- Coral reefs take tens of thousands of years to become established.Diver over coral reef image by DJ from Fotolia.com
Evidence of coral reefs in the fossil record dates back 400 million years, and shows that tens of thousands of years must pass for an outcrop of corals to grow into a well-established coral reef. Coral reefs form on the edges of islands or continents and are protected by the landmasses. Corals reproduce by releasing larvae that anchor to the rocks of the shoreline. Corals also require clear shallow waters, so they receive enough sunlight for photosynthesis. The rock formations off the coasts of islands and continents provide the base formations for coral reefs. - Fringe reefs form close to the coastline.Hanauma Bay image by mdanielj from Fotolia.com
Fringe reefs are the most abundant type of coral reefs, occurring closest to coastlines and protecting the coast from erosion and wave damage. Fringe reefs project outward from the central landmass. The reefs are shallow closest to the shore and gradually become deeper as they stretch away from the shore, finally dropping off into deep, open ocean. At low tide, the reef at the coastline becomes very shallow or emerges from the water completely. Hardy, flat corals are well-adapted to the inland areas of fringe reefs. As the reef stretches further off the coastline and becomes deeper, macroalgae and plant life dominate the reefscape and water may be silty due to the rising and falling tide. At the deeper outer edges of fringe reefs, the water clears and a diverse range of corals begin to appear. Fringe reefs can be thought of as the first stage of reef formation. - Barrier reefs are located further offshore than fringe reefs.maledives image by nici.cat from Fotolia.com
Barrier reefs are also known as "ribbon" reefs. Barrier reefs are located further offshore than fringe reefs. The rocky formations surrounding the coastline create a calm, often sandy lagoon between the reef and the shore. Barrier reefs cause waves to break far away from the coastline and are much larger than fringe reefs. Barrier reefs become deeper as they move further away from the shore. According to the National Oceanic and Atmospheric Administration (NOAA), "massive corals and encrusting coralline algae thrive, small fish inhabit the many holes and crevices on the reef, and many larger fish including sharks, jacks, barracudas and tunas patrol" these reef channels. Barrier reefs can be considered the second stage of reef formation. Barrier reefs are farther offshore because the main landmass is contracting; they are larger than fringe reefs because barrier reefs have had more time to grow. - Atolls are most abundant in the Pacific Ocean where volcanic islands are also common.lagoon image by Ahmed Zahir from Fotolia.com
Atoll reefs are circular reefs that were once fringe reefs. According to NOAA, "as a fringe reef expands, the interior landmass usually begins to subside. When the island completely subsides beneath the water leaving [just] a ring of coral with an open lagoon in its center, it becomes an atoll." An atoll reef takes tens of millions of years to form. The calm, shallow lagoon in the center of an atoll reef is likely to contain patch reefs (also known as "platform" reefs), which are scattered circular reef plates. Atolls are most abundant in the Pacific Ocean, where volcanic islands are also common, and atoll reefs are some of the most well-established and bio-diverse reefs in the world.
