Florida’s aquifers — a geologic history

To understand how the limestone aquifer that underlies all of Florida was formed, we need to look back 175 million years to the Mesozoic era and the breakup of the supercontinent called Pangea. 

Earth’s solid surface is called the lithosphere and is made up of tectonic plates. These plates are very thin compared to Earth’s volume and are always in motion, causing continental drift. Tectonic plates float on top of Earth’s asthenosphere, which is the superheated uppermost rock layer in Earth’s interior. Scientists describe Earth’s tectonic plates as thinner than an apple peel is to an apple. As the plates move over the Earth’s asthenosphere, they pull apart, crash together, and slide past each other, causing earthquakes, volcanoes, and mountain building.

Geologists, specifically paleontologists, discover the story of Earth’s past through the study of the rock record. Paleontologists use geologic principles to figure out what the rocks and geologic landforms are telling us about the past. Here are two principles that can help us understand Florida’s geologic history.

Uniformitarianism -The physical and chemical laws that operate today have operated in the same manner throughout Earth's history. “The present is the key to the past.” If we look at today’s geologic processes and what they produce, we can assume the same processes were operating thousands, millions, or even billions of years ago. 

For example, when we find limestone deposits, we assume they were formed in a similar way as we observe today, in warm shallow seas. 

Superposition - In an undisturbed sequence of sedimentary rocks, the oldest layer is on the bottom, and the youngest layer is at the top. 

For example, Florida has “layer cake" geology, which is the easiest type of rock record to interpret. No volcanoes, no mountain building, and no faults, just a sequence of horizontal layers of sediments deposited over geologic time.

The breakup of the supercontinent Pangaea began when the Mid-Atlantic Ridge rift zone began to separate Pangaea into the individual continents we know today. This rift zone became wider and wider, creating the Atlantic Ocean and moving North America apart from Africa and South America. The bedrock underlying Florida is made up of metamorphic and igneous rocks, which are remnants of ancient Pangea. These rocks share an origin with rocks in Africa that were separated from North America when the Atlantic Ocean was formed, and are approximately 500 million years old. 

From about 175 million to 15 million years ago, the area that makes up Florida was a warm, shallow sea; limestone, dolomite, and shell deposits accumulated over this long time period. There were essentially no terrestrial sediments that reached this shallow sea, causing Florida’s pure limestones to be almost entirely made of carbonates. This is known as the Great Florida Carbonate Platform. The carbonate layer is 1-5 kilometers thick and includes the shells of marine organisms, microscopic plankton, and vast areas of ancient coral reefs.

Starting approximately 30 million years ago, terrestrial sediments originating from the Appalachian Mountains started to cover Florida’s shallow sea with clays, silts, and sands, creating the modern landform of the Florida peninsula.

In the Cenozoic era, the final process that created the water-bearing rocks of Floridan Aquifer System, known as karst topography was formed. For many years sea levels changed during Earth’s ice ages, when water is locked up in massive glaciers worldwide, it causes sea level to be lower, at times doubling the size of the Florida peninsula.

During these periods, limestone was exposed at the land surface. As rain falls through the atmosphere, it becomes slightly acidic. When this water filters into the ground, it slowly dissolves the calcium carbonate in the underlying limestone, creating pores in the rock. Over thousands of years, these pores coalesce into massive cavities that fill with water.

As this karst topography was formed, so was our modern Floridan Aquifer System, which currently provides us with high-quality artesian water. 

Geologic time is hard to fathom, and it is even harder to summarize in a short article. There are geologic events not mentioned in this brief description of how the Florida peninsula and our amazing Floridan Aquifer System came to be.

A video about the creation of the Florida Aquifer System is below.