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Unformatted text preview: EXERCISE #3 PRELAB GEOLOGY 240 week of Sept. 22-26, 2008 MATERIALS NEEDED: Highlighter pen, colored pencils STRESS AND FAULTING Plate tectonic interactions occur when plates push against each other, slide past one another, or pull away from each other. These actions of pushing, sliding and pulling are the forces that cause deformation of the Earths lithosphere. In the upper crust, the deformation can be in the form of folding, cracking and faulting, to name a few. This lab will focus on the interrelationship between the forces operating at plate boundaries and the faults that result. I. Stress Stress is simply the force per unit area. For example, you can calculate the stress you impose on the ground when standing on one foot. Its simply your weight in pounds (e.g. 130 lbs.) divided by the area of your foot (e.g. 7 inches times 3 inches roughly), giving your stress in pounds per square inch (e.g. ~ 6 lbs./sq.in). In the case of the Earth the stresses are measured in newtons per square meter. Forces are vectors, which means stress can vary depending on the direction. It is this variation in stress that results in the different types of faults that occur in the crust. So, what forces are acting on rocks beneath the surface of the Earth? As mentioned above, the movement of plates imposes forces on the rocks in the crust. In order to think about stress in the Earth we need to put on our 3-D glasses. It is helpful to envision stress acting on a block of ice or granite. As stated previously, the amount of stress varies depending on the direction. The normal stress acting on a plane is the stress that acts perpendicular, or 90 O , to that plane. Shear stress is simply the stress acting parallel to that plane. Shear and normal stress are just two directions of stress on a plane but actually there are stresses acting in every direction on a point in the Earth....
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