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FLORIDA SINKHOLES- The posted coordinates will take you to
the very spot where a sinkhole occurred on December 8, 2002
according to FCIT. This reported sinkhole is based on data gathered
by the Florida geological Survey (FGS) and the Florida Department
of Evironmental Protection (FDEP). Broward Maps! The hole has since been filled
but will be a good place to learn about Florida sinkholes.
Sinkholes are most commonly thought of as physical depressions or
holes in the surface of the land. Not all sinkholes, however, are
as visible or dramatic as a home or roadway falling into the
ground. Many times, sinkhole activity never manifests itself on the
surface of the land, making it harder to detect. The entire state
of Florida sits on top of thousands of feet of limestone. Limestone
is a porous rock, capable of cracking, breaking and dissolving. The
most significant factor in the development of sinkholes is the
dissolution of the limestone underlying Florida by acidic waters.
As water moves through the soil, it becomes more acidic as it
reacts with living and decaying plant matter. This water reaches
the underlying limestone, it slowly begins to dissolve the rock and
creates voids and cavities. The soil resting on top of the
limestone then collapses or subsides into the caverns and voids and
causes sinkholes. A rapid increase in sinkhole activity can be
attributed to the development of land, which changes the
earth’s surface as well as the weight or load on the soil
beneath it. The creation of retention ponds, new buildings and
homes, roadways, and changes in ground water levels can all trigger
sinkhole activity.
SINKHOLES are considered a type of land subsidence, because
they involve a vertical downward movement of the land surface.
There are three types of sinkholes, each of which forms in a
different way. Collapse sinkholes are the most hazardous to humans
because of how suddenly they can form. There are 3 types of
sinkholes.
TYPE 1: SOLUTION SINKHOLE
Solution sinkholes form where soluble bedrock (i.e., limestone,
dolomite, marble, and rock salt) is exposed at the land surface and
thus subjected to weathering by dissolution. Surface water collects
in natural depressions and slowly dissolves a sinkhole.
TYPE 2: SUBSIDENCE SINKHOLES
Subsidence sinkholes are similar to solution sinkholes, except that
the soluble bedrock is covered by a thin layer of soil and/or
sediment. Surface water infiltration dissolves cavities where the
bedrock is most intensely fractured, and the overlying sediment
gradually moves downward into the expanding cavity.
The formation of a subsidence sinkhole is illustrated below. Note
that the geologic conditions include soluble bedrock (such as
limestone) covered by relatively thin deposits of sediments. This
type of sinkhole tends to form naturally and is not greatly
affected by human activities.
(1) Initially the bedrock contains fractures and small cavities
that have formed by dissolution, but no land subsidence has
occurred.
(2) Small cavities and cracks grow larger through time, and
sediments from above are slowly carried down by infiltrating
rainwater to fill the voids forming in the bedrock.
TYPE 3: COLLAPSE SINKHOLE
Collapse sinkholes form when surface materials suddenly sink into a
subsurface cavity or cave. The cavities form slowly over time, as
groundwater moves along fractures in soluble bedrock and enlarges
them through dissolution, and the actual collapse can occur in two
different ways:
a. When a cavity gets sufficiently large, the "roof" becomes too
thin to support the weight of any overlying rock or sediment, so it
collapses into the cavity.
b. Caves are sometimes able to support the weight of overlying
sediments because they are filled groundwater. However, if
groundwater levels are lowered, then the overlying sediment will
first erode and then collapse into the dewatered cavity.
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.
(1) There is no evidence of land subsidence, but small- to
medium-sized cavities have already formed in the bedrock.
(2) Cavities in the bedrock continue to grow larger but remain
filled with groundwater. Water pressure helps to support the
thinner, weaker roof of a large cavity.
(3) As groundwater levels drop naturally during the dry season (or
are drawn down by pumping), the cavities become dewatered. The
weight of overburden now exceeds the strength of the cavern roof,
and the overburden collapses into the cavern, forming a
sinkhole.
Let's Make our own SINKHOLE
MATERIALS
Styrofoam cup
Scouring pad
Bowl with small amount of water
Sugar in a cup
Sand in a cup
Scissors
Half piece of construction paper
Tape
MAKING SINKHOLE
-Using your thumb, make a hole about the size of a quarter in the
bottom of the cup.
-Cut a circle the size of the bottom of the cup from the scouring
pad.
-Place this circle into the bottom of the cup.
-Make a tube by rolling up the piece of construction paper.
-Tape it so that it does not unroll.
-Place the tube inside the center of the cup.
-Pour the sugar inside the tube.
-Pour the sand on the outside of the tube – make sure to go
all the way around to fill the cup.
-Carefully remove the paper tube by pulling it straight up.
-Place a thin layer of sand over the sugar.
-Look at your bowl – the water in the bowl represents
groundwater.
-Place the cup with the sugar and sand in the water.
-You may need to lift the cup gently for your sinkhole to
form.
CLICK HERE FOR VIDEO TUTORIAL
Credit for Cache: (Send via
email. Do Not Post answers in log)
1) Survey the area at
the posted coordinates, what evidence is there that a sinkhole
developed?
2) Do you think the SINKHOLE you created is the same that occurred
here in 2002? Explain.
3) Using the experiment as a guide, describe how the sinkhole
formed at this site?
4) How long did it take for the SINKHOLE to form in the
experiment?
5) OPTIONAL - post a photo of you and your SINKHOLE in your
log.