Folding and Faulting Notes

Folding

Folding occurs due to three main reasons:

  • Compression: This happens when two plates collide (convergent boundary).
  • Tension: This happens when two plates separate (divergent boundary). As plates pull apart, the Earth's crust stretches until it can no longer sustain the tension, causing it to snap.
  • Shearing: This happens when two plates slide past each other, applying shearing forces on the plates.

Elements of a Fold

  • Anticline: The peak or crest of a fold.
  • Syncline: The trough or valley between folds.
  • Limbs: The sides of the fold, which can vary in length.

Periods of Folding (Orogenies)

  • Caledonian Orogeny: Occurred approximately 400 million years ago (mya).
    • The Eurasian and North American plates collided.
    • Resulted in the formation of the Bluestack and Maamturk mountains on the west coast of Ireland, as well as the Wicklow mountains on the east coast.
    • Mountains exhibit a Southeast-Northwest (SE-NW) trend.
    • These mountains are now smaller due to extensive weathering and erosion over a long period.
  • Armorican Orogeny: Occurred approximately 250 mya.
    • The Eurasian and African plates collided.
    • Resulted in the formation of the MacGillycuddy Reeks and Galtee mountain ranges.
    • Mountains exhibit a West-East (W-E) trend.
    • Munster Ridge and Valley landscape: This landscape was formed as folding thrust layers of limestone upwards, starting the creation of anticlines. Weathering eroded the limestone on the anticlines, leaving behind sandstone mountain peaks and limestone in the valleys.
  • Alpine Orogeny: Occurred approximately 35 mya.
    • Caused by various collisions, including:
      • Nazca and South American plates (Andes Mountains).
      • Eurasian and African plates (Alps Mountains).
      • Indian and Eurasian plates (Himalayas Mountains).
    • These are the newest mountain ranges in the world.
    • Mount Everest, the highest mountain globally, is located in the Himalayas.

Faulting

Types of Faults

  • Normal Fault:
    • Occurs where plates separate along parallel fault lines.
    • Causes a section of land between the faults to drop, forming a rift valley (graben).
    • Example: East African Rift Valley.
    • Compression along fault lines can also cause a block of land (horst) to be thrust upward, creating a block mountain.
    • Example: Ox Mountains in County Sligo, Ireland.
  • Reverse Fault:
    • Occurs where compression causes one plate to be thrust on top of another.
    • Results in shortening of the Earth’s crust.
    • If the angle of the fault is less than 45∘45^{\circ}, it is known as a thrust fault.
  • Tear Fault (Strike-Slip Fault):
    • Occurs when plates slide past one another horizontally.
    • Creates horizontal faults.

Landforms of Faulting: East African Rift Valley

  • Magma plumes beneath the crust cause the crust to buckle and fracture, forming fault lines.
  • When fault lines run parallel, a rift valley can form between them.
  • In Africa, the plates are slowly moving apart, creating a flat-floored, steep-sided valley.
  • The valley's width ranges from 4040 to 100100 km.
  • Eventually, the continent will split, forming a new sea.