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The Florida Keys National Marine Sanctuary is a U.S. National Marine Sanctuary in the Florida Keys. It includes the third-largest coral barrier reef in the world. It also has extensive mangrove forest and seagrass fields. The Marine Sanctuary includes a 2,800 square nautical mile (9,600 km²) area surrounding the Keys and reaching into the Atlantic Ocean, Florida Bay and the Gulf of Mexico. The Sanctuary was established in 1990.
The Florida Keys are the exposed portions of an ancient coral reef, with very little sand. The northernmost island arising from the ancient reef formation is Elliott Key, in Biscayne National Park. North of Elliott Key are several small transitional keys, composed of sand built up around small areas of exposed ancient reef. Further north, Key Biscayne and places north are barrier islands, built up of sand.
The Florida Keys have taken their present form as the result of the drastic changes in sea level associated with recent glaciations or ice ages. Beginning some 130,000 years ago the Sangamon interglacial raised sea levels to approximately 25 feet (7.5 m.) above the current level. All of southern Florida was covered by a shallow sea. Several parallel lines of reef formed along the edge of the submerged Florida plateau, stretching south and then west from the present Miami area to what is now the Dry Tortugas. This reef formed the Key Largo limestone that is exposed on the surface from Soldier Key (midway between Key Biscayne and Elliott Key) to the southeast portion of Big Pine Key and the Newfound Harbor Keys. The types of coral that formed Key Largo limestone can be identified on the exposed surface of these keys.
Starting about 100,000 years ago the Wisconsin glaciation began lowering sea levels, exposing the coral reef and surrounding marine sediments. By 15,000 years ago the sea level had dropped to 300 to 350 feet below the contemporary level. The exposed reefs and sediments were heavily eroded. Acidic water, which can result from decaying vegetation, dissolves limestone. Some of the dissolved limestone redeposited as a denser cap rock, which can be seen as outcrops overlying the Key Largo and Miami limestones throughout the Keys. The limestone that eroded from the reef formed oolites in the shallow sea behind the reef, and together with the skeletal remains of bryozoans, formed the Miami limestone that is the current surface bedrock of the lower Florida peninsula and the lower keys from Big Pine Key to Key West. To the west of Key West the ancient reef is covered by recent calcareous sand. Wikipedia |
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