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.