Structural Geology Overview

Study Notes

Structural geology aims to understand the architecture of Earth's crust, focusing on the physical components and structures that form due to forces and stresses.
It helps interpret Earth's history and guides economic ventures:
- Oil exploration
- Mineral deposit discovery
- Construction projects

Geological contacts are formed at the same time as the rocks they are contained within.
- Bedding planes and intrusive igneous contacts are examples of primary structures.
Other structures form after the rocks they are found in.
- Faults are examples of secondary (or tectonic) structures.
Geological contacts can be either primary (sedimentary or igneous) or secondary.
An unconformity is a primary contact where younger rocks lie on or cross-cut older rocks.
- Unconformities are discordant, meaning they interrupt previously existing structures.
Dykes are intrusive bodies that are also discordant.
Sills are concordant structures, parallel to the surrounding layers.

Surfaces within rock bodies that divide them into blocks are called discontinuities.
- These include joints, fractures, and veins.

Folds are shapes defined by bedding planes, etc.
Boudins are sausage-shaped structures formed in deformed veins.
Ripple marks and other features on surfaces are linear.
- Examples include ripple crests, striations, and other features on fault planes.

Graded bedding shows a pattern of grain size, with coarser grains at the bottom and finer grains at the top.
Flow lineation is a primary structure where phenocrysts are aligned by lava flow.
Gneissosity is a secondary (metamorphic) structure that shows mineral banding.
Cleavage is a secondary structure that is generally discordant.
- These grain patterns are usually small-scale.

Many structures are essentially flat.
Planar structures can be horizontal, vertical, or inclined.
The strike is the compass direction of a horizontal line contained within a dipping plane.
- The horizontal line is known as the strike line (or line of strike).