The coral reef develops from coral animals that secrete lime to form their skeletons, and from algae, plants that also make limy encrustations.
Coral develops as large colonies of individual animals.
As the coral colonies die, new colonies are built upon them, and so a coral limestone evolves, made up almost entirely of strongly cemented limy skeletons. Island coral generally grows on volcanic islands built up by shield volcanoes . Shield volcanoes can evolve to be the largest of the Earth's landforms. They are found on continents as well as in oceans. In the ocean environment, examples of island volcanos are the Hawaiian volcanoes.