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Chapter 8

GLG110 Textbook Notes Exploring Geology 2nd Ed Chapter 8

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Earth Sciences
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[GLG Chapter 8 Text Notes] 1 Chapter 8: Crustal Deformation • Structural geology: a study of earth’s architecture • Deformation: is a general term that refers to all changes in the original form or size of a rock body; can produce changes in the location and orientation of rock; most crustal deformation occurs along or near plate margins o Force: is any influence that tends to put stationary objects in motion or change the motions of moving bodies  Stress: the amount of force applied to a given area; aka how concentrated the force is • Differential stress: when stress is applied unequally from different directions o Differential stress that shortens rock body is known as compressional stress; this stress, when associated with plate collisions, tends to shorten and thicken Earth‘s crust by folding, flowing and faulting o This stress can also cause rock to shear; shearing is similar to slippage that occurs between individual playing cards when the top of the deck is moved relative to the bottom (transform faults) • Tensional stress: when stress tends to elongate or pull apart a rock unit o Where plates are being rifted apart (divergent), tensional stresses tend to lengthen those rock bodies located in the upper crust by displacement along faults  Strain: whereas stress is the force applied to a given area, strain is the visible result of that force; strained bodies do not retain their original configuration during deformation • When rocks are subjected to greater stress than their own strength, they begin to deform, usually by flowing (folding) or fracturing (faulting) • Rocks near the surface, where temperatures and confining pressures are low, tend to behave like a brittle solid and fracture once their strength is exceeded; called brittle failure or brittle deformation [GLG Chapter 8 Text Notes] 2 • Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing; e.g. something like modelling clay • Rock type matters on how it will deform; for example, crystalline rocks composed of minerals that have strong internal molecular bonds tend to fail by brittle fracture; by contrast, sedimentary rocks are weakly cemented and metamorphic rocks that contain zones of weakness are more susceptible to ductile flow • Time (geologic) of during which the rocks are subjected stress also may affect how it will turn out • The processes of deformation generate features at many different scales; all of these phenomena, from the largest folds in the Alps to the smallest fractures in a slab of rock, are referred to as rock structures o Geologic mapping is most easily accomplished where sedimentary strata are exposed o Strike is the compass direction of the line produced by the intersection of an inclined rock layer or fault, with a horizontal plane o Dip is the angle of inclination of the surface of a rock unit or fault measured from a horizontal plane o During mountain building, flat-lying sedimentary and volcanic rocks are often bend into a series of wavelike undulations called folds; like the folds that would form if you were to push in a sheet of paper  Anticline is most commonly formed by the upfolding, or arching, of rock layers  Synclines are commonly found in association with anticlines are downfolds, or troughs  Ways to describe folds: basic folds = symmetrical, asymmetrical fold = overturned, an ove
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