Coral reefs form when generations of hard corals build limestone skeletons faster than waves, organisms and seawater break them down. New coral larvae settle on hard surfaces, grow into colonies and add to the framework left by earlier generations. The modern Great Barrier Reef began developing after the last ice age, as rising seas flooded Australia’s continental shelf.
From polyp to reef
A coral polyp is a small animal related to anemones and jellyfish. Many reef-building corals live in colonies of connected polyps. Each polyp lays down calcium carbonate, mainly in a crystal form called aragonite, beneath its living tissue. Over time, those skeletons make the branches, plates and domes we recognise as hard coral. Most shallow reef-building corals also host microscopic algae. The algae use light to make sugars, supplying energy the coral can use to grow. Coral polyps can also catch small prey with their tentacles. NOAA coral tutorial · Reef Authority coral bleaching guide
A larva settles. After sexual reproduction, a coral larva can drift before attaching to suitable hard substrate. On the Great Barrier Reef, many species release eggs and sperm during mass spawning, although reproduction varies by species. A successful settler becomes a polyp.
The colony grows. Many corals multiply by budding, adding genetically identical polyps as the skeleton extends. The shape matters. Branches can extend quickly and create shelter, while massive corals build dense domes that grow more slowly.
The reef accumulates. Skeletons persist after polyps die. New corals can settle on this foundation. Crustose coralline algae can cement parts of the framework, and fragments and carbonate sediment can accumulate in gaps. Sponges, grazing fish, waves and other processes also wear the structure away. The reef builds upward or outward when more material accumulates than is lost. NOAA reef formation · AIMS reef spawning research
How long does it take?
A coral colony can grow visibly over a few years. Building a reef framework usually takes centuries to millennia, depending on the starting surface, sea-level history and the balance between growth and erosion. NOAA gives approximate colony growth rates of 0.3–2 cm a year for massive corals and up to 10 cm a year for branching forms. Those figures describe growth of individual corals. A branch can extend that far without adding the same height to the reef as a whole. NOAA reef formation and growth rates
How scientists measure reef growth. A carbonate budget estimates the mass of reef material produced and removed per square metre each year. Researchers survey the organisms that build or erode the reef, then apply measured rates. That gives an estimate of net carbonate production. To estimate vertical growth, they must also account for how much sediment the reef retains and how loosely the material packs. A reef can still produce limestone yet deepen relative to sea level if the water rises faster than the framework builds. Perry and colleagues on reef growth and sea-level rise
Conditions for growth
Light and depth. Many tropical reef builders rely on light reaching their symbiotic algae. Depth, suspended sediment and water clarity determine how much light reaches a colony. Some coral reefs also grow in deeper, cooler water using different energy sources.
Stable conditions and room to settle. A hard surface gives a larva somewhere to attach. Loose rubble can shift and dislodge new settlers. Heavy sediment can bury surfaces or force corals to spend energy clearing their tissues.
Water chemistry and temperature. Marine heatwaves can disrupt the partnership between corals and their algae. Ocean acidification reduces the availability of carbonate ions needed to build skeletons and can change the balance between construction and erosion. NOAA coral tutorial · Coral bleaching on the Great Barrier Reef
Reef shapes
| Reef type | How to recognise it |
|---|---|
| Fringing reef | Grows close to the shore of a mainland or island. Look for shallow coral extending seaward from the coast. |
| Barrier reef | Lies offshore with a lagoon or expanse of water between the reef and land. The Great Barrier Reef includes many separate reefs across a continental shelf. |
| Atoll | A ring or near-ring of reef surrounding a lagoon. In the classic volcanic-island model, coral keeps growing as the island gradually subsides. |
| Patch reef | A smaller, isolated reef rising from a lagoon or shelf, with sand or other habitat around it. |
A coral cay is a low island made from reef-derived sediment deposited by waves and currents. A cay can sit on a reef platform, while other islands in the region are older continental landforms with reefs around them. A visitor may see both types of island on the same stretch of coast. NOAA reef shapes · UNESCO Great Barrier Reef description
The Great Barrier Reef’s origins
During the last ice age, lower sea levels exposed large parts of the continental shelf. As ice melted, the shoreline and the band of suitable reef habitat moved. Cores drilled from submerged reefs preserve records of those shifts. A 2018 study reconstructed 30,000 years of reef development and identified five episodes when reef growth ceased at the studied sites, largely associated with sea-level and environmental changes. The modern reef system emerged around 9,000 years ago, and parts grew over older reef foundations. Local reefs have different histories. University of Sydney summary of the reef-core study
What the cores revealed about where reefs grow. At the sampled sites, falling seas exposed reefs to air around 30,000 and 22,000 years ago. Later, as seas rose, reef development shifted landward and was interrupted around 17,000 and 13,000 years ago. In this context, a reef moves when new colonies establish in newly suitable places while older sites stop growing. The existing reef framework stays where it formed. University of Sydney reef-core study
You can see the result on a snorkel trip. The shallow crest takes the force of incoming waves. Seaward, the reef slope falls into deeper water. Behind the crest, sheltered areas can hold sand, patch reefs and seagrass. Those habitats are part of the same system and offer different places for animals to feed, shelter and breed. Great Barrier Reef snorkelling guide