustatic)
Darwin Point relative sea-level changes
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- Four Types of Coral Reef Microdocs, Stanford Education. Retrieved January 10, 2010.
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- Types of Coral Reefs at www.coral-reef-info.com. Retrieved 2 Feb 2018.
- Maldives Atolls at www.mymaldives.com. Retrieved 2 Feb 2018.