Rock Deformation and the Rock Cycle: A Comprehensive Study

Posted on Jan 6, 2025 in Geography

1. Deformation of Rocks

Geothermal Energy – Plate Tectonics – Stress – Deformation

1.1. Types of Stress

Stress is a force applied over an area. It can be classified into:

  • Confining stress (stress is equal from all directions)
  • Differential stress (stress is not equal from all directions)
    • Compressional stress, produced by opposite, convergent forces
    • Tensional stress, produced by opposite, divergent forces
    • Shear stress, produced by opposite, parallel forces

As the stress is applied to a rock, it begins to deform or to strain. A strain is a change in the shape and/or volume of a rock. Three successive stages of deformation (strain) can be differentiated:

  • Elastic deformation: It’s reversible. It disappears as the stress disappears.
  • Ductile deformation: The deformation is irreversible. Rocks do not break.
  • Brittle deformation: The deformation is irreversible. Rocks break.

An observable tectonic structure has to be:

  1. Permanent (no elastic), and
  2. It has to change the original shape (no confining stress)

Rock behavior is controlled by compositional and environmental factors:

  • Composition of the rock: There are elastic, ductile, and brittle materials
  • Environmental factors:
    • Confining pressure: At high confining pressure, brittle behavior is delayed.
    • Temperature: At high temperatures, ductile behavior is favored.
    • Time: At slow strain rates, ductile behavior is favored.

1.2. Brittle Deformation: Fractures

When a material undergoes brittle deformation, it breaks. If there is a relative displacement along the fracture, a fault appears. If not, it is a joint.

The main elements in a fault are:

  • Fault jump: Vector displacement.
  • Fault plane: Fracture surface.
  • Fault scarp: A long cliff along the fault.
  • Slickensides: Parallel striations on the fault plane as one block moves relative to the other.
  • Blocks: The two pieces of rock at both sides of the fault plane.

In dip-slip faults there are:

  • Hanging wall block: The block above the fault
  • Foot wall block: The block below the fault

The main types of faults are:

  • Dip-slip faults: Inclined fault plane. Displacement along the dip direction.
    • Normal faults: The hanging wall moves downwards.
    • Reverse faults: The hanging wall moves upwards.
  • Strike-slip faults: Vertical fault plane, horizontal fault jump

1.3. Ductile Deformation: Folding

When a material that is being compressed undergoes ductile deformation, it bends, forming folds.

Elements of a fold:

  • Hinge: Is the part of greatest curvature in the fold.
  • Limbs: Are the sides of the fold.
  • Fold axis: Is the line connecting all points on the hinge of one stratum.
  • Axial plane: Is the plane connecting all successive fold axes.
  • Plunge: Is the angle that the fold axis makes with a horizontal line.

Types of folds:

According to the Curvature

  • Anticline
  • Syncline
  • Monocline

According to the Axial Plane

  • Upright fold
  • Inclined fold
  • Overturned fold
  • Recumbent fold

According to the Angle Between Limbs

  • Open fold
  • Closed (tight) fold
  • Isoclinal fold

2. The Rock Cycle

2.1. External Processes

The external processes are those directly or indirectly caused by solar energy:

  • Weathering
  • Erosion
  • Transportation
  • Sedimentation or deposition
  • Lithification (Compaction, dehydration, and cementation)