"Atolls: Hot spots not only punch holes in the crust, but the also raise it up and heat it. As the plate moves away, it cools and subsides. The mountains also subside, and also erode. If the mountains are formed in a tropical climate a reef will form around it's edges. If the coral growth can keep up with the relative sea level rise from the subsidence, then the reef will survive while the mountain sinks and eventually disappears below water level. If that happens you will get a circular ring of reef with an open lagoon in the center like the atoll above. At one point there was a volcano in the center of that lagoon. It's a cool example of biology and geology interacting and biology winning, so to speak."