Coral Reefs- The Great Barrier Reef

2.1 Geomorphological Processes

The Great Barrier Reef has two clear Geomorphological (lithosphere) processes that occur in this ecosystem: 1) Earth Movements and 2) Weathering and Erosion.

Earth Movements

The following are Earth Movements that have affected the Great Barrier Reef in recent geological times.

Subsidence

Subsidence is where the lithosphere sinks either through the settling of sediment or a great weight has been placed on it . The Great Barrier Reef has sunk twice- 15 million years ago and 4 to 5 million years ago. When the land subsides coral reefs are able to form as they are an aquatic ecosystem. However, if the land subsides too far coral can die as one of the OPTIMAL CONDITIONS FOR CORAL GROWTH is depth of water. The rate of subsidence and sea level changes has determined reef growth over time.

Hydro-isostasy

Isostasy is a type of subsidence where the lithosphere is pushed down through weight but spings back up after the weight is gone. Hydro-isostasy is a where the lithosphere is depressed by the weight of the overlaying water or ice and rises again when that weight is removed (like a sponge cake)

During ice ages the underwater land around The Great Barrier Reef was exposed, the land rose as the pressure of the water was released. The Great Barrier Reef has undergone many periods of hydro-isostasy caused by the weight of the water and release when the water levels were low.

Rifting

In geology, a rift is a place where the Earth's crust is being pulled apart. They often form valleys. Around 95 million years ago the rifting caused Townsville and Queensland troughs to join close to the edge of the continent. These two deep rift basins have determined the shape of the continental shelf and the present distribution and shape of The Great Barrier Reef as the reef doses not go beyond the continetal shelf. Thus, rifting is linked to the depth of water required for optimal coral growth.

Continental Drift

Continental Drift is the theory that the continents of the Earth move gradually over a layer of liquid rock (molten rock) underneath the earth's surface.

Continental Drift has influenced the growth rate of coral in The Great Barrier Reef since the Australian continental plate first drifted into tropical waters over 20 million years ago. Reefs first appeared in what is now the northern area (Cape York) and last appeared in the South as Australia moved northwards. Therefore continetal drifting has allowed for the reefs to move to warmer waters, another key optimal growth factor.

Weathering and Erosion

Chemical Weathering

Since much of the coral reef is made of CALCIUM CARBONATE(lime) it is subject to chemical weathering. This is where the coral's lime shells are broken down by chemical reactions.
Limestone is chemically dissolved by acid that is made by the mixture of rainwater and carbon dioxide from the atmosphere to form CARBONIC ACID. This then is deposited on to the limestone which then weathers down.
Over, time, the coral surfaces are eroded by the Carbonic Acid in the rain to form rills and channels.

Mechanical Weathering

Also, since coral reefs are located in the dynamic environment of seas and oceans, the coral can be subject to many forms of mechanical weathering
Large waves and strong winds that are associated with cyclones may break apart and move large areas of coral. These large blocks of corals that are broken apart are called BOMMIES
Another type of mechanical weathering is biological. Coral browsers, such as parrotfish, molluscs, sponges and other species, also physically break apart the coral and move them as they browse. Worm species tunnel through corals using acid secretions, weakening the structure.