AS Structural Geology

Question 1

Bed

Answer 1

a unit of sedimentation, which can vary considerably in thickness.

Question 2

Bedding planes

Answer 2

mark a break between beds. They represents a break in sedimentation, a change in the composition or grain size, or a change in colour of the sediments.

Question 3

Dip

Answer 3

the maximum inclination of a bed measured from the horizontal using a clinometer.

Question 4

True dip

Answer 4

the actual angle of dip measured at right angles to the strike.

Question 5

Apparent dip

Answer 5

a dip that is measured to be less than the maximum inclination.

Question 6

Strike

Answer 6

the horizontal line on a bedding plane, measured as a bearing from north using a compass. It is at right angles to the dip.

Question 7

Fault

Answer 7

a fracture in a rock along which there has been an observable amount of displacement.

Question 8

Fault plane

Answer 8

a plane of fracture, along which the rocks have been displaced.

Question 9

Upthrow

Answer 9

the side of the fault where the movement is upwards, in relation to the other side.

Question 10

Downthrow

Answer 10

the side of the fault where the movement is downward, in relation to the other side.

Question 11

Throw

Answer 11

the vertical displacement of rocks along the fault plane.

Question 12

Footwall

Answer 12

the side of the fault that lies below the fault plane, if the fault is not vertical.

Question 13

Hanging wall

Answer 13

the side of the fault that lies above the maximum inclination of the fault plane as measured from the horizontal.

Question 14

Slickensides

Answer 14

the striations and polishing found on a fault plane indicating the direction of relative movement.

Question 15

Fault breccia

Answer 15

composed of fragments produced by rocks fracturing during faulting. It is found along the fault planes.

Question 16

Fault gouge

Answer 16

composed of very finely ground particles, produced by grinding of rock during faulting. These are often fused together due to frictional heat and found along fault planes.

Question 17

Mylonite

Answer 17

a rock produced by dynamic recrystallisation of minerals on a fault plane.

Question 18

Anticline

Answer 18

an upright fold with the oldest rocks in the core.

Question 19

Antiform

Answer 19

an upward closing fold.

Question 20

Syncline

Answer 20

a downward pointing fold with the youngest rocks in the core.

Question 21

Synform

Answer 21

a downward closing fold.

Question 22

Fold limb

Answer 22

the section of a fold between one hinge and the next, forming the side of a fold.

Question 23

Hinge

Answer 23

the line along which there is a change in the amount and/or direction of dip, forming the most sharply curved part of the fold.

Question 24

Crest

Answer 24

the highest point of a folded bed.

Question 25

Trough

Answer 25

the lowest point of a folded bed.

Question 26

Axial plane

Answer 26

a plane that joins the hinges of all the beds. It bisects the fold.

Question 27

Axial plane trace

Answer 27

the outcrop of the axial plane at the Earth’s surface.

Question 28

Plunge

Answer 28

the angle of dip of the axial plane from the horizontal.

Question 29

Dome

Answer 29

an anticline which dips away from the centre in all directions.

Question 30

Basin

Answer 30

a syncline which dips towards the centre from all directions.

Question 31

Overfolds

Answer 31

folds which have both fold limbs dipping in the same direction but by different amounts.

Question 32

Recumbent folds

Answer 32

have axial planes and fold limbs which are close to horizontal and always less than 30 degrees.

Question 33

Nappes

Answer 33

recumbent folds that are broken along thrust planes.

Question 34

Isoclinal

Answer 34

folds have parallel limbs that are nearly vertical and in very tight folds.

Question 35

Stress

Answer 35

the force applied to rocks.

Question 36

Strain

Answer 36

a change in the shape or volume of a body as a result of applied stress.

In cases where vertical stress is applied to perfectly elastic materials:

Strain = the change of length of line ÷ the original length of line

Question 37

Competent

Answer 37

rocks are strong and brittle and tend to joint and fault.

Question 38

Incompetent

Answer 38

rocks are weak and plastic and tend to fold and develop cleavage.

Question 39

Tension

Answer 39

the force trying to pull rocks apart, the Earth’s crust will be lengthened.

Question 40

Compression

Answer 40

the force trying to push rocks together, the Earth’s crust will be shortened.

Question 41

Shear forces

Answer 41

are those which act along a plane in the rock and promote sliding along that plane.

Question 42

Joint

Answer 42

a fracture in competent rocks along which no observable movement has occurred.

Question 43

Tectonic joints

Answer 43

form as a result of folding and cause tension joints parallel to the axial plane and cross joints on the limbs.

Question 44

Cooling joints

Answer 44

form as a result of contraction on cooling of igneous rocks.

Question 45

Unloading joints

Answer 45

are often horizontal as well as vertical and form as a result of lower pressure near to the surface.

Question 46

Cleavage

Answer 46

planes of weakness in incompetent sedimentary rocks (e.g. shale, made from clay minerals) and low-grade metamorphic rocks (e.g. slate) along which these rocks will split. It forms perpendicular to o max in folds, so is at an angle to bedding planes.