16-Structural Geology

Question 1

Stress and strain in rocks

Answer 1

Stress= force applied over unit area
Strain = how force manifests itself in the rock

Question 2

Result of compression, extensional, and shear on rocks

Answer 2

Compression = crustal rocks pushed together, leads to horizontal shortening/vertical lengthening
Extensional = crustal rocks pulled apart, leads to horizontal lengthening/vertical shortening
Shear = rocks pushed in opposite directions, leads to rocks breaking into sub-parallel blocks

Question 3

3 types of deformation(elastic, plastic, brittle)

Answer 3

Elastic = temporary changes, recovers when stress is removed, stores energy
Plastic = permanent change, in rocks results in folding
Brittle = loss of cohesion due to stress, in rocks results in faults or fractures

Question 4

What does brittle or ductile deformation depend on

Answer 4

-type of stress
-amount of stress
-temperature
- strength of material (lithology)
-length of time rock is subjected to stress (strain rate)
-confining pressure (depth)

Question 5

What depths do rocks experience brittle vs ductile behaviour

Answer 5

Rocks become more ductile with depth
Transition zone (6-15km) depending on lithology, fluid present
Few rocks exhibit brittle behaviour below 30km

Question 6

Folding

Answer 6

Deformation event resulting from horizontal compression of rock layers by internal forces of the earth along plate boundaries
-plastic-ductile deformation
-no breaking, rocks remain intact
-occurs over millions of years

Question 7

Faulting

Answer 7

Process by which rocks break and move along a fracture
-brittle deformation
-fracturing, rock layers too rigid to bend/fold
-Occurs in seconds
-associated with earthquakes

Question 8

Types of faults and forces that cause them

Answer 8

Normal- tensional forces
Reverse-compression forces
Strike slip- shearing forces
**thrust faults are low angle reverse faults(<30 degrees)

Question 9

Engineering hazard- fault zones

Answer 9

Represent major zones of weakness
Provide pathway for increased fluid flow (eg groundwater)

Question 10

Rock joints

Answer 10

Involve fracture along which there has been no observable movement of one side relative to the other
3 types
1. Tectonic joints: tectonic stresses induce tensile failure
2. Unloading joints: uplift and erosion removes the overlaying rock thereby reducing the compressive load and allowing rock to expand laterally
3. Cooling joints: cooling of hot magma/lava introducing stresses that induce tensile failure of the rock