Structural geology

Question 1

What are the two components of dip?

Answer 1

Direction of tilt using:
Compass direction / degrees of dip
And angle

Question 2

What is strike?

Answer 2

Is the structural trend on the surface. It is the “grain” of the rocks. Strike is two opposite compass directions both at perpendicular to the dip direction (NE- SW) ETC

Question 3

What is the preferred method for recording dip and strike data?

Answer 3

3 - digit compass bearing followed by 2 - digit dip angle, use the left hand rule

Question 4

What is the difference between stress and strain?

Answer 4

Stress = Force applied
Strain = Resultant deformation of the stress

Question 5

What is the difference between competent and incompetent?

Answer 5

Competent rocks are hard and brittle and tend to joint and fault
Incompetent rocks are weak and plastic and tend to fold or develop cleavage

Question 6

What are 4 deformation types?

Answer 6

Plastic - changes shape when force is removed
Elastic - change of shape reverts back to what it originally was when force is removed
Ductile - Plastic
Brittle

Question 7

What are 4 stresses?

Answer 7

Compressional - Inwards
Tensional - Outwards
Shear- Side by side
Confining- all directions

Question 8

How do you calculate strain?

Answer 8

Strain = change in length/ original length

Question 9

What is an antiform and a synform?

Answer 9

1. Folded rock that bends upwards
2. Synform is folded rock that bends downwards

Question 10

What is a syncline and anticline?

Answer 10

Syncline = Younger rocks are in the core of the fold
Anticline= Older rocks are in the core of the fold

Question 11

What are the four interlimb angles?

Answer 11

Gentle = 188-120 Degrees
Open = 120-70 degrees
Closed = 70-30 degrees
Tight = < 30 degrees

Question 12

What is a fold limb?

Answer 12

The section between two “hinges” forming the sides of a fold.
Hinge = the line where there is a change in dip

Question 13

What is the axial plain?

Answer 13

A plain that joins the hinges of all beds and bisects the fold

Question 14

What are four fold orientations?

Answer 14

Upright horizontal
Horizontal overturned
Plunging
Recumbent

Question 15

What are recumbent, nappes and isoclinal folds?

Answer 15

Recumbent = Folds that have its axial plain and fold limbs horizontal
Nappes = Recumbent folds that are broken along a fault
Isoclinal = Folds that have parallel limbs and are nearly vertical

Question 16

What are domes and basins?

Answer 16

Dome - An anticline which dips from the centre in all directions
Basin - A syncline which dips towards the centre in the same direction but by different amounts

Question 17

What is cleavage?

Answer 17

Describes how a mineral breaks along its weak points where the atoms are aligned.

Question 18

What is slaty cleavage?

Answer 18

Where the slate minerals line up vertically , and are easily split parallel to the clay minerals.

Question 19

What are tension joints?

Answer 19

Where the crest of a fold opens up.

Question 20

What are two other types of jointing?

Answer 20

-Pressure release jointing
-Columnar jointing

Question 21

What is pressure release jointing?

Answer 21

Found where the rock type is uniform and recent hills/valleys. Rocks removed by erosion unlock the underlying rocks, causing them to expand which creates joints parallel to the ground surface.

Question 22

What is columnar jointing?

Answer 22

Form when lava cools and contracts steadily and slowly. Tensional forces are created during this and hexagons can tesselate across the surface which creates the joints.

Question 23

Define:
Throw
Hanging wall
Foot wall

Answer 23

Throw = Vertical movement
Hanging wall = Wall that overhangs the fault
Foot wall = wall that is under the fault plane

Question 24

What fault is caused by tensional forces?

Answer 24

Normal faults - creating a gap in the beds. Also known as a “dip slip fault”

Question 25

What fault is created from compressional forces?

Answer 25

Reverse fault - Creates a repetition of beds and an overhanging wall

Question 26

What fault is created from shear forces?

Answer 26

Tear faults = No throw, hanging wall or footwall. Also known as “strike fault”

Left movement = Sinistral
Right movement = Dextral

Question 27

What is a thrust fault?

Answer 27

A low angle reverse fault

Question 28

What is fault breccia?

Answer 28

Broken rock from either side of a fault creating a rubble zone

Question 29

What is slickensides?

Answer 29

Small scratches where the rocks move past eachother.

Question 30

What is an unconformity?

Answer 30

Represents a break in time where no sediment was depsoited

Question 31

What is the difference between Graben and Horst

Answer 31

Grabren = Area that forms the downthrow between two faults
Horst = Area that forms the upthrow between two faults
G H
/-\______/——\

Question 32

What are transform faults?

Answer 32

Faults created at conservative plate margins

Question 33

What is fault gouge?

Answer 33

Fine particles of rock caused by the grinding of rock in faults. If the gouge’s heated up and pressurised it becomes hard and forms mylonite

Question 34

How do angular unconformities form?

Answer 34

1. Beds of young rock at top, old at bottom
2. Tilt/folding of rock
3. Erosion of tilted rock
4. Deposition of new sediment
5. Uplift and erosion expose the unconformity