Florida Geological Survey - Hazards - Sinkholes
Sinkholes are a fact of life in
Florida. They occur because the entire state of Florida is underlain by
limestone, a type of rock that is slowly dissolved by weak natural acids
found in rain and in the pore spaces in soil. The formation of sinkholes
is explained in detail in the Geology Topics section of our website.
There are more sinkholes in some parts of Florida than
in others. Map Series 110,
which may be accessed from this website, depicts four types of
sinkhole terrain in Florida. Where limestone bedrock is thinly
covered, depending on the area, there are relatively few to a
moderate number of sinkholes. They are usually shallow and broad and
develop gradually and are called “solution sinkholes.” In other
areas the limestone is covered by sand that is easily permeable to
water and incohesive because it contains little clay. Sinkholes in
these areas tend to be few and small and develop gradually even
though the sand may range in thickness from 30 to 200 feet. They are
referred to as “cover-subsidence sinkholes.”
Sinkholes become more of a problem in areas where
sediments that lie above the limestone are mainly clays mixed with
sand. Clay causes these sediments, which also range in thickness
from 30 to 200 feet, to be cohesive. They are not very permeable to
water. In these areas sinkholes are most numerous. They vary in size
and may form suddenly. In a few areas of Florida over 200 feet of
sediments cover the underlying limestone. These sediments are
cohesive because of the clay and layers of limestone they contain.
Although there are not many sinkholes in these areas, the ones that
occur are deep and wide. These types of sinkholes are referred to as
“cover-collapse sinkholes” because cohesive layers of sediment
collapse into underground cavities when they form.
The abrupt formation of sinkholes may follow extreme
rain producing events such as tropical storms or hurricanes. This is
because the weight of a large amount of rain water at the earth’s
surface may bring about the collapse of an underground cavity if its
limestone “ceiling” has become thin. This tendency for sinkholes to
form following events that produce large amounts of rainfall is made
worse in times of drought. During periods of drought underground
cavities that might normally be filled with water may be only
partially filled. These cavities are less likely to bear the weight
of flood waters without collapsing.
Sinkholes are part of Florida’s natural environment.
Surface water flows through sinkholes into the underlying limestone
layers of the Floridan aquifer system, the major source of drinking
water in Florida. Because they provide a direct connection from the
surface to the aquifer system, it is very important to protect
sinkholes from pollution. This may be as simple as making sure that
sinkholes do not become convenient sites for dumping. The use of
conservation buffers (strips of land maintained in permanent
vegetation) around sinkholes can help protect groundwater from
fertilizers and pesticides that might otherwise enter sinkholes on
farm lands. The use of fencing around sinkholes may help protect
groundwater from animal waste.
People who live in Florida or who are moving into the
state should be aware of sinkholes and the possible problems they
pose for homeowners. The section of this website that addresses
frequently asked questions concerning sinkholes contains useful
information on practical issues associated with sinkholes. Topics
important to homeowners and prospective homeowners, such as sinkhole
disclosure laws and insurance issues, are covered.
These links will provide additional information
concerning sinkholes in Florida:
Click on any picture to enlarge.