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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
Syncline
•Limbs point up on either side of the axis
•Youngest rock is in the center of the fold
Stress
Stress
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
Foot
Wall
Stress
Hanging Wall
Tensional Stress
Hanging wall moves down
Results in extension of crust
Stress
Reverse Fault
Hanging Wall
Stress
Stress
Foot
Wall
Compressional Stress
Hanging wall moves up
Results in shortening of the crust