Introduction to Structural Geology

What is Structural Geology?

• This field studies the architecture of the Earth's Crust, examining its physical components and structures.
• It focuses on how these structures form in response to forces and stresses applied to the Earth's crust.
• Understanding rock structures helps interpret Earth's history and aids in economic ventures, like:
  • Oil exploration
  • Mineral deposits
  • Construction projects

Types of Geological Structures

• The main categories are:
  • Geological Contacts:
    • Primary Structures: formed at the same time as the rocks they are found in (e.g., bedding planes, igneous intrusive contacts).
    • Secondary Structures: formed after the rocks they are found in (e.g., faults), also known as tectonic structures, formed by rock deformation.
  • Discontinuities Within Rock Bodies: surfaces that divide rocks into blocks (e.g., joints, fractures, veins).
  • Shapes and Features:
    • Folds: shapes defined by layers like bedding planes, etc.
    • Boudins: sausage-shaped structures in deformed veins.
    • Linear Features: features on surfaces (e.g., ripple crests on bedding planes or striations on fault planes).
  • Patterns on the Grain Scale:
    • Graded Bedding: a grain-size pattern showing coarsest grains at the bottom and finest ones at the top of a bed.
    • Flow Lineation: mineral alignment caused by lava flow.
    • Mineral Banding: like gneissosity, a grain-distribution pattern.
    • Cleavage: a secondary structure usually discordant to other features.

Understanding Geological Contacts

• Unconformity: a discordant contact between younger and older rocks, where the younger rocks cross-cut the older ones.
• Discordant Structures: structures that cross-cut or interrupt other structures (e.g., faults, dykes).
• Concordant Structures: structures that are parallel to other surfaces (e.g., sills).

Measurement of Structures

• Planar Structures: structures that are essentially flat (e.g., bedding planes, faults, veins, cleavages).
• Strike: the compass direction of the horizontal line in a dipping plane.
• Dip: the angle of inclination of the plane from the horizontal.

Examples of Structures

• Bedding Planes: represent layers of sediment deposited over time.
• Intrusive Contacts: formed when magma or lava intrudes into existing rock.
• Faults: fractures in the Earth's crust where rocks have moved past each other.
• Folds: bends in rock layers caused by compressional forces.
• Joints: fractures in rocks where there has been no movement.
• Veins: filled fractures in rocks, often with minerals.
• Ripple Marks: undulating bedforms produced by currents (water or wind), and can be symmetrical or asymmetrical.
• Striations: scratches on fault planes, caused by the movement of rocks along the fault.
• Graded Bedding: a primary structure that indicates deposition by currents, where the coarsest grains are at the bottom and the finest grains are at the top.
• Flow Lineation: a primary structure created by the flow of lava, where mineral crystals are aligned in the direction of flow.
• Gneissosity: a secondary structure created by metamorphism, where minerals are banded or layered.
• Cleavage: a secondary structure that often cuts across other structures.
• Dykes: discordant tabular intrusive bodies.
• Sills: concordant tabular intrusive bodies.

Explore the fascinating field of Structural Geology, which focuses on the architecture of the Earth's crust and the various structures it contains. This quiz covers the formation of rocks, categories of geological structures, and their importance in understanding Earth's history and economic applications. Test your knowledge on the forces that shape the Earth's surface.