HKDSE Geography/M1/Faulting

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Faulting - a displacement of rocks along a line of weakness in the rock strata


[Note: For those who don't know, 'displacement' is a fancy geography term saying the blocks have 'moved'.]

The line along which the two rocks slide is the 'fault or fault line. We will look at faults, then at the landforms associated with faulting.

Faults[edit]

Structure[edit]

TODO: Diagram

  • Fault/Faultline: The line along which the two rock blocks slide
  • Fault plane: The plane between the two blocks
  • Downthrow: The sinking block
  • Upthrow:The rising block
  • Footwall: The face of the sinking block touching the rising block
  • Hanging wall: The exposed lateral face of the rising block
  • Fault scarp: A 'step' on the ground caused by a fault

Classification[edit]

  • Normal fault: Formed by tensional force. One block slides down.
  • Reverse fault: Formed by compressional force. One block slides up.
  • Tear/transform fault: Formed by lateral/shear force. The rocks slide past each other. (There is no vertical displacement.)

Associated Landforms[edit]

Rift Valleys (Grabens)[edit]

Rift valley - A flat-bottom valley with steep fault-scarps and block mountains on both sides


Rift valleys are long, elongated and have uniform width. They are formed between two faults. They may be formed by normal or reverse faulting:

  • When the two faults tilt towards each other, under compressional force, the two outer blocks will be displaced upwards as upthrow. The central block will be left as downthrow and form the rift valley.
  • When the two faults tilt away from each other, under tensional force, the central block will sink as downthrow, forming the rift valley. The side blocks will be upthrow. If this process repeats, it may form parallel faults, and a set of parallel faults forms a step fault.

Block Mountins (Horsts)[edit]

Like folding, faulting can also form mountains.

Block mountain - A flat-top mountain with steep fault-scarps and rift valleys on both sides


Block mountains are long, elongated and have uniform width. They are formed betweem two faults. They may be formed by normal or reverse faulting:

  • When the two faults tilt away from each other, under compressional force, the central block will be pushed upwards as upthrow. The side blocks will be left as downthrow.
  • When the two faults tilt towards each other, under tensional force, the two side blocks will sink as downthrow, leaving the outer blocks as upthrow, i.e. the block mountains.
River Rhine

The Franco-German portion of the River Rhine is formed in the middle of a series of parallel faults which form a step fault. On the one side is the Vosges of France; on the other, the Black Forest of Germany. Both are notable block mountiains.