The Pacific islands are often classified as continental and oceanic types, but scientists are not always agreed on the meaning of the terms. The geologists commonly consider an island to be continental if it contains rocks like gneiss, schist, or granite, believed to have been formed under conditions of great heat and pressure that occur only on the continents or along their borders. By this definition New Caledonia and the larger islands of Fiji would be continental islands because of the metamorphic or deep-seated igneous rocks found there. The oceanic islands would be those remote from continents and would be essentially of volcanic and coralline origin. The biologist bases his division of islands into continental and oceanic on floristic and faunistic evidence. If floras and faunas closely resemble those of a near-by continent the island is called continental, whereas if there is dissimilarity in life forms, including some endemic (local) species on the island, it is considered oceanic.
In the central and south Pacific there is a striking contrast between the "high" islands of volcanic origin and the "low" islands built of reef limestone. Special interest has been aroused by the curious coral atolls, which typically are of a ring shape around a central lagoon.
Reefs are made by different types of lime-secreting organisms, including: (1) polyps, which are the most important coral-forming animals and the chief source of coral reefs; (2) the soft corals, Millepora and Heliopora, which are varieties of hydroids; and (3) some of the bryozoans, which, although of small importance today, in rocks of the Paleozoic era made up more than half the volume of some limestone beds.
The remains of the lime-secreting animals are consolidated into rock by deposits made by organisms and chemical precipitation from solution. The chief living and growing part of a coral reef is the outer side, which is but a small part of the total volume of the reef. On the inner side of the reef there is relatively little living coral; rather there are likely to be many boulders broken off by waves from the outer edge. Erosion of material from the reef and its distribution seaward by the waves make a slope on which corals can establish themselves and grow, thereby extending the reef oceanward.
Coral reefs are called fringing when they form a platform, which may be hundreds of feet wide, between the shore and deep water. A barrier reef is at a distance from the shore, from which it is separated by a lagoon that may be several miles wide or only a few hundred feet. Barrier reefs are commonly interrupted by passages or channels, sometimes deep enough for boats. Sometimes fringing and barrier reefs grade together. The Great Barrier Reef of Australia extends more than 1200 miles along nearly the whole length of Queensland to reefs off the coast of New Guinea. The barrier reef of New Caledonia extends beyond the limits of the island and is almost 400 miles long.
Atolls vary greatly in size but are similar in their low elevation and narrow width of land surrounding a central lagoon. Many atolls are only a few miles in diameter, but a few are huge affairs as much as 50 to 100 miles across. Kwajalein in the Marshall Islands is about 90 miles long in a curving line by 20 miles wide. Atolls as small as 1 to 2 miles in diameter are rare. Atolls are rarely circular; generally one diameter is longer than another. For example, Canton Island ( Fig. 9 ), one of the Phoenix Islands and used as an airbase, has a central lagoon that measures about 4 miles by 9 miles. Eniwetok in the Marshall group is a near circle with a diameter of 20 miles. Many atolls are highly irregular in shape; Butaritari in the Gilberts and Christmas in the Line Islands are examples. Jaluit, in the Marshalls, is triangular in shape and about 12 by 30 miles in dimensions. Very few atoll rings are unbroken. Most of the rings are composed of many motus (islets) connected by reefs, some of which are permanently submerged though others are submerged only at high tide. The majority of the motus are on the windward side of atolls. The land is everywhere narrow compared to the width of the lagoon--a few yards or hundreds of feet for width of the land compared to miles of lagoon water. Since the ground above sea level consists of narrow strips, the area of land in an atoll is inconsiderable compared with that of its lagoon. Manihiki, one of the Cook Islands, is a typical atoll with a lagoon about 5 miles across and has a land area of 2 square miles. Names are commonly given to the motus or fragmental islets composing an atoll as well as to the atoll itself. Wake Island has three islets: Wake, Wilkes, and Peale. Tarawa has eight islets, and Funafuti has a score or more of islets, each of them named, around its 10by-15-mile lagoon.
Because of their low altitude the problem of drinking water is acute on atolls, and any ground water available is in scant supply and often brackish in quality. The potable water is merely a thin lens of reasonably fresh water floating on the denser salt water that impregnates the rock at and below sea level.
Numerous raised atoll islands are known. The central lagoon has disappeared from such islands, although it may be represented by a hollow, and there is a greater land area, deeper soil, more vegetation, and usually more adequate and fresher ground water than on an atoll of the same diameter. Makatea ( Aurora) in the Tuamotus, Nauru, Ocean, Johnston, Baker, Howland, and Marcus are examples of such islands. Given the same rainfall, man finds living conditions much more favorable in raised islands than on atolls. Some atolls have been only partly raised, and in Wake a portion of the lagoon has survived. Mangaia, one of the Cook Islands, has a central hill of basalt surrounded by a cavernous terrace of raised coral reef.
Some high islands of volcanic or other origin contain limestone as the result of the raising of coral reefs--proof that the land has been elevated. Guam is an example.
Some authorities have applied the term complex island to types like Truk in the Carolines in which volcanic islands rise from the lagoon inside the reefs and islets of an atoll rim, but others have called this type an almost-atoll.