Deformation, lectures 9 & 10

Question 1

Deformation takes place on a range of spatial scales…

2 examples

Answer 1

1. atomic level in single crystals

2. regional scale within mountain belts

Question 2

Deformation takes place on a range of temporal scales

2 examples

Answer 2

1. seconds-minutes during seismic activity

2. Ma for mountain building (logarithmic scale used)

Question 3

Equation for stress

Answer 3

force/area

same as pressure

Question 4

Compression stress

Answer 4

acts to shorten an object

Question 5

Tension stress

Answer 5

acts to lengthen an object

Question 6

Normal stress

Answer 6

acts perpendicular to a surface (compression or tension)

Question 7

Shear stress

Answer 7

acts parallel to a surface

Question 8

The Earth is…

Answer 8

dynamic

Question 9

The Earth is dynamic which means that

Answer 9

deformation occurred in the past and the present

Question 10

Two types of compression stress

Answer 10

hydrostatic/uniform

directed/differential

Question 11

Hydrostatic/uniform stress

type of compression stress

Answer 11

Uniform in all directions

Question 12

Directed/differential stress

type of compression stress

Answer 12

isn’t uniform in all directions

Question 13

Strain

Answer 13

A measure of the extent of deformation, eg: a measure of the change in shape or volume due to stress.

Question 14

Strain

change in dimension…

Answer 14

change in dimension relative to the original dimension

Question 15

Two types of deformation

Answer 15

plastic

brittle

Question 16

Plastic deformation

Answer 16

results in folding

eg: folded dyke

Question 17

Brittle deformation

Answer 17

results in fracturing

eg: fault in sedimentary rock

Question 18

Faults and folds

Answer 18

Geologic features resulting from deformation, provide a record of the forces which operated in the past

Question 19

Benefits of interpretation of geologic structures

3

Answer 19

1. helps to understand the history of the Earth
2. aids exploration of natural resources
3. helps to select appropriate sites for infrastructure

Question 20

Sources of information on deformation processes

4

Answer 20

1. field observations
2. lab experiments
3. computer simulations
4. theoretical calculations

Question 21

Three factors which affect how minerals and rocks respond to stress

Answer 21

1. temp. and pressure
2. minerals/rock type
3. strain rate (rate of deformation)

Question 22

Temperature and pressure

factors which affect how minerals and rocks respond to stress

Answer 22

more likely to deform under high temps.

Question 23

Mineral/rock type

factors which affect how minerals and rocks respond to stress

Answer 23

• hard igneous/metamorphic rocks tend to behave as brittle material (faults)
• softer sedimentary rocks tends to deform in a ductile manner (folds)

Question 24

Strain rate (rate of deformation)
(factors which affect how minerals and rocks respond to stress)

Answer 24

• rapid deformation may result in brittle failure/faulting

- slow deformation in plastic deformation/folding

Question 25

Dip and strike of strata

Answer 25

used to describe the orientation of deformed strata

Question 26

Strike

Answer 26

The direction of a line formed by the intersection of a horizontal plane with an inclined plane, such as a rock layer.

Question 27

What is strike direction relative to?

Answer 27

North, | compass used

Question 28

Dip

Answer 28

A measure of the maximum angular deviation of an inclined plane from the horizontal

Question 29

What is dip measured as relative to?

Answer 29

perpendicular to the strike direction

Question 30

Classification of folds

Answer 30

based on geomtery and symmetry, and orientation of fold axis

Question 31

Monocline

Answer 31

A step-like fold in strata with a zone of steeper dip in approximately horizontal sequence

Question 32

Mechanism of formation of monocline | 2 steps

Answer 32

- Differential compaction over underlying structure at edge of basin and greater compatibility of the basin fill - Reactivation if underlying fault coupled with uplift/ subsidence

Question 33

Non-plunging fold

Answer 33

fold axis is horizontal

Question 34

Plunging fold

Answer 34

fold axis is inclined

Question 35

Features of a plunging fold

Answer 35

- plunging anticline or plunging syncline - all rocks dip away from the axis - oldest rocks are exposed in the centre

Question 36

Dome and basin

Answer 36

circular or oval equivalents of antiform and synform respectively

Question 37

Dome

Answer 37

beds dip away from a central point

Question 38

Basin

Answer 38

beds dip in towards a central point

Question 39

Classification of joints

Answer 39

based on geometry or process of formation

Question 40

Classification of faults

Answer 40

based on orientation of fault plane ...

Question 41

Joints

Answer 41

Fractures along which little or no movement has occurred or movement is perpendicular to the surface.

Question 42

Faults

Answer 42

Fractures along which blocks on opposite sides of the fracture plane have moved.

Question 43

GPS & LPS | faults

Answer 43

greatest principle stress axis | least principle stress axis

Question 44

Two types of joints

Answer 44

tensional | cooling

Question 45

Cooling joints

Answer 45

commonly occur in basaltic lavas

Question 46

Antiform/anticline

Answer 46

an 'n-shaped/up-arched' fold

Question 47

Synform/syncline

Answer 47

A 'u-shaped/down-arched' fold

Question 48

Symmetrical/upright fold

Answer 48

Axial plane is vertical and each limb dips at the same angle

Question 49

Asymmetrical fold

Answer 49

Axial plane is inclined and limbs dips at different angles

Question 50

Overturned fold

Answer 50

one limb has been rotated so much that it is now upside down, common in mountain ranges

Question 51

Recumbent fold

Answer 51

Axial plane is horizontal, common in...