Mountain Building Folding and Faulting
Stress in the Crust • Stress from plate motions causes crustal rocks to deform • Rocks near the surface are cool and rigid. They will tend to fault (break) • Rocks deeper within the Earth are warm and more ductile. They will tend to fold (bend)
Stress Directions • The pressure that causes plate motions is in horizontal directions • Therefore, STRESS within the rocks must also be horizontal
Stress Directions • Compression • Rocks are being squeezed • Result is ‘shortening’ • Tension • Rocks are being stretched • Result is ‘lengthening • Shear⇄ • Rocks move in opposite directions and are distorted
Folds • Occur deep within earth where rocks are ductile. • Horizontal stress will cause rocks to bend upwards or downwards. • Folds are usually caused by compressional stress. • All folds contain an axial plane and limbs.
Anticline • Limbs point down on either side of the axis • Oldest rock is in the center of the fold Stress Stress Limb Limb
Syncline • Limbs point up on either side of the axis • Youngest rock is in the center of the fold Stress Stress Limb Limb
Faults • Occur near the surface where rocks are cool and brittle. • Stress causes the rocks to ‘strain’ and break. • Movement (up or down) must occur along either side of the fault. • Stress that creates faults can be compressional, tensional, or shear.
Parts of a fault Fault Plane – the plane where movement occurs Hanging Wall – Above the fault plane Footwall – Below the fault plane
Normal Fault Tensional Stress Hanging wall moves down Results in extension of crust Foot Wall Hanging Wall Stress Stress
Reverse Fault Hanging Wall Stress Stress Foot Wall Compressional Stress Hanging wall moves up Results in shortening of the crust