1. Archive


Second of two parts

Florida is almost surrounded by ocean. It is one of only two states with a coastline along two different oceanic basins _ Alaska is the other. Closeness to the ocean has major consequences for how Floridians live and for what Florida is like. An underground ocean is just one of these consequences.

Traveling this sandy peninsula would make this point clearly.

Suppose you decided to drive "coast to coast" across Florida from Cocoa Beach to St. Petersburg. Your car would begin by climbing slightly from the Atlantic seashore onto the ridges and low hills in the center of the state extending north from Lake Okeechobee. Then it would "descend" to the Tampa Bay region as the hills get smaller until you reach the shoreline of the Gulf of Mexico.

If you had a guidebook, it might tell you that you had passed near Florida's region of springs _ Blue Spring, Alexander Spring, Juniper Spring, and so on. Many are huge, "first magnitude" springs _ 65-million gallons of fresh water a day or more. (Florida has about twice as many first magnitude springs as any other U.S. state.)

A topographic map would show that you "descended" no more than about 300 feet while driving from the center of the state to the "West Coast." A road map would put the total "coast-to-coast" distance you traveled at about 100-150 miles.

The interesting thing is that, if you could get out of your car in St. Petersburg and climb into a little submarine that moved along the sea bed, you could continue to travel to the west for another 100-150 miles and not drop much more than another 300 feet or so.

Out to the west of us is a very broad, shallow part of the Gulf of Mexico which is about as large in area as the entire state of Florida. It is called the West Florida Continental Shelf, and it is one of the widest and largest continental shelves in the world. At its westernmost edge, called a shelf break, the sea bed drops steeply to the deep Gulf of Mexico some 2.5 miles below the elevation of Tampa Bay's beaches.

If the ocean were removed, we would clearly see that the state of Florida is located on top of the eastern half of a tremendous plateau that rises thousands of feet from the floors of the ocean basins on either side. It is called the Floridan Plateau.

The "West Coast" of Florida actually lies somewhere near the center _ and not the western edge _ of this plateau. Geologists have learned that the plateau is composed almost exclusively of limestone rock, which is made of a very important substance called calcium carbonate.

One way to think of the Floridan plateau is that it is like a huge "cake" in which the layers are forms of limestone, and the "icing" is mostly the quartz sand that is found in our gray (not brown or red) soils and our beautiful beaches. This "icing" also contains some clays and other pieces of rock debris washed down from Georgia and Alabama onto the limestone, beginning about 20-million years ago during the Miocene Epoch of geological time.

All of these observations and facts are clues that should lead us to suspect that Florida has an "underground ocean" and give us some idea of what that means for Floridians.

Why? Because of the nature of calcium carbonate. The limestone layers of the Floridan plateau were formed from tiny pieces of calcium carbonate shells that piled up on top of each other on the sea bed over many millions of years. This random piling-up process created holes between the pieces. Calcium carbonate is well known for dissolving in water _ either fresh water or sea water _ which can make new holes, make hollow channels, or make the old holes bigger.

Also, in a pile as large as the Floridan Plateau, the weight of all that material can cause calcium carbonate to develop small breaks or fractures. So we need to modify our "layer-cake" model of the plateau a bit by imagining that each limestone "layer" has an internal structure a little like a piece of Swiss cheese _ the holes tend to be connected to each other by lots of tiny channels and fractures.

Water can flow through these layers rather easily because of all these connected open spaces. All that is needed is for portions of the layers to come into contact with fresh water (or sea water) under conditions where the combination of the weight of the water and the overlying air pressure is large enough, and the water will be forced into the "Swiss Cheese" structure of the layers and begin to flow within them.

One of the most important portions of the plateau that is heavily riddled with this structure is called the Floridan Aquifer. It is a series of mostly limestone layers just below the sand-clay "icing" of the "cake." At places where the "icing" gets a little thin or is missing, rain water has its most direct access to the aquifer to "recharge" it with fresh water.

That access, plus the very high rainfall in Florida and southern parts of Georgia and Alabama (more than 50 inches per year), makes the upper part of the Floridan aquifer one of the most extensive and productive sources of fresh water in the world. It supplies the water flowing out of most of Florida's first magnitude springs and is the principal source of drinking water for Florida's burgeoning cities.

But what about the rest of the Floridan Plateau, such as in the lower part of the aquifer and deeper in the plateau as well as under the ocean out to the west on the West Florida Continental Shelf? That's where we encounter the underground "ocean," and the way it was discovered is a fascinating story.

Some years ago, wells were drilled in south Florida for oil exploration and waste disposal. Part of this operation involved measuring the temperature and collecting samples of the rock layers being drilled. Francis Kohout, a geologist with the U.S. Geological Survey, got a look at the drilling results and noticed that the temperatures of the rock layers in the Floridan aquifer were a bit unusual. Normally, the temperature of the surrounding rock increases as a drill grinds deeper and deeper. This is because the interior of the Earth is much hotter than the surface, and the flow of heat up through the rocks to the atmosphere sets up a predictable temperature gradient, from high in the interior to low on the surface.

The rocks in these wells were different: The increases in temperature down through the Floridan aquifer were very small. At some sites, there was even a temperature decrease. These "strange" rocks were largely limestones, and they often contained flowing salt water. Below them, more normal gradients of increasing temperature with depth were found. Only the aquifer rocks were strangely cool.

Kohout's explanation for this was that ocean water from around the Floridan Plateau was penetrating and "cooling" the rocks of the Floridan Aquifer, especially those in its lower portion some 2,000-3,000 feet below sea level.

Sea water can act as a coolant because all deep water in the major ocean basins is cold. The polar regions of the Earth are the only source of deep and bottom waters in the major oceans, which therefore have a year-round temperature between 30 and 35 degrees F. Surface temperatures in the ocean around Florida average about 75-80 degrees F.; so around 2,000-3,000 feet below sea level the ocean has an intermediate temperature of about 45 degrees F., which could easily cool off deep rocks that would otherwise reach temperatures of 100 degrees F. or more.

But this scheme would work only if the cold sea water from 2,000-3,000 feet could penetrate the Floridan Aquifer and then flow through it. If some sea water merely penetrated and stayed put, heat from the Earth's interior would eventually warm it until any cooling effect was overwhelmed.

Is there any evidence that sea water is "flowing" through the Floridan Aquifer beneath Florida and the Floridan Plateau? Yes ... at least some. First, there appears to be salt water at the bottom of the aquifer in at least the southern half of Florida. It has a chloride concentration similar to what is found in sea water. So sea water is present in deep layers of the plateau, and probably underlies all of Florida. (The fresh water on which we depend so heavily "floats" on this sea water.)

But is this sea water flowing? Temperature trends suggest that it is.

At the bottom of the Floridan Aquifer some 3,000 feet below the west coast of Florida, the temperature is over 100 degrees F. Remember that the west coast lies near the center of the Floridan Plateau, as far removed as possible from the places along the outer edge of the plateau where cold, deep sea water might enter the plateau.

Out near the east coast of Florida and in the Florida Keys, which lie along the plateau's outer edge, temperatures at the bottom of the underlying aquifer are only about 60-80 degrees F. So there must have been penetration by sea water all around the outer edge of the plateau. It would be very unlikely that this penetration occurred only once, at exactly the time when the test wells were drilled. So penetration must be a "permanent" process, and the sea water that penetrates has to "go" somewhere, which means a "flow" beneath the Floridan Plateau.

There is one more bit of evidence for deep, flowing sea water: warm, salty springs. A few miles south of Sanibel Island is a place on the West Florida Continental Shelf where warm water comes out of vents in the sea bed. It is called Mud Hole Submarine Spring, and it has been studied by Kohout and other investigators, including some of us in the Marine Science Department of the University of South Florida.

This spring lies along the center-line of the Floridan plateau, and seems to be fed from a sinkhole or some other large cavity that penetrates deep within the limestone of the plateau because the water that comes out has a temperature of 98.6 degrees F. (human body temperature). The shallowest depth at which you can find water that hot inside the plateau is around 2,000-3,000 feet near its center. In addition, the water flowing out of the spring has the chemical composition of sea water from 2,500 feet deep in the Gulf of Mexico. So it seems to be heated sea water, and, since it flows out naturally, it had to have moved into the aquifer from the ocean and then "flowed" within it in order to pick up the heat.

If you travel along the center line of the Floridan plateau to the north-northwest of Mud Hole Submarine Spring, you will first pass over Sanibel Island and then onto mainland Florida. Eventually, after about 50 miles, you will come to a "spa" called Warm Mineral Spring near Venice, Fla. This is a deeply penetrating sinkhole with warm "brackish" water flowing out of it.

Visitors come to swim and soak in the water, which feels very good on cold, winter days. The water is brackish because its salt content is roughly halfway between fresh water and sea water, such as comes out of Mud Hole Submarine Spring. Its temperature is halfway between those other waters as well _ about 85 degrees F. or so. Warm Mineral Spring is also famous because of ancient human relics found in cavities in its walls. Humans have known about it for hundreds to thousands of years.

These two springs are clear examples of sea water flowing out of the Floridan Plateau somewhere near its center. In one case essentially "pure" sea water comes out. In the second case sea water mixed about half-and-half with underground fresh water emerges.

In both cases the salty water is warm, meaning that it had to have been deep within the plateau. These two springs act as "escape" ports for the sea water that penetrates the plateau from the surrounding ocean. There are reports of other such springs around Florida, and even some off shore. Whether there are enough of them to account for all the sea water that should be entering the deeper layers of the Floridan Aquifer is not known for certain.

What we do know for certain is that the fresh water which is so vital to the human activities in Florida is a "bubble" in porous limestone layers of the upper Floridan Aquifer more or less directly beneath the state on the eastern side of the Floridan Plateau. This bubble essentially "floats" above salt water that appears to have come from the ocean into limestone layers in the plateau that are deeper than those containing the fresh water. Sea water surrounds this bubble on each side as well, either in the ocean proper or within limestone layers that underlie the ocean.

Normally, the fact that sea water is denser than fresh water at the same temperature keeps the sea water from rapidly displacing any fresh water floating above it. However, some of the sea water within the plateau is heated, which makes the density difference less _ so much so that it can rise into and mix with fresh water beneath Florida in parts of the plateau where the cavities and channels are properly arranged. (Hot water rises just like hot air rises.)

Also, the pumping of fresh water out of the upper aquifer faster than it can be replaced by rainfall will reduce the volume and the downward and outward pressures exerted by the bubble. Then the pressure exerted by the ocean around the plateau will force sea water upward and horizontally into cavities formerly occupied by fresh water. Unless technically and economically viable alternate sources of fresh water are found, the shrinkage of the bubble will ultimately limit human activities possible within the state of Florida. The recently proposed "drastic" watering restrictions are probably only the beginning.

Fantastic voyage

Hey there kids. Professor Wasser here! I'm floating out more than 100 miles in the Gulf of Mexico with my trusted and very small lab assistant Smedley. Smedley is about to embark on a most excellent adventure. He's going to follow this water here on its route to, well, I won't spoil it for you. We'll keep in touch by radio. Ready Smedley?

Hey! Professor. I'm at 2,500 feet. It's getting cold and is very dark down here! What's happening?

Relax Smedley. You've dropped down into sea water that came partly from the polar regions of the ocean. It's also dark because all sunlight is absorbed in the upper few hundred feet of the ocean. Now start moving horizontally to the east.

Now there's this big wall rising up in the distance. It's getting closer. Can I get through?

Oh, you are a nervous type Smedley. The wall is the western flank of the Floridan Plateau. It's made mostly of limestone and has holes that you can enter because you're so small.

Hey professor. I'm inside the plateau. There's things in my way, but I don't think I can dodge them all!

Sure you can! You're in tiny fractures and channels inside the plateau. The things in the way are pieces of plateau limestone. Just be careful and maneuver around them.

Heeeeeeeeyyyyyy. Professor. Holy cow, it's getting hot in here!!!

You bet! It should be over 100 degrees F. You've now reached the central interior of the Floridan Plateau where the effects of geothermal heating are the worst. You may not have noticed, but the temperature has been rising all along your path after you entered the plateau, sort of like you were moving along a hot plate. You should be almost directly under the West Coast of Florida.

What is this strange map I'm passing?

This map, Smedley, explains your route and the hydraulic forces that are pushing you along right now. Florida, as we scientists see it, actually extends 100 to 150 miles out in the Gulf of Mexico -- that's the West Florida Shelf. Sea water warmed by the earth pushes against cooler freshwater on top in the Floridan Aquifer, sometimes causing salt water intrusion.

I think I'm rising. The channel is widening too? What's happening? Get me outta here.

Chill out! The hot sea water you're in is less dense than the cold sea water around it, both outside and inside the plateau. That means there's a buoyant force that's lifting you toward the surface, and there's also a wider set of channels inside the limestone that are your escape route. You should be out soon.

Whew! It's getting cooler. I can breathe again. Where am I?

It's as I expected. You've now reached an area where the hot sea water you're riding in has started to mix with the cooler fresh water trapped beneath Florida. You're very close now!

Whoa! Who is that? It's a man. He's in the water with me. He's speaking German! Am I still in America?

Oh, my brave assistant. Calm down! He's a tourist. Where would we be without them? He's soaking in Warm Mineral Spring, which is where the water you've been riding in comes back to the surface on land. Your trip is over. Good job!