The Geology of Dry Tortugas National Park: Coral Islands on the Florida Platform

Dry Tortugas National Park lies about 113 km (70 mi) west of Key West, between the Gulf of Mexico and the Atlantic Ocean. Colorful coral reefs surround a cluster of seven small islands and a central lagoon, and the park is known for its marine life, nesting bird colonies, military history and tales of pirates and sunken treasure. It can be reached only by boat or seaplane.

A presidential proclamation created Fort Jefferson National Monument in 1935 to preserve the Dry Tortugas islands within the federal military reservation of islands, keys and banks set aside in 1845. Fort Jefferson, on Garden Key, is described as the largest masonry structure in the Americas. In 1992 Congress expanded the monument and renamed it Dry Tortugas National Park.

Deep roots

Florida's geologic history reaches back to the Precambrian, more than 541 million years ago. Precambrian and Cambrian igneous rocks lie thousands of meters beneath the carbonate platform, forming Florida's "basement." Above them are younger Ordovician, Silurian and Devonian sedimentary rocks, laid down in a basin created by the rising Appalachian Mountains as the supercontinent Pangaea came together. By 300 million years ago the continents had joined as Pangaea, and what is now Florida was part of the ancient landmass called Gondwana.

The Dry Tortugas story proper begins about 200 million years ago, when Pangaea broke apart, the modern Atlantic opened, and the Florida Platform began to build up.

The Florida Platform

The park sits near the southwestern edge of the Florida Platform, a broad, flat carbonate platform of which the Florida peninsula is only the exposed part. The submerged portion reaches depths of about 90 m (300 ft); beyond that, the sea floor drops abruptly to more than 3,000 m (10,000 ft).

Limestone and dolomite more than 15,000 feet thick make up the platform beneath the park, and these rocks are still forming on its submerged part.

During the Pleistocene (2.6 million to 11,700 years ago), sea level rose and fell again and again as glaciers advanced and retreated. In glacial times, with more of Earth's water locked up as ice, much of the platform was exposed and its rock eroded. In warmer interglacial times the platform was largely underwater, and coral reefs grew around the edge of a small Florida peninsula. When the sea later fell to its present level, the tops of those reefs were left exposed — they are the Florida Keys.

Building the reefs

About 6,000 years ago the sea rose high enough to flood the southern Florida Platform. The lowest ground, including the area east of the park, flooded first, cutting the Dry Tortugas off from the mainland early on. The sea kept rising through the Holocene until the continental shelf was drowned to its present extent, leaving only a small fraction of the keys above water.

As sea level steadied, modern corals settled on the raised foundation of older Pleistocene reefs, the Key Largo Limestone. Waves, exposure and organisms broke the reefs down into large amounts of carbonate sand, which collected in the deeper water around the old reef foundation. Seagrass and other underwater plants spread over the calmer sandy areas behind the reefs, and gradually the Dry Tortugas of today took shape.

Loggerhead Light on Loggerhead Key warns mariners of the keys and the shallow, reef-fringed carbonate banks. USGS photo by Don Hickey.

Fossils

Every geologic layer in the park contains fossils, from the Holocene reefs and island sediments at the surface down to the Pleistocene limestone below — corals, mollusks and other invertebrates, microfauna and the remains of calcareous algae. All fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Part of the park's digital geologic map. National Park Service, Geologic Resources Inventory.

The NPS Geologic Resources Inventory report and map for the park, and an NPS soil survey, are available through the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Dry Tortugas National Park, Florida," citing National Park Service (2006), Robbin (1984), Murphy (1993), Davis (1997) and Toscano and others (2010).