Geological Background

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Aerial View of the Florida Keys
In order to understand the ecosystem of the Florida Keys, one must understand teh general geology and physical repercussions of the geophysical formations.

The Florida Keys Region extends from Soldier Key, just south of Key Biscayne in Dade County, to the Dry Tortugas.  Geologically, the Florida Keys are divided into two groups based upon the limestone that forms the islands.  The Upper Keys are comprised of Miami Oolite.  The upper Keys were
a once live, thriving and dense forests of many corals and other marine organisms - flora and fauna. As one would expect the coral forests were of various densities and heights; however, they are of less height the farther south one goes except for Key West. These have heights from 10 to 18 feet. The Low Coral Keys are from five to 10 feet. As the glaciers reformed taking water from the ocean, sea level dropped, the coral forests died and collapsed into the now known  islands.  Big Pine Key signals the beginning of the Oolite Keys. Oolite is compacted small egg shaped deposits of calcium carbonate and in this area lays on top of Key Largo Limestone.


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Map of the Florida Keys
Narrow, natural beaches begin to occur along Upper Matecumbe Key and can be found on Lower Matecumbe Key, Long Key, Toms Harbor Keys, Grassy Key, Crawl Key, Little Crawl Key, Vaca Key, and Boot Key. There are 2.6 miles of critically eroded beaches in this subregion.

Carbonate sand beaches fronting the Straits of Florida include Little Duck Key, Missouri Key, Ohio Key, Bahia Honda Key, Spanish Harbor Key, Big Pine Key, Newfound Harbor Keys, Ramrod Key, Sugarloaf Key, Boca Chica Key, and Key West. On the Gulf of Mexico side of the islands, narrow carbonate sand beaches occur on Bahia Honda Key, the Content Keys, Sawyer Key, Marvin Key, Snipe Point, and Mud Key. There are 5.1 miles of critically eroded beaches in this subregion. Erosion is attributed to tropical storms and hurricanes, and to natural geomorphic changes caused by the pattern of littoral transport of sediments in this area. However, natural shoreline fluctuations are exaggerated by interaction with coastal protection structures, such as seawalls and revetments, which are located at the ends of many small pocket beaches in this region. The most erosive storms in recent years were Hurricane Georges in 1998, which caused extensive property damage throughout the Keys and significant beach erosion at many locations, and Hurricane Irene in 1999.


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Gelogical map of the Florida Keys soils
The Florida Keys form a crescentic chain of small limestone islands which extend from near Miami to Key West, a distance of about 150 miles. They are made of two main formations of Pleistocene age—the Key Largo Limestone- anelevated coral reef rock and the Miami oolitic Limestone.  The Key Largo Limestone is the surface rock of the Upper Keys, and the Miami Limestone covers the Lower Keys. A contact found at Big Pine Key shows that the oölitic limestone overlaps the Key Largo, and core borings show that the Key Largo underlies the oölitic cover for the entire area of the Lower Keys.The Key Largo, therefore, extends for the total length of the Keys. A core placed at Dry Tortugas, 70 miles west of Key West,encountered it at 30 feet below sea level. The Key Largo has a maximum thickness of over 200 feet.



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Cross-section of the Coral Reefs
The fact that the Keys are located about 5 miles from the seaward edge of the Florida coral reef platform creates an interesting problem. The remnants of an ancient outer reef similar to that found today bordering the living coral reef tract may have become established as a line of patch reefs in the back reef zone of a pre-existing platform which extended seaward some distance.

To the east of the Florida Keys lies the only living coral reef in the continental United States. It is located between four and seven miles offshore, running parallel to the Keys from Key Largo to the Dry Tortugas. This living marine marvel, rising as high as a few feet below the surface of the water and descending to depths near the Gulf Stream, protects the Keys from the waves of the pounding Atlantic surf and hence from the development of sand beaches.

Florida's coral reefs came into existence 5,000 to 7,000 years ago when sea levels rose following the Wisconsin Ice Age. Reef growth is slow; estimates range from one to sixteen feet every 1,000 years.

Stony corals are the major reef architects. Polyps, the living portion of the coral, extract calcium from seawater and combine it with carbon dioxide to construct the elaborate limestone skeletons that form the reef backbone. Coral polyps are united into colonies. An individual colony grows one-half to seven inches a year, depending on the species. Corals start life as free-living larvae that later settle on the sea floor and develop into massive, sedentary limestone formations.

Though reef corals are classified as animals, there is, in fact, a complex of microscopic plants that lives within the animal tissues (a symbiotic relationship). The animals benefit from the energy that the plants provide through photosynthesis. The plants are protected within the coral tissues and gain nutrients from animal wastes. These tiny plants are called zooxanthellae and are responsible for much of the color seen in reef corals.