Geo-pic of the week: Cleavage

The mineral in the above pictures is calcite, a common mineral in earth’s crust that is the main component of the sedimentary rock limestone.  The stack of samples (top) exhibit a physical characteristic known as cleavage.  The cleavage of calcite causes it to break into a rhombus-shape (see picture).

Cleavage is the tendency of a crystalline substance, such as a mineral, to break along parallel planes that reflect the internal arrangement of the atoms in the crystal.  All crystals, by definition, have a uniform atomic arrangement.  To illustrate this property, I’ve included a second picture (bottom), borrowed from Dr. Cathy Sutton, that shows an extremely magnified calcite crystal.  The repeating rhombus-shapes in the picture are individual calcite molecules.  Basically, cleavage is the outward expression of the internal structure of a mineral.

The samples on the left were collected from Midwest Lime Quarry, Batesville, Arkansas.

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.