*Larvae attach themselves and secrete limestone cup around themselves like in any reef.

*Soft part of animal called polyp will spend rest of its life in the external skeleton.

*Polyps reproduce by budding; thousands form the reef.

*When sea floor under volcano peak sinks, the peak sinks too which widens the distance between island and reef.

*When reef is separated by water from volcano peak, it is called a barrier reef. The circular coral belt around lagoon turns reef into island. When this island sinks below sea level an atoll is born.