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 Floridan aquifer system. The sediments and water contained in the unsaturated zone, surficial aquifer system, and the confining layer above the Floridan 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.

Collapse Sinkholes
Collapse sinkholes are the most dramatic of the three sinkhole types; they form with little warning and leave behind a deep, steeply sided hole. Collapse occurs because of the weakening of the rock of the aquifer by erosion and is often triggered by changes in water levels in the surficial and Floridan aquifers. The progression of a collapse sinkhole is shown below.

(1) No evidence of land subsidence, small- to medium-sized cavities in the rock matrix. Water from surface percolates through to rock, and the erosion process begins.

(2) Cavities in limestone continue to grow larger. Note missing confining layer that allows more water to flow through to the rock matrix. Roof of the cavern is thinner, weaker.

(3) As groundwater levels drop during the dry season, the weight of the overburden exceeds the strength of the cavern roof, and the overburden collapses into the cavern, forming a sinkhole.

Subsidence sinkholes
The progression of a subsidence sinkhole is shown below. Rainwater percolates through overlying sediments and reaches the limestone, dissolving the rock and gradually weakening its structural integrity. Gradually subsiding sinkholes commonly form where slow dissolution takes place, mostly along joints in the limestone. These sinkholes tend to form naturally and are not greatly affected by human activities.

(1) Initially the limestone contains fractures, but no subsidence has occurred. Potentiometric surface may coincide with the water table.

(2) Small cavities and cracks grow larger as time progresses, and water moving through the rock erodes the rock matrix. Sediments carried by the water fill the voids in the rock.

(3) Sediments from the upper layers continue to fill in the openings in the limestone, causing a depression at the land surface. If water collects in the depression, a new lake is formed

source: Hydrology of Central Florida Lakes--A Primer