These elegant whorls of color are atolls, enclosed coral reefs almost always surrounding a lagoon. Generally speaking, atolls are the products of volcanic islands that have eroded away.
In general the process is believed to work like this. Coral forms off shore from volcanic islands in tropical latitudes, developing a barrier reef that's separated by a growing lagoon. But over time, while the surrounding ocean wears away the main body of the island, the coral ring remains. When the island ultimately disappears from view, the remaining lagoon is left with a protective atoll.
One year, 900 locations, thousands of coral reefs. That's the tally of NASA's Landsat 7 satellite as it continues to deliver cutting edge images and information about the Earth. Data being presented this week at an international conference in Indonesia is the first assessment of the physical condition of major reefs from the the new Landsat 7 collection of images. More than 5000 coral reef scenes have been amassed in the first year of Landsat 7's operation. In that collection, many reefs have been seen more than once, offering scientists an opportunity to study seasonal variations as well as other changes in the reefs caused by hurricanes and climate change.
For more information, see the accompanying press release