Coral reefs and Types of coral reefs

Coral reefs and Types of coral reefs

Subject : Geography

Section: Oceanography

Coral reefs are some of the most diverse ecosystems in the world. Coral polyps, the animals primarily responsible for building reefs, can take many forms: large reef building colonies, graceful flowing fans, and even small, solitary organisms.

Types of coral reefs

• Fringing reefs grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reef.
• Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest points, they can reach the water’s surface forming a “barrier” to navigation.
• An atoll is a ring-shaped coral reef, island, or series of islets. The atoll surrounds a body of water called a lagoon. Sometimes, atolls and lagoons protect a central island. Channels between islets connect a lagoon to the open ocean or sea.

How do Atolls develop?

• Atolls develop with underwater volcanoes, called seamounts.
• Formation of Fringe Reefs:
  • First, the volcano erupts, piling up lava on the seafloor. As the volcano continues to erupt, the seamount’s elevation grows higher, eventually breaking the surface of the water. The top of the volcano becomes an oceanic island.
  • In the next stage, tiny sea animals called corals begin to build a reef around the island. The type of corals that build reefs are called hermatypic corals, or hard corals. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate).
  • This coral reef, called a fringing reef, surrounds the island just below the ocean surface. The thin, shallow strip of water between the fringing reef and the island is the lagoon.
• Formation of Barrier Reef:
  • Over millions of years, the volcanic island erodes and sinks to the seafloor. This process is called subsidence. The seamount erodes into the sea, its top made flat by the constant pounding of powerful ocean waves. As it subsides, the flat-topped seamount is called a ‘guyot’.
  • As the island subsides to become a guyot, its ring-shaped fringing reef turns into a barrier reef. A barrier reef is farther from the shore and has a deeper lagoon. The barrier reef protects the lagoon from the harsh winds and waves of the open ocean.
  • Subsidence brings slight differences in ocean chemistry that change the reef radically. The outer, ocean-facing side of the reef remains a healthy marine ecosystem. Corals on the inner, lagoon-facing side, however, begin to slowly decay. The algae that corals need to survive face much more competition for fewer nutrient resources. The limestone decays, changing the color of the lagoon from deep ocean blue to bright teal.
• Atoll formation:
  • In the final stage of an atoll’s formation, ocean waves break apart pieces of the limestone reef. They pound, break, and erode the coral into tiny grains of sand.
  • This sand and other material deposited by waves or wind pile up on the reef. This material, including organic matter such as plant seeds, forms a ring-shaped island or islets. This is an atoll.
  • Hermatypic corals only live in warm water. An island that is located where ocean temperatures are just warm enough to support hermatypic corals is said to be at “Darwin Point,” named after Charles Darwin. The famous naturalist was the first to outline how atolls form.

Patch reefs:

• Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf.
• They usually occur between fringing reefs and barrier reefs. They vary greatly in size, and they rarely reach the surface of the water.

Pseudo-Atolls:

• The island nation of Bermuda is sometimes called the world’s “northernmost atoll.”Bermuda is a pseudo-atoll, a ring of islands that look like an atoll, but have several distinguishing characteristics: high elevations, mostly submerged reefs, and a wide reef-front terrace (the broad, sloping shelf running from the island’s highest elevation to the sea).