Clift Islands in the Coral Seas

Abstract

The great majority of reef-encircled islands in the Coral Seas are of maturely dissected form, with slopes of moderate declivity that descend to an indented shoreline, where the tapering spurs end in salient points and the widening valleys open into branching bays, thus proclaiming that the islands have suffered partial submergence since they were dissected. It is generally agreed by those who accept this interpretation of insular relief in relation to shoreline pattern that the encircling barrier reefs have grown upwards from their beginning as fringing reefs during the subsidence which gave the shoreline its indented outline. Reasons for this conclusion were given in a previous article 'The Origin of Coral Reefs' in these PROCEEDINGS 1, 146-152, 1915; they have been more fully stated in 'A Shaler Memorial Study of Coral Reefs' recently published in the American Journal of Science, 40, 223-271, 1915. The object of the present article is to call attention to certain exceptional reef-encircled islands, Reunion, Tahiti, and New Caledonia, in the Coral Seas, which present clift shore lines, and to suggest an explanation by which their exceptional features may be simply accounted for. If all the islands of the Coral Seas were clift like these three, they would support either the theory of veneering barrier reefs on wave-cut platforms, or the Glacial-control theory of coral reefs. But clift islands are exceptional. Some means of explaining them must be found which will not be in discord with the theory of coral reefs that is supported by the features of non-clift islands, namely, Darwin's theory of intermittent subsidence. It will appear from what follows that the suggested means of explaining the exceptional clift islands of the Coral Seas gives unexpected support to this theory. Imagine a young volcanic island, from which detritus is washed so abundantly into the sea that corals and the associated reef-making organisms can establish themselves if at all, only in short and discontinuous fringing-reef patches around its shore. So long as the island continues to grow by eruption, its height and perimeter will increase; its earlier fringing reef patches will be buried under newer lava flows, agglomerate beds and ash falls. The completed volcanic island will have a shoreline of simple pattern without pronounced salients and reentrants, because the materials ejected from the summit crater are spread rather equably on all sides of the cone, and thus give it a nearly circular base. The sim-