Rock Deformation

Question 1

Where do rocks get deformed?

Answer 1

Where tectonic forces exert pressure (stress) on rocks -> Mostly at or near plate boundaries

Question 2

what is stress?

Answer 2

force applied per unit area

Question 3

what does stress cause?

Answer 3

strain

Question 4

what does uniform stress (=confining pressure) do to rocks

Answer 4

squeezes rock (no shape change)

Question 5

what does differential stress (=directed pressure; unequal application of stress) do to rocks?

Answer 5

distorts rock

Question 6

what is yield stress?

Answer 6

amount of stress needed to deform the rock (to make it “yield”)

Question 7

what does yield stress depend on?

Answer 7

rock strength

Question 8

what is strain?

Answer 8

change in shape of body by deformation

Question 9

what are the two types of strain?

Answer 9

(1) Brittle deformation - breaks/fractures (faults, joints)
(2) Ductile deformation - bends/flows (folds)
Type depends on composition, P, T, strain rate.

Question 10

when are rocks stronger?

Answer 10

at higher pressure (greater depths)

Question 11

when are rocks weaker?

Answer 11

at higher temperatures

Question 12

deformation at shallower depths…

Answer 12

brittle

Question 13

deformation at greater depth…

Answer 13

more ductile

Question 14

what are the three types of differential stress?

Answer 14

tensional stress -> EXTENSION
compressional stress -> COMPRESSION
shear stress -> Lateral shift

Question 15

what are some example of geological structure formed by deformation?

Answer 15

Brittle deformation
- Joints
- Faults
Ductile deformation
- Folds

Question 16

what are joints?

Answer 16

fractures with no movement

Question 17

what do joints result from?

Answer 17

may result from cooling -> contraction (e.g. basalt columns)
OR
pressure unloading -> sheeting

Question 18

what are faults?

Answer 18

fractures along which displacement has occurred (one side moves relative to the other)

Question 19

what does a sudden, rapid slip on fault plane lead to?

Answer 19

earthquakes

slip of cm to 10s of m in one event

Question 20

what are the types of faults?

Answer 20

Strike-slip faults
-Movement is horizontal (to left or right)
-Parallel to the strike of the fault
Dip-slip faults
-Movement up or down
-Parallel to the dip of the fault surface
-e.g. Normal faults and Reverse (thrust) faults

Question 21

San Andreas is a transform plate boundary, strike-slip fault. is it left or right lateral?

Answer 21

right lateral

Question 22

how is the type of dip-slip fault defined?

Answer 22

the relative motion (slip) of:
hanging wall vs. footwall
(i.e. does the hanging wall move up or down)

Question 23

what happens at a normal fault?

Answer 23

Hanging wall (upper block) moves DOWN the dip of the fault surface

• Caused by extensional stress
• Leads to crustal thinning

Question 24

what happens at a reverse (thrust) fault?

Answer 24

Hanging wall (upper block) moves UP the dip of the fault surface

• Caused by compressional stress
• Leads to crustal thickening, shortening

Question 25

what is a thrust fault?

Answer 25

reverse fault with dip angle ≤ 45°

Question 26

what are folds?

Answer 26

Warping of rock layers at all scales (micro to mega)

• Shortening – contraction
• Form over long time periods as rocks yield to compressional stress – ductile behaviour
• Mountain belts – regional folding of rocks, and many folds can be refolded, overturned etc

Question 27

what is an example of ductile extensional deformation?

Answer 27

boudins