Florida's carbonate limestone dissolves away, leaving a pockmarked surface. Sinkholes are created when such underground features collapse.USGS


- The faster caverns and sinkholes form, the faster Florida rises out of the sea.

- A model tests the idea and finds that it all adds up.

- The rate of uplift is still far to slow to offset sea level rise projected for this century.

Caverns and sinkholes could keep parts of Florida out of the water as sea levels rise -- at least in the very, very long term.

It turns out that as the limestone there dissolves away to make the Swiss cheese-like ground, underfoot, the earth there gets lighter and buoys higher on the Earth's mantle, say geologists.

It's a very slow-motion version of what's happening in Alaska, Tierra del Fuego and Greenland as glaciers melt and mountains rebound upwards, relieved of all that weight. This "isostatic rebound" idea also finally explains the longstanding mystery of how some Florida limestones -- filled with marine fossils -- managed to get so high above sea level.

In some places the geologically recent fossils are in ridges 250 feet above sea level. This would not be a big deal in a setting where crustal plates are smashing together and pushing rocks up. But that's exactly the opposite of Florida's geology, which is very sleepy and tectonically passive.

"Either sea level was that high or there is some kind of uplift," said geologist Peter Adams at the University of Florida at Gainesville. Adams is the lead author of a paper on the matter in the June issue of the journal Geology.

Since tectonic collision can't be called on to explain the matter, Adams decided to put the cavern-ridden rebound idea -- first proposed in 1984 by his co-author Neil Updyke -- to the test with a model.

The numerical model combined estimated sea level fluctuations over more than a million years, the length of time the land has been out of the water, rainfall, cave-formation rates and isostatic uplift to see if the idea has any legs.

"I'm just seeing if mechanically those ridges fit with the ages," said Adams.

The model suggests the ages of north Florida's Trail Ridge, Penholoway Terrace and Talbot terrace are 1.44 million, 408,000, and 120,000 years old respectively. That basically matches the fossil evidence in the rocks, Adams said.

"They did a good job with this," commented veteran Florida geologist Thomas Scott, formerly of the Florida Geological Survey and author of the book "Roadside Geology of Florida." The model is a good "proof of concept" study, he said.

With the heights and times worked out, it's a simple matter to calculate the uplift rate: 0.047 millimeters per year. While that might seem awfully slow, it's almost twice as fast as had been previously thought and implies that the rotting way of limestone is happening at a rate more than three times faster than expected.

Still, with sea level rise clocked at rates of more than 3 millimeters per year, it's only over the very long haul that uplift will help Florida.

"We're not going to be saved by this," said Adams. "We're still drowning."