Coral-Reefs

Coral-Reefs. Many species of coral (q.v.) are widely distributed, and are found at all depths both in warmer and colder seas. Lophohelia prolifera and Dendrophyllia ramea form dense beds at a depth of from 100 to 200 fathoms off the coasts of Norway, Scotland and Portugal, and the “Challenger” and other deep-sea dredging expeditions have brought up corals from great depths in the Pacific and Atlantic oceans. But the larger number of species, particularly the more massive kinds, occur only in tropical seas in shallow waters, whose mean temperature does not fall below 68° Fahr., and they do not flourish unless the temperature is considerably higher. These conditions of temperature are found in a belt of ocean which may roughly be indicated as lying between the 28th N. and S. parallels. Within these limits there are numerous reefs and islands formed of coral intermixed with the calcareous skeletons of other animals, and their formation has long been a matter of dispute among naturalists and geologists.

Coral formations may be classed as fringing or shore reefs, barrier reefs and atolls. Fringing reefs are platforms of coral rock extending no great distance from the shores of a continent or island. The seaward edge of the platform is usually somewhat higher than the inner part, and is often awash at low water. It is intersected by numerous creeks and channels, especially opposite those places where streams of fresh water flow down from the land, and there is usually a channel deep enough to be navigable by small boats between the edge of the reef and the land. The outer wall of the reef is rather steep, but descends into a comparatively shallow sea. Since corals are killed by fresh water or by deposition of mud or sand, it is obvious that the outer edge of the reef is the region of most active coral growth, and the boat channel and the passages leading into it from the open sea have been formed by the suppression of coral growth by one of the above-mentioned causes, assisted by the scour of the tides and the solvent action of sea-water. Barrier reefs may be regarded as fringing reefs on a large scale. The great Australian barrier reef extends for no less a distance than 1250 m. from Torres Strait in 9.5° S. lat. to Lady Elliot island in 24° S. lat. The outer edge of a barrier reef is much farther from the shore than that of a fringing reef, and the channel between it and the land is much deeper. Opposite Cape York the seaward edge of the great Australian barrier reef is nearly 90 m. distant from the coast, and the maximum depth of the channel at this point is nearly 20 fathoms. As is the case in a fringing reef, the outer edge of a barrier reef is in many places awash at low tides, and masses of dead coral and sand may be piled up on it by the action of the waves, so that islets are formed which in time are covered with vegetation. These islets may coalesce and form a strip of dry land lying some hundred yards or less from the extreme outer edge of the reef, and separated by a wide channel from the mainland. Where the barrier reef is not far from the land there are always gaps in it opposite the mouths of rivers or considerable streams. The outer wall of a barrier reef is steep, and frequently, though not always, descends abruptly into great depths. In many cases in the Pacific Ocean a barrier reef surrounds one or more island peaks, and the strips of land on the edge of the reef may encircle the peaks with a nearly complete ring. An atoll is a ring-shaped reef, either awash at low tide or surmounted by several islets, or more rarely by a complete strip of dry land surrounding a central lagoon. The outer wall of an atoll generally descends with a very steep but irregular slope to a depth of 500 fathoms or more, but the lagoon is seldom more than 20 fathoms deep, and may be much less. Frequently, especially to the leeward side of an atoll, there may be one or more navigable passages leading from the lagoon to the open sea.

Though corals flourish everywhere under suitable conditions in tropical seas, coral reefs and atolls are by no means universal in the torrid zone. The Atlantic Ocean is remarkably free from coral formations, though there are numerous reefs in the West Indian islands, off the south coast of Florida, and on the coast of Brazil. The Bermudas also are coral formations, their high land being formed by sand accumulated by the wind and cemented into rock, and are remarkable for being the farthest removed from the equator of any recent reefs, being situated in 32° N. lat. In the Pacific Ocean there is a vast area thickly dotted with coral formations, extending from 5° N. lat. to 25° S. lat., and from 130° E. long, to 145° W. long. There are also extensive reefs in the westernmost islands of the Hawaiian group in about 25° N. lat. In the Indian Ocean, the Laccadive and Maldive islands are large groups of atolls off the west and south-west of India. Still farther south is the Chagos group of atolls, and there are numerous reefs off the north coast of Madagascar, at Mauritius, Bourbon and the Seychelles. The Cocos-Keeling Islands, in 12° S. lat. and 96° E. long., are typical atolls in the eastern part of the Indian Ocean.

Diagram showing the formation of an atoll during subsidence. (After C. Darwin.) The lower part of the figure represents a barrier reef surrounding a central peak.

A, A, outer edges of the barrier reef at the sea-level; the coco-nut trees indicate dry land formed on the edges of the reef.

L, L, lagoon channel.

A’, A’, outer edges of the atoll formed by upgrowth of the coral during the subsidence of the peak.

L’, lagoon of the atoll.

The vertical scale is considerably exaggerated as compared with the horizontal scale.

The remarkable characters of barrier reefs and atolls, their isolated position in the midst of the great oceans the seemingly unfathomable depths from which they rise their peaceful and shallow lagoons and inner channels, their narrow strips of land covered with coco-nut palms and other vegetation, and rising but a few feet above the level of the ocean, naturally attracted the attention of the earlier navigators, who formed sundry speculations as to their origin. The poet-naturalist, A. von Chamisso, was the first to propound a definite theory of the origin of atolls and encircling reefs, attributing their peculiar features to the natural growth of corals and the action of the waves. He pointed out that the larger and more massive species of corals flourish best on the outer sides of a reef, whilst the more interior corals are killed or stunted in growth by the accumulation of coral and other debris. Thus the outer edge of a submerged reef is the first to reach the surface, and a ring of land being formed by materials piled up by the waves, an atoll with a central lagoon is produced. Chamisso’s theory necessarily assumed the existence of a great number of submerged banks reaching nearly, but not quite, to the surface of the sea in the Pacific and Indian oceans, and the difficulty of accounting for the existence of so many of these led C. Darwin to reject his views and bring forward an explanation which may be called the theory of subsidence. Starting from the well-known premise that reef-building species of corals do not flourish in a greater depth of water than 20 fathoms, Darwin argued that all coral islands must have a rocky base, and that it was inconceivable that, in such large tracts of sea as occur in the Pacific and Indian oceans, there should be a vast number of submarine peaks or banks all rising to within 20 or 30 fathoms of the surface and none emerging above it. But on the supposition that the atolls and encircling reefs were formed round land which was undergoing a slow movement of subsidence, their structure could easily be explained. Take the case of an island consisting of a single high peak. At first the coral growth would form a fringing reef clinging to its shores. As the island slowly subsided into the ocean the upward growth of coral would keep the outer rim of the reef level with or within a few fathoms of the surface, so that, as subsidence proceeded, the distance between the outer rim of the reef and the sinking land would continually increase, with the result that a barrier-reef would be formed separated by a wide channel from the central peak. As corals and other organisms with calcareous skeletons live in the channel, their remains, as well as the accumulation of coral and other debris thrown over the outer edge of the reef, would maintain the channel at a shallower depth than that of the ocean outside. Finally, if the subsidence continued, the central peak would disappear beneath the surface, and an atoll would be left consisting of a raised margin of reef surrounding a central lagoon, and any pause during the movement of subsidence would result in the formation of raised islets or a strip of dry land along the margin of the reef. Darwin’s theory was published in 1842, and found almost universal acceptance, both because of its simplicity and its applicability to every known type of coral-reef formation, including such difficult cases as the Great Chagos Bank, a huge submerged atoll in the Indian Ocean.

Darwin’s theory was adopted and strengthened by J. D. Dana, who had made extensive observations among the Pacific coral reefs between 1838 and 1842, but it was not long before it was attacked by other observers. In 1851 Louis Agassiz produced evidence to show that the reefs off the south coast of Florida were not formed during subsidence, and in 1863 Karl Semper showed that in the Pelew islands there is abundant evidence of recent upheaval in a region where both atolls and barrier-reefs exist. Latterly, many instances of recently upraised coral formations have been described by H. B. Guppy, J. S. Gardiner and others, and Alexander Agassiz and Sir J. Murray have brought forward a mass of evidence tending to shake the subsidence theory to its foundations. Murray has pointed out that the deep-sea soundings of the “Tuscarora” and “Challenger” have proved the existence of a large number of submarine elevations rising out of a depth of 2000 fathoms or more to within a few hundred fathoms of the surface. The existence of such banks was unknown to Darwin, and removes his objections to Chamisso’s theory. For although they may at first be too far below the surface for reef-building corals, they afford a habitat for numerous echinoderms, molluscs, crustacea and deep-sea corals, whose skeletons accumulate on their summits, and they further receive a constant rain of the calcareous and silicious skeletons of minute organisms which teem in the waters above. By these agencies the banks are gradually raised to the lowest depth at which reef-building corals can flourish, and once these establish themselves they will grow more rapidly on the periphery of the bank, because they are more favourably situated as regards food-supply. Thus the reef will rise to the surface as an atoll, and the nearer it approaches the surface the more will the corals on the exterior faces be favoured, and the more will those in the centre of the reef decrease, for experiment has shown that the minute pelagic organisms on which corals feed are far less abundant in a lagoon than in the sea outside. Eventually, as the margin of the reef rises to the surface and material is accumulated upon it to form islets or continuous land, the coral growth in the lagoon will be feeble, and the solvent action of sea-water and the scour of the tide will tend to deepen the lagoon. Thus the considerable depth of some lagoons, amounting to 40 or 50 fathoms, may be accounted for. The observations of Guppy in the Solomon islands have gone far to confirm Murray’s conclusions, since he found in the islands of Ugi, Santa Anna and Treasury and Stirling islands unmistakable evidences of a nucleus of volcanic rock, covered with soft earthy bedded deposits several hundred feet thick. These deposits are highly fossiliferous in parts, and contain the remains of pteropods, lamellibranchs and echinoderms, embedded in a foraminiferous deposit mixed with volcanic debris, like the deep-sea muds brought up by the “Challenger.” The flanks of these elevated beds are covered with coralline limestone rocks varying from 100 to 16 ft. in thickness. One of the islands, Santa Anna, has the form of an upraised atoll, with a mass of coral limestone 80 ft. in vertical thickness, resting on a friable and sparingly argillaceous rock resembling a deep-sea deposit. A. Agassiz, in a number of important researches on the Florida reefs, the Bahamas, the Bermudas, the Fiji islands and the Great Barrier Reef of Australia, has further shown that many of the peculiar features of these coral formations cannot be explained on the theory of subsidence, but are rather attributable to the natural growth of corals on banks formed by prevailing currents, or on extensive shore platforms or submarine flats formed by the erosion of pre-existing land surfaces.

In face of this accumulated evidence, it must be admitted that the subsidence theory of Darwin is inapplicable to a large number of coral reefs and islands, but it is hardly possible to assert, as Murray does, that no atolls or barrier reefs have ever been developed after the manner indicated by Darwin. The most recent research on the structure of coral reefs has also been the most thorough and most convincing. It is obvious that, if Murray’s theory were correct, a bore hole sunk deep into an atoll would pass through some 100 ft. of coral rock, then through a greater or less thickness of argillaceous rock, and finally would penetrate the volcanic rock on which the other materials were deposited. If Darwin’s theory is correct, the boring would pass through a great thickness of coral rock, and finally, if it went deep enough, would pass into the original rock which subsided below the waters. An expedition sent out by the Royal Society of London started in 1896 for the island of Funafuti, a typical atoll of the Ellice group in the Pacific Ocean, with the purpose of making a deep boring to test this question. The first attempt was not successful, for at a depth of 105 ft. the refractory nature of the rock stopped further progress. But a second attempt, under the management of Professor Edgeworth David of Sydney, proved a complete success. With improved apparatus, the boring was carried down to a depth of 697 ft. (116 fathoms), and a third attempt carried it down to 1114 ft. (185 fathoms). The boring proves the existence of a mass of pure limestone of organic origin to the depth of 1114 ft., and there is no trace of any other rock. The organic remains found in the core brought up by the drill consist of corals, foraminifera, calcareous algae and other organisms. A boring was also made from the deck of a ship into the floor of the lagoon, which shows that under 100 ft. of water there exists at the bottom of the lagoon a deposit more than 100 ft. thick, consisting of the remains of a calcareous alga, Halimeda opuntia, mixed with abundant foraminifera. At greater depths, down to 245 ft., the same materials, mixed with the remains of branching madrepores, were met with, and further progress was stopped by the existence of solid masses of coral, fragments of porites, madrepora and heliopora having been brought up in the core. These are shallow-water corals, and their existence at a depth of nearly 46 fathoms, buried beneath a mass of Halimeda and foraminifera, is clear evidence of recent subsidence. Halimeda grows abundantly over the floor of the lagoon of Funafuti, and has been observed in many other lagoons. The writer collected a quantity of it in the lagoon of Diego Garcia in the Chagos group. The boring demonstrates that the lagoon of Funafuti has been filled up to an extent of at least 245 ft. (nearly 41 fathoms), and this fact accords well with Darwin’s theory, but is incompatible with that of Murray. In the present state of our knowledge it seems reasonable to conclude that coral reefs are formed wherever the conditions suitable for growth exist, whether in areas of subsidence, elevation or rest. A considerable number of reefs, at all events, have not been formed in areas of subsidence, and of these the Florida reefs, the Bermudas, the Solomon islands, and possibly the Great Barrier Reef of Australia are examples. Funafuti would appear to have been formed in an area of subsidence, and it is quite probable that the large groups of low-lying islands in the Pacific and Indian oceans have been formed under the same conditions. At the same time, it must be remembered that the atoll or barrier reef shape is not necessarily evidence of formation during subsidence, for the observations of Karl Semper, A. Agassiz, and Guppy are sufficient to prove that these forms of reefs may be produced by the natural growth of coral, modified by the action of waves and currents in regions in which subsidence has certainly not taken place.

See A. Agassiz, many publications in the Mem. Amer. Acad. (1883) and Bull. Mus. Comp. Zool. (Harvard, 1889-1899); J. D. Dana, Corals and Coral Islands (1853; 2nd ed., 1872; 3rd ed., 1890); C. Darwin, The Structure and Distribution of Coral Reefs (3rd ed., 1889); H. B. Guppy, “The Recent Calcareous Formations of the Solomon Group,” Trans. Roy. Soc. Edinb. xxxii. (1885); R. Langenbeck, “Die neueren Forschungen über die Korallenriffe,” Hettner geogr. Zeitsch. iii. (1897); J. Murray, “On the Structure and Origin of Coral Reefs and Islands,” Proc. Roy. Soc. Edinb. x. (1879-1880); J. Murray and Irvine, “On Coral Reefs and other Carbonate of Lime Formations in Modern Seas,” Proc. Roy. Soc. Edinb. (1889); W. Savile Kent, The Great Barrier Reef of Australia (London, W. H. Allen & Co., 1893); Karl Semper, Animal Life, “Internat. Sci. Series,” vol. xxxi. (1881); J. S. Gardiner, Nature, lxix. 371.

(G. C. B.)

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