Final answer:
The solid structure of a coral reef is mainly composed of calcium carbonate.
Step-by-step explanation:
The solid structure of a coral reef is composed mainly of calcium carbonate. Calcium carbonate is a chemical compound that is abundant in the ocean and is the primary component of the hard skeleton of corals. It is formed through the process of calcification, where corals absorb calcium carbonate from the water to build their intricate structures.
Learn more about Composition of coral reefs