Geological Structures

Question 1

Define structural geology.

Answer 1

Structural geology is the study of the processes that result in the formation of geologic structures and how these structures affect rocks, from microscopic to global scales.

Question 2

What are geological structures?

Answer 2

Deformations or features in rocks caused by tectonic forces, such as folds, faults, and joints.

Question 3

What are the main causes of geological structures?

Answer 3

Tectonic forces from plate movements that fold, fracture, or shear rocks.

Question 4

How does structural geology relate to natural resources?

Answer 4

Many resources such as oil, gas, and ores accumulate along or near structural features like folds and faults.

Question 5

What is a tectonic plate boundary?

Answer 5

3D surface or zone across which there is significant relative movement between lithospheric plates.

Question 6

Explain the two main mechanisms of plate movement.

Answer 6

Convection: Hot, less dense rock rises, cools, and sinks, transferring heat.
Conduction: Direct transfer of heat through contact without movement of material.

Question 7

What happens along plate boundaries due to these movements?

Answer 7

Deformation such as stretching, folding, and shearing occurs.

Question 8

What is mechanical deformation?

Answer 8

Any change in shape, size, or volume of a rock body due to applied stress.

Question 9

Where does most crustal deformation occur?

Answer 9

Along plate margins.

Question 10

Define stress and strain.

Answer 10

Stress is the force applied per unit area; strain is the change in shape or size due to that stress.

Question 11

List the three types of stress.

Answer 11

(a) Compressional (shortens rocks)
(b) Tensional (stretches rocks)
(c) Shear (slides rocks past each other)

Question 12

What are the three types of strain (deformation)?

Answer 12

1. Elastic: Reversible.
2. Brittle: Breaks rocks permanently.
3. Plastic (ductile): Permanent but without fracture.

Question 13

When does a rock behave ductilely?

Answer 13

Under high temperature, high pressure, and low strain rate.

Question 14

When does a rock behave brittlely?

Answer 14

Under low temperature, low pressure, or high strain rate.

Question 15

List four factors influencing rock deformation.

Answer 15

Temperature, confining pressure, strain rate, and rock composition.

Question 16

How does confining pressure affect rock behavior?

Answer 16

High pressure prevents fractures; low pressure promotes brittle failure.

Question 17

How does the presence of water affect deformation?

Answer 17

Water weakens chemical bonds and promotes ductile flow.

Question 18

Give examples of brittle and ductile minerals.

Answer 18

Brittle: Quartz, olivine, feldspar.
Ductile: Mica, clay, calcite.

Question 19

What is “strike”?

Answer 19

The compass direction of the line formed by the intersection of a tilted rock layer with a horizontal plane.

Question 20

What is “dip”?

Answer 20

The angle between the tilted bedding plane and the horizontal plane.

Question 21

State the difference between dip direction and dip angle.

Answer 21

Dip direction = orientation; Dip angle = steepness.

Question 22

What is the right-hand rule in geology?

Answer 22

A convention that defines strike and dip using the right hand; not universal but commonly used.

Question 23

Define a fold.

Answer 23

A bend in rock layers caused by ductile deformation under stress.

Question 24

What is Pumpelly’s rule?

Answer 24

Small-scale folds mimic larger regional folds.

Question 25

Define limbs:

Answer 25

Flanks of the fold.

Question 26

Define hinge line and fold axis.

Answer 26

Hinge line is the line of maximum curvature in a single layer; fold axis describes the 3D fold orientation.

Question 27

What is the axial surface and axial plane?

Answer 27

Axial surface: connects all hinge lines Axial plane: planar surface of axial surface

Question 28

What is plunge?

Answer 28

The angle of inclination of a fold axis with the horizontal.

Question 29

Based on tightness, folds are:

Answer 29

Open (gentle limbs), Tight (steep limbs), Isoclinal (parallel limbs).

Question 30

Based on axial plane orientation:

Answer 30

Upright (vertical), Overturned (inclined), Recumbent (horizontal).

Question 31

Based on symmetry:

Answer 31

Symmetrical, Asymmetrical, Overfold, Recumbent.

Question 32

Based on bed thickness:

Answer 32

Thickly-bedded, concentric folds (constant bed thickness) Thinly-bedded, similar folds (constant vertical thickness)

Question 33

What are the scales of folds?

Answer 33

Microscopic, Mesoscopic, and Macroscopic.

Question 34

Define a fault.

Answer 34

A fracture in rock where displacement has occurred parallel to the fault surface.

Question 35

Differentiate between a fault and a joint.

Answer 35

Fault involves displacement; joint does not.

Question 36

Define fault plane, fault scarp, and fault line scarp.

Answer 36

Fault plane: Surface along which movement occurs. Fault scarp: Steep slope due to fault displacement. Fault line scarp: Eroded version of a fault scarp.

Question 37

What is the difference between hanging wall and footwall?

Answer 37

Hanging wall lies above the fault plane; footwall below it.

Question 38

How are faults classified by dip angle?

Answer 38

Low (<45°), Moderate (30–60°), Steep (>60°), Vertical (90°).

Question 39

What are the main types of fault movements?

Answer 39

Dip-slip, Strike-slip, and Oblique-slip.

Question 40

What is a normal fault?

Answer 40

Hanging wall moves down relative to footwall due to tensional stress; common at divergent boundaries.

Question 41

What is a reverse fault?

Answer 41

Hanging wall moves up relative to footwall due to compressional stress; common at convergent margins.

Question 42

What is a thrust fault?

Answer 42

A low-angle reverse fault (<45° dip).

Question 43

Define allochthon and autochthon.

Answer 43

Allochthon: Rock mass displaced from its origin. Autochthon: Rock mass in original position.

Question 44

Define klippe and window.

Answer 44

Klippe: Isolated allochthon block. Window: Hole exposing underlying autochthon.

Question 45

Define graben and horst.

Answer 45

Graben: Down-dropped block between normal faults. Horst: Uplifted block between faults.

Question 46

What is a strike-slip fault?

Answer 46

Horizontal displacement parallel to the strike.

Question 47

Differentiate between sinistral and dextral faults.

Answer 47

Sinistral (left-lateral); Dextral (right-lateral) movement when viewed across the fault.

Question 48

What is an oblique-slip fault?

Answer 48

Movement with both strike-slip and dip-slip components.

Question 49

Define rake angle and net slip.

Answer 49

Rake: Angle between slip line and strike line; Net slip: total movement along fault plane.

Question 50

Parallel faults:

Answer 50

Multiple faults running in same direction.

Question 51

Step faults:

Answer 51

Parallel faults with uniform downthrows forming steps.

Question 52

Radial faults:

Answer 52

Faults radiating outward from a point (volcanic regions).

Question 53

Peripheral faults:

Answer 53

Circular or arcuate faults enclosing a region.

Question 54

En echelon faults:

Answer 54

Overlapping short faults arranged in a staggered pattern.

Question 55

Compression causes normal faults.

Answer 55

False

Question 56

Tension causes normal faults.

Answer 56

True

Question 57

Strike-slip faults involve vertical movement.

Answer 57

False

Question 58

Thrust faults are common in mountain belts.

Answer 58

True

Question 59

Folds are formed by brittle deformation.

Answer 59

False

Question 60

Joints show measurable displacement.

Answer 60

False

Question 61

Shear stress produces slippage and translation.

Answer 61

True

Question 62

Mechanical deformation =

Answer 62

Change in form/size of rocks.

Question 63

Stress results in _____.

Answer 63

Strain

Question 64

High strain rate →

Answer 64

Brittle failure.

Question 65

Plastic deformation is _____.

Answer 65

irreversible

Question 66

Hinge line connects points of _____.

Answer 66

maximum curvature

Question 67

Anticline folds arch _____, synclines _____.

Answer 67

upward, downward

Question 68

Fault scarp forms by _____.

Answer 68

vertical displacement

Question 69

Graben forms by _____.

Answer 69

tensional forces

Question 70

Strike-slip faults are _____ in motion.

Answer 70

horizontal

Question 71

Define hinge:

Answer 71

Line of maximum curvature.

Question 72

Define crest:

Answer 72

Highest point.

Question 73

Define trough:

Answer 73

Lowest point.