Atolls - in fact atolls are formations of ancient origin, often million years old. They take the shape of ring-shaped reefs enclosing a shallow lagoon and dropping away to get depth on the outsides. Usually atolls start their life as fringing reefs around volcanic islands. As time goes by the water levels rises, while at the same time the volcanoes wear off. Most atolls are found in the Pacific and Indian Ocean, there are also several in Philippine waters, such as the Tubbataha Reefs and the Cagayan Islands.
Finging Reefs - can be found in shallow waters near to land. Typically they
extend to depths of 15-45 m (50 -150 ft), depending on factors such as the profile and depth of the seabed. Good examples in the Philippines are the islands of Camiguin and Siquijor. These islands areseparated from the nearest main land by very deep-water channels, so deep that even during the last ice age they were not connected to these main lands. Another example is for instance Catanduanes, a much larger island.Barrier Reefs - this type of reef can be found along edges of islands or continental shelves, they are substantial in size. The major difference between fringing reef and a barrier reef is that a wide and deep lagoons separates the last from the
shore. The outer edge of the barrier reef drops away deeply to the ocean floor beyond. Initially these reef were formed in shallow water, but if water level increases they built progressively upwards so that their living top parts stay close to the water surface. A nice example in Philippine waters of such a reed is the West Coast of Palawan Island.
Patch reefs, shoals - There are many places where reefs exists as isolated mounds or hillocks on the seabed. They vary in size from a few meters in diameter to thousands of meters. Some even emerge above the sea water and are topped by sand cays. They are found in usually rather shallow water and can be a serious obstruction to shipping. Many are, for instance, found in the China Sea.
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