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)
The pressure that causes plate motions is
in horizontal directions
Therefore, STRESS within the rocks must
also be horizontal
– Rocks are being squeezed
– Result is ‘shortening’
– Rocks are being stretched
– Result is ‘lengthening
– Rocks move in opposite directions and are
Occur deep within earth
where rocks are ductile.
Horizontal stress will
cause rocks to bend
upwards or downwards.
Folds are usually
All folds contain an
axial plane and limbs.
•Limbs point down on either side of the axis
•Oldest rock is in the center of the fold
•Limbs point up on either side of the axis
•Youngest rock is in the center of the fold
Occur near the surface where rocks are
cool and brittle.
Stress causes the rocks to ‘strain’ and
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
Below the fault plane
Hanging Wall Stress
Hanging wall moves down
Results in extension of crust
Hanging wall moves up
Results in shortening of the crust