Sinkholes in the News

The exposed land mass that constitutes the Florida peninsula is only part of a larger, mostly submerged carbonate platform that is partially capped with a sequence of relatively insoluble sand and clay deposits. Siliciclastic sediments (sand and clay) were deposited atop the irregular carbonate surface, creating a blanket of unconsolidated, relatively insoluble material that varies in composition and thickness throughout the State. In west-central Florida, the relation between the carbonate surface and the mantling deposits plays an important role in the circulation and chemical quality of ground water and the development of landforms.
Sinkhole development depends on limestone dissolution, water movement, and other envi-ronmental conditions. Limestone dissolution rates (on the order of millimeters per thousand years) are highest in areas where precipitation rates are high. Cavities develop in limestone over geologic time and result from chemical and mechanical erosion of material (Ford and Williams, 1989) .
There appears to be an in-creasing frequency of sink-holes, although the statistics may be affected by report-ing biases.

Dissolving carbonate rocks create sinkholes and other features

The soluble limestones and dolomites that constitute the carbonate rocks are sculpted by dissolution and weathering processes into a distinct geomorphology known as karst. Features characteristic of karst terranes are directly related to limestone dissolution and ground-water flow and include sinkholes, springs, caves, disappearing streams, internally drained basins, and subsurface drainage net-works.
Dissolution cavities can range in size from tiny vugs to gigantic caverns. As these enlarging voids coalesce and become hydraulically interconnected, they greatly enhance the movement of ground water, which can perpetuate further dissolution and erosion.

On a local scale, the caverns and cave networks can form extensive conduit systems that convey significant ground-water flow at very high velocities (Atkinson, 1977; Quinlan and others, 1993). On a regional scale, the many interconnected local scale features can cre-ate a vast system of highly transmissive aquifers that constitute a highly productive ground-water resource.
Changes in sea level helped develop karst terranes

Karst is well-developed in the carbonate rocks throughout the
Florida carbonate platform. Throughout recent geologic time, fluctuationsdin sea level have alternately flooded and exposed the platform, weathering and dissolving the carbonate rocks. During the Ice Ages, an increased proportion of the Earth's water was frozen in polar ice and continental glaciers, lowering sea level along the Florida peninsula by 280 to 330 feet as recently as 18,000 years ago. The sea-level low stands exposed the great carbonate platforms of the Gulf of Mexico and the Caribbean Sea to karst processes. The lower sea-level stands were accompanied by lower ground-water levels (Watts, 1980; Watts and Stuiver, 1980; Watts and Hansen, 1988), which accelerated the development of karst. With the melting of the ice, sea levels and ground-water levels rose and many of the karst features were submerged. Examples of these flooded features include the "blue holes" found in the Bahamas, the cenotes of the Yucatan, the springs of Florida, and numerous water-filled cave pas-sages throughout these terranes. Many of the numerous lakes and ponds of west-central Florida formed as overburden materials settled into cavities in the underlying limestone.

Mining exposed this typical karst limestone surface,
which is riddled with disso-lution cavities.
Karst is an important part of the ground-water plumbing

At present, in west-central Florida, most of the soluble bedrock is below the water table. As ground water flows through the rock, geochemical processes continually modify both the rock and the chemical composition of the ground water. In many areas within the platform, the carbonates continue to dissolve, further enlarging cavities and conduits for ground-water flow.
Fractures, faults, bedding planes and differences in the mineral composition of the car-bonate rocks also play a role in the development, orientation, and extent of the internal plumbing system. Lineaments (linear features expressed in the regional surface terrain and often remotely sensed using aerial photography or satellite imagery) are often associated with locations of sinkholes and highly transmissive zones in the carbonate platform (Lattman and Parizek, 1964; Littlefield, and others, 1984).

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