deform - Deformation of Rock EENS 1110 Physical Geology...

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This page last updated on 21-Feb-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Deformation of Rock Mount Everest is the highest peak on Earth at 29,028 feet above sea level. The rock at the top of the peak is a marine limestone, deposited on the sea floor about 450 million years ago! This is an amazing fact that begs the question - how did that rock get there? In this discussion we will try to answer that question. The topics we will cover include: z Review of Stress and Strain z Brittle Deformation – Faults and Joints z Ductile deformation – Folds z Mountain Building Processes Stress and Strain We start our discussion with a brief review of the concepts of stress and strain. Recall that stress is a force acting on a material that produces a strain. Stress is a force applied over an area and therefore has units of Force/area (like lb/in 2 ). Pressure is a stress where the forces act equally from all directions. If stress is not equal from all directions then we say that the stress is a differential stress. Three kinds of differential stress occur. 1. Tensional stress (or extensional stress) , which stretches rock; 2. Compressional stress , which squeezes rock; and 3. Shear stress , which result in slippage and translation. When rocks deform they are said to strain . A strain is a change in size, shape, or volume of a material. We here modify that definition somewhat to say that a strain also includes any kind of movement of the material, including translation and tilting. Deformation of Rock 2/21/2011 Page 1 of 16
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Stages of Deformation When a rock is subjected to increasing stress it passes through 3 successive stages of deformation. z Elastic Deformation -- wherein the strain is reversible. z Ductile Deformation -- wherein the strain is irreversible. z Fracture - irreversible strain wherein the material breaks . We can divide materials into two classes that depend on their relative behavior under stress. z Brittle materials have a small or large region of elastic behavior but only a small region of ductile behavior before they fracture. z Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture. How a material behaves will depend on several factors. Among them are: z Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle. z Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner. z
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This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.

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deform - Deformation of Rock EENS 1110 Physical Geology...

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