The Florida Keys

The Florida Keys are a coral cay archipelago located off the southern coast of Florida, forming the southernmost portion of the continental United States. They begin at the southeastern coast of the Florida peninsula, about 15 miles (24 km) south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry Tortugas. The islands lie along the Florida Straits, dividing the Atlantic Ocean to the east from the Gulf of Mexico to the northwest, and defining one edge of Florida Bay. At the nearest point, the southern part of Key West is just 90 miles (140 km) from Cuba. The Florida Keys are between about 23.5 and 25.5 degrees North latitude. 

The climate of the Keys is defined as tropical savanna according to Köppen climate classification. More than 95 percent of the land area lies in Monroe County, but a small portion extends northeast into Miami-Dade County, such as Totten Key. The total land area is 137.3 square miles (356 km2). As of the 2010 census the population was 73,090 with an average density of 532.34 per square mile (205.54/km2), although much of the population is concentrated in a few areas of much higher density, such as the city of Key West, which has 32% of the entire population of the Keys. The US Census population estimate for 2014 is 77,136. 

The city of Key West is the county seat of Monroe County. The county consists of a section on the mainland which is almost entirely in Everglades National Park, and the Keys islands from Key Largo to the Dry Tortugas. 

The Keys were originally inhabited by Calusa and Tequesta Native Americans. They were later found and charted by Juan Ponce de León in 1513. De León named the islands Los Martires ('The Martyrs') as they looked like suffering men from a distance. "Key" is derived from the Spanish Cayo, meaning small island. For many years, Key West was the largest town in Florida, and it grew prosperous on wrecking. The isolated outpost was well located for trade with Cuba, the Bahamas, and was on the main trade route from New Orleans. Improved navigation led to fewer shipwrecks, and Key West went into a decline in the late nineteenth century. 

The Florida Keys are the exposed portions of an ancient coral reef. 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 Sangamonian Stage 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 (110 m) 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. 

Just offshore of the Florida Keys along the edge of the Florida Straits is the Florida Reef (also known as the Florida Reef Tract). The Florida Reef extends 270 km from Fowey Rocks just east of Soldier Key to just south of the Marquesas Keys. It is the third-largest barrier reef system in the world.