During the last Ice Age, when sea levels were lower and the Bahama shelf was exposed land. Rainwater drained down the cliffs, carving gullies in soft rocks. Modern sea levels submerge the region, with pale blue water on the shallow shelf, darkening to deep blue in the submarine canyon.
Tongue of the Ocean, a deep canyon on the Bahamas Platform.
The submarine canyon in the top right is the Tongue of the Ocean. The dark blue is truly deep water, up to 3,000 meters depth. The pale blue waters of the shelf are are less than 15 meters deep. The shallow waters have limited circulation with the open ocean, and warm easily. This leads to evaporation, increased salinity, and precipitation of minerals. The high salt level inhibits the growth of corals that are otherwise common in calm, warm, shallow water.
Oolite limestone from Brazil. The rock is 11 centimeters tall by 12 centimeters long, and the ooids are 0.8 centimeters diameter on average. Photography credit: Eurico Zimbres
Single-celled marine organisms produce aragonite with their shells, a calcium carbonate mineral. This pairs with directly precipitated minerals to form ooids, small spherical grains of limestone. With time, compaction, and compression, the ooids lithify into limestone. Despite their different origin, these limestones are chemically similar to limestones formed by coral reefs.
Tongue of the Ocean licking past the West Bank of Andros Island.
The Tongue of the Ocean is the southern portion of the Great Bahama Canyon. The canyon is more than 225 kilometers long, and up to 37 kilometers wide at its deepest point. The rock walls rise 4,285 meters from the canyon floor up to the Bahama Platform. The canyon was carved during the last Ice Age when more of the world's water was locked on land in gigantic ice sheets. This lower sea level exposed much of the shelf as dry land, eroded into gullies and small canyons as rainwater drained off the shelf into the canyon below. As the Ice Age ended and the ice sheets melted, water returning to the oceans rose sea levels, flooding the shelf.
Photography credit: NASA. NM23-739-093 (top) was captured in 1997, and STSO29-90-9 (bottom) was captured in 1989. For astrophotography quickies, check out nearby Florida, a rocket during re-entry, or using satellites to track slope instability. Want something more longer? Try boomerangs in space, or what happens when a companion-star goes supernova.