Eduqas A-level Geology · G2

Rock deformation

50 exam-style questions with answers for G2 Rock deformation in Eduqas A-level Geology (spec A480QS). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.

All 50 questions and answers

Question 1 · Spec G2, Key Idea 1 · Brittle and ductile
State three conditions that control whether a rock deforms in a brittle or ductile way. [3 marks]
Answer:
Temperature (1); confining pressure (1); strain rate (1).
Question 2 · Spec G2, Key Idea 1 · Brittle and ductile
Explain why high temperature favours ductile deformation. [2 marks]
Answer:
Minerals can deform and recrystallise more easily at high temperature (1); allowing permanent change of shape without fracturing (1).
Question 3 · Spec G2, Key Idea 1 · Brittle and ductile
Explain how high confining pressure reduces brittle failure. [2 marks]
Answer:
It holds cracks closed and stops them growing (1); so the rock is more likely to deform in a ductile way (1).
Question 4 · Spec G2, Key Idea 1 · Strain rate
Explain why slowly applied stress can produce ductile deformation. [2 marks]
Answer:
Slow deformation allows time for creep and recrystallisation (1); rapid stress does not, so the rock fractures (1).
Question 5 · Spec G2, Key Idea 1 · Competence
Define a competent layer. [1 mark]
Answer:
A strong layer that resists deformation and tends to fold or fracture rather than flow, such as sandstone or limestone.
Question 6 · Spec G2, Key Idea 1 · Competence
Explain why a competent sandstone bed may fold while mudstone around it flows. [2 marks]
Answer:
The beds have different mechanical properties (1); so the weaker, incompetent mudstone deforms more easily around the stronger bed (1).
Question 7 · Spec G2, Key Idea 1 · Stress
A force of 2.0 × 10⁶ N acts on an area of 0.20 m². Calculate the stress in MPa. [2 marks]
Answer:
Stress = force ÷ area = 2.0 × 10⁶ ÷ 0.20 = 1.0 × 10⁷ Pa (1); = 10 MPa (1).
Question 8 · Spec G2, Key Idea 1 · Strain
A bed shortens from 100 m to 80 m. Calculate the percentage shortening. [2 marks]
Answer:
Shortening = 20 m (1); 20 ÷ 100 × 100 = 20% (1).
Question 9 · Spec G2, Key Idea 1 · Deformation experiments
Explain how removing the load distinguishes elastic from plastic deformation. [2 marks]
Answer:
If the sample returns to its original shape, the deformation was elastic (1); if it stays deformed, the deformation was plastic (permanent) (1).
Question 10 · Spec G2, Key Idea 1 · Fold geometry
Define the amplitude of a fold. [1 mark]
Answer:
The height of the fold from the median surface to the hinge (half the vertical distance from crest to trough).
Question 11 · Spec G2, Key Idea 1 · Fold geometry
Define the wavelength of a fold. [1 mark]
Answer:
The distance between two adjacent equivalent hinges (e.g. from one anticline hinge to the next).
Question 12 · Spec G2, Key Idea 1 · Fold tightness
A fold has an interlimb angle of 25°. Another has 120°. State which is tighter and explain why. [2 marks]
Answer:
The 25° fold (1); because a smaller interlimb angle means the limbs are closer together (1).
Question 13 · Spec G2, Key Idea 1 · Fold types
Describe an isoclinal fold. [1 mark]
Answer:
A fold with limbs that are parallel (interlimb angle close to 0°).
Question 14 · Spec G2, Key Idea 1 · Fold geometry
Define the axial plane of a fold. [1 mark]
Answer:
The surface that joins the hinge lines of all the folded layers.
Question 15 · Spec G2, Key Idea 1 · Fold types
Distinguish between upright and inclined folds. [2 marks]
Answer:
An upright fold has a vertical axial plane (1); an inclined fold has a tilted axial plane, with limbs dipping at different angles (1).
Question 16 · Spec G2, Key Idea 1 · Fold types
Describe an overturned fold. [2 marks]
Answer:
The axial plane is strongly inclined (1); and one limb has been rotated past vertical, so both limbs dip in the same direction and one is upside down (1).
Question 17 · Spec G2, Key Idea 1 · Fold types
Describe a recumbent fold. [1 mark]
Answer:
A fold with a horizontal or nearly horizontal axial plane.
Question 18 · Spec G2, Key Idea 1 · Plunging folds
Define the plunge of a fold. [1 mark]
Answer:
The angle between the fold axis (hinge line) and the horizontal.
Question 19 · Spec G2, Key Idea 1 · Fold terminology
Distinguish between an antiform and an anticline. [2 marks]
Answer:
An antiform is any fold that closes upwards (1); an anticline has the oldest rocks in its core (1).
Question 20 · Spec G2, Key Idea 1 · Fold terminology
A fold closes downwards but has the oldest rocks in its core. Name it. [1 mark]
Answer:
A synformal anticline.
Question 21 · Spec G2, Key Idea 1 · Plunging folds
Describe the outcrop pattern of a plunging fold. [2 marks]
Answer:
Beds form a V or U shape on the map where they close around the hinge (1); because the inclined axis makes the folded beds intersect the ground surface in a curve (1).
Question 22 · Spec G2, Key Idea 1 · Cleavage
Explain how cleavage helps interpret a fold. [2 marks]
Answer:
Slaty cleavage forms parallel to the axial plane of folds (axial planar cleavage) (1); so its orientation shows the attitude of the axial plane (1).
Question 23 · Spec G2, Key Idea 1 · Cleavage
Explain why cleavage must be distinguished from bedding before measuring dip. [2 marks]
Answer:
Cleavage is a tectonic fabric that can cut across bedding at an angle (1); measuring cleavage by mistake would give the wrong bedding orientation and structure (1).
Question 24 · Spec G2, Key Idea 1 · Stereonets
State what a pole represents on a stereonet. [1 mark]
Answer:
The line at right angles (normal) to a plane.
Question 25 · Spec G2, Key Idea 1 · Stereonets
A plane dips at 60°. Calculate the plunge of its pole. [1 mark]
Answer:
90 − 60 = 30°.
Question 26 · Spec G2, Key Idea 1 · Stereonets
A bed dips 40° due east. State the trend and plunge of its pole. [2 marks]
Answer:
Trends west (270°) (1); plunges 50° (1).
Question 27 · Spec G2, Key Idea 1 · Stereonets
State what a tight cluster of bedding poles on a stereonet shows. [1 mark]
Answer:
The beds have very similar orientations (e.g. one fold limb or uniformly dipping beds).
Question 28 · Spec G2, Key Idea 1 · Fieldwork
Explain why bedding should be measured at several sites. [2 marks]
Answer:
One site may show only local variation or one fold limb (1); more sites show whether the measurement is representative of the structure (1).
Question 29 · Spec G2, Key Idea 1 · Stress and faults
State the orientation of the principal stresses that produce normal faults. [2 marks]
Answer:
Maximum principal stress (σ₁) is vertical (1); minimum principal stress (σ₃) is horizontal, causing extension (1).
Question 30 · Spec G2, Key Idea 1 · Stress and faults
State the orientation of the principal stresses that produce reverse faults. [2 marks]
Answer:
Maximum principal stress (σ₁) is horizontal (1); minimum principal stress (σ₃) is vertical, causing shortening (1).
Question 31 · Spec G2, Key Idea 1 · Stress and faults
State the orientation of the principal stresses for strike-slip faults. [2 marks]
Answer:
The intermediate stress (σ₂) is vertical (1); σ₁ and σ₃ are both horizontal (1).
Question 32 · Spec G2, Key Idea 1 · Faults
Distinguish between a thrust and a reverse fault. [2 marks]
Answer:
Both have the hanging wall moved up relative to the footwall due to compression (1); a thrust has a low dip angle, less than 45° (1).
Question 33 · Spec G2, Key Idea 1 · Fault rocks
State what slickensides show. [1 mark]
Answer:
The direction of movement on the fault plane.
Question 34 · Spec G2, Key Idea 1 · Fault rocks
Distinguish between fault gouge and fault breccia. [2 marks]
Answer:
Fault gouge is fine-grained, clay-rich crushed rock (1); fault breccia is made of angular rock fragments (1).
Question 35 · Spec G2, Key Idea 1 · Fault displacement
Distinguish between throw and net slip. [2 marks]
Answer:
Throw is the vertical displacement on a fault (1); net slip is the total displacement measured along the fault plane (1).
Question 36 · Spec G2, Key Idea 1 · Fault displacement
A dip-slip fault has 20 m of net slip on a plane dipping 30°. Calculate the throw. [2 marks]
Answer:
Throw = slip × sin(dip) = 20 × sin 30° (1); = 10 m (1).
Question 37 · Spec G2, Key Idea 1 · Strike-slip faults
Looking across a fault, the far block has moved to your right. Name the type of movement. [1 mark]
Answer:
Dextral (right-lateral).
Question 38 · Spec G2, Key Idea 1 · Fault reactivation
State the evidence that a fault has moved more than once. [2 marks]
Answer:
It displaces rocks or structures formed after its first movement (1); or has slickensides showing different directions of movement (1).
Question 39 · Spec G2, Key Idea 1 · Inversion
Explain how a normal fault can become a reverse fault. [2 marks]
Answer:
The fault formed during extension (1); later compression reactivates it with reverse movement (basin inversion) (1).
Question 40 · Spec G2, Key Idea 1 · Refolding
State the evidence for a fold being refolded. [1 mark]
Answer:
The axial plane or axial planar cleavage of the first fold is itself folded, giving an interference pattern.
Question 41 · Spec G2, Key Idea 1 · Dip
Explain why true dip is the maximum dip of a bed. [2 marks]
Answer:
It is measured perpendicular to strike, down the steepest slope of the bed (1); any other direction gives a smaller apparent dip (1).
Question 42 · Spec G2, Key Idea 1 · Dip
A bed strikes north-south and dips east. State the direction of the true dip section. [1 mark]
Answer:
East-west (perpendicular to strike).
Question 43 · Spec G2, Key Idea 1 · True thickness
A bed has a vertical thickness of 100 m and dips at 60°. Calculate its true thickness. [2 marks]
Answer:
True thickness = vertical thickness × cos(dip) = 100 × cos 60° (1); = 50 m (1).
Question 44 · Spec G2, Key Idea 1 · True thickness
A bed dips 30° under flat ground with an outcrop width of 120 m. Calculate its true thickness. [2 marks]
Answer:
True thickness = outcrop width × sin(dip) = 120 × sin 30° (1); = 60 m (1).
Question 45 · Spec G2, Key Idea 1 · Dip
A 40 m thick bed has an outcrop width of 80 m on flat ground. Calculate its dip. [2 marks]
Answer:
sin(dip) = 40 ÷ 80 = 0.5 (1); dip = 30° (1).
Question 46 · Spec G2, Key Idea 1 · Outcrop width
State two reasons why outcrop width can change without the true thickness changing. [2 marks]
Answer:
Any two from: changes in dip (1); slope of the ground (1); folding or faulting repeating the bed (1).
Question 47 · Spec G2, Key Idea 1 · Fieldwork
State three pieces of evidence for a fault rather than a fold. [3 marks]
Answer:
Any three from: displaced beds (1); a discrete fault plane (1); fault breccia or gouge (1); slickensides (1).
Question 48 · Spec G2, Key Idea 1 · Fold interpretation
Explain why fold shape cannot tell us the size of the original stress. [2 marks]
Answer:
Fold shape depends on competence, temperature, pressure and strain rate (1); so similar folds can form under different stress conditions (1).
Question 49 · Spec G2, Key Idea 1 · Sequence of events
An older cleavage is folded and then cut by an undeformed dyke. State the order of events. [3 marks]
Answer:
Formation of the cleavage (1); folding of the cleavage (1); intrusion of the dyke (1).
Question 50 · Spec G2, Key Idea 1 · Fieldwork
State three ways an investigation based on one poorly exposed fault could be improved. [3 marks]
Answer:
Any three from: map the fault along its length (1); take more orientation measurements (1); look for displaced marker beds (1); look for fault rocks or slickensides at other exposures (1).

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