How does barrier reef form?

Barrier reefs begin as fringing reefs along the shores of a volcano. Then over millions of years the volcano sinks lower into the sea and the sea level rises around the volcano. The coral grows upwards to keep from getting too far from the sunlight at the sea surface. The outward side of the coral reef grows fastest, since ocean currents bring in the plankton that the corals feed on. The water on the landward side of the reef is still and there is less oceanic plankton. Here the reef is unable to grow fast enough to keep up with the rising sea level and is eventually drowned. A lagoon develops between the reef and the land, resulting in the characteristic barrier reef shape.