Can you explain why atolls extend from the surface of the ocean down to great depths into the sea? Corals, which make up these ring-shaped structures, can only live where water is warmed and illuminated by sunlight, no more than approximately 45 meters deep. How do you explain an atoll that extends thousands of meters deep?

A  Corals deep in the ocean can survive because they are warmed by seamounts instead of the sun.  The atoll consists of layers of coral reef alive at the surface and near seamount vents.

B  The coral reef once formed around an active volcano.  The reef became a barrier reef, with hard skeletons of previous colonies acting as the scaffolding for new ones on top.  The volcano gradually sank into sea, moving with the plate it rested on, as the coral continued to grow upward, leaving the atoll as a ring structure at the top.

C  Sea currents circulate nutrients to the reef deeper than 45 meters and provide a rigid force to keep the coral reef at the surface.

D  Pieces of coral from other reefs are carried farther out into the ocean, where they act like seeds and start new coral reefs.  These corals quickly grow up into the warmer parts of the ocean where they thrive and form round atolls as a means of trapping warmer water around them.