Coral Cay and Motus

• Coral reefs


  • Can only live in warmer waters near the tropics
  • Maximum ocean depth of 100 metres as light does not penetrate much beyond this depth
• Reasons why coral reefs exist where they do

  • Volcanic action built underwater mountains close to the surface
  • Uplift near a subduction zone raised the floor of the ocean
  • Shallower shelf areas existed around large continental islands
• Volcanic islands


  Acquire fringing reefs, which may develop into barrier reefs and atolls as the volcanic core subsides and is eroded
• Even the large Pacific atolls with their rim of islands have volcanic foundations
• The pattern of island development depends on both erosion and subsidence
• Volcanic foundations may subside or sink as they move on their oceanic plate, or may be uplifted hundreds of metres by violent earth movements at subduction zones
• How low islands of the Pacific coral reefs form

  1. Reefs produce the materials

  2. Waves break over the reef and push the materials above sea level

  3. If materials are mostly sand, a coral cay will form

  4. If materials are coarse rubble, a motu will form
• Waves


  • Size depends on constancy of wind direction, strength of wind, and distance of open ocean over which the wind is blowing
  • When waves move into shallow water, they slow down, become shorter and closer together, and are bent by the sea floor (wave refraction)
  • Waves become higher as they approach a shallow shore, and eventually collapse and break as surf
• How waves relate to the formation of coral cays


  1. Wave refraction and other changes take place only very close to the reef

  2. Waves can pick up sand-sized and gravel-sized particles

  3. Waves that have broken on the reef edge can continue to travel across the shallow water of the reef flat and lagoon, carrying sediment along their path

  4. Finer sediment of the reef flat can be swept towards a local point on the reef, depending on the direction of wave approach, the shape of the reef, and the size of the reef
• How motus form


  1. Very large waves from tropical cyclones are needed to carry coarse sediment

  2. Water draining off the reef flat or from the lagoon can form gaps through the motu ridge, producing shallow passes
• Coral cays and motus

  • Sand cays without vegetation are very unstable, and can move tens of metres in a 24-hour period or up to 100 metres in less than a month
  • Increase in size of the coral cay or motu helps stabilise it
  • Vegetation helps bind the loose sediment together and adds organic material
  • Freshwater lens or brackish water lens can form under the island
  • Cementation of sand particles and coral rubble into beachrock and coral conglomerate helps protect against erosion
  • Guano from seabirds can leach down and combine with the sands or shingle to form cay sandstone, stabilising the inner, older sections of reef islands