Structural Geology Exam 2

Fall 2026

Deformation Bands

Develop in porous rocks, usually wider than ordinary fractures. No large offset and are single structures. Can have dilation bands, shear bands, or compaction bands that show how the grains responded to stress.

Granular Flow

Deformation in unlithified/loosely consolidated grains where grains rotate and frictionally slide against each other without breaking

Cataclastic Flow

Ductile movement of consolidated rock involving micro-fracturing and grain crushing. Grains are reduced in size and angular fragments are seen such as breccia.

Joints

Narrow extension fracture, form parallel to maximum principal stress (sigma 1). No displacement parallel to fracture surface

Orthogonal Joint Array

90-degree angles between master set and cross-joint set

Conjugate Joint Array

Not 90-degree angles between master set and cross-joint set

Systematic Joint Array vs Nonsystematic Joint Array

Systematic joint arrays have a subparallel orientation and regular spacing. Nonsystematic joint arrays do not share a common orientation and are irregularly spaced

How are joints formed?

Mostly from strain (outer-arc extension), unloading (exfoliation jointing), cooling (columnar jointing). Also some form from burial (hydrofracturing)

What determines joint spacing?

Bed thickness, lithology, tensile strength, magnitude of strain.

(Thinner layers of strong rock will develop more fractures than thicker layers)

Strain-Induced Jointing - Buckling

Rock is compressed and joints form perpendicular to the maximum stress vector

Strain-Induced Jointing - Doming

Rock is uplifted and fractures parallel to uplift (vertically)

Strain-Induced Jointing - Plateau Uplift

Rock is uplifted from below and vertical fractures form on sides of plateau

Exfoliation Jointing

Surface parallel fracture sets that appear as sheets. Caused by contraction during cooling, poisson effect (rock expanding vertically and contracting horizontally), membrane effect (expansion due to increase in curvature of layer). Sigma 1 horizontally and sigma 3 vertically.

Joint Morphology - Plumose Structure

“Feather-like” appearance created from ribs and hackles, shows how the fracture migrated through a rock. Hackles: show propagation , Ribs: show hesitation

Joint Morphology - Fringe

Fringe: edge of the joints where fracture morphology stops. Hackles can be twisted at the fringe, forming en echelon fractures.

Veins

Fractures filled with minerals. Arrays can be parallel or stockwork (random)

En echelon (sigmoidal) veins

Veins are semi-parallel, not perfectly straight. Form as a result of simple shear and show shear direction (all new veins form relative to the shear axis and all previously formed veins will rotate)

Four types of vein fill

Blocky, stretching, elongate blocky, fibrous (shows what direction the fracture opened in, grows perpendicular to crack)