Not sure?  Look it up! EEB/MCB 182.
Lectures by W. M. Schaffer.


Darwin's Theory of Coral Atoll Formation.
 

As described in lecture and in your text, colonial corals secrete calcareous shells. Together with other marine invertebrates (encrusting algae, sponges, etc.), these animals often form extensive structures called coral reefs that are home to fantastically diverse assemblages of plants and animals. Sometimes the reefs grow in shallow water just off shore. These structures are called fringing reefs. In other cases, reefs are found far out to sea. Sometimes they completely surround an island, forming what is known as a barrier reef. In other cases, there is no island at all, just a strip of coral ringing a body of water called a lagoon. All three types of reefs are common in the South Pacific. (Click here for examples.)

The existence of coral atolls in deep waters was a problem for Victorian biologists who knew that the polyps cannot grow at depths below 120 feet. To resolve this dilemma, Darwin proposed a characteriztically uniformitarian solution. Inspired by his firsthand observation of an earthquake in South America and his deduction that the Andes mountains are still rising, and the nearby seafloor, sinking, Darwin imagined a balance between reef expansion and island subsidence.

According to Darwin, coral would grow along the edges of a newly formed volcanic island, thereby forming a fringing reef (Figure 1).

As the island began to sink, a lagoon would open between the island and the reef, which would now qualify as a barrier reef (Figure 2). Eventually, the island would disappear entirely, leaving an atoll, i.e., a lagoon ringed by coral. To support his theory, which, with minor modifications, is accepted to this day, Darwin searched for (and found!) dead corals on lagoon bottoms below the 120 foot limit. This finding he interpreted as confiming his supposition that the sea bed surrounding volcanic islands is often subsiding.

Figures and original captions (below) reproduced from The Voyage of the Beagle.

 

Figure 1. Transition from fringing to barrier reef.

 

Figure 2. Transition from island surrounded by barrier reef to an atoll.