by
David L. Ozsvath
Department of Geography/Geology
University of Wisconsin—Stevens Point
The Orlando metropolitan area is situated in a region where sinkholes can develop unexpectedly as the land surface collapses into underground cavities (void spaces that range in size from small openings to large caverns). These cavities occur in the limestone bedrock, which is covered by 90 to 150 feet of sandy clay and an additional 30 to 60 feet of sand (see geologic cross section in Figure 1). The limestone bedrock also serves as an aquifer, called the Floridan Aquifer, for the residents of this area.
Figure 1: Geologic cross-section showing entry points to groundwater contamination. Source: USGS. The Suwannee River Basin Pilot Study.
At least 140 new sinkholes formed in the Orlando area (Orange and Seminole Counties) between 1961 and 1986, with an average of 11 new sinkholes appearing each year (Wilson and Beck 1992). Data from Wilson and Beck (1992) show that the appearance of new sinkholes is not equally distributed throughout the year (see Figure 2).
Figure 2: Number of new sinkholes in Orange and Seminole Counties, Florida by month of the year for the years 1961 through 1986. Note that the month of the year is represented with a number, starting with January as 1 and ending with December as 12.
| Aquifer | Cavities |
| Collapse | Sand |
| Bedrock | Clay |
| Limestone | Sinkhole |
Date Posted: 12/04/00 mb. Last revised 09/08/05 nas.
Image Credit: Photograph by A. S. Navoy, U.S. Geological Survey. Used with permission of the U.S. Geological Survey.
This file is also available in Adobe Portable Document Format (PDF). 
Originally published at http://www.sciencecases.org/abyss/abyss.asp
Copyright © 1999–2009 by the National Center for Case Study Teaching in Science. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.
