Structural Geology – Week 8-9 Lecture

20 question-and-answer flashcards covering key terms and concepts from the Structural Geology lecture, including stress-strain relationships, deformation types, folds, faults, joints, and mapping conventions.

What does structural geology study?

The deformation of rocks, including the geometry, processes, and history of folds, faults, joints, and other structures.

How do stress and strain differ in geology?

Stress is the force per unit area applied to a rock, whereas strain is the resulting change in shape or volume (deformation) of the rock.

What is Young’s modulus?

The ratio of stress to strain in a material; it measures stiffness—high values indicate stiff materials like steel, low values indicate less-stiff materials like lead.

Name the three main kinds of differential stress that act on rocks.

Compressional stress, tensional stress, and shear stress.

How does confining pressure differ from differential stress?

Confining pressure acts equally in all directions, while differential stress is unequal and greater in one or more directions.

List four factors that control whether a rock deforms brittly or ductilely.

Temperature, confining pressure, strain rate, and rock composition (including presence of water).

At roughly what depth in the crust does the brittle–ductile transition occur?

Around 15 km beneath Earth’s surface.

Describe brittle deformation and where it is common.

Rocks fracture or crack when stress exceeds yield strength; it dominates at shallow crustal levels where temperatures and pressures are low.

Describe ductile (plastic) deformation and where it is common.

Rocks flow without fracturing when stress exceeds yield strength; it dominates deeper in the crust and mantle where temperatures and pressures are high.

What is an anticline?

An upward-convex fold in which limbs dip away from the hinge and the oldest rocks occupy the core.

What is a syncline?

A downward-concave fold in which limbs dip toward the hinge and the youngest rocks occupy the core.

Define a monocline.

A simple fold in which otherwise horizontal strata are bent in one direction so both limbs remain horizontal—essentially half of an anticline or syncline.

In fold geometry, what are the limbs, hinge, fold axis, and axial plane?

Limbs are the sides of the fold; the hinge is the tightest curved part; a fold axis connects successive hinge points; the axial plane contains the fold axis and splits the fold as symmetrically as possible.

How does a normal fault differ from a reverse (thrust) fault?

In a normal fault the hanging wall moves down relative to the footwall; in a reverse or thrust fault the hanging wall moves up relative to the footwall.

Which tectonic setting typically produces normal faults?

Tensional environments such as divergent plate boundaries.

What characterizes a strike-slip (transform) fault?

A steep to vertical fault plane along which blocks move horizontally past each other; e.g., the San Andreas Fault.

What are grabens and horsts?

A graben is a down-dropped block between two inward-dipping normal faults; a horst is an uplifted block between two outward-dipping normal faults.

What is a joint, and how does it differ from a fault?

A joint is a fracture with little or no displacement along the plane, usually occurring in sets; a fault is a larger fracture with significant movement of rock blocks.

Define strike and dip.

Strike is the compass direction of a horizontal line on a planar surface; dip is the angle and direction the surface inclines from the horizontal, measured perpendicular to strike.

On international geologic maps, what colour is conventionally used for limestone?

Blue