GEOL 21062 Lecture Notes - Lecture 21: Diffusion Creep, Plate Tectonics, Grain Boundary
31 Jan 2020
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Rocks are deformed due to directional forces acting on them. These are essentially the results of plate tectonic (and we will discuss the driving forces for that later in the course). Uniform relates to pressure, which we think about in metamorphism. 3 types of differential stresses (draw a picture for each): compression results in shortening, extension results in pulling apart, shearing pushes two sides apart. Explain the sigma 1,2,3 notation for major, minor and intermediate stresses. Changes in a body"s dimensions are measured in strain: Definition of shear strain: = tan (opposite/adjacent) Stresses can result in two types of deformation: recoverable (elastic) and permanent (ductile, brittle) Related to breaking of rocks: minerals break apart mainly along pre-existing weak zones such as microfractures. Make an experiment of two students breaking chalk along extension and compression. Ask for an explanation as to why chalk is so much weaker in tension than in compression. Explain that this is true for rocks, too.
