Comprehensive Study of Rock Deformations: Types, Classification, Field Recognition, and Geological Importance – DGMS Examination Guide

Question 1 (a) What is rock deformation? 

  Answer: Rock deformation refers to the process involving the alteration of the original shape, size, or volume of a rock mass. This happens when rocks experience stress exceeding their internal strength, causing displacement of points within the rock relative to each other or an external reference frame. It typically occurs due to tectonic activities, gravity, temperature variations, or confining pressures within the Earth's crust.

(b) Enumerate various types of deformation and their effects. 

  Answer: Rock deformation includes several fundamental types:

1. Elastic Deformation:
• Temporary deformation.
• Rock returns to original shape upon stress removal.
2. Brittle Deformation:
• Permanent deformation characterized by fracturing.
• Common in shallow depths with low temperature and confining pressure.
• Results in faults and joints.
3. Ductile Deformation:
• Permanent deformation characterized by bending and folding without fracturing.
• Occurs under conditions of high temperature, pressure, and slow strain rate.
• Results in fold structures.

Effects:

• Formation of geological structures like folds, faults, joints, and unconformities.
• Alteration of rock mechanical properties (strength, permeability).
• Control of fluid movement (groundwater, hydrocarbons).
• Impact on civil engineering projects (stability of slopes, dams, tunnels, buildings).

(c) Briefly explain the different types of forces responsible for rock deformation. Answer:
Main forces responsible include:

• Compressive Forces:
• Pushes rocks together, causing shortening and thickening.
• Common at convergent plate boundaries.
• Produces folds, reverse faults, and thrust faults.
• Tensional Forces:
• Pulls rocks apart, causing stretching and thinning.
• Common at divergent plate boundaries.
• Produces normal faults and fault-block mountains.
• Shear Forces:
• Moves rock bodies horizontally past each other.
• Common at transform boundaries.
• Produces strike-slip faults.

Question 2
Differentiate between the (a) Anticline vs Syncline Folds

Anticline	Syncline
Convex upward arch-like folds.	Concave upward trough-like folds.
Older rocks at center.	Younger rocks at center.
Limbs dip away from fold axis.	Limbs dip toward fold axis.

Differentiate between the (b) Plunging vs Non-Plunging Folds

Plunging Folds	Non-Plunging Folds
Fold axis inclined at an angle to horizontal.	Fold axis nearly horizontal.
Produces V-shaped outcrop patterns.	Produces parallel, linear outcrop patterns.

Differentiate between the (c) Domes vs Basins

Domes	Basins
Anticlinal structures dipping outward.	Synclinal structures dipping inward.
Oldest rocks at center.	Youngest rocks at center.
Suitable for oil/gas traps.	Suitable for groundwater accumulation.

Differentiate between the (d) Brittle vs Ductile Rock Deformation

Brittle	Ductile
Rock fractures or breaks.	Rock bends or flows without fracturing.
Low temperature & confining pressure.	High temperature & confining pressure.
Sudden release of energy.	Slow deformation over time.

Question 3 (a) Anatomy of Folds with neat sketches: 

 The primary parts of a fold are:

• Limbs: Sides of the fold.
• Hinge: Point of maximum curvature.
• Fold Axis: Imaginary line along hinge points.
• Axial Plane: Divides fold symmetrically into two limbs.
• Crest (anticline) and Trough (syncline): Highest and lowest points, respectively.

(b) Folds and Field Recognition:

• Definition: Curved or wavy geological structures resulting from plastic deformation of rocks under compressive stress.
• Field recognition criteria:
• Repetition of rock layers.
• Opposing dip directions on limbs.
• Curved bedding planes visible at outcrops.

Question 4 (a) Forces causing fault genesis:

• Tensional force: Creates normal faults.
• Compressive force: Creates reverse/thrust faults.
• Shear force: Creates strike-slip faults.

(b) Classification of faults with illustrations:

• Dip-slip Faults:
• Movement along fault dip (vertical).
• Includes normal and reverse faults.
• Strike-slip Faults:
• Horizontal displacement.
• Movement parallel to fault strike.
• Oblique-slip Faults:
• Combination of dip-slip and strike-slip movements.

Question 5
Differentiate between the following:

Criteria	Explanation
Dip-slip vs Strike-slip:	Dip-slip involves vertical displacement; strike-slip involves horizontal displacement.
Normal vs Reverse:	Normal: hanging wall moves down; Reverse: hanging wall moves up.
Thrust vs Overthrust:	Thrust faults dip <45°; Overthrust faults dip <10°.
Primary vs Secondary Joints:	Primary forms during rock formation; secondary forms post-formation due to tectonic stresses.

Question 6 Short Notes:

• Columnar & Sheet joints:
• Columnar: Hexagonal prisms from cooling volcanic rocks.
• Sheet: Parallel surfaces from unloading/exfoliation, commonly in granites.
• Classification of faults:
• Based on slip (dip-slip, strike-slip, oblique-slip).
• Based on movement (normal, reverse, thrust).
• Drag fold & Ptygmatic fold:
• Drag folds: minor folds near larger faults.
• Ptygmatic folds: chaotic, irregular folds indicating plastic conditions.

Question 7 (a) Field Identification Criteria:

• Folds: Opposite dips, repeated beds, curving layers.
• Faults: Fault scarps, offset features, breccia zones.

(b) Fold Classification:

• Genesis: Anticlines, Synclines, Drag, Ptygmatic.
• Geometry: Symmetrical, Asymmetrical, Overturned, Recumbent, Plunging, Non-Plunging, Dome, Basin.

Question 8 (a) Unconformities & Formation:

• Definition: Erosional/non-depositional surfaces indicating breaks in geological records.
• Formation: Occurs via erosion, tectonic uplift, subsidence, or sedimentation breaks.

(b) Conditions for Joints Development:

• Tensile strength exceeded by stress.
• Cooling and contraction.
• Tectonic stresses.
• Unloading and erosion.

Question 9 (a) Types & Properties of Unconformities:

• Disconformity: Parallel beds with erosional gaps.
• Nonconformity: Sedimentary rocks above igneous/metamorphic rocks.
• Angular unconformity: Tilted beds overlain by flat beds.

(b) Field Recognition of Unconformities:

• Abrupt changes in rock type.
• Missing geological record.
• Conglomerates or erosional surfaces.