Donaldson Cave at Spring Mill State Park formed in the Mitchell Plain, an area where karst topography is present. This cave passageway was cut open and exposed to the surface by a perpendicular cave passageway. Click for a larger image (IUPUI Earth Sciences).
Most people have heard the term cave and sinkhole before, but you may not have understood, or noticed that caves and sinkholes only occur in certain areas. Karst landscapes are areas where weathering can dissolve exposed bedrock at the surface or near surface, creating sinkholes, caves, and other features you will learn about.
Because Indiana has two areas that exhibit karst landscapes, you will complete this lab to learn more about these landscapes. The Exercise Overview is divided into the following sections:
- Location and Cause of Karst in Indiana
- Karst Features within Indiana
- Identifying Karst Features
The Overview is followed by the directions for completing your workbook exercise.
BEFORE continuing this overview, please read the following pages in your lab book:
Pages 225-227: Please read all of these pages to familiarize yourself with the features of Karst Landscapes.
Pages 231-232: Read these pages to undestand where and why we find karst landscapes.
Background on Karst
Karst topography forms when earth material can dissolve in the presence of water. In most of the world, limestone and dolomite is the common sedimentary rock that dissolves in rainwater and groundwater. Limestone and dolomite contain the mineral calcite (and related minerals), which dissolves when in contact with acid.
While weakly acidic rainwater can form a large cave, it is still hundreds of times less acidic than a Coke or Pepsi. The Phosphoric Acid in Coke and Pepsi gives cola a very high acidity. Your teeth are made of a mineral similar to calcite---and not brushing your teeth allows these acids to eat away at your teeth in the same fashion that rain water eats away at limestone.
Pouring a strong acid like lemon juice or hydrochloric acid dissolves the calcite so quickly you can see it yourself---it bubbles as the solid calcite is turned into the gas carbon dioxide. Rain water and groundwater are not acidic enough to cause this quick reaction-instead the dissolution happens very slowly on the molecular level, taking decades to centuries to thousands of years to eat away several feet of rock. Since most rainwater and groundwater is naturally slightly acidic (air and water pollution from humans has made both even more acidic), the action of water dissolving these rocks can be a constant through geologic time.
Not all limestone and dolomite dissolves in the presence of water-some contain enough impurities or other characteristics that prevent or slow down the dissolution. Additionally, other rock types exist on earth that can form karst landscapes, but none of these are found in the Midwest.