How does an atoll form?
The formation of atolls begins with an underwater volcano breaking the surface of the ocean. This volcanic island provides a shallow spot for coral to grow as fringe just around the base of the volcano under the water line. Over time the volcano’s summit erodes (after the volcano goes extinct) due to rain and ocean waves. Also, plate tectonics can make the volcano sink lower and glacial melting can make the water level get higher. As all of these processes occur, the volcano sinks, but coral keeps growing in a ring around the volcano. At first, the volcano’s coral ring becomes a barrier island, but as the volcano completely disappears the result is an atoll–a ring island that appears to be completely alone in the ocean.
