11. Eleuthera Island, Exuma Sound, Bahamas

This photograph provides a rare opportunity to observe a natural chemical laboratory at work in one of the few places on Earth where limestones of a quite different sort from those forming the Barrier Reef are actually in the process of formation. Eleuthera Island itself is little more than a sandbar rising just a few meters above sea level, but it separates the deep, dark blue waters of the Atlantic on the right from the 10-meter (33-feet) shallows of the Great Bahama Bank (left), where details of the topography of the sea bottom are clearly visible through the clear waters.

The shallow waters are warm and become extremely salty. Crystals of aragonite, a calcium carbonate mineral, form into oolites (small grains of limestone) as the tidal currents swirl back and forth. Lithification of the carbonate sands produces an oolitic limestone. Although the water is warm and clear, corals do not live in the shallows, probably because of the salt content. The rocks resulting from this process therefore have a quite different origin from those formed from coral reefs, although chemically they are very similar.

STS-1, April 1981. Picture #1-12-0322.

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