Water and Limestone React to Create Karst Potential in Iowa

This video shows a 2.0 M sample of aqueous hydrochloric acid being dropped onto a sample of limestone rock. There is a chemical reaction that occurs, as noted by the gas formation (sizzling sound) and gas that is visibly produced. Limestone consists primarily of calcium carbonate, which reacts with an acidic solution to form water and carbon dioxide. This chemical reaction breaks down the rock in a process called chemical erosion.

In Northeast Iowa, the landscape is dominated by winding rivers, gentle rolling hills, and limestone-based geology called karst topography. As Iowa’s acidic water enters the ground and interacts with the limestone, a reaction occurs which can be a cause for sinkholes, a common karst feature. Water is an incredibly constructive and destructive force that shapes our planet as it interacts with its various landscapes. This standard suggests that water influences earth’s surface processes to form specific landscapes within the rock cycle, including karst topography.

Students should wonder what different types of interactions water can have on its surface, both physically and chemically. They should notice how water moves through different elevations, rock bed types, and different temperatures through the use of stream tables, porosity/permeability based labs, and geologic maps.

Video

Northeast

Iowa Core Standard

HS-ESS2-5

Plan and conduct an investigation of the properties of fluids (i.e. wind and water) and their effects on Earth materials and surface processes.

Driving Question

What can the reaction between limestone and acidic water tell us about the potential for karst features, such as sinkholes, in Northeast Iowa?

Probing Questions

As you watch the video, how do you know that a reaction is occurring?
What are all of the holes and caverns within the limestone sample?
What is hydrochloric acid and what do we use it for? How can we get acid naturally?
What do you notice about the speed of the reaction?

Classroom Suggestions

Students could:

Replicate a field test by using a weak (1.0 M) solution of hydrochloric acid (ensure safety goggles are used) on a sample of limestone. They can record their reaction and their observations. This could be the driving phenomenon to introduce questions such as “how do we know if something is reacting?” This phenomenon can be returned to as students investigate chemical weathering. Note: This could also be done as a teacher demonstration if needed.
Learn where limestone comes from, including several ages of limestone development such as Devonian, Pennsylvanian, Mississippian, and Silurian. Use online interactive maps to show what the Earth looked like at these time periods (linked in the reference table below). Have students put a star on their hometown in Iowa (or nearby states). They will see that their city could have been under a shallow sea. Shallow sea environments are dominated by shelled marine life and corals, both of which consist of calcium carbonate. When the seas retreat, the marine life gets buried, and cements into limestone. The reaction between the acidic water and the calcium carbonate is what causes the chemical reaction demonstrated in the video.
Learn how water gets acidic. Consider using a stream table and do a hands-on experiment of how water moves through the ground and shapes the surface of the Earth. Use different materials in your stream table such as sand, gravel, pebbles, etc. until students understand porosity and permeability of different materials. Have students test the pH of the water as it starts in the stream table then as it goes through the different materials in the stream table.

Resources

Iowa DNR | AFO Siting:This online database models where karst potential features are located within the state of Iowa.
Iowa Geologic Survey | Living in Karst: This is a publication of the potential for karst in Northeast Iowa. Included in the guidebook are maps of karst potential.
Iowa Geologic Survey | Bedrock Geology of Northeast Iowa: This is a detailed geologic map of Northeast Iowa’s bedrock geology through geologic time.
U.S. Geological Survey (USGS) | National Geologic Map Database: An interactive map that includes all types of geologic types in the United States of America.
Dinosaur Database | Ancient Earth: An online simulation that models what Earth looked like throughout several millions of years and different ages.
Iowa Science Phenomena | Karst Topography & Spring-Fed Waterfalls: This related phenomenon features Grannis Creek Falls. A moderate-sized, spring-fed stream in the karst topography of the Driftless Region of northeast Iowa, it is a dramatic example (for Iowa) of fractured bedrock creating cool spring fed streams characteristic of the driftless landscape.

Contributors

Contributed by Michaela Moel as part of the University of Iowa Geology Project.