Final answer:
Coral reefs are formed by coral organisms secreting calcium carbonate skeletons, which grow over time to create diverse reef systems like fringing islands and atolls.
Explanation:
Coral Reefs: Formation and Composition
Coral reefs are built by marine invertebrates known as corals. These tiny animals secrete calcium carbonate skeletons around themselves, which accumulate over time, forming the reef structure. The process of reef formation involves the gradual growth and accumulation of these skeletons, creating diverse reef systems such as fringing islands and atolls.
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