Geological Processes: Faults, Folds, Metamorphism, and More

Posted on Mar 25, 2025 in Geology

Geological Processes: An Overview

Faults and Fractures

Faults: Fractures with displacement of blocks. Without displacement, they are called jointing.

Elements of a Fault:

  • Fault Plane: Fracture separating displaced blocks.
  • Hanging Wall: The block above the fault plane.
  • Footwall: The block below the fault plane.
  • Slickensides: Scratches caused by friction.
  • Fault Throw: Vertical displacement between blocks.

Types of Faults:

  • Normal Fault: Hanging wall moves down relative to the footwall.
  • Reverse Fault: Hanging wall moves up relative to the footwall.
  • Strike-Slip Fault: Blocks move horizontally.
    • Left-Lateral: Displacement is to the left.
    • Right-Lateral: Displacement is to the right.

Folds

Elements of a Fold:

  • Fold Axis: Intersection between the axial plane and any layer.
  • Hinge: Zone of maximum curvature.
  • Limb: Side of the fold.
  • Axial Plane: Plane connecting the hinges.
  • Stratum: Layers of the fold.
  • Core: Innermost part of the fold.

Fold Geometry:

  • Antiform: Fold with limbs that dip away from the hinge.
  • Synform: Fold with limbs that dip toward the hinge.
  • Neutral: Neither antiform nor synform.

Axial Plane Inclination:

  • Upright: Vertical axial plane.
  • Inclined: Tilted axial plane.
  • Overturned: Axial plane inclined beyond vertical.
  • Recumbent: Horizontal axial plane.

Limb Opening:

  • Gentle: Wide angle between limbs.
  • Open: Moderate angle between limbs.
  • Tight: Small angle between limbs.
  • Isoclinal: Parallel limbs.

Fold Types:

  • Anticline: Oldest rocks in the core.
  • Syncline: Youngest rocks in the core.

Petrological Cycle

Sedimentary Rock (Metamorphosed) > Metamorphic Rock (Anatexis) > Magmatic or Igneous Rock > Sediments (Diagenesis) > Sedimentary Rock.

Magma and Diagenesis

  • Magma: Molten silicate rock.
  • Diagenesis: Processes transforming sediments into sedimentary rocks:
    • Compaction
    • Cementation
    • Water Loss
    • Formation of New Minerals

Metamorphism and Anatexis

  • Metamorphism: Mineralogical and structural changes in solid rock due to pressure and temperature. Example: Slate and Marble.
  • Anatexis: Partial melting.

Metamorphic Rock Types

  • Filonian
  • Plutonic
  • Volcanic

Example of Change: Slate > Shale > Gneiss (Increasing Temperature and Pressure)

Rock Texture

The relationship of shape and size between rock components.

Textures:

  • Phaneritic (Grainy): Plutonic rocks, solidified within the Earth’s crust, have crystals.
  • Porphyritic: Volcanic rocks, solidify externally. Vitreous matrix with microlites.
  • Vitreous (Glassy): Filonian rocks, homogeneous texture.

External Geological Agents

  • Atmosphere
  • Wind
  • Living Things
  • Waters
    • Sea
    • Continental
      • Underground
      • Surface
        • Rivers
        • Lakes
        • Glaciers

External Processes

  • Weathering
  • Erosion
  • Transportation
  • Sedimentation

Internal Processes: Magmatism and Tectonic Plates

Fusion may be due to:

  • Temperature Increase: Produced by friction.
  • Pressure Decrease: Reduces the melting point.
  • Presence of Fluids: Reduces the melting temperature of rocks.

Types of Metamorphism

  • Contact: Increased temperature.
  • Regional: Increased pressure and temperature.
  • High Pressure: Increased pressure.

Pressure can be directed or lithostatic.

Isostasy

Flotation equilibrium between the lithosphere and plastic mantle. If mass increases, the lithosphere tends to sink. If reduced, it tends to rise.

Theory of Contractions

  • The Earth was once molten.
  • As the crust cooled, ridges formed. This theory evolved into the geosyncline theory.

Geological Eras

Precambrian, Paleozoic (Primary), Mesozoic (Secondary), Cenozoic (Tertiary)

Types of Volcanoes

  • Hawaiian: Low viscosity and gas content.
  • Stratovolcanoes (Vulcanian): Intermediate viscosity.
  • Pelean: High viscosity and gas content.

Seismic Risk

Hazard (probability of risk) + Exposure (people and materials) + Vulnerability (percentage of loss).

Seismic Belts:

Circum-Pacific, Mediterranean.

Detrital Sediments

Sediment from a detrital source, transported in solid state.

  • Grain size > 2 mm: Conglomerates
    • Puddingstone (Rounded)
    • Breccia (Sharp Edges)
  • Between 2 mm and 1/16 mm: Sandstones
  • Less than 1/16 mm: Clay

Non-Detrital Sediments

  • Calcite: Calcium carbonate (Limestone). Reacts with sulfuric acid.
  • Marl: Rock halfway between limestone and clay.

Conglomerate or sandstone subjected to pressure and temperature becomes quartzite.