Characteristics, Distribution + Formation

Question 1

Characteristics

Answer 1

• calcium carbonate structures, made up of reef-building stony corals
• limited to the depth of light penetration, so only occur in shallow water
• dependence on light also means reefs are only found where the surrounding water contains relatively small amounts of suspended material
• live only in tropical seas = temperature, salinity + lack of turbid water
• shelter more than 25% of all known marine life

Question 2

Origin of reefs

Answer 2

• 1842 = Charles Darwin + ‘Dana’ explained the process of reef formation
• mostly caused by subsidence e.g. coral reefs grow upwards from submerging features
• this leads to fringing reefs being replaced by barrier reefs as the land submerges
• growth is more vigorous on the edges due to wave action, creating the lagoon within
• this theory applies well to GBR + Indonesia but no evidence elsewhere

Question 3

Formation of reefs

Answer 3

• coral is formed by a symbiotic relationship between polyp + zooxanthellae (photosynthesising plant)
• the zooxanthellae gains shelter from the polyp, the polyp gets energy from the photosynthesis
• the polyps skeleton is calcium carbonate + forms the reef

Question 4

How is a fringing reef formed?

Answer 4

• coral reefs that are attached to the shoreline or run closely parallel
• a small lagoon may form between the reef and land - however a deep + wide lagoon makes it a barrier reef

Question 5

How is a barrier reef formed?

Answer 5

• a coral reef that is completely separate from the shoreline by a lagoon
• they also run parallel to the coastline but are separated by much deeper + wider lagoons

Question 6

How are atolls formed?

Answer 6

• circular coral reefs with a lagoon in the middle
• form from a fringing reef surrounding a volcanic island
• as sea levels rise, or the volcanic island sinks, the reef continues to grow
• this leaves behind a ring of coral, that once surrounded the island, with a shallow lagoon in the middle