Mass Wasting in Yellowstone

Yellowstone National Park has had several instances of mass wasting in its history. Mass wasting is the “the down slope movement of angular slope material”. The term “landslide” encompasses all types of rapid mass wasting (Mass Wasting lecture).  The landslides in Yellowstone have received quite a bit of media attention as they have closed roads and damaged property. There are several reasons why the Yellowstone area is prone to landslides.
 (Picture showing cars on a road in the middle of a debris flow. Picture courtesy of the National Park Service)

The first reason why Yellowstone has rapid mass wasting is because of gravity. This force acts on every object on earth, including Yellowstone’s mountains and hills. Gravity causes the colluvium or angular slope material to slide downward.

(Diagram showing the effect of gravity on a slope. The gravity causes stress on the slope, which leads to a landslide. Picture courtesy of Google Images)

Another important factor to consider in determining why an area has mass wasting is whether the slope is permeable or impermeable. Impermeable slopes do not absorb water, and therefore do not have a lot of vegetation. Permeable slopes, like those in Yellowstone, absorb water and are heavily vegetated; they usually lead to slower mass wasting phenomenon like creep. This occurs when the slope gradually moves downhill as a result of the soil swelling and shrinking as it gets wet and dries out. This has occurred in Yellowstone, causing damage to hiking trails and trees.

(Example of a permeable hillside. The hillside has a lot of vegetation; showing that the soil absorbs water. Picture courtesy of Google Images)

 ( An Example of an impermeable surface. The rock is unable to absorb the water. Picture courtesy of Google Images)

( A diagram of creep's effect on trees. Picture courtesy of Google Images)

( A real life example of creep. Picture courtesy of Google Images)

 

     The permeability of the slope explains why Yellowstone has had landslides after forest fires and heavy rain. A forest fire can kill thousands of trees; the roots of the trees help absorb water, as well as anchor the soil to the hillside. Without the stability of the tree roots, the hillside is in danger of collapse. When the rain fall is heavy the soil on the hillside becomes over saturated, and turns into a mudslide or a debris flow, both of which are highly viscous. The difference between the two is in their contents. A mudslide has a lot of mud, with very few rocks. A debris flow has high amount of mud as well as a high amount of rock debris. Both forms have caused damage to roads, hiking trails, and property in Yellowstone National Park.
( Example of a mudslide after a forest fire. The trees closest to the camera show fire damage on their trunks.  This picture also shows damage to a road in the path of the mudslide. Picture courtesy of Google Images)


 
             Sources:

1.      Mass Wasting Lecture

2.      http://books.google.com/books?id=3WFIbcFe7ioC&pg=PA34&lpg=PA34&dq=landslides+in+yellowstone+after+a+fire?&source=bl&ots=q2MfKYR7Hv&sig=xNfiVoNr8sPOlNK9VFsQjA73y58&hl=en&sa=X&ei=9650UOKdA-i0igK9hIGwDg&ved=0CDUQ6AEwBA#v=onepage&q=landslides%20in%20yellowstone%20after%20a%20fire%3F&f=false

3.      http://wapi.isu.edu/envgeo/EG4_mass_wasting/yellowstone_landslide_2004.htm