The Origin of Atolls

Authorities differ in their ideas about the origin of coral atolls. Among the leading theories is Charles Darwin's. His theory supposes three stages. (1) The coral builds a fringing reef around a tropical high island, which if volcanic would customarily have an oval shape. (2) The island slowly sinks, and the coral reef grows upward as fast as the rate of sinking to form a barrier reef separated from the shore by a lagoon. (3) Continued subsidence of the island causes it to disappear by submergence, and the growth upward of the coral perpetuates the reef, which becomes an atoll with a lagoon occupying the former land area whose outline is reflected in the roughly oval rim of land. The late William Morris Davis was a strong supporter of Darwin's theory and pointed out, as Dana had previously done, that many islands surrounded by barrier reefs show drowning of the lower parts of valleys. This condition is best explained by submergence of the island. A boring at Funafuti found nothing but coral rock to depths of more than 1000 feet, which is below the limit of growth of reef corals and thus supports Darwin's theory of subsidence. In 1947 a drillhole on Bikini atoll was carried to a depth of 2556 feet, wholly in calcareous materials, some of which carried fossils of the Tertiary age. From geophysical evidence Ladd and Tracey state that the depth to the basement on which Bikini is built would be about 8000 feet. If coral has grown to such a thickness as the area slowly sank, it appears that a profound sinking of the ocean floor has taken place in this part of the Pacific.

Examples of high islands with barrier reefs whose coastlines are embayed, possibly by subsidence, are New Caledonia and some of the Fiji, Society, and Cook islands. However, not all the Pacific area has sunk. Stearns found evidence of both rising and sinking in Hawaii, and Fiji has raised beaches and reefs.

Another theory, which received support from Alexander Agassiz, is that of Sir John Murray, who believed that coral grew upward from shallow submarine mounds or banks that were supposed to represent the accumulated debris or skeletons of lime-secreting organisms. The maximum growth would be on the outside of the reef, and the foundation on which the reef grows would in part be talus broken from the reef itself. Murray explained the central lagoon as formed largely by solution because the coral toward the center of the lagoon would lack food and die and the dead coral could be dissolved to form the lagoon. No change in sea level is necessary according to this theory. Murray suggested that around high islands the reefs grew outward from the shore on the talus and that the lagoons were formed by sea water dissolving the inner, dead portion of the reef. However, from studies of the Florida keys, Vaughn concluded that lime deposition rather than solution was taking place and that, because of the absence of dissolved carbon dioxide in the sea water, solution of the coral limestone by virtue of free carbon dioxide would be impossible.

It has been suggested by Daly that during the glacial period the level of the sea was lowered when millions of cubic miles of ice were locked up on the land in the vast continental glaciers. The temperature of ocean water might then have been too low for coral to live in areas where it now flourishes, and without their protective coral reefs certain islands could be eroded away below sea level by the waves, especially since storms may have been more frequent and powerful in the tropics then than now. When the glaciers melted, the ocean rose perhaps about 200 feet, the banks were submerged more deeply, river mouths were drowned, and where islands had been truncated by marine erosion the corals were reestablished as the water became warm enough for them, and the coral reefs then grew upward from the banks. Daly considered the coral deposits to be merely a veneer on top of the eroded surface of other rocks, but the borings on Funafuti and Bikini do not support his hypothesis. On the other hand the extreme flatness of the floor of the lagoons is what would be expected from marine planation.

It seems probable that not all coral atolls are formed in the same way, and that at least some of the curious islands have been formed by other methods or combinations of them.

No comments: