How Are Coral Reefs Formed?

Reef-Building Corals

There are four taxa (formally classified groups) of corals that substantially assist in the deposition of calcium carbonates in the reef-building process. These are:

Scleractinian corals

Octocorals (a very few) Fire corals (Hydrocorals)

Stylasterine corals (Hydrocorals)

Shallow marine waters have high concentrations of calcium (Ca++) and bicarbonate (HCO3-) ions. Coral animals (polyps) have the ability to combine these two chemicals to form calcium carbonate (CaCO3), from which their protective skeletons are fashioned.

Crystal structure of calcite and aragonite - two forms of calcium carbonate common in modern coral reefs.

Interestingly, calcium carbonate is produced by different types of reef-building marine life in one of two alternate mineral forms: calcite and aragonite. Both of these minerals have the same chemical composition, but differing crystal structures (see photos, above).

Modern reef-building corals mainly produce aragonite, which is more stable in seawater. After the polyps die, they leave behind their aragonite skeletons. Over time, the accumulated minerals contained in the multitudes of these skeletal remains become the large, solid structures we call coral reefs.

How Are Coral Reefs Formed?

Here we have provided but a brief overview of the story of coral reef formation. A great variety of organisms assist the two primary types of reef builders (discussed above) in the reef-building process by depositing lesser amounts of carbonate particles.

Octocorals adorn a shallow Caribbean reef. 

It is also noteworthy that on many shallow reefs - particularly on shallow reefs of the Greater Caribbean region - a substantial superficial layer (epibiota) consisting of sponges, octocorals, and other invertebrate animals and macroalgae grows from the reef base.

Together, these biotic assemblages add substantially to the structural complexity and biological diversity of coral reefs.

The processes involved in the formation of different types of coral reefs are discussed on another page of our site.