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

Footwall

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.