Florida's Geologic History

   The Earth is more than 4.5 billion years old!  During this unimaginable expanse of time our planet has undergone drastic changes.  For the earliest part of Earth’s history the planet was a molten inferno.  As the planet slowly cooled, rocks and minerals began to form and continents and ocean basins took shape.  Earth’s continents today look nothing like they did hundreds of millions of years ago.  In fact, the continents move around through a process called plate tectonics.  Continental crust (the thin outer skin of our planet) sits on top of hot rock material, called the mantle, which behaves like cold syrup .  As the plates shift they can collide, causing mountain ranges and deep ocean trenches, and they can slide past one another along long faults, like the San Andreas fault, and they can spread apart as seen along mid-ocean ridges.  Plate tectonics describes the processes involved in plate motions and allows geologists to understand how Earth’s plates, of which Florida is a part, came to be.

   The geological history of Florida can be traced back to the Paleozoic Era, 540 – 251 million years ago (mya) based upon rock core samples retrieved from thousands of feet below the surface.  These rocks, referred to as basement rocks, consist of igneous and metamorphic suites overlain by sandstones and shales.  These sequences of rocks record the events that were taking place as the Laurentian and Gondwanan  landmasses  were converging to create the supercontinent of Pangea. As these and other smaller landmasses converged they would create the foundation for the accumulation of vast thicknesses of carbonate (limestone) which would eventually become the Florida Platform.

   During the early Mesozoic Era (251 – 65.5 mya) the supercontinent of Pangea began to rift and break apart.  At this time, Florida was located between what would later become the continents of Africa, South America and North America.  In fact, as North America separated from Africa a small portion of the African plate remained “stuck” to North America and that provided some of the foundation upon which Florida now rests.  Geologists can tell this by looking at the chemistry and fossil assemblage of Florida’s basement rocks.  During the later part of the Mesozoic Period, Florida’s landmass was beneath a warm, shallow ocean.  As marine organisms died and sank to the ocean floor they began to accumulate in great thickness.  This sediment would later become limestone.

   The end of the Mesozoic Era was brought about by a great cataclysm – a large meteor impact in what is now the Yucatan Peninsula.  This event, it is thought, was catastrophic enough to cause mass extinction and probably brought about the demise of the dinosaurs.  Although there are rocks of this age in Florida, there is no evidence of the impact in the rock record here.  Whatever debris might have been deposited on the Florida Platform was eroded away at a later time.

   During the Cenozoic Era (65.5 mya – present) Florida slowly took its current shape.  Warm, tropical oceans still covered the state until the Late Oligocene Epoch (28.4 – 23 mya).  Limestone comprised of the skeletons of billions of small creatures called foraminifera accumulated.  Large, voracious whales roamed our shallow seas hunting other marine vertebrates.  Small patch reefs formed in the warm, clear, shallow waters of Florida during this time.  Also during this time period a marine current, very similar to the Gulf Stream, swept across northern Florida and scoured the sea floor.  This current deflected sediment that was being eroded and transported from the main land.  This is the reason why limestones from this time period in Florida are so pure (up to 99% calcium carbonate).

   At the end of the Oligocene Epoch, sea levels dropped and Florida emerged from the sea.  The first fossils of terrestrial vertebrates come from this time period and include animals like bats, horses and carnivores.  From this point on at least some portion of Florida would remain above sea level.  It is primarily the interaction between land and the ocean that has sculpted the landforms of Florida.  Karst refers to features that have formed as the result of dissolution of rock material.  Limestone, which underlies all of Florida, is able to be dissolved by slightly acidic rain water.  Over geologic time (millions of years) large pore spaces, conduits and caverns can form.  As the land surface collapses into these voids, sinkholes form.  Sinkholes are a prominent feature in the Florida landscape.  Other karst features include springs, air caves and disappearing streams.

   Throughout the end of the Oligocene and into the Miocene, sea-levels fluctuated and clays and sands became common deposits.  The Miocene Epoch (23 – 5.3 mya) was a time of unique conditions across Florida.  In the Early Miocene, the Appalachians were uplifted, erosional rates increased, and continental siliciclastic sediments filled the Gulf Trough. Siliciclastic sediments began encroaching southward upon the carbonate depositing environments. Large deposits of phosphorite accumulated as cool, nutrient-laden ocean water bathed Florida.  These deposits are mined today and account for a significant portion of the phosphate produced in the United States.  Unique creatures also existed in Florida at this time.  Large sharks patrolled the near-shore marine environments preying on whales.  Horses, saber-toothed cats and elephants roamed the land.  Many of these creatures left behind their bones in Miocene deposits.  A prized fossil from this time period is the tooth from the giant extinct Caracharodon megalodon shark.  These teeth can exceed six inches in length and belonged to an animal that may have been fifty feet long! 

   The Pliocene Epoch (5.3 – 2.6 mya) was an important time for land animals in Florida.  North America became connected to South America and allowed animals from both continents to travel freely between them.  North America became the home to animals like sloths, giant armadillos and llamas that migrated north over the newly formed connection.  Ocean currents were also interrupted and the Gulf of Mexico became isolated from the influence of the Pacific.  The exchange of flora and fauna from South America is known as the Great American Interchange.  Sea levels were fluctuating and marine deposits (limestone and shell beds) were accumulating in south Florida.  Some of the most diverse molluscan faunas in the world accumulated in southwest Florida during this time.

   The Pleistocene Epoch (2.6 mya – 10,000 years ago), also known as the Ice Age, was a time of extreme climate and sea-level change.  Sea levels were as much as 300 feet lower and as much as 100 feet higher than today.  As the giant continental glaciers advanced and retreated, sea-levels responded by falling and rising.  During warm, interglacial periods, sea levels were sufficiently high to allow marine limestones to accumulate.  During glacial periods, sea levels were much lower and erosion and dissolution of limestone occurred.  Giant ice-age mammals roamed Florida at this time and included some of the largest land mammals to have ever existed.  Some of these animals include mammoths, mastodons, giant lions, Dire wolves, saber-tooth cats, giant sloths and giant beavers.  At the end of the Pleistocene another animal arrived in Florida – man.  This also coincided with the demise of the giant ice-age mammals.  Many large mammals went extinct at the end of the Pleistocene either as the result of climate change, human hunting or a combination of both.