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Reefs

From NASA's Visible Earth collection:This very clear view of the nearly 1800 kilometer-long Great Barrier Reef off the coast of Australia is seen in this SeaWiFS image.
From NASA's Visible Earth collection:This very clear view of the nearly 1800 kilometer-long Great Barrier Reef off the coast of Australia is seen in this SeaWiFS image.

What are reefs and how have they changed over time?

The word reef comes from the Old Norse Word “rif” which meant any rib of land that stuck out in the water. Today different people use the word "reef" in different ways. To sailors and on nautical maps, it refers to any obstruction on the seafloor that might cause a ship to run aground. Fishermen refer to any build up on the seafloor where fish might congregate. However for biologists reefs are organic structures build up by living animals from the sea floor. A reef is a structure built by organisms that rises above the surrounding seafloor. Reefs throughout geologic history share a common set of environmental features. Reefs are built in warm shallow seawater in the tropics and subtropics. Reefs occur only in waters that are relatively free of suspended, land-derived sediment, which allows sunlight to penetrate to the reef surface, permitting photosynthetic organisms to live. Reefs are characterized by high biodiversity and include five major types of organisms.

Photo Gallery

Underwater features are actually more striking than surface features in this Moderate Resolution Imaging Spectroradiometer (MODIS) image, acquired on January 24, 2004, by NASA’s Terra satellite. The Great Bahama Bank and Little Bahama Bank are a brilliant blue in contrast to the black of the surrounding ocean. The Banks are shallow coral reefs that reflect light through the ocean, giving it a bright blue color. The ocean becomes suddenly deep on the western edge of the Grand Bahama Bank, and here it becomes an inky black. Coral reefs are also visible around the Florida Keys and in the Golfo de Batabanóuth of Cuba. Near land, sediment may also be coloring the water. Like coral reefs, sediment reflects light through the water, giving it a bright color. A few fires were burning when this image was acquired, and they have been marked with red dots. The water around the islands appears lighter in color, then suddenly changes to dark blue, because the islands of the Bahamas sit atop submerged carbonate platforms with sudden drop-offs at their edges. These platforms are known as the Bahama Banks. The water above the platforms is shallower and thus appears lighter blue than the significantly deeper waters beyond the drop-offs. The Bahama Banks are submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The waters of the Bahama Banks are very shallow; on the Great Bahama Bank they are generally no deeper than 25 meters, hence their light blue color in the image. The slopes around them however, such as the border of the Tongue of the Ocean in the Great Bahama Bank, are very steep. These slopes are visible in the image as the lines that seem to radiate out from the dark circle. This dark blue curve is the deep waters of the Tongue; in that particular area the waters reach 1300m in depth – a stark contrast to the 25m depth above the slopes.
courtesy of NASA, CC BY
From NASA's Visible Earth collection:This very clear view of the nearly 1800 kilometer-long Great Barrier Reef off the coast of Australia is seen in this SeaWiFS image.
courtesy of NASA, CC BY