Adapted from Project Underground, A Natural Resource Education Guide
Sinkholes are natural depressions in the land caused when limestone and soils dissolve. They form when groundwater removes rock underground. They can form by slow gradual sinking or by sudden collapse of an underlying hole.
Sinkholes are common in about one quarter of the U.S. You can usually identify them as circular or oval low spots in fields that may gather standing water after rains. They can be small or larger than a football field. A sinkhole of any size indicates there was a cavity in the bedrock near the surface. Sinkholes are evidence of a subsurface groundwater, either in the past or present. Formation of a new sinkhole or continued collapse of an existing sinkhole, indicates present day groundwater.
People can affect the location and rate at which sinkholes form. One way sinkholes form is by the removal of large amounts of water from the ground for human use, livestock, or irrigation. This may lower the water table rapidly. Because of the loss of the water, the land surface can collapse into holes already formed in the underlying limestone.
- 8 oz. foam cup
- scouring pad or very thin sponge
- empty 2-liter soda bottle
- piece of paper
- Make a hole about the size of your thumb in the bottom of the foam cup.
- Cut a circle the size of the cup bottom from a thin scouring pad. Place this circle in the bottom of the cup.
- Place a column of sugar in the center of the cup and surround it by sand. To do this, make a tube by rolling up a piece of paper and place it in the center of the cup. The paper tube should be about the same height and one half the diameter of the cup. Fill the inside of the tube with sugar and the outside of the tube with sand (the sand should be between the paper tube and the sides of the cup). Carefully remove the paper tube. Place a thin layer of sand over the sugar.
- Cut the bottom off a two-liter soda bottle at about the same height as the foam cup to create a dish. Fill it about one-third full of water. This will symbolize groundwater.
- Place the cup with the sugar and sand in the water. Watch as the water fills into the cup and the sugar dissolves and runs out. A sinkhole is formed in the cup as the surface sand sinks into the area where the sugar dissolved. (You may need to remove the cup from the dish of water to allow the water to drain out of the cup and the sinkhole to form).
- What natural process is demonstrated as the sugar is dissolved by the water and the surface sinks?
- What type of rock does the sugar represent?
- What characteristics must a rock have to be suitable for forming sinkholes and caves?
- What does the water in the dish represent?
- Why did the sinkhole form only over the sugar deposit?
Sinkholes are divided into two categories:
- Bedrock Collapse: This sinkhole forms when the underground rock dissolves enough to totally collapse, bringing surface rock and soil along with it.
- Surface Arch Collapse: This sinkhole forms when the bedrock has dissolved and voids form beneath the surface rocks and soil. The surface soil continues to erode away into the underground voids. Eventually the soil arch can no longer support itself and collapses into the dissolved bedrock empty spaces.