Mountain Building

1
Mountain Building

• Folding and Faulting

2
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)

3
Stress Directions

• The pressure that causes plate motions is in
  horizontal directions
• Therefore, STRESS within the rocks must also be
  horizontal

4
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

5
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.

6
Anticline

• Limbs point down on either side of the axis
• Oldest rock is in the center of the fold

Stress
Stress
Limb
Limb
7
Syncline

• Limbs point up on either side of the axis
• Youngest rock is in the center of the fold

Stress
Stress
Limb
Limb
8
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.

9
Parts of a fault
Fault Plane the plane where movement occurs
Hanging Wall Above the fault plane
Footwall Below the fault plane
10
Normal Fault
Foot Wall
Hanging Wall
Stress
Stress

• Tensional Stress
• Hanging wall moves down
• Results in extension of crust

11
Reverse Fault
Hanging Wall
Stress
Stress
Foot Wall
Compressional Stress Hanging wall moves
up Results in shortening of the crust