Deformation-Of-The-Crust

Deformation

  • Deformation: The process in which rocks change in shape, size, location, tilt, or break due to squeezing, stretching, or shearing.

  • It is the dominant process in the formation of mountain belts.

Stress

  • Stress: Force applied to an object; the force per unit area.

  • Causes materials (rocks) to deform; the response can include folding, faulting, or fracturing.

  • There are four types of stress:

    1. Confining Stress: Uniform stress exerted in all directions.

    2. Compression: Force directed toward the rock.

    3. Tension: Force pulling rocks apart.

    4. Shearing: Forces sliding past each other.

Confining Stress

  • Definition: Stress exerted by the weight of overlying materials in the crust.

  • Uniform and does not cause deformation but increases with depth.

  • Example: A deeply buried rock that cannot move or deform.

Strain

  • Strain: The resulting change in the rock due to stress; can be a change in size, shape, or volume.

  • Responses to stress:

    • Elongation (stretching)

    • Shortening (contraction)

Stages of Deformation

  1. Elastic Deformation: Reversible strain; effects disappear when stress is removed.

  2. Ductile Deformation: Irreversible strain occurs after reaching elastic limit.

  3. Fracture: Permanent deformation resulting in rock breakage.

Classification of Materials Under Stress

  • Brittle Materials: Small region of ductile behavior but can exhibit large elastic behavior.

  • Ductile Materials: Large ductile behavior region, smaller elastic behavior.

Factors Affecting Material Behavior

  1. Temperature

  2. Confining Pressure

  3. Strain Rate

  4. Mineral Composition

Influence of Temperature and Pressure

  • Ductile behavior increases with depth (e.g., at 15 km) due to high temperature and pressure, known as the Brittle-Ductile Transition.

  • Earthquakes occur above this zone.

Types of Differential Stress

  1. Compressional Stress: Directed towards each other, causes rocks to shorten and thicken.

    • Leads to folding and faulting; prominent in convergent plate boundaries.

    • Fault: Fracture along which two rock masses slide past each other.

  2. Tensional Stress: Directed away from each other, causes rocks to elongate and thin.

    • Common in divergent plate boundaries (e.g., mid-ocean ridges).

  3. Shearing Stress: Forces directed towards each other but not along the same axis; leads to lateral displacement.

    • Results in strike-slip faults.

Geological Structures from Deformation

  • Fractures: Broad term for any rock break; classified into joints (small cracks with no movement) and faults (segments with movement).

Orientation Terms

  • Dip: Angle at which a rock layer or fault plane inclines relative to a horizontal surface.

  • Strike: Compass direction of a horizontal line on the inclined plane.

  • Strike and dip are always perpendicular to each other.

Types of Faults

  1. Normal Fault: Rock units are pulled apart; hanging wall moves down.

  2. Reverse Fault: Results from horizontal compression; hanging wall moves up.

  3. Thrust Fault: A reverse fault with low angle of inclination (<30°).

  4. Strike-Slip Fault: Rocks slide horizontally; minimal vertical displacement.

    • Left-Lateral: Opposite side moves left.

    • Right-Lateral: Opposite side moves right.

Fault Expression in Landscapes

  • Fault Scarp: Step in landscape created by fault movement during earthquakes.

  • Grabens: Blocks lowered relative to others due to faulting.

  • Horsts: Elevated blocks relative to others due to faulting.

Folds

  • Folds are bends in rock layers from compressional forces; deform without breaking.

  • Types:

    1. Anticline: Upward arch shape; oldest rocks in center.

    2. Syncline: Downward trough shape; youngest rocks in center.

    3. Monocline: Step-like bend in flat-lying layers.

Types of Plate Boundaries

  1. Divergent: Plates move apart; mid-ocean ridges are examples.

  2. Convergent: Plates collide, resulting in earthquakes and deformation; subduction may occur.

    • Oceanic-Continental: Oceanic plate subducts under continental.

    • Continental-Continental: Collision leads to mountain range formation.

  3. Transform: Plates slide past each other; results in transform faults (e.g., San Andreas Fault).

Tectonic Setting of the Philippine Archipelago

  • The Philippine Sea plate has convergent boundaries that contribute to geological activity.

  • Subduction of the Pacific plate beneath the Philippine Sea plate and the Eurasian plate leads to unique tectonic features.

  • The Philippine Fault is a significant fault that extends for 1200 km, indicating complex interactions with neighboring tectonic plates.