Conditions for good reef building
- Close to sea level but not above it: very few coral grow below a depth of 30m because they rely on a symbiotic relationship with photosynthesising dinoflagellates called zoocanthellae. Sunlight is rapidly filtered by sea water such that below 30m it becomes increasinhlu difficult to photosynthesize.
- Clear waters with little sediment which could clog the polyps and block sunlight from reaching the zooxanthellae.
- fully marine environment with salinity between 30-40 parts per thousand and a temperature between 23 and 27 degrees C (tropical seas).
- High enery levels needed to incorporate lots of oxygen in the water and sufficient wave actions, or currents, to cirulate sufficient nutrients to the filter feeding corals through upwelling.
- previously supported reef systems during the Devonian, forming the hrizontal strata of the Kimberlly System
- but for much of Palaeozoic and Mesozoi the surrounding waters have been too cool to provide a suitable environment for growth
By following Australia's northward drift in the fossil record through the Cenozoic it is possible to reconstruct that enrivonments in which coral could and could not survive.
Early in the Cenozoic Australia was too southerly to provide the warm waters necessary to sustain coral and it was not until 24 million years ago that the continent entered the tropics.
- Coral spread quickly but the heavy erosion of the mountains create the Great Divide filled the waters with too much sediment and the various deltas and estuaries clogged the polyps of early settling coral.
- over time the Coral Sea formed and this helped alleviate the sediment problems as a vast counter clockwise spially current was established
- also as a result of this new cirulation upwelling of well mixed, nutrient rich, agiated waters from deep below began along the East Coast which has helped support coral life.
Types of Reef
There are three main typed of reef in the world's modern oceans;
- Fringing reefs
- Barrier reefs
- Atoll reefs
Fringing reefs directly meet the land and form along the coast with no lagoon water between them and the shore. As a result in tidal regions some parts of the coral may be above sea level during low tide. e.g. Maldives, many parts of the Caribbean.
Barrier reefs are further out to sea than fringing reefs and will normally have a lagoon of backwater separating them from the land directly. As a result they are always submerged and will not be exposed by low tides
e.g. the Great Barrier Reef, Australia
Atolls are the remains of sunken volcanoes which existed far offshore millions of years ago, generally created as a result of hotspot activity, creating a low profile shielf volcano with surrounding shallow seas.
The immense weight of the volcano means that gradually it begins to sink, pushing the Moho down below it.
The small fringe reef evolves through a barrier reef (with a small lagoon between it and the remaining parts of the volcano) to an atoll reef as the volcano sinks entirely.
The rate of coral growth must meet the rate of subsidence and can be as rapid as 10-60cm a year
Global distribution of coral reefs
Most coral reefs are between the tropics (30 degrees North to 30 degrees South)
Found on the continental shelf
Close the the land if they are fringe or barrier reefs and out at sea if an atoll
It is assumed that the distribution of palaeo reefs would ave been controlled by the same factors as today limiting their distribution in similarly narrow bands.