There are several theories about how coral structures are formed. Charles Darwin originally described 3 major types of coral reefs.
The first of these types is the fringing reef, an area along the shore where coral colonies have been able to grow. Fringing reefs occur close to land and often extend out to sea for long distances. The second type of reef is a barrier reef, a well defined coral zone separated from land by a lagoon. The lagoon is a shallow area with a sandy floor, patch reefs, and patches of seagrass. An atoll is the third type of reef. An atoll is a ring-like formation of reefs with a lagoon inside the ring.
According to Darwin, barrier reefs begin as fringing reefs along the shores of a volcano. Over millions of years, the volcano sinks lower into the sea and the sea level rises around the volcano. The coral grows upwards to keep from getting too far from the sunlight at the sea surface. The outward side of the coral reef grows fastest since ocean currents bring in the plankton that the corals feed on. The water on the landward side of the reef is still and there is less oceanic plankton. Here the reef is unable to grow fast enough to keep up with the rising sea level and is eventually drowned. A lagoon develops between the reef and the land, resulting in the characteristic barrier reef shape. The volcano continues sinking until it disappears under the sea surface. The result is an atoll, a ring of coral reefs surrounding the submerged, extinct volcano. Eventually sand is trapped by the reefs and sandy islands, called cays, appear. The word "keys", as in Florida Keys, derives from the Spanish word "cay."
The Reefs of Bermuda
Bermuda has been classified as an atoll, but more correctly it should be called a pseudoatoll. This is because the process that formed Bermuda is different than the process that forms an atoll. In Bermuda, sands have collected, been compressed, and turned into rock by a process known as lithification. The land formations of Bermuda are much taller than the formations of the true Pacific atolls.
Look at these two views of Bermuda islands. The one on the left was taken from space and shows the patterns of green and white on land due to trees and houses. The patterns of blue indicate the depth of water with the lightest color the shallowest water. (If you click on the island you can view a larger image.)
Compare the actual photo with the drawing. The faint lines around the platform are the depth contours of 200 and 1000 meters. The Terrace Reef Zone is indicated in black and the Rim Reef Zone is is grey. BBSR is located on the island of St. George at the eastern end of the country.
The Bermuda platform is the area incorporati ng the islands, the lagoon, the rim area and the main terrace of Bermuda.
A cross section of the Bermuda atoll shows the location of these different reef formations. The lagoon is called the North Lagoon and has a depth that ranges between 10 and 20 meters. The waters inside the North Lagoon are calmer and are home to numerous individual patches of corals called patch or lagoonal reefs. One of the disadvantages of the low wave environment is the problem of increased amounts of silt falling on the corals. These reefs which come in many different shapes and sizes, are surrounded by sand. Branching corals, such as Madracis, Oculina, the fire coral Millepora alcicornis, and soft corals such as the sea rod Pseudoplexaura, flourish in this calmer environment.
The rim reefs create a circle along the edge of the lagoon and are formed on the raised rim of the Bermuda platform. They range in depth between 2 and 10 meters. North Rock is an example of a rim reef area. Many different hard corals, especially massive boulder corals such as Brain and Star corals are abundant here, as well as sea rods and sea fans such as the large Gorgonia.
"Boilers" occur on the southern and southeastern side of the island and are part of the rim reef system. They are small, generally rounded reefs that extend to the sea surface and have waves continuously breaking over them. This makes the water look as though it is boiling. The picture here was taken while scouting for sites to put the underwater camera seaward of the rim area.