Origin of coral reefs and islands

Abstract

Dana interprets the previous work on coral reefs on Pacific islands as supporting Darwin9s (1842) theory of subsidence of coral reef islands. He states that the island of Hawaii, although growing little coral, has undergone more subsidence than the coral reef end of the chain, the volcanic activity more than keeping pace with the subsidence to yield a submarine continuation of the shield form as revealed by deep soundings fairly close to the island. These define a 4 degrees 309 slope, inferred to be that of a constructional shield. Dana emphasizes the distinctions that must be made to interpret coral deposits between porous beach deposits made by subaerial denudation, and the compact coral reef rock made underwater by living organisms. He suggests that firm evidence of subsidence could be obtained by identifying beach deposits underlying coral reef rock and suggests core drilling of atolls to find such evidence. In part 2, Dana considers the following objections to Darwin9s theory: I. Darwin9s insufficient knowledge of the facts bearing on the subject. II. Subsidence not ordinarily a fact because methods of producing barrier reefs and atolls have been brought forward that do not require its aid. III. The occurrence of cases of elevation in regions of atolls and barrier reefs inconsistent with the subsidence theory. IV. No ancient coral reefs in the geological series have the great thickness attributed by the subsidence-theory to modern reefs. V. Other methods of explanation and their supporting evidence. Under III, he cites Professor Karl Semper (1868, Zeitschrift Wissenschaft Zoologie, v. 13, p. 558), who objects to subsidence on islands that have reefs raised from 200 to 250 feet above sea level as "a cumbrous and entirely hypothetical series of upward and downward movements." Semper also argues that the strongest wind waves are too feeble to break off and lift large masses of reef, some 10 feet thick. Dana counters: "But the difficulty does not exist in fact: for earthquakes may have made the waves.," citing the proximity of the Pelew island to the tectonically and volcanically active areas to the west, including the Philippines and Krakatau. Dana concludes by evaluating all counter-claims to subsidence, finding none to irreconcilably conflict with Darwin, and much to support him.