Coral atoll

“ The coral reefs that grew around the shore line of the islands still remain after the islands have washed away, and thus the living reefs now mark the contours of the islands as they were. The currents flowing in and out of openings in the reef-rim have deepened the lagoons, but nevertheless there are many coral heads growing in the lagoon of every coral atoll. ”

“ As far as the actual reef of coral is concerned, there is not the smallest difference in general size, outline, grouping, and even in quite trifling details of structure, between a barrier and an atoll. The geographer Balbi has well remarked that an encircled island is an atoll with high land rising out of its lagoon; remove the land from within, and a perfect atoll is left.
But what has caused these reefs to spring up at such great distances from the shores of the included islands? It cannot be that the corals will not grow close to the land ”

“ The case of the schooner-channel choked up with coral in an interval of less than ten years, in the lagoon of Keeling atoll, should be here borne in mind. We may also infer, from the trouble which the inhabitants of the Maldiva atolls take to root out, as they express it, the coral-knolls from their harbours, that their growth can hardly be very slow. ”

“ As the reef approaches the surface the corals of the inner area are smothered by silt and starved, and their Submarine Volcanic Peak hard parts are dissolved and scoured away; while those of the circumference, with abundant food supply, nourish and build the ring of the atoll. Atolls may be produced also by the backward drift of sand from either end of a crescentic coral reef or island, the spits uniting in the quiet water of the lee to inclose a lagoon. ”

“ The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds, since the coral growth is there slower than where the waves constantly renew the polyps’ food supply. The conical pedestal rising from the depths is frequently a submarine volcanic cone or island, though any submerged peak may be crowned by an atoll. For the theory of atoll formation see Coral-reefs. ”

“ There are atoll-islands where ships can enter, through some break in the ring, into the inner circle. They find themselves in a perfect harbor, in a sheltered lake which no wave can ever enter, yet deep enough and wide enough to hold all the navies of the world. Round about on every side there are the dazzling beaches which are composed of coral sand, and crowning these there is the peaceful cocoanut-palm, and a lower jungle of dense tropical vegetation. On landing and exploring the woods and shores, nothing can be seen but coral. The whole island is a ring of this purely marine product ”

“ It is unfortunate that the upraised coral-islands in the Pacific have not been examined by a geologist. The cliffs of Elizabeth Island, in the Low Archipelago, are eighty feet high, and appear, from Captain Beechey’s description, to consist of a homogeneous coral-rock. From the isolated position of this island, we may safely infer that it is an upraised atoll, and therefore that it has been formed by masses of coral, grown together. ”

“ In time our coral reef may become a beautiful tropical island fringed with waving trees and plants, and inhabited by man.
Fig. 2.—An Atoll.
Circular islands seldom form complete rings. There is generally an opening into the lake on the side most sheltered from the wind. A safe harbor in mid-ocean is thus made, in which vessels may take shelter, but it requires an expert navigator to pass the perils at its entrance. To anchor on the outer shore would be impossible. In Fig. 2 is a pretty little coral island with ships in its lagoon. ”

“ Some of the atolls, in the larger archipelagoes, stand so near to each other, and have such an evident relationship in form, that they compose little sub-groups: in the Caroline Archipelago, one such sub-group consists of Pouynipete, a lofty island encircled by a barrier-reef, and separated by a channel only four miles and a half wide from Andeema atoll, with a second atoll a little further off. In all these respects an examination of a series of charts will show how perfectly groups of atolls resemble groups of common islands. ”

“ In this course we have seen, that in the lapse of ages encircling barrier-reefs are occasionally converted into atolls, the name of atoll being properly applicable, at the moment when the last pinnacle of encircled land sinks beneath the surface of the sea. We have, also, seen that large atolls during the progressive subsidence of the areas in which they stand, sometimes become dissevered into smaller ones; at other times, the reef-building polypifers having entirely perished, atolls are converted into atoll-formed banks of dead rock ”

“ In the Pacific, the Loo Choo Islands, in latitude 27° N., have reefs on their shores, and there is an atoll in 28° 30′, situated N.W. of the Sandwich Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In the southern hemisphere coral-reefs do not extend so far from the equatorial sea. In the Southern Pacific there are only a few reefs beyond the line of the tropics, but Houtmans Abrolhos, on the western shores of Australia in latitude 29° S., are of coral formation. ”

“ I have stated that the outer margin of the Maldiva atolls consists of living corals (some of which, if not all, are of the same species with those at Keeling atoll) , and here the surf is so tremendous, that even large ships have been thrown, by a single heave of the sea, high and dry on the reef, all on board thus escaping with their lives.
Ehrenberg [6] remarks, that in the Red Sea the strongest corals live on the outer reefs, and appear to love the surf; he adds, that the more branched kinds abound a little way within, but that even these in still more protected places, become smaller. ”

“ There are some atoll-formed reefs, rising to the surface of the sea and partly dry at low water, on which from some cause islets have never been formed; and there are others on which they have been formed, but have subsequently been worn away. In atolls of small dimensions the islets frequently become united into a single horse-shoe or ring-formed strip; but Diego Garcia, although an atoll of considerable size, being thirteen miles and a half in length, has its lagoon entirely surrounded, except at the northern end, by a belt of land, on an average a third of a mile in width. ”

“ The southern group consists of seven or eight islands and islets on the atoll, which is the skeleton of what some day, according to the history of coral reefs, will be a continuous island. North Keeling has no harbor, is seldom visited, and is of no importance. ”

“ From this masses of coral, sand, stones or large boulders, are always falling on to the foundation slopes, forming successive sloping layers indicated by the dotted lines, upon which fresh growth takes its rise [57] . When the coral reef has become of some breadth [116] (and atoll rings may be 30 miles or more across) a boring at the edge might descend for a thousand fathoms and never meet with the original foundations, but would pass only through recent corals fallen from the shallow zone. ”

“ There are three great classes of coral reefs—atoll, barrier, and fringing. Now, the utmost depth at which corals can construct reefs is between twenty and thirty fathoms, so that wherever there is an atoll a foundation must have originally existed within a depth of from twenty to thirty fathoms from the surface. The coral formation is raised only to that height to which the waves can throw up fragments and the winds pile up sand. The foundation, such as a mountain peak, therefore, must have sunk to the required level, and not have been raised, as has hitherto been generally supposed. ”

“ He led the way toward the wreck, Floyd and Isbel following.
The coral islands of the Pacific may be roughly divided into two classes: compound islands—that is to say, islands made of solid land and surrounded by a coral ring or breakwater, and simple islands or atolls—that is, simple rings of coral inclosing lagoons.
Then we have occasionally a third variety, an atoll island in whose lagoon one finds several islets. ”

“ Darwin's great work upon the coral reefs, you will see a very beautiful chart of the world, which he prepared with great pains and labour, showing the distribution on the one hand of the reefs, and on the other of the volcanoes; you will find that in no case does the atoll accompany the volcano, or the volcano burst up among the atolls. It is most instructive to look at the great area of the Pacific on the map, and see the great masses of atolls forming in one region of it a most enormous belt, running from north-west to south-east ”

“ Although the limit of depth, at which each particular kind of coral ceases to exist, is far from being accurately known; yet when we bear in mind the manner in which the clumps of coral gradually became infrequent at about the same depth, and wholly disappeared at a greater depth than twenty fathoms, on the slope round Keeling atoll, on the leeward side of the Mauritius, and at rather less depth, both without and within the atolls of the Maldiva and Chagos Archipelagoes ”

“ The inhabitants of the extensive Paumoto group of islands give a striking example of the dreary monotony of life on small coral islands. Indeed, coral atolls are lacking in pretty nearly all the features that are necessary for a high degree of civilization; nature, therefore, reacts, with the result that the human life of this region is in a condition of savagery. Many of the natives are cannibals.
The natives of Australia are a race that seems to be separate and distinct in itself. ”

“ If they settle about an island on all sides of which the conditions for their growth are equally favorable, they will raise a wall all around it, thus encircling it with a ring of Coral growth. The Athols in the Pacific Ocean, those circular islands inclosing sometimes a fresh-water lake in mid-ocean, are Coral walls of this kind, that have formed a ring around a central island. This is easily understood, if we remember that the bottom of the Pacific Ocean is by no means a stable foundation for such a structure. ”

“ There is considerable beauty in a small coral reef, when seen from a ship's mast-head, at a short distance, in clear weather. A small island with a white sand-beach and a tuft of trees, is surrounded by a symmetrically oval space of shallow water, of a bright grass-green colour, enclosed by a ring of glittering surf as white as snow; immediately outside of which is the rich dark blue of deep water. All the sea is perfectly clear from any mixture of sand or mud. It is this perfect clearness of the water which renders navigation among coral reefs at all practicable ”

“ These animals cannot exist at a greater depth in the sea than about ten fathoms, and as the Coral Islands often rise with great steepness from a sea more than three hundred fathoms deep, it would seem that a great alteration must have taken place in the depth of the ocean since the time when these little architects commenced their labours. Throughout the whole range of the Polynesian and Australasian islands, there is scarcely a league of sea unoccupied by a coral reef, or a coral island ”

“ On the weather-side, exposed to the long sweep of the ocean-rollers, there are but short, black-looking reefs backed by irregular piles of loose, flat, sea-worn coral, thrown up and accumulating till its surface is brushed by the pendant leaves of the cocoanuts, only to be washed and swirled back seawards when the wind comes from the westward and sends a fierce sweeping current along the white beaches and black coral rocks alike. ”

“ The expanse of water inside when not occupied by land usually exhibits a depth of from 15 to 30 fathoms, with numerous sunken patches of coral, and several reefs which partially dry at low-water. The shores of the islands also are generally protected by fringing coral reefs, the largest of which is that extending off the west and south side of Piron Island to a distance of seven or eight miles, with a well defined border towards Coral Haven. ”

“ Like Mangaia, Mitiaro, and Atiu, this island is of mixed volcanic and raised-coral origin—the pinnacle of a submerged peak, ringed with millions of tons of coral, and without any lagoon worthy of the name. The polyps have built a sort of platform around the land, low inshore and highest—as seems usually the case—just before it drops off into the sea. Breaching across the outer ridge, the surf fills a narrow belt of shallows between it and the shore; the result is a miniature edition of a lagoon—a place of rocky pools where children wade knee-deep, on the lookout for crayfish and baby octopus. ”

“ The meridian sun was pouring down its fiercest rays, and he was thinking of returning to his tent and the grateful shade of the palm-trees, when, just as he had rounded the jagged spur of a particularly ugly-looking coral reef, he suddenly saw before him a deep, dark line of perfectly smooth water, over-arched by a natural bridge of grayish-white limestone, and flowing, as it seemed to him, directly under the island. ”

“ In the eastern half of the Pacific, the Sandwich Islands are all fringed, and almost every naturalist who has visited them, has remarked on the abundance of elevated corals and shells, apparently identical with living species. The Rev. W. Ellis informs me, that he has noticed round several parts of Hawaii, beds of coral-detritus, about twenty feet above the level of the sea, and where the coast is low they extend far inland. Upraised coral-rock forms a considerable part of the borders of Oahu; and at Elizabeth Island [8] it composes three strata, each about ten feet thick. ”