Mass Wasting and Weathering

Mass Wasting and Weathering

As you drive through Yellowstone National Park, you can observe many different forms of mass wasting, as well as both physical and chemicalweathering of the landscape.  For example,  the picture below portrays beautiful columnar basalt with talus piled at  the base. Columnar basalt, a type of mafic igneous rock,  forms from the solidification of lava in a vertical feature called a dike.

Pile of talus from columnar basalt.  The countless joints, or fissures, in between the columns are very conducive to weathering.  Water will deposit in the fissures, freeze, and expand each joint a little at a time and eventually cause parts of the rock to break off.

Yellowstone has an abundance of water features allowing numerous ways for water to interact with the environment.  One way is depicted below where the overflow of water is channeled through certain areas, and over time erodes the soil away creating rills.

Rills are formed by the overflow of surface water that erodes the soil away.  They are always  located in hills where gravity carries water at a fast rate that allows it to carry the soil as it flows.

One type of physical weathering observed in the park includes alveoli, seen in the picture below.  It is a type of cavernous weathering caused by tiny salt crystals which exert pressure in the rock and eventually form these small imperfections in the rock.

The tiny holes and imperfections on the rock surface are alveoli.  Salt crystals within the rock exert pressure and eventually create the pores in the rock.

Minerva Terrace is an excellent place to see both signs of physical and chemical weathering.  Notice the abundance of white in the picture.  This is the salt deposit found within the rocks.  Salt can cause alveoli and tafoni to form over time.  Towards the middle right you notice that some of the rock is a reddish color.  The rock is probably rich in iron minerals and has become oxidized over time.  The green tints towards the top left and center left of the picture are mosses and lichens that attach to the rock with tiny microscopic “arms” and help break down the rock over time.  The white structure in the center of the picture also shows signs of pressure release because it seems as if large slabs of the rock have fallen off.  This happens due to quick erosion of rocks that were formed under great pressure.

Oxidation of rock occurs in environments where there is an excess of oxygen.  Iron will change from Fe^2- to Fe³⁺ changing the color of the rock to a deep red rust color.¹

References:

1. http://www.tulane.edu/~sanelson/geol111/sedrx.htm

2. Geography 1202 Class Notes

3. http://commons.wikimedia.org/wiki/File:Columnar_basalt_at_Sheepeater_Cliff_in_Yellowstone.JPG