Sinkhole Formation in Iowa

The karst landscape, found in certain parts of Iowa, can lead to sinkholes like this one in Knoxville forming. This is the result of limestone erosion occurring underground over time which eventually leads to the collapse of the ground as the rock erodes away. One of the causes of sinkholes in Iowa is the presence of carbonic acid (H₂CO₃) in rainwater reacting with limestone (CaCO₃) underground. Increasing the concentration of H₂CO₃ in the water increases the rate of reaction with limestone (CaCO₃). Sinkholes are also more prevalent in the summer when there is more ground water and higher temperatures. Additionally, concentrations of carbonic acid are increasing in rainwater because of another chemical reaction taking place. In the atmosphere carbon dioxide (CO₂) reacts with water (H₂O) to form carbonic acid. Increasing the concentration of carbon dioxide in the atmosphere is increasing the rate at which carbonic acid is being produced.

Teachers can consider connecting this phenomenon with other physical science standards, including HS-PS1-6 and HS-PS1-7. Bundle with HS-PS1-7 to explore chemical reactions between limestone and carbonic acid as well as carbon dioxide with water. Or if you are not bundling, present HS-PS1-7 as precursor to using this phenomenon. HS-PS1-6 which covers concepts of equilibrium also connects to this phenomenon well because multiple reactions in this process are represented by equilibrium. Carbon dioxide reacting with water in the atmosphere to form carbonic acid is an equilibrium reaction as well as the carbonic acid reacting with water to form hydronium and bicarbonate.

Image

Knoxville

Iowa Core Standard

HS-PS1-5

Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs

Driving Question

What may cause the formation of the sinkhole?

Probing Questions

What do you notice about the location of Iowa sinkholes? What is it about this area of Iowa that makes it conducive for sinkhole formation?
What is limestone and why is that important?
How does water impact sinkhole formation?
When did the sinkhole near Knoxville form? How would that play a role?

Classroom Suggestions

Students could:

Take a field trip to a sinkhole in your area or review photos from a sinkhole that is most familiar to your students.
Conduct a lab to see how acids of varying concentrations react with limestone (calcium carbonate) at different rates. They can then try changing the temperatures of the solutions to see the effect temperature has on the rate of reaction. The teacher could also do this demonstration.
Conduct a lab testing how carbon dioxide mixes with water and makes the water more acidic. This can be done with litmus paper, pH paper or an acid base indicator. They can blow bubbles into water to measure the change in acidity in one of the above mentioned ways.
Construct explanations that correlate the patterns of reaction rates with temperature and concentration data to collision theory.
Design solutions to reduce sinkholes. (Note: This would likely involve reducing the amount of carbon dioxide in the atmosphere or somehow reducing the amount of carbonic acid in ground water.)
Explore cave formation in Iowa, particularly if your school is located near one of Iowa’s cave systems. There are several cave systems in Iowa that have formed by the erosion of limestone bedrock, including Maquoketa Caves, Spook Cave, Postville Caves, Devil’s Backbone Caves, Linn County Caves, Decorah Ice Cave. (Note: This may garner more interest from students.)

Resources

KCCI News | Officials halt Knoxville sinkhole repairs as they investigate the cause: This video gives a nice vantage point so we can see the magnitude of the sinkhole that is formed.
Iowa Department of Natural Resources (Iowa DNR) | Karst Geology: This shows a map of the karst geology and the location of sinkholes in the state. It shows that the majority of the karst geology and sinkholes are located in northeast Iowa.
Iowa Department of Natural Resources (Iowa DNR) | Maquoketa Caves State Park: This is the DNR website and you can find a lot of pictures and additional information about the Maquoketa Caves.
Iowa PBS | FIND Iowa: The Creation of the Maquoketa Caves: This video shows some images of the Maquoketa Caves and provides some additional information about their formation. It doesn’t get into any of the chemistry of what is happening but may spark students to ask questions.

Contributors

Submitted by Jed Dettmering.

Photos provided courtesy Iowa Department of Agriculture and Land Stewardship (IDALS).