Over geologic time the central volcano becomes extinct, followed by erosion and subsidence beneath the sea surface, leaving the coral reefs as a ring around (or cap on) the submerged island remnant.
Layers of frozen sea water, known as sea ice, cap the Arctic Ocean. The ice grows dramatically each winter, and melts just as dramatically each summer.
北冰洋的冰层由结冰的海水构成，这些冰层在每年的冬天急速冻结，又在每年的夏天迅速消融。
3
Allen also may order BP to open the cap and let oil and gas flow via pipelines to vessels on the sea surface where it will be separated, stored and burned.