Answer:
Coral reefs are built by millions of coral polyps, small colonial animals resembling overturned jellyfish. They excrete an exoskeleton made of calcium carbonate, which forms their distinctive shapes as a colony. Thousands of coral colonies build up over time to form the reef.
After an extensive period of time, the reef may die off, perhaps because the conditions of the environment change for the worse for the corals. Since reefs often form in conditions where there is a lot of water movement (because the water is more oxygenated in these areas), if the colony is no longer being sustained by living polyps the colonies break up. Reefs can still break up when the coral is still alive if the conditions are severe. Sedimentation and cementation occurs as calcium carbonate precipitates out of the water and of the exoskeletons of the coral.