Darwin’s theory of the formation of coral reefs

Darwin's cross-section explaining coral reef formation. CUL DAR 44, f. 24

It was in the tropical Pacific that Darwin realised he could revise the common understanding of coral formations as well as the taxonomy of coral animals. As he confided to his diary, ‘It is my opinion, that besides the avowed ignorance concerning the tiny architects of each individual species, little is yet known, in spite of the much which has been written, of the structure & origin of the Coral Islands & reefs.’ In his ‘Coral Islands’ manuscript of late 1835, Darwin argued that atolls are formed when islands sink due to the gradual subsidence of the earth’s crust; coral reefs that had originally formed in the shallow waters along the island shore are forced to grow upward so that their tops remain near the surface. Coral Reefs (1842) later became his first monograph and Darwin’s coral theory became the key to his meteoric rise in the science of geology.