Shaping the Earth’s Crust Already talked about plate tectonics

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

Stephen Marshak

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

Shaping the Earth’s Crust Already talked about plate tectonics