Charles Darwin visited Tuamotu in French Polynesia on the H.M.S. Beagle in 1835 and described the formation of coral reefs:

  • ​fringing reefs, barrier reefs and atolls represent succession through geologic time resulting from upward growth of coral on a sinking volcano
  • ​an oceanic, emergent volcano becomes dormant, coral grows on its rim and forms a fringing reef (top images)
  • the volcano sinks, coral continues growing upward, forming a barrier reef with a lagoon between the island and the reef (center images)
  • ​the volcano completely disappears below sea level and coral forms a ring surrounding the lagoon, with islands on the lagoonal side (lower left)
  • ​islands and patch reefs form within the lagoon through deposition of coral rock and coral sand (bottom left and right)

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Land

CORAL REEFS (red dots above) occur in the tropics and subtropics between 30 degrees North and South latitude

  • ​Found in 109 countries; 7.6% in Caribbean; 11.3% in Indian Ocean; 80% in Pacific Ocean
  • ​The oldest coral reefs occurred about 500 million years ago; modern reefs began growing on old reef structures between 8,000-20,000 years ago
  • ​​Shallow water tropical reefs occupy 0.089% of the world's ocean, to about 100 m depth
  • ​There are also mesophotic coral reefs (40-150 m) and deep sea coral reefs

Wave action, currents, tides, temperature and other factors create many different zones found in a coral reef ecosystem 

Coral island  atoll

CORAL REEF ECOSYSTEMS SUPPORT FOUR DIFFERENT COMMUNITIES

  • Seagrass bedsare found in protected bays and lagoons and sandy areas at the base of the reef
  • Mangroves forests are found on the shoreline in protected areas with limited circulation 
  • Sand/mud/rubble flats are found close to shore, in deep lagoons, channels, and at the base of the reef
  • Coral reefs ​are found in clear, salty water with low nutrients and temperatures of 16-30 degrees Celsius

Volcanic Island

True atoll

Island atoll

REEF FEATURES