Deformation & Mountain Building

Deformation and Mountain Building Notes

Deformation

  • Definition: A change in a rock’s position, shape, and/or volume from the application of stress.

  • Foliation in Metamorphic Rocks: Requires pressure to create foliation.

  • Structural Geology: A dedicated field of geology that studies how rocks deform.

  • Stress: The forces acting on the surface of a material. Types of stress include:

    • Tensional Stress
    • Compressional Stress
    • Shear Stress

  • Strain: A measure of a rock’s change in shape or volume as a result of applied stress.

Types of Stress

  1. Tensional Stress:

    • Forces act outward from the rock.
    • Causes strains to become longer and thinner, often seen at divergent plate boundaries.

  2. Compressional Stress:

    • Forces act inward on the rock.
    • Causes strains to become shorter and thicker, typically associated with convergent plate boundaries.

  3. Shear Stress:

    • Forces act parallel and opposite on different rock faces.
    • Causes strains to become tilted, often observed at transform boundaries.

Responses to Stress

  • Elastic Deformation:

    • Temporary change in position, shape, or volume. Rock returns to original shape when stress is removed (e.g. earthquakes).

  • Plastic/Ductile Deformation:

    • Permanent bending, flowing, and smearing of rock under stress. Rock does not return to original shape.

  • Brittle Deformation:

    • Rock breaks or fractures under stress. Does not revert to original shape.

Factors Influencing Deformation

  • Rock Type:

    • Different minerals and rock structures react to stress uniquely.

  • Temperature & Pressure:

    • Higher temperatures and pressures can lead to ductile deformation.

  • Time:

    • Longer periods under stress can result in more significant deformation.

Orientation of Geologic Structures

  • Strike: The orientation of an imaginary horizontal line on a plane of a geologic structure, measured relative to North.
  • Dip: The angle of a geologic feature relative to a horizontal plane.

Fractures

Types of Fractures

  1. Joints:

    • Rock fractures with no appreciable movement (e.g., columnar, exfoliation).

  2. Faults:

    • Fractures with significant movement characterized by:
      • Normal Faults: Hanging wall moves down relative to footwall (tensional stress).
      • Reverse Faults: Hanging wall moves up relative to footwall (compressional stress).
      • Strike-Slip Faults: Horizontal movement where the fault can be either right-lateral or left-lateral.

Folds

  • Fold Anatomy:
    • Fold Hinge: Line joining points of greatest curvature in each layer.
    • Axial Plane: Imaginary surface dividing a fold into two halves.

Types of Folds

  1. Anticline: A fold where rock layers arch upward, oldest rocks are at the core.
  2. Syncline: A fold where rock layers flex downward, youngest rocks are at the core.
  3. Plunging Folds: Hinge slopes downward, creating a zigzag pattern on the surface.
  4. Dome: Circular bulge with layers dipping away from the summit.
  5. Basin: Bowl-shaped structure with layers dipping inward towards the center.

Plate Tectonics & Mountain Building

  • Orogeny: Process of mountain building through tectonic forces.

Four Main Ways to Create Mountains

  1. Hot Spots: Volcanoes formed above hot mantle plumes.
  2. Mid-Ocean Ridges: Diverging tectonic plates create new oceanic crust, often leading to subsidence.
  3. Subduction: Collision of tectonic plates causing magma generation and volcanic mountain formation.
  4. Collision: Interaction between continents, often leading to orogenesis (e.g., Himalayas).

Specific Examples

  • Andes Mountains: Formed from subduction of oceanic crust under continental crust.

  • Himalayas: Created from the collision of the Indian Plate with the Eurasian Plate, making them the youngest and highest mountains.

  • Appalachians: Formed through multiple episodes of collision and subduction over 600 million years, showcasing diverse geological features.

Review Questions

  • What is the difference between stress and pressure?
  • Describe how brittle and ductile rocks respond to stress.
  • What are strike and dip, and how are they measured?
  • Differentiate between normal and reverse faults.
  • Discuss how mountains are formed, providing examples like the Himalayas and the Andes.