In this lecture we start out with a lesson in mechanical engineering, then apply that knowledge to the study of the mechanical properties of the Earth. Stress and Strain
More on Young's modulus. Basic descriptive terminology: Structural geologists are preoccupied with the orientation of things in space. To describe the orientations of lines and planes, three terms are used.
Fractures: Fractures are simply cracks, the traces of brittle deformation of rocks.
In the field, faults almost never appear as in the nice clear block diagrams. Weathering and erosion immediately attack any topographic expression of faults, like in the image above. Once that happens, faults are inconspicuous and reveal their presence only indirectly. Some keys:
Folds: Folds and associated structures are the record of ductile deformation of rocks.
What type of stress is uniform stress?One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress.
What stress of a fault occurs when slabs of rock are being pulled apart from each other?Tension stress pulls rocks apart. Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries.
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