So, what is an Atoll? it is a wonderful example of geologic evolution. If you look back at some of the photos on our blog from last July/August when we were in the Marquesas, you will see tall islands that jut out of the ocean. There are almost no beaches as those were volcanos that grew from the ocean floor millions of years ago.

Once the volcanos have stopped growing, all the organisms that make up a reef start to grow around the island and at the same time, the volcanic mountains slowly erode away. After millions of years you end up with and island like Tahiti and the Society Islands: A mountainous island with an "alluvial plain" around it, a lagoon and a reef surrounding the outside.

After millions of years the mountains continue to erode away until all that is left is the ring of the reef. On the ring of reef tiny islands form from the bits of sediment, drift wood, and coconuts. These are called Motus. Inside the atolls the depths are from 200 feet to 0 with coral heads growing up from the bottom.

The photo is of Kaukura as we were flying to Apataki on the Twin Otter.