Fringing reefs, also known as shore reefs, are the most common type of reefs. They are relatively young reefs that are found close to the shore in shallow lagoons. They sometimes grow around oceanic islands and are usually attached to the island or other land mass. Because of their proximity to the land, fringing reefs are the most susceptible of all the types of reefs to human activities, coastal development and pollution. They grow from two to seven millimeters a year.
Barrier Reefs grow parallel to the shore and are similar in appearance to fringing reefs. However, barrier reefs can be significantly larger than fringing reefs. Also, barrier reefs exist further out from the shore than fringing reefs. In fact, barrier reefs are sometimes a great distance from the coastline. Barrier reefs usually have several channels that afford access to the lagoon and the land mass beyond it.
According to the National Oceanic and Atmospheric Administration, coral that continues to grow around a volcanic island after the island has subsided completely under the water forms an atoll. Atolls are relatively circular or oval and have a central lagoon. They are thought to be the final stage in the development of the coral reef and, depending on their sizes, can take from 100,000 to 30,000,000 years to fully form.
In the 1830s, Charles Darwin theorized that coral reefs were formed in a three-stage process. In the first stage, a fringing reef forms in the shallow waters near a tropical island. The second stage occurs over time as the reef grows outward away from the land. In this stage, the fringing reef develops into a barrier reef. The final stage occurs if the island completely submerges under the water. The reef that is left -- which is usually shaped like the island it surrounded -- then surrounds an oceanic lagoon. Darwin also theorized that under each of these lagoons should be a bedrock base, a theory later proven to be correct. This base is the remains of the original island.
