Post #2: Mechanical Weathering

Mechanical Weathering, the breakdown or decay of rock, is responsible for most of the current landscaping of the earth. Most people believe that Mechanical Weathering is powered solely by the wind, this is not the case. Joints, or fractures in the rocks create fissures that are the key to all types of mechanical weathering. There are six types of Mechanical Weathering; Frost, Pressure Release, Salt, Thermal Expansion and Contraction, Expansion and Contraction from Wetting and Drying, and Pressure Roots. Frost, Salt, and Root Mechanical Weathering are the most common in Glenwood Springs.

In Frost Weathering, water goes into fissures then expands and contracts inside it. This increasingly expands the size of the fissure, as shown in figure 1. Frost Weathering occurs mainly in High altitudes with numerous freeze/thaw cycles. The location of Glenwood Springs, nestled in the Rocky Mountains with our mild yet stormy winters, provides the perfect conditions for Frost Weathering.  

Figure 1: A diagram of the process of Frost Weathering.  ("Mechanical weathering processes," )

On my many trips to Glenwood Springs I have seen trees growing out of rocks and wondered how that was possible. What I now know is that I was witnessing a form of weathering; Pressure of Root Weathering to be more precise. Plant roots can spread into fissures in rocks expanding the fissure as the plant grows or even breaking the rock.

Salt Weathering occurs in two processes, Crystallization and Heating and Cooling. Crystallization Salt Weathering occurs when salt particles crystallize in the fissures, and porous areas of rocks. These crystals exert an outward force on the affected rocks causing them to expand and weaken and eventually break apart. Heating and cooling of the salt crystals cause the crystals to expand and contract, also exerting pressure on the rock. Cavernous Salt Weathering creates Tafoni, craters in the surface of a rock. An example of these craters is shown in figure 2. Salt Weathering formed the Vapor Caves in Glenwood Springs, seen in figure 3.

Figure 2: A sample picture of Tafoni. (Boxerman, 2008)

 

 

Figure 3: A picture of the Glenwood Springs cavern. You can see Tafoni in the upper portion of the picture as well as the smooth texture of the rock.  (Alden, 2010)

 

Wind does play a part in Mechanical Weathering; wind blows small granules of rock and salt spreading salt to other rocks. It helps send ice crystals deep into fissures during storms and helps to deliver seeds to fertile ground. Wind also smooth’s out rocks giving them the shiny silky look found in some caves.

 

Resources

Alden, A. (2010). Glenwood caverns: Geology of glenwood springs, colorado. Retrieved from http://geology.about.com/od/geology_co/ig/glenwood-springs-CO/glenwood-caverns.htm

Boxerman, J. (2008). Tafoni_8: Tafoni-from coastal northern california. Retrieved from http://tafoni.com/Gallery/Pages/Northern_California.html

Dr. Allen, C. (2012). Decay of earth’s rocks (weathering): the key to our beautiful earth. (University of Colorado Denver )Retrieved from http://clasfaculty.ucdenver.edu/callen/1202/Battle/Destroy/Wx/Wx.html

Mechanical weathering processes. (n.d.). Retrieved from http://itc.gsw.edu/faculty/bcarter/physgeol/weather/mechwth.htm