Geo-pic of the week: Solution sinkholes

A sinkhole is an area of ground that has no external surface drainage.  Water that enters a sinkhole exits by draining into the subsurface.  Many people are leery of sinkholes because of the damage they sometimes cause.  Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake. 

Not every kind of sinkhole is the dangerous kind though.  The picture above shows a solution sinkhole.  Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression.  Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground. 

In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone.  These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.

Geopic of the week: Travertine on the Buffalo River

Travertine is a common feature in the northern Ozarks and along the Buffalo River due to the abundance of soluble limestone there.  Common in caves (stalactites, stalagmites), travertine forms by the precipitation of minerals from ground water.  In the example above, it formed on the face of a bluff, giving the bluff a melted appearance.

Like limestone, travertine is composed of the mineral calcite which dissolves if exposed to acid.  When rain falls, it picks up CO₂ from the atmosphere and soil, and becomes slightly acidic.  It then flows underground through the bedrock dissolving some limestone along the way.  When the groundwater re-surfaces at a spring or seep, The pressure drops, forcing the CO₂ out of the water.  The loss of CO₂ lowers the waters acidity; It can no longer hold the calcite in solution, and calcite precipitates as the sedimentary rock travertine.

For more views of travertine click here