Geology Chapter: Folding and Orogeny

Folding

• Folding is a bend or curve in the Earth's crustal rocks.
• It occurs when two continental plates collide, forcing land upwards.
• Compression is the process that leads to folding.
• Orogeny describes mountain-building processes.
• Tectonic movements cause plates to buckle, forming fold mountains.

Types of Folding

• Simple Fold: Two limbs of equal steepness. Compression is applied evenly on both sides.
• Asymmetrical Fold: One limb is steeper than the other, due to unequal compression on either side.
• Overfold: One limb is significantly pushed over the other limb, as compression continues.
• Recumbent Fold: Folds lie almost horizontally on each other.
• Overthrust Fold: One limb eventually lies on top of the other limb. The compression force is very strong to cause this to happen.

Periods of Orogeny

• Caledonian Orogeny: 400 million years ago, including the Wicklow Mountains, Scottish Highlands, and Appalachian Mountains. This occured when the North American and Eurasian plates collided..
• Armorican Orogeny: 250 million years ago, including the Munster Ridge & Valley. This period occurred when the African and Eurasian plates collided, creating anticlines and synclines.
• Alpine Orogeny: 30 million years ago, including the Alps and Himalayas; did not affect Ireland.

Faulting

• A fault is a crack or fracture in the Earth's crust.

• Pressure and tension cause movement along fault lines.

• Movement can be vertical or horizontal and occurs in areas of weakness in the crust.

• Three main types of faults:

  • Normal Fault: One side of the fault slips downwards relative to the other. Caused by tension from plates pulling apart (divergent boundaries).
  • Reverse Fault: Land is compressed and pushed upwards, opposite to a normal fault. Caused by convergent plate boundaries.
  • Tear Fault: Plates slide past each other horizontally. Caused by the movement of plates in a transform boundary.

Case Studies

• Munster Ridge and Valley: Formed from Old Red Sandstone, limestone, and shale; features include trellis drainage patterns in the synclines. This area experienced a desert climate in the past (380 MYA).
• African Rift Valley: Weakened by magma plumes that cause immense tension between plates, creating huge fault lines that formed a valley with steep sides. This is an example of a diverging plate boundary. The widening of this valley continues to increase by 4m per year.

Doming

• Doming is an upward bulge in sedimentary rocks due to magma rising.
• Sedimentary rocks are deformed more easily compared to other rock types. Lava rising does not necessarily crack the rock layers, but can cause the surface to bulge.

Examples of Faults

• San Andreas Fault (Tear Fault): This tear fault is located in California.
• Clew Bay, Co. Mayo (Normal Fault): Provides an example of normal fault formation creating a rift valley (graben).
• Ben Bulben (Normal Fault): An example of a normal fault.