Florida, the Sunshine State, provides the continental United States a unique geographic feature that attracts over 100 million visitors each year. Its population growth since the end of World War 2 has been spectacular. It has gone from the second least populated state to the third most popular state in just 75 years. Situated on a peninsula surrounded by the Atlantic Ocean on the east and by the Gulf of Mexico on the west, its coastline stretches 1,350 miles, second only to Alaska. It is also the flattest state in the Union and contains 27 first magnitude freshwater springs, the most of any other state. Also, it is the only contiguous state with a tropical environment, and its geologic history is unique as well.
The Florida peninsula consists of a porous plateau of karst limestone sitting atop a continental bedrock feature called the Florida platform. The emergent land that is Florida has formed fairly recently in geologic time as the Gulf of Mexico trough was filled in with sedimentary rocks, primarily from the Mississippi River and by the deposit of carbonate sediments across the platform. The carbonates developed a karst topography, characterized by sinkholes and caves caused by the dissolution of soluble rocks by fresh water. Hence the large number of freshwater springs.
The Florida platform is much wider than the actual emergent peninsula and on the west side plunges over 9000 feet down to the abyssal plain of the Gulf of Mexico, over 100 miles west of the current coastline. On the eastern side, across a 2500 foot deep Miami Terrace, the Little Bahamas Bank extends eastward until it begins to drop rapidly to about 4500 feet, and then plunges over the Blake escarpment almost 12000 feet to the Atlantic abyssal plain. The geology of Florida reveals a three-fold history of the development of this portion of the North American Continental Plate that contributes to the unique setting of this fascinating state. A technique such as borehole logging enables geologists to rediscover the uniqueness of this state.
The first stage of development of the Florida Platform occurred more than 160 million years ago (geologic time) when a supercontinent called Gondwana, containing present day South America, Africa, Arabia, Madagascar, India, Australia and Antarctica, collided with a supercontinent called Laurasia, containing Asia, Europe, and North America, to form the even larger supercontinent called Pangaea. At that time, Florida was sandwiched between the Americas and Africa, but the volcanic and sedimentary rocks in the platform are more closely related to the rocks in Senegal, Africa than to those in the North American plate. When Pangaea began to break apart, some of the African plate stayed connected to the American plate and went with it as the Atlantic basin began to develop.
The second stage occurred as the North American plate moved westward carrying the Florida platform with it. Warm, shallow seas covered the platform while a strong, west-to-east current between Georgia and Florida kept siliciclastic sediments from flooding it from the eroding Appalachian Mountain chain to the north. Much like today in the eastern Gulf of Mexico, these clear, sediment-free, marine waters allowed carbonate rocks to be deposited across the platform. This massive deposition of carbonate rocks lasted from approximately 160 million years to just 23 million years ago and accounts for the development of the karst topography and the many fresh-water springs in the state.
The third stage started about 23 million years ago when siliciclastic sediments began to flood into the platform from the north and to build up the Florida Peninsula as we know it today on top of it. The principle geologic layer of this period was the Hawthorn Group of sand, silt and clay which covered the underlying carbonate rocks. Being much less permeable than the carbonates, these sediments produced an impermeable layer that trapped the fresh water in the Floridan Aquifer. This aquifer has provided drinking water for almost 10 million people, as well as water for industrial and agricultural uses. Furthermore, these clastic sediments contain an abundance of fossil remains from both terrestrial and marine environments. Saber-toothed tigers, camels, rhinos, tapirs, giant sloths, elephants, horses, dugongs, whales and sharks (mainly their teeth) have left their remains behind in these rock layers. These layers have important ramifications for climate change studies today because they have recorded short term changes in sea level across the platform, during the past 23 million years. Specifically, they demonstrate a series of high stand conditions, when the sea level is relatively high, that alternate with low stands, when the sea level is relatively low. These changes have been correlated with continental glacial periods, with the low stands developing when glacial activity was high and high stands when the glaciers melted.
This latter stage of siliciclastic invasion is still occurring today over most of the state. Only the far south portion, especially the Florida Keys, continue to produce conditions where coral and algal reefs can still form. Sea level changes that are recorded across the Florida Peninsula by the distribution of clastic and marine sediments are important indicators of the potential widespread impact of global warming in the future. The unique geologic history of Florida provides the appropriate background for the unique geographic features that make Florida an interesting and exciting state to visit.
One caveat, however. Be sure to check the tropical weather forecasts if you plan a visit, especially during hurricane season. Although no hurricanes made landfall in Florida during the 2020 season when the United States was hit by 12 named storms, the most in 104 years, 41% of all hurricanes that have struck the US since 1851 have made a landfall there. Since records began to be kept, the state has been hit by more than twice the number of hurricanes than the second leading state, Texas.