According to the National Oceanic and Atmospheric Administration, coral reefs are formed when coral larvae attach themselves to rocks or other hard surfaces found under water along the edges of islands and along coasts. Additionally, plants may also accumulate within the reef. The process is slow and aided by limestone deposits, which settle on the plant and animal life.
Several types of animals and plants call the reef ecosystem home, and many contribute to the habitat's construction. However, there are relatively few species that are responsible for building the framework on which the other animals and plants take refuge. They include hard corals and coralline red algae. Both coral and algae are able to extract calcium and bicarbonate from seawater and combine them into a substance called limestone, which is essential for constructing the skeleton of the reef.
There are three major types of coral reefs, including fringing, barrier and atoll. The most common type of reefs are fringing reefs, which grow in shallow waters close to the coastline. Barrier reefs are larger and separated from land by a lagoon. Atolls are ring-shaped formations and are located near the water's surface. Atolls normally form around underwater islands or inactive volcanoes.