111-35 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Notes Rock Deformation Lecture Goals : A) Confining pressure and rock deformation B) Elastic versus permanent deformation C) Types of deformation Reference: Press et al., 2004, Chapter 11; Grotzinger et al., 2007, Chapter 7 (p 154-163) A) Confining pressure and rock deformation In an earlier lecture dealing with metamorphism, we discussed the role of pressure in changing the orientation of minerals. That type of directed pressure is usually associated with compression at or near convergent plate boundaries. It is more properly called stress and in particular, compressive stress since that is the type of force that operates at that type of boundary. There are other types of stress at other plate boundaries. For example, divergent plate boundaries are sites of tension or tensile stress and transform plate boundaries are sites of shear or shear stress . There is, however, another type of pressure that we need to discuss. It isn’t so much “directed” as it is uniform. It is the pressure associate with rocks as they get buried by successive layers of sediment and sedimentary rock. This overlying material is usually called overburden and if you recall, we previously discussed it when we were discussing how compaction produces shale from fine-grained sediment. The pressure that squeezes the water out of the sediment and compacts it into shale is said to be isostatic as it is equal in all directions. Some geologists (yours truly included) call this type of isostatic force the confining pressure . The best way to visualize confining pressure is to imagine a cube of sedimentary rock that is being subjected to equal compression from 3 different directions. It stands to reason that the deeper the rocks are buried, the more intense the confining pressure. The volume of rock pictured above will decrease the deeper the rocks are buried (see cartoon at the top of the next page). However, the volume will always remain cubical because confining pressure is isostatic.
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GY 111 Lecture Notes D. Haywick (2008-09) 2 At this point, we need to introduce an important term; rock deformation . It is defined as any change in the volume, shape or attitude 1 of a rock body. So the volume change that occurs when confining pressure increases is an example of rock deformation. But I have to tell you, identifying volume changes in rocks is really tricky. You can really only do it if there is some means by which to gauge the original volume of the rock. If you started off with a oolitic limestone or a quartz arenite sandstone which both consist of evenly sized and spaced sedimentary particles, volume loss could be recognized by squashed grains 2 . But what if the starting rock was a granite or a shale? Volume loss in these rocks is very difficult to recognize. Recognizing change in shape or attitude can be a much easier means by which to recognize rock deformation, but once again, you need to know the starting shape/attitude of the rock body. Once again, the best rocks to use to illustrate
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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111-35 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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