The Florida Keys are a distinctive chain of islands curving southwest from the southern tip of the Florida peninsula, separating the Atlantic Ocean from the Gulf of Mexico. This unique geographic feature spans a significant distance. The formation functions as a single, connected entity, sharing a singular history and environment. This often prompts questions about its formal classification. Understanding the nature of this chain, from its ancient material composition to its modern-day layout, helps define its place among the world’s island groups.
Defining an Archipelago
An archipelago is formally defined as a group or chain of islands closely scattered in a sea or other body of water. The Florida Keys perfectly fit this geographic description, making them a recognized archipelago. The chain is composed of over 800 named islands, or “keys,” a term derived from the Spanish word cayo, meaning small island. These keys stretch in a gentle arc for approximately 120 to 150 miles, beginning near Miami and extending all the way to the Dry Tortugas.
Coral Cay Classification
The Florida Keys are specified as a coral cay archipelago, a type formed by the accumulation of sediment on top of coral reefs. This designation confirms that they are indeed an archipelago. While many of the islands are small and uninhabited, their collective arrangement establishes the Keys as a cohesive island group. The concentration and proximity of these landmasses meet the criteria for a true chain of islands.
The Unique Geological Origin of the Keys
The formation of the Florida Keys is tied directly to the dramatic fluctuations in sea level that occurred during the Pleistocene Epoch. Unlike volcanic archipelagos, this island chain was built by the growth and exposure of ancient marine life, not erupting magma. The islands are primarily composed of two distinct types of limestone, both formed underwater when sea levels were much higher.
Key Largo Limestone
The bedrock of the Upper and Middle Keys is Key Largo Limestone, which is fossilized coral reef rock. This material is evidence of a time when living coral reefs flourished in the shallow marine environment, creating a massive, elevated structure. When glacial periods caused global sea levels to drop significantly, these ancient reefs were exposed to the air, forming the initial landmasses.
Miami Limestone
The Lower Keys, in contrast, are mainly surfaced with Miami Limestone, which is oolitic limestone. Oolite consists of millions of tiny, spherical calcium carbonate pellets, called ooids, which accumulated in the shallow, current-swept waters behind the main reef tract. This oolitic formation overlaps the Key Largo Limestone in the Lower Keys, demonstrating the sequence of deposition that built the islands.
Structure and Extent of the Island Chain
The Florida Keys archipelago extends for approximately 125 miles from the mainland, following a distinctive arc that first curves southwest and then turns westward. This crescent shape is a reflection of the underlying geological structures and the boundaries of the Florida Straits. Geographically, the chain is commonly divided into three main regions: the Upper Keys, the Middle Keys, and the Lower Keys.
Geographic Divisions
The Upper Keys, including Key Largo and Islamorada, are closest to the mainland. The Middle Keys are centered around the city of Marathon, while the Lower Keys include Key West, the southernmost inhabited island. The entire chain is connected by the Overseas Highway, U.S. Route 1.
The Overseas Highway
This highway utilizes 42 separate bridges to span the water gaps between the major islands, effectively linking the entire archipelago. The longest of these spans is the Seven Mile Bridge, which connects the Middle Keys to the Lower Keys. This connection allows for continuous travel across the islands.