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.
During the Holocene Epoch (10,000 years ago –
present), sea level reached its current elevation.
Human populations expanded and shaped the landscape
to suit their needs. The Everglades of south
Florida formed and thick layers of peat were
deposited. The Keys became and
new coral reefs began to grow. Our modern climate
developed and Florida took the shape we are all used
to seeing – a long peninsula.