How sinkholes form is not an overnight event. Sinkholes are part of the slow, natural process of erosion in Florida’s limestone/karst terrain that forms over thousands of years. These common geologic phenomena generally occur where the limestone is within a few hundred feet of the land’s surface.
How Sinkholes Form
Rainfall percolating, or seeping, through the soil absorbs carbon dioxide and reacts with
decaying vegetation, creating a slightly acidic water. Water continues to move through spaces
and underground cracks, slowly dissolving limestone and creating a network of cavities and
voids. As the limestone dissolves, pores and cracks are enlarged and provides a large passage
for more acidic water. Sinkholes are formed when the land surface above collapses or sinks into
the cavities or when surface material is carried downward into the voids.
Drought, along with resulting high groundwater withdrawals, can make conditions favorable for
sinkholes to form. In addition, heavy rains after draughts trends faster towards the karst layer
and the process to create a sinkhole begins.
If such sinkhole forms in an urban or suburban area, sinkhole can be fatal, as it recently
happened in Seffner.
The whole State of Florida is geologically hazardous. Sinkholes usually don’t collapse at once,
it happens in slow motion. You can see walls crack, strain, and complain as the earth begins
to slowly give way under the house. In many cases, residents have enough time to gather
valuables and evacuate to the safety.
“Losing a house to a sinkhole is very common, losing life is uncommon,” said retired University
of Florida geologist Tony Randazzo. “Most people will have some time warning of the pending
doom or catastrophic collapse. But there apparently were no warning signs of what happened at
the Bush house. That would be very scary.”
It is not yet clear what caused the Seffner sinkhole, however, but geologists say the area, which
is part of heavily populated I-4 corridor that crosses Florida’s midriff from Tampa to Daytona, is
particularly prone to sinkhole collapses.
A brochure issued by the Southwest Florida Water Management District lists several sinkhole
warning signs, including slumping trees or fence posts; the formation of small ponds in areas
where water has not collected before; wilting of small, circular areas of vegetation; and structural cracks in walls. www.swfwmd.state.fl.us/hydrology/sinkholes/brochure.pdf
Despite the efforts to spot sinkholes before they occur, there are still plenty of unpleasant – and
occasionally tragic – surprises, and there is no way of ever predicting where a sinkhole is going
to occur. 1
