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)


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


  • 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