Karst Topography
What is Karst Topography?
Regions characterized by sinkholes, sinkhole ponds, lost rivers and underground drainage are
said to have Karst Topography. Some karst regions include thousands of caves, although
evidence of caves large enough for human exploration is not a required characteristic of karst.
What is a sinkhole?
Sinkholes are depressions that form in the surface because carbon dioxide in the water has
dissolved some of the rock beneath the soil. A pond or lake may form when a sinkhole is deep
enough to meet the water table.
What type of bedrock dissolves more easily?
Limestone is a common bedrock that dissolves more easily than some other types of rocks.
What acid found in soil dissolves the limestone?
Limestone is dissolved by the carbonic acid found in groundwater.
How is carbonic acid formed?
Rainwater forms a weak solution of carbonic acid when carbon dioxide from the air and from
decaying vegetation in the soil dissolves in it.
Water works its way through the "vadose" zone above the water table through joints and cracks in the limestone bedrock. The
slightly acidic water erodes the carbonate-rich rock.
Small passages grow into caverns as water continues to dissolve and carry away rock.
If the cavern ceiling grows thin enough, or it otherwise can't support the weight of the rock and soil above, it collapses into a
sinkhole.
What happens to the porosity of this rock?
Porosity is created in limestone and existing porosity is increased.
Porosity is a measure of how much of a rock is open space. This space can be between grains
or within cracks or cavities of the rock.
Permeability is a measure of the ease with which a fluid (water in this case) can move through
a porous rock.
Tom Grace, TERC
Microscopic structure of shale, sandstone, and limestone with water in
pore spaces. Note differences in scale among views of each rock type.
How are caves/caverns formed?
Rainwater containing carbonic acid seeps into the ground. Limestone dissolves, forming
underground caves.
How do stalactites and stalagmites form?
When groundwater drips from the roof of a limestone cave, it slowly deposits calcite.
Stalactites hang like icicles from the roof along the routes of the dripping water. Stalagmites
from on the cave floor.
What mineral is typically present in the rocks of karst areas?
Calcite
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Cave Formation 4:12