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I. Deformation. Shaping the Earth’s Crust Already talked about plate tectonics Large-scale movement of Earth’s lithosphere Structural geology: study of crustal deformation & mountain building. I. Force, Stress, and Strain Stress : the force applied per unit area Stress = σ = F/A
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I. Deformation Shaping the Earth’s Crust Already talked about plate tectonics Large-scale movement of Earth’s lithosphere Structural geology: study of crustal deformation & mountain building
I. Force, Stress, and Strain • Stress: the force applied per unit area • Stress = σ = F/A • (Force = Mass x Acceleration) • Use example of an axe, sharp side vs dull side and hockey • B. Strain: the distortion or deformation. • The change in size or shape of a rock • The deformation that occurs due to stress
II. Types of Differential Stress Stress applied unequally in different directions 1. Compressional 2. Tensional 3. Shear
II. Types of Differential Stress Stress applied unequally in different directions A.Compression: stress that shortens a rock body, pushes it together Associated with plate collisions, convergence Causes rocks to crumple, thicken vertically and shorten laterally
II. Types of Differential Stress Stress applied unequally in different directions B. Tension: stress that stretches or extends rocks Associated with divergent plate boundaries Causes rocks to thin vertically and lengthen laterally
II. Types of Differential Stress Stress applied unequally in different directions C. Shear: stress that pushes rocks past each other in parallel but opposite directions Associated with transform plate margins Slices rocks into parallel blocks, breaks and displaces preexisting rocks & structures (deck of cards example)
III. Types of Deformation When a rock is subjected to stress, they can strain in different ways A. Elastic deformation = recoverable When minor stress is applied, a rock may strain slightly, but then return to its original shape after the stress is removed Rubber band strains, but returns to its original shape, rocks usually won’t take as much stress as a rubber band
III. Types of Deformation B. Brittle deformation Rocks crack or rupture Faults The actual atomic bonds within the rock are broken along the zone of max stress = where the crack appears Occurs under conditions of low temp and pressure Also occurs when stress is applied quickly, a quick swing of a hammer
III. Types of Deformation C. Plastic (ductile) deformation Folds An irreversible change in shape or volume that occurs without the rock breaking Atoms rearrange themselves on a microscopic scale, from areas of maximum stress to areas of lower stress (i.e. toothpaste oozes away from pressure) Occurs under conditions of higher temp and pressure Sometimes a rock begins experiences ductile deformation but if the stress increases it may fail by brittle deformation Draw folds caused by horizontal compression
III. Types of Deformation C. Plastic (ductile) deformation Folds An irreversible change in shape or volume that occurs without the rock breaking Plastic (ductile): low rates of strain, exceed yield point Brittle: high rates of strain, rocks break or fracture, exceed yield point Silly Putty Example: Elastic, roll into ball and bounce
IV. Factors affecting rock deformation • A. Heat (blacksmith heats metal to work with it, so its ductile) • 1. low temperature brittle • 2. high temperature ductile/plastic • B.Pressure • 1. low pressure brittle • 2. high pressure ductile
IV. Factors affecting rock deformation C.Time dependent (just as with silly putty) 1. less time brittle 2. more time (same stress) ductile D. Rock Type or composition 1. Some mineral bonds are simply stronger than others (all other factors being equal) Salt is weak ductile basalt is strong brittle 2. The presence of water helps rocks be more ductile