Atolls– Distribution, Development and

Architecture –

Nicole SchulzTUBAF Institute for Geology

01/24/2007

Abstract

Whether a volcanic island becomes an atoll or a guyot depends on physical

conditions like water temperature, erosion, salinity, sedimentation and sinking

rate. Reefs occur in different sizes and shapes, resulting from the hydrological

and geological conditions that occur in different areas of the tropics. The most

common types of reefs are grouped into three categories: atoll, barrier and fringe

reef. So the distribution of atolls is not very equal at all. Atolls require strong

light and warm waters and are limited in the existing seas to tropical and near-

tropical latitudes. A large percentage of the world’s atolls are contained in the

central and southwestern Pacific and in parts of the Indian Ocean and a number

are found, mostly on continental shelves, in the Caribbean area.

1 Introduction

Atolls are annular reefs with deep cen-tral lagoons (see fig.5) (e.g.Scoffin,1987, Darwin, 1842). They arethe product of the growth of tropi-cal marine organisms, so these islandsare only found in warm tropical wa-ters. Atolls are also a monitor of

eustasy because they provide a sub-siding platform of carbonate deposi-tion that preserves a record of sea-level changes (Quinn & Matthews,1990). Volcanic islands located be-yond the warm water temperaturerequirements of reef building (her-matypic) organisms become gyuots asthey subside and are eroded away at

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the surface. An island that is locatedwhere the ocean water temperaturesare just sufficiently warm for upwardreef growth to keep pace with the rateof subsidence is said to be at the Dar-win Point. For the North Pacific thisPoint is at 28°N on the Hawaiian is-land chain at Kure atoll (see fig:4)(Scoffin, 1987). North of this pointthe coral reefs are drowned and cold-water carbonates veneer them whilethe volcanic island chain is submerg-ing. Islands more polar evolve to-wards seamounts or guyots; islandsmore equatorial evolve towards atolls(see Kure Atoll). A lot of geologi-cal facts can be explained on the ex-

ample of Enewetak Atoll. Thereforemost parts of the chapters Develop-ment and Architecture refer to thisatoll.

2 Distribution

The distribution of atolls (fig:1) islimited to warm tropical and subtrop-ical waters of oceans and seas and isnot uniform. Most of the atolls canbe found in the Pacific Ocean (e.g.Marshall Islands, Coral Sea Islands,Kiribati) and the Indian Ocean (e.g.Outer Islands of the Seychelles, Ma-lidives) (Paul S. Kench, 2006).

Figure 1: The distribution of some examples of atolls. The distribution of atollsaround the globe is instructive: most of the world’s atolls are in the Pacific Ocean(with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, CoralSea Islands, and the island groups of Kiribati and Tuvalu) and Indian Ocean (theMaldives, the Laccadive Islands, the Chagos Archipelago and the Outer Islands of theSeychelles). The Atlantic Ocean has no large groups of atolls, other than eight atollseast of Nicaragua.

Atolls are reduced or absent at thelimits of tropical areas (fig:2), suchas the west coasts of South Americaand Africa, because these are areasof upwelling and the existence of coldcurrents, such as in Peru and SouthAmerica. The Atlantic Ocean has no

appreciable groups of atolls, only inthe region of Bermuda.

The northernmost atoll of theworld is Kure Atoll at 28°24’N, alongwith other atolls of the Northwest-ern Hawaiian Islands. The southern-most atoll of the world is Elizabeth

Atolls – Distribution, Development and Architecture 3

Reef at 29°58’S in the Tasman Sea,which is part of the Coral Sea Is-lands Territory. The next southerlyatoll is Ducie Island in the PitcairnIslands Group, at 24°40’S. Bermudais sometimes claimed as the northern-most atoll at a latitude of 32°24’N. Atthis latitude coral reefs would not de-velop without the warming waters ofthe Gulf Stream. However, Bermudais what is termed a pseudo-atoll be-cause its general form, while resem-bling that of an atoll, has a very dif-ferent mode of formation. The clos-

est atoll to the Equator is Aranuka ofKiribati, with its southern tip just 12km North of the Equator (Darwin,1842).

The largest atolls are found inthe Maldives: Huvadhoo Atoll, hav-ing an area of 2800 km2. Largeatolls are also found in the TuamotuArchipelago. In most cases, the landarea of an atoll is very small incomparison to the total area. Thelargest atoll in the world in terms of’land area’ above sea-level is Kiriti-mati (321.37 km2).

Figure 2: The dynamics of warm currents. Atolls develop exactly between the 20 ℃thermoclines (modified after www.naturlink.pt).

3 Development

Already Charles Darwin was con-cerned about the development ofatolls. To his subsidence theory(Darwin, 1842) atolls develope fromfringing reefs, which are formedaround a volcanic island. When a vol-canic island is build on the oceaniccrust, the lithosphere starts to sagand a wide (tens of kilometers) moatis created around the island. Anyatolls in this zone of depression would

subside more rapidly than normal.Because the lithosphere behaves elas-tically and the subsidence is accom-panied by a much less pronouncedperipheral bulging, any atolls in thiszone of arching would suffer a verti-cal movement and maybe raise oversea-level (Scoffin, 1987).

The fossils identified in the Enewe-tak Atoll drill cores throughout theentire reef thickness were varietiescharacteristic of shallow water only(Wiens, 1959). In reefs rocky sub-

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strate areas can be found, wheremany sessile organisms attach them-selves, and areas of sand, which re-quire different forms of adaptation, inthe same way that there are zonesof strong hydrodynamism and oth-ers that are calm, where currents areminimal. This great diversity of habi-tats is one of the reasons for the greatdiversity of live on the reefs.

A volcanic island begins to sub-side in the course of time (fig:3).When the island becomes an atoll,gradually all traces of basalt aresubmerged beneath the sea surface.Through the process of global warm-ing, glacial melting and/or island sub-sidence, the level of the sea gradu-ally rises ( Guozhong, 1998; Quinn& Matthews, 1990) relative to theseabed and water begins to overtakethe island. Scott & Rotondo(1983) said that it seemed clear thatthe life cycle of Bikini and Enewe-tak Atoll was one of subsidence of

volcanic mounds, reef building, emer-gence during the Miocene, and ero-sion and growth during the fluctuat-ing sea level of the Pleistocene. Ear-liest parts of the reef can be found indepths of 4600 feet below the presentsurface. This can be explained in twodifferent ways. One is that the islandfoundation was subsided since thefirst corals started forming. The otheris that the sea-level rose (Wiens,1959).

If the subsidence does not oc-cur very rapidly, the corals willcontinue to push upward and out-ward well after the volcanic islandis completely submerged (Fager-strom, 1987). Under favorable con-ditions the reefs grow upwards at apace equal to that of the foundationsubsidence, and as the volcanic conesubsides, become separated from thecoast by a progressively deepening la-goon. This is called the barrier reefstage (Scoffin, 1987).

Figure 3: The standard theory of atoll formation states that a volcanic island forms indeep tropical waters, giving coral polyps a foundation to grow on. In time, the volcanobecomes dormant and the island begins to subside (middle). Coral reefs, originallyfringing the edges of the island, become a barrier reef outlining the contour of theoriginal coastline. After the original island slips entirely beneath the waves, all that isleft is a coral atoll. (modified after the Images by Robert Simmon, NASA GSFC)

Quinn & Matthews (1990) men-tioned that the subsidence rate of theEnewetak Atoll is about 39 meters permillion years. Scoffin (1987) saidthat the initial rates of subsidence

(first 2 Ma) of mid-ocean ridges are0.2 mm per year and for mature oceancrust the rate is 0.02 mm per year.Mid-ocean volcanic islands subside asthey cool, and the coral reefs that

Atolls – Distribution, Development and Architecture 5

fringe the shores develop into atolls.Where the island used to be, as-

sorted detritus and dead coral willpile up on the inside of the coral ringand fill the void. For Scoffin (1987)it is possible that the surface ’dish’morphology was enhanced by fresh-water dissolution when the limestonewas exposed to the atmosphere dur-ing times of lowered sea-level.

Another theory says that atollsbuild from cone-shaped reefs, onwhich the corals in the center die offbecause of insufficient water supplyand only the corals at the border con-tinue growing. So the structure be-comes cone-shaped.

3.1 Sedimentation

In Purdy & Gischler (2005) theauthors describe the lateral fillingprocesses of the lagoon by reef-derived sediment in stages and theaggradation and progradation of car-bonate sediments.

The lateral filling is a result of thebilateral progradation that results aswaves breaking on a beach with themajor return flow of water seawardand the minor short-lived advance-ment of the strand line shore ward.They named the table reefs of thesouth seas, which are small coral reefareas without a central island or la-goon, as an example for complete in-filling. For some atolls it is evidentthat the filling process is not neces-sarily dependent on the area and/ordepth of the lagoon. Sand produc-tion, wind-induced current transport,leeward and windward rates of reefdebris production are equal.

The dominant filling factor is thedifference in the production of reefperimeter debris. Since the sea-levelonly rose to its present level of 3 to

6 ky ago, it follows that the presentextent of infilling is minimal becauseprior to that time, the more rapidrate of sea-level rise would have in-sured that vertical sediment aggrada-tion exceeded lateral accretion by awide margin (Purdy & Gischler,2005; Woodroofe et al., 1994).Karst development might be expectedto have occurred several times onatolls and limestone shelves duringperiods of low sea-level in the Pleis-tocene (Scoffin, 1987). The car-bonate production would be greatestin interglacial (re-advance of the sea)on the elevated rims and prominentknolls. The remains of an ancientatoll as a hill in a limestone area iscalled a reef knoll.

3.2 Limiting factors

Reefs are limited by depth and donot develop at depths greater than50–70 meters, most of them beingfound above 25 meters (Guozhong,1998). This restriction is related tothe need for light in coral develop-ment for photosynthesis by zooxan-thoma. If this factor was limited,photosynthesis would be reduced andwith it the capacity of the corals tosecrete calcium carbonate. Quinn& Matthews (1990) used for theirmodel a optimal lag depth of 8 me-ters. The lag depth is the water depthneeded to initiate carbonate sedimen-tation.

Sedimentation, which is related tofresh water drainage, is also consid-ered as a limiting factor in coral de-velopment. Most corals do not sup-port high levels of sediments, not onlybecause these block feeding struc-tures, but because they also reducewater luminosity, making photosyn-thesis difficult. In general, coral

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reef development is greater in areasthat are subjected to strong hydrody-namism (wave action), because thisis a continual source of oxygen andfood, without sedimentation. Con-tinual growth on the outer edge (ofthe reef) by the coral keeps the reefclose to the surface, but in its interior,due to conditions of calm water andhigher sedimentation, this is not pos-sible, which causes the developmentof the lagoon. Sedimentation ratesthat do not keep pace with sea-levelchange result in the preservation ofadditional subaerial exposure surfacesby allowing subsequent intermediatesea-level stillstands tobe recorded asdepositional events.

Salinity is another factor that re-stricts reef development. Typicalcorals are intolerant of salinity lev-els below 25% and above 32%. Onthe Atlantic coast of South America,reefs are absent, due to the discharge

of fresh water from the Amazon andOrinoco rivers.

Coral exposure is also important.Mucus secretion can prevent dehydra-tion, but only when exposure occursfor a short periode, and corals willnot survive being out of the water formore than two hours. Mucus is a vis-cous, slippery substance that consistschiefly of mucin, water, cells, and in-organic salts and is secreted as a pro-tective lubricant coating by cells andglands of the mucous membranes.

When the plates movement even-tually carries the atoll into colderwater, where the corals cannot sur-vive, the rate of carbonate depostionmay become slower and the coral’sgrowth cannot keep up with the is-lands subsidence (Scoffin, 1987).When the coral dies, only a guyot (aflat-topped submerged seamount) re-mains of the former high volcanic is-land (see fig: 4).

Figure 4: Schematic illustration of the Hawaiian-Emperor chain migrating on thePacific plate towards the Kamchatka trench. At a latitude of 28°N (the Darwin Point)reef growth slows to a rate that cannot keep up with subsidence, and guyots result(Scoffin, 1987).

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4 Architecture

Wardlaw et al. (1991) describesthe Enewetak Atoll (fig: 5) as fol-lowed: It is circumscribed by a bar-rier reef and a chain of small sand is-lands are bordering an acorn-shapedlagoon. These islands are concen-trated on the northeastern, easternand southern sides of the atoll. The

barrier reef is cut by three passagesor channels that lead from the openocean into the lagoon. But also wa-ter spills over the reef into the lagoonbecause the barrier reef flats are shal-low. The reefs of the atoll ring areflat, pavementlike areas, large parts ofwhich, particularly along the seawardmargin, may be exposed at times oflow tide.

Figure 5: Enewetak Atoll, Republic of the Marshall Islands – with location of mainpassages and channels, the rim, islands and the coral pinnacles MACK and OSCAR,(modified after http:/eed.llnl.gov/mi/enewetak.php)

The reefs vary in width from nar-row ribbons to broad bulging areasmore than a mile (1.6 km) across.The structures form a most effectivebaffle that robs the incoming wavesof much of their destructive power,and at the same time brings a con-stant supply of refreshing sea water

with oxygen, food, and nutrient saltsto wide expanses of the reef (Parker& Parker, 1994). Figure 6 showsthe effect of water flow around thereef. It points out that the corals onthe leeward side of the atoll get lessnutrients, grow slower and thereforebuild a thinner reef.

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Figure 6: The flow of water around the reefs influencing the growth of atoll. Forexample, prevailing winds can agitate water on one side of the island, stimulating coralgrowth. The red arrows in the image indicate the prevailing wind direction. On thelee side of the island the corals do not receive as many nutrients, resulting in slowergrowth and a thinner reef. ( Image by Robert Simmon, NASA GSFC)

Emery et al. (1954) divides thelagoon in four bathymetric features:1) lagoon terrace, 2) lagoon basin, 3)coral ’knolls’ or pinnacle reefs, 4) reefchannels or passages.

The rim is formed by the lagoonterrace. The surface of this terrace ispockmarked with scattered, irregulardepressions of 3–4 meters and the wa-ter depth varies from 15 to 22 meters.The lagoon’s water column is isohalinand isotherma and has a salinity of34.4 parts per thousand. The tem-perature averages 27 to 29 ℃ .

The inner slope of the lagoon ter-race is less than 1.5 degrees andseperates the terrace from the lagoonbasin. The floor of the lagoon basinis marked by several thousand coralpinnacle reefs and patch reefs as theterrace and is elliptical, gently undu-lating. This lagoon basin has a depthof 55 meters. This shows that theEnewetak lagoon is relatively deep incomparison with other Pacific atolls(Wiens, 1962).

The main marine connection on thewindward side is the east channel (seefig: 5) and is 1.5 km wide and 55meters deep. But the subtidal envi-

ronments and biotic communities ofthe Enewetak lagoon are still poorlyknown. After Colin (1986) the sub-tidal area can be divided into: 1) alagoon-margin and 2) a deep-lagoonenvironment.

The benthic samples from theEnewetak Atoll can be subdividedinto eight facies: granule, granule–sand, sand–granule, granule–sand–mud, sand, muddy sand, sand–mud,and mud (Wardlaw et al., 1991).

5 Discussion

Lot of theories exists for the develop-ment of atolls and reefs. For exam-ple the Glacial Control Theory or theMurray Theory (more in Engeln,1942).

Reef-building corals thrive onlywhere a number of specific environ-mental conditions are satisfied (En-geln, 1942). It seemed to be clearthat the island-types result primar-ily from the subsidence of volcanic is-lands due to the tangential motion ofthe lithospheric plate (e.g. Scott &Rotondo, 1983). The almost com-plete absent of atolls north of 28°N

in the Pacific is considered to be dueto the combined action of rapid sub-aerial erosion of basalts and this tan-gential component.

Around the 1950s, bore holes weredrilled in the Marshall Islands (Eni-wetok atoll), to a depth of 1283meters, where volcanic rock (olivinebasalt) was encountered, thus con-firming Darwin’s theory, a hundredyears later. This theory connectsthe three types of reef (atoll, barrierand fringe reef) in an evolutionary se-quence, but does not explain, how-ever, all the barrier and fringe reefs.

Some of the studies of atolls havebeen made because of the long-termeffects of the radionuclides in the sed-iments on the food chain and theirconsequent impact on relocated hu-man populations after nuclear tests(Wardlaw et al., 1991 and Ward-law & Henry, 1990).

In Paul S. Kench (2006) the au-thors described the effect of tsunamison the system of atolls as minor. Mor-phological and sedimentary evidencesuggests that although tsunamis dogenerate both erosional and deposi-tional signatures.

To the actual state of research thestructure and development of atolls isalready not clear. Alone the fact thatnearly every atoll build up in anotherway because of the different condi-tions makes the research for a univer-sal model complicate. So a combina-tion of several theories may providethe general answer that is sought forthe atoll-reef problem.

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Engeln, O.D. (1942): Geomorphol-ogy. The Macmillan Company,New York.

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