What is a coral reef made of?

A coral reef is a rock built by living things. Its framework is limestone, calcium carbonate, laid down mainly by reef-building or hermatypic stony corals in the order Scleractinia. Each coral colony is made up of hundreds to many thousands of small animals called polyps, each sitting in a cup-shaped skeleton that it secretes beneath itself. As the colony grows, it adds new skeleton on top of the old, so a living coral is a thin skin of tissue over a block of stone that may be decades or centuries old. When the coral dies, the skeleton remains, and new corals settle and grow on top of it.

Corals are not the only builders. Coralline red algae lay down sheets and crusts of limestone that bind loose material together. Green algae of the genus Halimeda produce calcified segments that pile up as sand. Molluscs, foraminifera, sea urchins and other animals add shells and spines. Over time, waves and grazing animals break the skeletons into rubble and sand that fill the gaps in the framework, and chemical cementation turns the whole mass into solid rock. A reef is therefore the combined product of many organisms and many centuries, and the living community on its surface is only the top layer of a much thicker structure.

The world’s coral reefs together cover about 284,300 square kilometres, just under 0.1 percent of the ocean’s surface, yet they are home to at least a quarter of all marine species. More than 90 percent of that area lies in the Indo-Pacific, from the Red Sea to the central Pacific, and less than a tenth in the Atlantic and Caribbean.

How does a coral polyp make limestone?

A coral polyp is a simple animal related to sea anemones and jellyfish, in the class Anthozoa of the phylum Cnidaria. It has a ring of tentacles around a mouth and a body only a few cell layers thick. The layer of cells at the base of the polyp, in contact with the skeleton, pumps calcium and carbonate ions into a tiny space between tissue and skeleton and removes acid, raising the concentration of carbonate until crystals of aragonite, a form of calcium carbonate, grow on the skeleton’s surface. Proteins secreted by the coral guide the growth of these crystals and help shape the intricate walls, plates and spines characteristic of each species.

This process, called calcification, needs a great deal of energy, and most of that energy comes from sunlight. Inside the polyp’s tissues live single-celled algae called zooxanthellae, dinoflagellates of the family Symbiodiniaceae, at densities averaging around a million cells per cubic centimetre of coral. They photosynthesise and pass sugars, glycerol and amino acids to their host, providing up to 90 percent of the coral’s nutritional needs. Corals with zooxanthellae calcify much faster in the light than in the dark, and without their symbionts reef-building corals would grow too slowly to form large structures. This partnership is why reef-building corals live only in the sunlit zone, mostly in water shallower than about 50 m, and why clear water favours reef growth.

Corals also feed as animals. Many extend their tentacles at night to catch passing zooplankton, which supplies nitrogen and phosphorus that sunlight cannot provide. The coral shares some of these nutrients with its algae, completing a tight internal recycling loop.

How does a new reef begin?

Every reef starts with a coral larva finding somewhere to settle. Most reef-building corals reproduce by mass spawning, releasing eggs and sperm into the water on a few nights each year, timed by water temperature, the lunar cycle and the time of day. On the Great Barrier Reef this happens a few nights after a full moon in late spring. Fertilised eggs develop into tiny swimming larvae called planulae, which drift for days to weeks before settling. Other corals brood their larvae internally and release them ready to settle.

A planula needs a hard, clean surface to settle on, and it is choosy. Many coral larvae are strongly attracted to crustose coralline algae, the pink crusts that cover much of the rock on healthy reefs, and to chemical cues from the bacteria living on them. Once a larva attaches, it transforms into a single polyp and begins to lay down skeleton. It then grows into a colony by budding, dividing itself into new polyps again and again. Some colonies, such as massive Porites, can live for hundreds of years and grow several metres across.

A reef is born when enough colonies grow close together on a suitable foundation that their skeletons, and the debris between them, start to build up faster than they are worn away. That foundation might be the rocky shore of a volcanic island, a drowned hill on a continental shelf or the remains of an older reef.

How do coral colonies turn into solid reef?

Living corals grow upward and outward, but reefs are also being broken down all the time. Waves, storms, parrotfish biting at coral and algae, sea urchins scraping the rock, boring sponges, worms and molluscs that drill into skeletons all turn coral into rubble and sand. This process, called bioerosion, is not purely destructive. The fragments fall into the spaces between living colonies and fill them, and in lagoons they accumulate as sand flats. Parrotfish alone produce large quantities of the fine white sand found on many tropical beaches.

Coralline algae then bind the rubble. Their mineral deposition is slower than that of corals, but they tolerate heavy wave action and grow over loose fragments, cementing them in place. On the most exposed parts of a reef facing the open ocean, coralline algae often form a tough pink ridge that takes the full force of the waves and protects the corals behind it. Over time, seawater percolating through the reef deposits additional carbonate cement in the pores, turning loose sediment into hard limestone.

Whether a reef grows or shrinks depends on its carbonate budget: the balance between how much calcium carbonate is produced by corals, algae and other calcifiers, and how much is removed by erosion and dissolution. A healthy reef produces more than it loses and slowly builds upward. A damaged reef with little living coral may lose more than it gains and gradually flatten.

What conditions do coral reefs need to form?

Reef-building corals are fussy about where they live, which is why reefs are found only in a narrow band of the world’s oceans. They need warm water. Shallow-water reefs form only in a zone extending roughly 30 degrees north and south of the equator, the optimum temperature for most reefs is about 26 to 27 °C, and few reefs exist where water falls below about 18 °C. Bermuda, at about 32°N, has the northernmost reef system in the Atlantic only because the Gulf Stream carries warm water past it.

They need light, so most reefs grow in water less than 50 m deep, and the clearest water allows the deepest growth. They need full-strength seawater and little sediment, which is why reefs are rare near the mouths of large rivers such as the Amazon and Ganges, whose fresh, muddy water would smother corals. And surprisingly, they need water that is low in nutrients. Coral reefs flourish in tropical seas that are nutrient-poor and clear, and they are rare along the west coasts of the Americas and Africa, where cold, nutrient-rich upwelling currents such as the Peru, Benguela and Canary Currents both cool the water and favour plankton and seaweeds over corals.

Some reefs push these limits. Corals in the Persian Gulf survive winter temperatures of about 13 °C and summer temperatures of about 38 °C, and the world’s southernmost reef, at Lord Howe Island off eastern Australia, and the northernmost, in a bay of Tsushima Island in the Korea Strait, lie well outside the usual tropical belt. Deep-water corals in cold seas as far north as Norway can also form reefs, but these grow without zooxanthellae, slowly, in darkness.

How did Darwin explain the formation of atolls?

Atolls, rings of coral reef enclosing a lagoon in the middle of the ocean, puzzled naturalists for centuries. Corals can only grow in shallow, sunlit water, yet atolls rise from ocean floor that may be thousands of metres deep. During the voyage of HMS Beagle in the 1830s, Charles Darwin developed an explanation, which he published in 1842 in The Structure and Distribution of Coral Reefs, his first scientific monograph.

Darwin proposed that atolls form in three stages as a volcanic island slowly sinks. First, corals grow around the shores of the young island, forming a fringing reef. As the island and the sea floor beneath it gradually subside, the corals keep growing upward to stay in shallow water, so the reef stays at the surface while the island sinks. The gap between the reef and the shrinking island deepens into a lagoon, and the fringing reef becomes a barrier reef. Finally, the island disappears completely beneath the sea, and only the ring of reef remains, an atoll enclosing a lagoon. Darwin predicted that if anyone drilled through an atoll, they would find a thick cap of coral limestone resting on volcanic rock, the remains of the original island.

Darwin’s theory rested on two observations: that reef-building corals only thrive in shallow, agitated tropical water, and that the land in many ocean regions appeared to be sinking. In his time there was no way to test the idea directly, and rival explanations were proposed, including the view that atolls grew on submerged platforms or on the rims of drowned volcanic craters.

How was Darwin’s theory proved?

The decisive test came more than a century later, at Enewetak Atoll in the Marshall Islands. In 1951 and 1952, as part of the preparations for nuclear weapons testing there, United States geologists drilled deep boreholes through the atoll. They found more than a kilometre of shallow-water reef limestone before reaching the volcanic basalt of the underlying seamount at roughly 1,300 to 1,400 m below sea level. Because corals can only have grown near the surface, the thickness of the limestone showed that the volcano had sunk well over a kilometre while the reef kept pace above it, exactly as Darwin had predicted. The seamount beneath Enewetak formed in the late Cretaceous, and its present depth is due to a general subsidence of the whole region rather than to erosion.

Plate tectonics later explained why ocean islands sink. Volcanic islands form on hot, buoyant oceanic crust, often over hotspots. As the plate carries them away, the crust cools, becomes denser and subsides, while the weight of the volcano also presses down on the plate. In the Hawaiian chain, the islands get progressively older, lower and more eroded toward the northwest, until they become atolls and finally drowned seamounts. The point at which reef growth can no longer keep up with subsidence and the reef drowns is called the Darwin Point; in the Hawaiian chain it lies near the northwestern end of the archipelago, around Kure Atoll.

Why are most living reefs less than 10,000 years old?

Darwin’s subsidence explains atolls, but changes in sea level have shaped all reefs. During the last ice age, so much water was locked in ice sheets that sea level was about 120 m lower than today, and most of today’s reef areas on continental shelves were dry land. As the ice melted after about 20,000 years ago, the sea rose and flooded the shelves. Corals colonised the newly submerged surfaces, often on top of older reefs from previous warm periods, and grew upward to keep pace with the rising water. Reefs that grew too slowly were left behind in deep water and drowned.

The Great Barrier Reef illustrates this history. Reefs have grown on the Queensland shelf on and off for hundreds of thousands of years, each time the sea flooded it. In the most recent episode, sea level was about 60 m lower than today by 13,000 years ago, and coastal hills had become offshore islands. As the sea continued to rise, it covered most of these islands, and corals overgrew them. Sea level there has changed little over the last 6,000 years, and the living reef structure is estimated to be about 6,000 to 8,000 years old. As a result, most coral reefs in the world today are less than 10,000 years old, even though many sit on foundations that are far older.

The geologist Reginald Daly emphasised this role of sea-level change in his “glacial control” theory in the early twentieth century, arguing that ice-age sea levels had planed off platforms on which modern reefs later grew. Today both ideas are accepted: subsidence explains the long-term growth of atolls over millions of years, while glacial sea-level cycles explain the shape and age of the living reef surface.

What are the main types of coral reef?

Most classifications recognise three classic reef types that follow Darwin’s sequence, plus a fourth group of platform reefs. A fringing reef grows directly along a shoreline or is separated from it by a narrow, shallow channel. It is the most common type and is usually less than 100 m wide; the fringing reefs of the Red Sea, which line almost the entire coast except for sandy bays, are among the best developed in the world. A fringing reef can take about 10,000 years to form.

A barrier reef is separated from the mainland or an island by a deep lagoon that can be several kilometres wide and 30 to 70 m deep. Barrier reefs take much longer to form and are much rarer. The Great Barrier Reef, with more than 2,900 individual reefs and 900 islands stretching for over 2,000 km off Queensland, is the largest; the Mesoamerican reef off Belize and the New Caledonian barrier reef are other major examples.

An atoll is a ring or near-ring of reef surrounding a central lagoon with no central island. Atolls are most numerous in the tropical Pacific and Indian Oceans, in places such as the Marshall Islands, French Polynesia and the Maldives, which consists of 26 atolls. An atoll can take up to 30 million years to develop. Platform reefs, also called bank or table reefs, grow wherever the sea bed rises close enough to the surface, on continental shelves or in the open ocean, and expand in all directions rather than only seaward. Small platform reefs within lagoons are called patch reefs.

How long have coral reefs existed?

Reefs are far older than modern corals. The first reefs, more than 500 million years ago in the Cambrian period, were built by microbes and by sponge-like animals called archaeocyathids. Coral reefs first appeared about 485 million years ago at the start of the Ordovician. The great Palaeozoic reefs were built by two extinct groups of corals, the rugose and tabulate corals, and reef building flourished in the Devonian and Carboniferous. After the mass extinction at the end of the Permian, those coral groups vanished, and the modern stony corals, the Scleractinia, appeared in the Triassic. In the Late Cretaceous, some of the largest reefs were built not by corals but by rudists, strange bivalve molluscs with one cone-shaped shell and one cap-like lid. Throughout this history, different organisms have led reef building at different times, and reefs have repeatedly collapsed during mass extinctions and recovered over millions of years.

Are coral reefs still forming today?

Yes, but their growth is under pressure. A healthy reef still adds new limestone each year, with individual corals extending by anything from a few millimetres in massive species to many centimetres in fast-growing branching corals. The reef surface as a whole grows much more slowly than the fastest corals, because erosion removes a large share of what is produced. Healthy tropical reefs can expand outward by about 1 to 3 cm a year.

Several modern changes are tipping the carbonate budget the wrong way. Coral bleaching caused by marine heatwaves kills corals and removes the main producers of new limestone. Ocean acidification, caused by the uptake of carbon dioxide, lowers the concentration of carbonate ions in seawater and makes calcification harder while speeding the dissolution of existing reef rock. Pollution, sediment and overfishing favour seaweeds over corals. A study published in Nature in 2018 by Chris Perry and colleagues found that most of the reefs they surveyed in the Caribbean and Indian Ocean were growing too slowly to keep pace with projected sea-level rise, which could leave reef crests deeper and less able to protect coastlines. Understanding how reefs form is therefore not just a question of geology: it is the basis for predicting whether they will continue to grow.