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Types of Sinkholes
The three general types of sinkholes -- subsidence, solution, and collapse -- generally correspond to the thickness of the sediments overlying the limestone of the Floridian aquifer system. The sediments and water contained in the unsaturated zone, surficial aquifer system, and the confining layer above the Floridian aquifer are all collectively referred to as overburden. Collapse sinkholes are most common in areas where the overburden is thick, but the confining layer is breached or absent. Subsidence sinkholes form where the overburden is thin and only a veneer of sediments is present overlying the limestone. Solution sinkholes form where the overburden is absent and the limestone is exposed at land surface.
I. COLLAPSE SINKHOLES
Collapse sinkholes are the most dramatic of the three sinkhole types; they form with little warning and leave behind deep, steep-sided holes. One mechanism for the formation of a collapse sinkhole is illustrated below. Notice that the geologic conditions include soluble bedrock (such as limestone) covered by relatively thick deposits of sediments. This type of sinkhole can form naturally but is often affected by human activities. The progression of a collapse sinkhole is illustrated in figures 1-3 below.
FIGURE 1 |
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There is no evidence of land subsidence, but small to medium size cavities have already formed in the rock matrix. Water from surface percolates through to rock, and the erosion process begins. |
FIGURE 2 |
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Cavities in the rock matrix continue to grow larger but remain filled with water. This water pressure helps to support the thinner, weaker roof of the enlarged cavity. |
FIGURE 3 |
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| As the water level drops during the dry season, or is lowered due to pumping it out, the weight of the overburden exceeds the strength of the cavern roof, and the overburden collapses into the cavern, forming a sinkhole. |
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What is a Sinkhole?
