Florida Keys Geography — Florida

The Florida Keys are a crescent-shaped archipelago of approximately 1,700 limestone islands extending roughly 220 miles southwest from the southern tip of the Florida peninsula to the Dry Tortugas, with the Atlantic Ocean to the southeast and Florida Bay and the Gulf of Mexico to the northwest. The Florida Museum of Natural History at the University of Florida documents the chain as 202 miles (325 km) in length, reaching to within 90 miles (145 km) of Cuba — placing Key West at the southernmost point of the contiguous United States. The islands are the fossilized remnants of ancient coral reefs and oolithic sand bars formed during the Pleistocene epoch, making them geologically distinct from any other island chain in the continental United States. The chain sits almost entirely within Monroe County, which by total area — approximately 3,738 square miles of land and water combined — is the largest county in Florida. At the 2020 U.S. Census, Monroe County recorded a population of 82,874, with more than 99% of county residents living on the Keys themselves.

The Florida Keys owe their existence to two distinct Pleistocene limestone formations identified in the geological literature. The Key Largo Limestone — an elevated fossil coral reef rock — forms the surface geology of the Upper Keys, from approximately Elliott Key south to Big Pine Key. The Miami Limestone, an oolithic rock formed from carbonate sand grains precipitated in warm shallow water, underlies the Lower Keys. A study published in the GSA Bulletin by Hoffmeister and Multer (1968), as cited in peer-reviewed geological literature, established that both formations date to the last interglacial period of the Pleistocene epoch, approximately 125,000 to 100,000 years ago. The Key Largo Limestone also underlies the Miami Limestone in the Lower Keys; a geological core drilled at the Dry Tortugas encountered Key Largo Limestone at 30 feet below sea level.

The NOAA Florida Keys National Marine Sanctuary 2011 Condition Report describes how rising sea levels beginning approximately 15,000 years ago flooded the ancient reef and sand formations, creating the modern island chain through a combination of marine inundation and tidal influence. The name key derives from the Spanish cayo, meaning small island, used by Spanish explorers and cartographers from the early 16th century onward. According to the Keys History Project, the first documented American use of the English spelling key for these islands appears in a 1744 shipwreck lawsuit. The northernmost islands in Miami-Dade County — including Virginia Key and Soldier Key — differ from the rest of the chain in that they are sedimentary in nature rather than solid limestone.

The Florida Keys rank among the lowest-lying land in the United States. Earth Magazine's geology feature on the Keys documents that the highest point in the entire chain is located on Windley Key — the site of Windley Key Fossil Reef Geological State Park — at only 5.5 meters (approximately 18 feet) above sea level. Most islands lie less than 2 meters above the surrounding water. The total land area of the Keys is approximately 137.3 square miles, situated within Monroe County.

The Monroe County Green Keys Initiative provides average elevation figures by subregion: the Upper Keys (Ocean Reef to Tavernier Creek) average 4.8 feet above sea level; the Middle Keys (Plantation Key to Knights Key and Marathon) average 4.29 feet; and the Lower Keys (Ohio Key to Stock Island) average 3.17 feet. These figures reflect a general trend of decreasing elevation from northeast to southwest along the chain.

The Florida Keys form a natural boundary between two major water bodies: the Atlantic Ocean lies to the southeast, and Florida Bay — a large, shallow inland sea averaging roughly 1.5 meters in depth — lies to the northwest. Tidal passes cut through the island chain, particularly through the Middle Keys, allowing water exchange between Florida Bay and the Atlantic that drives nutrient cycling across coral reef and seagrass ecosystems, as the NOAA 2011 Condition Report documents.

Running along the Atlantic flank of the Keys from the Miami area southwest to the Dry Tortugas is the Florida Reef Tract — documented by the Florida Museum of Natural History as the only coral reef system off the North American continent and the third largest barrier reef in the world, after Australia's Great Barrier Reef and Belize's Mesoamerican Reef. The Florida Keys National Marine Sanctuary, designated by Congress on November 16, 1990, protects 4,539 square miles of waters surrounding the Keys. NOAA's Sentinel Site Program characterizes the sanctuary as one of the largest marine protected areas in the United States, encompassing 2,896 square nautical miles. NOAA's Office of National Marine Sanctuaries reports that nearly 6,000 marine species inhabit the coral reefs, mangrove forests, seagrass beds, and other sanctuary habitats. The sanctuary's habitat framework identifies five distinct geographic regions: Florida Bay, the Atlantic nearshore environment, the southwest continental shelf, the Florida Reef Tract, and the Straits of Florida.

The Florida Keys are conventionally divided into five geographic subregions by NOAA and resource managers: the Upper Keys, Middle Keys, Lower Keys, Marquesas Keys, and Dry Tortugas. The Upper Keys — extending from Elliott Key, which lies within Biscayne National Park, southward to approximately Big Pine Key — rest atop exposed Key Largo Limestone and include Key Largo, the largest individual island in the chain by land area. The city of Islamorada, incorporated as a municipality in 1997, occupies a significant portion of this subregion.

The Middle Keys, centered on the City of Marathon, occupy an estuarine transition zone. The NOAA Condition Report notes that tidal passes through the Middle Keys historically created conditions less favorable for coral reef development along portions of this subregion's nearshore Atlantic shelf. The Lower Keys, from Big Pine Key to Stock Island, are underlain by Miami Limestone and are characterized by the lowest average elevations in the chain and a higher proportion of shallow tidal flats. Big Pine Key is the location where the geological contact between the Key Largo Limestone and the Miami Limestone occurs, as documented in analyses citing Hoffmeister and Multer.

The Marquesas Keys — an uninhabited atoll-like ring of mangrove islands approximately 25 miles west of Key West — and the Dry Tortugas, roughly 70 miles west of Key West, complete the chain's western arc. Fort Jefferson, a massive 19th-century coastal fortification on Garden Key in the Dry Tortugas, is administered as Dry Tortugas National Park. The Florida Fish and Wildlife Conservation Commission documents that native habitats across the Keys — including rockland hammock, mangrove swamp, and coastal rock barren and Keys tidal rock barren — support plant and animal species found nowhere else in the continental United States.

The Overseas Highway (U.S. Route 1) is the sole road connection linking the Florida Keys to mainland Florida and linking the islands to one another. Encyclopaedia Britannica documents that the highway features 42 bridges, including the Seven Mile Bridge, and is among the longest overwater roads in the world. The highway was completed in 1938, built largely over the former right-of-way of Henry Flagler's Florida East Coast Railway Overseas Railroad. Flagler's railroad was completed in 1912 and connected Key West to the Florida mainland for the first time by rail; it was heavily damaged and subsequently abandoned after the Labor Day Hurricane of September 1935. The highway's function as the exclusive evacuation corridor for a permanent resident population of more than 73,000 — as recorded in the 2010 census — as well as a large seasonal tourism economy, makes its physical geography a matter of sustained public safety and emergency management concern at the county and state level.

The combination of extremely low elevation, porous karst limestone substrate, and geographic isolation makes the Florida Keys one of Florida's most documented sea-level-rise vulnerabilities. The Monroe County official resilience planning page describes the county as ground zero for experiencing the impacts of global climate change and sea-level rise, citing routine nuisance flooding during extreme high tides. The Key West tide gauge, which has recorded water levels since 1913, provides one of the longest continuous sea-level records on the Gulf Coast. NOAA's National Weather Service Key West office reports that NOAA projections for local sea-level rise at the Key West gauge range from approximately 1.5 feet to 7 feet by 2100.

In June 2021, NOAA scientists presented sea-level rise science and updated projections directly to Monroe County commissioners, who subsequently passed resolutions reaffirming the use of projections from the Southeast Florida Regional Climate Change Compact — a four-county regional planning body comprising Monroe, Miami-Dade, Broward, and Palm Beach counties. Monroe County subsequently launched a Sea Level Rise Vulnerability Analysis focused on county-maintained road infrastructure. The Keys' geography also intersects with broader Florida-wide environmental policy: the health of Florida Bay, which lies between the Keys and the Everglades, is directly tied to freshwater inflow through Everglades restoration, linking Keys marine geography to statewide water management decisions administered through the South Florida Water Management District and related federal programs.