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Crust Deformation: Forces, Faults and Folds Deformation • The bending, tilting, and breaking of the earth’s crust • major cause of deformation = plate tectonics Isostasy • There are two forces involved in changing the earth’s crust: • 1. The force of the crust pressing down on the mantle Isostasy • 2. The force of the mantle pressing up on the crust. • The balancing of these forces is termed Isostasy! Isostatic Adjustments • The up and down movements of the crust to reach isostasy are called Isostatic Adjustments Isostatic Adjustments • Isostatic Adjustments create pressure causing rocks in that particular area to bend and deform. • Common cause of earthquakes! Stress • Both isostatic adjustments & plate mov’t cause stress in rocks that make up the crust. • Stress = the force that applies pressure to the rocks in the crust Three types of Stress or Forces • Compression: occurs when crustal rock is squeezed together Three types of Stress or Forces • Tension: occurs when rocks are pulled apart Three types of Stress or Forces • Shearing: occurs when rocks are being pushed in two opposite directions What happens because of stress??? • Folding & Faulting occurs. • When rock responds to stress by becoming permanently deformed w/o breaking, the result is folding. Faulting • Because of cooler temperatures & lower pressure near earth’s surface rocks respond to stress by breaking. Faulting • When there is no movement in the rocks along a break it is called fracture. • When rocks do move at a break it is called a fault. So what’s the REAL difference between folding (bending) and faulting (breaking)? folding faulting Fault terminology • Fault plane: the actual break in the rock Hanging wall Foot wall Fault terminology • Hanging wall: rock above fault plane that moves relative to rocks below the fault plane • Footwall: rock below the fault plane Three types of Faults • Normal fault: hanging wall moves down relative to fault plane Three types of Faults • Reverse fault: compression causes the hanging wall to move up relative to the foot wall Three types of Faults • Strike-slip fault: rock on either side of fault plane move horizontally; usually occur at transform fault boundaries.