How are coral reefs formed? Name two examples.

Coral reefs are formed from hard, limestone "skeletons" of coral. A brown algae living symbiotically with the coral help the coral to produce limestone. The algae provide sugars and oxygen that they produce by photosynthesis.

The structural foundation of individual coral reefs is formed by a multitude of marine animals and plants through the processes of slow accumulation and deposition of calcium carbonate (limestone) extracted from seawater.