Read Chapter 14 carefully in preparation for this work. Also read the material below as a supplement to the text. Questions related to this supplementary material will appear on the next exam

To begin our understanding of mass wasting processes, we must first understand the forces that control mass wasting. Gravity is of most importance. Consider a slope with the angle shown at the left, and a mass of material sitting on the slope. The force of gravity acting toward the center of the Earth is shown as "g". The component of gravity that holds the material on the slope is shown as gn. Note that this force is perpendicular to the slope. The force of gravity that pulls material down the slope is shown as gt. Note that this force is parallel to the slope. The bigger you make the slope angle, the larger gt becomes, and the smaller gn becomes. When gn is greater than gt (low slope angles), the slope is stable Itop diagram). But if the angle of slope is increased enough, then gt will become greater than gn, and the material will move down the slope (second diagram). Note here that slope angle is a very important factor in determining if mass wasting will occur or not.

The concept of the critical angle of a slope is therefore very important. The critical angle (or angle of repose) is a theoretical angle for a given slope, below which the slope will be stable (third diagram). That is to say, if the actual slope angle is less than the theoretical critical angle, then no mass wasting will occur. And if the actual slope angle is greater than the critical angle, then mass wasting will occur. The critical angle depends on such things as vegetation on the slope, the type of material on the slope, whether the slope is wet or dry, etc.

We must next go beyond the influence of the force of gravity and how slope angle affects gn and gt, and talk about another force that holds material on the slope. That force is friction. Let's write a new expression for the condition where a slope is stable.

Here we see that friction is another force that holds material on a slope. If we want to consider a situation where mass wasting will occur, if we reduce friction, and the slope angle is steep enough, the material will move down the slope. Consider how we might reduce friction. When we do this in an automobile engine, we add oil (a lubricant). If we want to rub something down, we add a liquid to reduce the friction. In nature, the liquid that reduces friction in material held on a slope by gravity will be water. In other words, if there is a lot of rainfall in a gal of time, the material on a slope may be lubricated to the point that friction is drastically reduced, and if the slope angle is steep enough, ma will occur. That's why we often get mass wasting here in southeastern Minnesota in the spring of the year, when we tend to have more rain.

Now, think about mass wasting as an equilibrium process. If a stable slope is made unstable (disequilibrium) by having the slope angle increased, and having friction reduced, then the conditions on the slope may exceed the theoretical critical angle for that slope, and mass wasting will occur. How are slope angles increased in nature? River erosion along the sides of a valley may cut into the base of the valley slopncrease the slope angles (removing material from the base of a slope increases slope angle). This sort of erosion also removes support from the base of the slope, so that it can fail. Mass wasting may occur as a result of this erosion. The mass wasting process is simply an attempt by the natural system to restore equilibrium. The downslope movement will continue until the slope is once again at or below the critical angle.

Humans also upset conditions of equilibrium on a slope. Consider what happens when we build highways, such as US 61 along the Mississippi River on the Minnesota side. Slopes are undercut in order to make space for the road, and slopes are therefore steepened. This results in a condition of instability. Mass wasting will occur to restore equilibrium. Downslope movement will continue until the critical angle is reestablished. Humans act in another way to unwittingly cause mass wasting. When we add fill to the top of a slope to make additional space for building homes, we steepen the slope (adding material to the top of a slope will steepen te slope). Mass wasting will ultimately occur to restore the slope to an angle less than critical. As a consequence, entire housing developments may be put at risk.

So, we see that mass wasting is simply a response by natural systems to restore the equilibrium condition. If humans upset the equilibrium, nature will respond, sometimes with devastating results. You should read in the text the specific case studies that document such devastation.

In the text, be certain to read about the specific types of mass wasting, and how these types of movement may be classified as different from one another. Classification of the processes usually depends upon two things. Rate of movement (from imperceptibly slow, to devastatingly fastm, such as in an avalanche), and mechanics of movement (whether or not the mass of moving material underwent flow - note that flow can occur in a solid mass, with just small amounts of water acting as a lubricant).

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