Some important MCQs with explanations related to Petrology for the Jharkhand Academic Council (JAC) 12th Exam -4

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1. What is a fold in structural geology?

a) A fracture in rock layers
b) A bend in rock layers
c) A fault plane
d) A joint in rock

Answer: b) A bend in rock layers
Explanation:
A fold occurs when rock layers are bent due to compressional forces. Examples include anticlines and synclines.


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2. What is the hinge line of a fold?

a) The line along which the fold is curved
b) The outer edge of the fold
c) The region of least curvature in the fold
d) A fracture along the fold

Answer: a) The line along which the fold is curved
Explanation:
The hinge line is the line that connects points of maximum curvature in a fold.


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3. Which of the following is a type of fault caused by tensional forces?

a) Normal fault
b) Reverse fault
c) Strike-slip fault
d) Thrust fault

Answer: a) Normal fault
Explanation:
Normal faults occur due to tensional forces, which cause the hanging wall to move downward relative to the footwall.


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4. What type of stress is responsible for the formation of reverse faults?

a) Tensional stress
b) Compressional stress
c) Shear stress
d) Elastic stress

Answer: b) Compressional stress
Explanation:
Reverse faults are caused by compressional stress, which pushes rocks together, forcing the hanging wall upward relative to the footwall.

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5. What is the difference between a joint and a fault?

a) Joints are smaller than faults.
b) Faults show movement; joints do not.
c) Joints are formed in igneous rocks, while faults are in sedimentary rocks.
d) Joints are horizontal, and faults are vertical.

Answer: b) Faults show movement; joints do not.
Explanation:
Joints are fractures in rocks with no displacement, while faults involve significant movement along the fracture.


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6. What is an anticline?

a) A fold where the oldest rocks are at the center
b) A fold where the youngest rocks are at the center
c) A vertical fracture in rocks
d) A fault plane

Answer: a) A fold where the oldest rocks are at the center
Explanation:
In an anticline, the rock layers arch upwards, and the oldest rocks are exposed at the core of the fold.


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7. What is the plunge of a fold?

a) The angle of dip of the fold limbs
b) The angle between the hinge line and horizontal plane
c) The angle of inclination of the fold axis
d) The vertical displacement along the fold

Answer: b) The angle between the hinge line and horizontal plane
Explanation:
Plunge refers to the angle at which the hinge line of a fold dips relative to the horizontal plane.


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8. Which type of fault results from horizontal shear forces?

a) Normal fault
b) Reverse fault
c) Strike-slip fault
d) Thrust fault

Answer: c) Strike-slip fault
Explanation:
Strike-slip faults are caused by horizontal shear forces, resulting in lateral movement of rock blocks. Examples include the San Andreas Fault.


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9. What is the dip of a rock layer?

a) The angle between a rock layer and the vertical plane
b) The angle between a rock layer and the horizontal plane
c) The direction of the fold axis
d) The line of intersection between two rock layers

Answer: b) The angle between a rock layer and the horizontal plane
Explanation:
The dip is the angle of inclination of a rock layer relative to a horizontal plane.


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10. What is the term for the compass direction of a horizontal line on a tilted rock surface?

a) Dip
b) Strike
c) Plunge
d) Hinge

Answer: b) Strike
Explanation:
The strike is the compass direction of a horizontal line on a rock surface, perpendicular to the dip direction.

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11. What is a thrust fault?

a) A low-angle reverse fault
b) A high-angle normal fault
c) A fault with no vertical movement
d) A horizontal joint

Answer: a) A low-angle reverse fault
Explanation:
A thrust fault is a type of reverse fault with a dip angle less than 45°, caused by compressional forces.


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12. What are axial planes in folds?

a) Planes that divide folds into two equal halves
b) Planes that separate joints
c) The surface along which faults occur
d) Planes of deposition of sediments

Answer: a) Planes that divide folds into two equal halves
Explanation:
The axial plane is an imaginary plane that divides a fold symmetrically into two limbs.


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13. What is a recumbent fold?

a) A fold with a nearly horizontal axial plane
b) A fold with a steeply inclined axial plane
c) A fold with no curvature
d) A fold formed by extensional forces

Answer: a) A fold with a nearly horizontal axial plane
Explanation:
Recumbent folds have axial planes that are almost horizontal, indicating extreme deformation.


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14. Which fault has vertical movement only?

a) Normal fault
b) Strike-slip fault
c) Oblique-slip fault
d) Transform fault

Answer: a) Normal fault
Explanation:
Normal faults involve vertical displacement of the hanging wall relative to the footwall due to tensional forces.

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15. What is the difference between brittle and ductile deformation?

a) Brittle deformation forms folds, while ductile deformation forms faults.
b) Brittle deformation occurs at high temperatures, while ductile deformation occurs at low temperatures.
c) Brittle deformation forms fractures, while ductile deformation forms folds.
d) Brittle deformation happens in deep crust, while ductile deformation happens in shallow crust.

Answer: c) Brittle deformation forms fractures, while ductile deformation forms folds.
Explanation:
Brittle deformation results in fracturing (e.g., faults), while ductile deformation leads to bending or folding of rocks without fracturing.


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