Structural Geology

17 marks·pp. 44-55·~55 min

Complete lesson with depth, context, and extra examples

Introduction and Stress-Strain

~6m

DEF

Structural Geology

The branch of geology that deals with the study of the architecture of the Earth's crust — rock deformation, origin, and mechanism of geological structures such as folds, faults, joints, and unconformities.

notes·notes p.1

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Stress

Stress is the pressure applied to rocks, expressed as force per unit area (F/A).

textbook·p.44

LIST

Types of Stress

Lithostatic stress: Equal pressure exerted on rocks from all directions due to the weight of overlying rocks. Similar to hydrostatic stress.
Directional/Differential stress: Unequal stress due to tectonic forces. Three types: (i) Tensional stress (stretching/extension), (ii) Compressional stress (squeezing), (iii) Shearing stress.

textbook·p.44

DEF

Strain

Strain is the deformation of rocks in response to stress.

textbook·p.44

LIST

Types of Deformation

Elastic deformation: For small directional stresses (less than yield strength), rock deforms like a spring (Hooke's Law). Deformation is not permanent — rock returns to original shape when stress is released.
Plastic deformation: When differential stress exceeds yield strength, rock flows permanently. Occurs in the lower continental crust and mantle.
Brittle deformation: Near the Earth's surface, if differential stress exceeds yield strength, rock fractures or fails with development of a weak zone/plane.
Ductile deformation: At depths greater than 10 km, enormous lithostatic stress and high temperature make rock softer, less brittle and more malleable, preventing fracture.

textbook·p.44-45

DEF

Outcrop

The exposure of solid rock on the Earth's surface.

notes·notes p.1

Strike and Dip of Beds

~6m

EXPLAIN

Attitude of Beds

The simplest form of deformation is tilting of initially horizontal sedimentary beds. The position of the bed on the ground surface is expressed as its attitude, quantified by Strike and Dip.

textbook·p.45

DEF

Strike

The direction of a line formed by the intersection of a horizontal plane with an inclined bedding plane. It is the direction of the line of intersection of a geological feature with a horizontal plane.

notes·notes p.2

DEF

Dip

The angle of inclination of a rock bed relative to the horizontal plane. Measured in a plane perpendicular to the strike direction. Expressed in degrees (0°–90°).

notes·notes p.2

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True Dip

The maximum angle of inclination of a bed, measured perpendicular to the strike direction. True dip is always greater than or equal to apparent dip.

notes·notes p.2

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Apparent Dip

The angle of inclination of a bed measured in any direction other than perpendicular to the strike. Apparent dip is always less than true dip. It is zero parallel to strike and increases to equal true dip when measured perpendicular to strike.

notes·notes p.2

NOTE

Orientation Table

Horizontal bed: Strike absent, Dip absent, Dip angle = 0°. Vertical bed: Strike present, Dip absent, Dip angle = 90°. Inclined bed: Strike present, Dip present, Dip angle >0° and <90°.

textbook·p.46

EXPLAIN

Brunton Compass

A Brunton Pocket Transit is a precision compass used to measure strike and dip of geological features. It uses the Earth's magnetic field and can be adjusted for declination angle. Strike is measured by leveling the compass along the plane; dip is measured by laying the compass perpendicular to strike and using the clinometer.

textbook·p.45-46

Folds

~14m

DEF

Fold

A wave-like bend or curve formed in rock strata due to compressive forces. Folds may occur singly, isolated, or as extensive fold trains of different sizes.

notes·notes p.3

LIST

Parts of a Fold

Axial plane: An imaginary plane that divides the fold into two halves as symmetrically as possible.
Axis/Hinge: An imaginary line along which the bedding plane has suffered maximum bending or curvature. It may or may not coincide with the axial plane.
Crest: The convex or up-arched (highest) part of the fold.
Trough: The concave or down-arched (lowest) part of the fold.
Limbs/Flanks: The sides of a fold. In a series of folds, the part between the crest of one fold and the trough of the adjacent fold.

notes·notes p.3

DIAGRAMParts / Elements of a Fold

Key parts of folds — anticlines (crests) and synclines (troughs) with their axial planes and limbs.

Must label:axial planeleft limbright limbcresttroughhinge line

DEF

Anticline

A fold that is convex upward. The limbs dip away from each other with reference to its axial plane. The oldest beds occur towards the core or centre of curvature.

textbook·p.47

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Syncline

A fold that is convex downward (opposite of anticline). The limbs dip towards each other with reference to its axial plane. The youngest beds occur towards the core or centre of curvature.

textbook·p.47

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Symmetrical Fold

A fold in which the axial plane is vertical and the two limbs dip at the same angle in opposite directions. Formed when compressive forces from both sides are equal (P₁ = P₂). One half is the mirror image of the other.

notes·notes p.4

DIAGRAMSymmetrical Folds (Anticline and Syncline)

In symmetrical folds, the axial plane is vertical and both limbs dip at equal angles.

Must label:axial planeleft limbright limb

DEF

Asymmetrical Fold

A fold in which the axial plane is inclined and the two limbs dip at different angles on either side of the axial plane. Formed when compressive forces from two sides are unequal (P₁ > P₂).

notes·notes p.4

DIAGRAMAsymmetrical Folds (Anticline and Syncline)

In asymmetrical folds, the axial plane is inclined and limbs dip at different angles.

Must label:axial planeleft limbright limb

DEF

Overturned Fold

A fold in which the axial plane is highly inclined and both limbs dip in the same direction at different angles. Formed when compressive force is very high on one side (P₁ ≫ P₂).

notes·notes p.5

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Recumbent Fold

A fold in which the axial plane is horizontal or nearly horizontal (dip angle approximately 0°). Formed when compressive force is extremely high on one side.

notes·notes p.5

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Isoclinal Fold

A fold in which the limbs dip at the same angle in the same direction — the limbs are parallel to each other. The axial plane is inclined.

notes·notes p.6

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Fan Fold

A fold in which the limbs dip towards each other (synclinal fan fold) or away from each other (anticlinal fan fold), resembling a fan. Crests and troughs are broad and rounded.

notes·notes p.6

DEF

Open and Closed Fold

Open Fold: Angle between limbs is obtuse (>90° but <180°). Bed thickness remains uniform. Closed Fold: Angle between limbs is acute (<90°). Beds are thicker at the hinge and thinner at the limbs.

notes·notes p.7

DEF

Chevron Fold

Folds with sharp, angular hinges and straight limbs.

notes·notes p.8

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Box Fold

A fold with a flat-topped crest and two vertical/steep limbs, forming a box-like shape.

notes·notes p.8

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Monoclinal Fold

A fold with a local steepening in horizontal or gently dipping strata. Only one limb is inclined.

notes·notes p.8

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Anticlinorium and Synclinorium

Anticlinorium: A large anticlinal structure consisting of a series of smaller folds (anticlines and synclines) superimposed on it. Synclinorium: A large synclinal structure consisting of a series of smaller folds superimposed on it.

notes·notes p.9

DEF

Plunging and Non-plunging Fold

Plunging fold: A fold whose axis is inclined to the horizontal plane. The angle of inclination is the plunge angle. Non-plunging fold: A fold whose axis is horizontal (plunge angle = 0°).

notes·notes p.10

DEF

Dome and Basin

Dome (Pericline): A double-folded structure where beds dip away from a central point in all directions, forming an inverted bowl shape. Basin: Beds dip towards a central point from all directions, forming a bowl shape.

notes·notes p.11

LIST

Significance of Folds

Mountain building: Folds create valleys, hills, and mountain ranges. The Himalayas are fold mountains.
Groundwater occurrence: Synclines provide favourable conditions for groundwater; artesian springs and wells owe their origin to synclinal structures.
Oil and Gas deposits: Anticlinal folds serve as good structural traps for oil and gas deposits.

textbook·p.48

DIAGRAMAnticlinal Trap (Oil and Gas)

Anticlinal folds serve as structural traps for oil and gas deposits.

Must label:gasoilwatercap rockreservoir rock

EXAMPLE

Mumbai High Field

Mumbai High (Bombay High) is an offshore oilfield in the Arabian Sea, ~160 km west of Mumbai coast. Discovered in 1974. The structure is a north-northwest to south-southeast trending doubly plunging anticline with a faulted east limb, 65 km long and 23 km wide. Operated by ONGC.

textbook·p.48

Faults

~14m

DEF

Fault

A planar fracture in a volume of rock across which there has been significant displacement along the fracture as a result of Earth movements. Energy release associated with rapid movement on active faults is the main cause of earthquakes.

notes·notes p.12

LIST

Parts of a Fault

Fault Plane: The fracture plane along which displacement occurs. Its intersection with the horizontal plane gives the strike of the fault.
Footwall: The rock block that lies below the fault plane.
Hanging wall: The rock block that lies above the fault plane.
Slip/Displacement: The relative movement of blocks along the fault plane. Total displacement is the net slip (may be strike-slip, dip-slip, or both).
Heave: The horizontal component of displacement.
Throw: The vertical component of displacement.

notes·notes p.12

DIAGRAMParts of a Fault

The hanging wall is above the fault plane; the footwall is below.

Must label:fault planefoot wallhanging wallarrow

DEF

Normal Fault

A fault in which the hanging wall moves downward relative to the footwall. Also called gravity fault. Results from tensional forces (stretching/extension). Lengthening of the crust occurs.

notes·notes p.13

CONFLICT

Normal fault dip angle

D1⚠ Unverified

Confirm with Vandana Miss. There are two answers here: the teacher's notes do not specify a particular dip angle range for normal faults (or suggest 0°-45°), while the textbook states normal faults have dips greater than 45° (p.49). Leave this for the students to ask the teacher.

notes·notes p.13

DIAGRAMNormal Fault (Tensional Forces)

In a normal fault, the hanging wall moves down. Caused by tensional forces.

Must label:fault planefoot wallhanging wallarrow

DEF

Reverse Fault

A fault in which the hanging wall moves upward relative to the footwall. Results from compressional forces. Shortening of the crust occurs.

notes·notes p.13

CONFLICT

Reverse Fault vs Thrust Fault

Confirm with Vandana Miss. There are two answers here: the teacher's notes treat reverse fault and thrust fault as synonymous, while the textbook splits them (reverse faults have dips >45°, thrust faults have dips <45°). Leave this for the students to ask the teacher.

notes·notes p.13

DIAGRAMReverse Fault (Compressional Forces)

In a reverse fault, the hanging wall moves up. Caused by compressional forces.

Must label:fault planefoot wallhanging wallarrow

EXPLAIN

Thrust Fault (Textbook detail)

According to the textbook, a thrust fault is a special case of reverse fault characterised by low dip angles (less than 45°).

textbook·p.50

DEF

Strike-Slip Fault

A fault in which the blocks slide past each other along the direction of strike. Displacement is horizontal, parallel to the strike of the fault plane. Also called lateral fault, trans-current fault, wrench fault, or tear fault.

notes·notes p.14

DEF

Dip-Slip Fault

A fault in which the displacement is vertical/inclined, parallel to the dip of the fault plane.

notes·notes p.14

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Oblique-Slip Fault

A fault in which displacement has both horizontal (strike) and vertical (dip) components.

notes·notes p.14

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Step Faults

A series of parallel normal faults occurring at regular intervals, with blocks displaced in the same direction, giving a step-like appearance.

notes·notes p.15

DEF

Horst and Graben

Horst: When normal faults with mutually diverging/converging fault planes occur, the central block is displaced upward relative to adjacent blocks. Forms block mountains. Graben: The central block is displaced downward relative to adjacent blocks. Forms rift valleys.

notes·notes p.17

DIAGRAMHorst and Graben

Horsts form block mountains; grabens form rift valleys.

Must label:horstgrabenfault plane

LIST

Additional Fault Classifications (Notes)

Strike Fault: Fault plane strike is parallel to the strike of inclined beds.
Dip Fault: Fault plane strike is parallel to the dip direction of inclined beds.
Oblique Fault: Fault plane strike is neither parallel to strike nor dip of the bedding plane.
Bedding Fault: Fault plane is parallel to the bedding plane.
Radial Fault: Multiple fault lines diverging from a central point.

notes·notes p.16-19

LIST

Significance of Faults

Faults cause damage to rocks — real hazards in mining and engineering works.
Fault breccia and fault gouge have low strength — poor foundation materials.
Earthquakes and landslides are triggered by faults.
Knowledge of faults helps in proper designing of large civil structures (dams, tunnels).
Many fault zones are suitable sites for mineralization (gold, silver, copper).
Faults may create lakes, swamps, hot water springs (sources of geothermal energy).
Some fault zones form potential oil traps.

textbook·p.50-51

Joints

~8m

DEF

Joint

A fracture or crack in a rock along which there has been little to no displacement. Joints divide rocks into blocks. Unlike faults, joints have no significant displacement along the fracture plane.

notes·notes p.20

NOTE

Joint vs Fault

Key difference: Fault has displacement; Joint has no displacement. Both are fractures in rock.

notes·notes p.20

LIST

Genetic Classification of Joints (Notes)

Tensional Joint: Formed due to pulling apart forces (tension).
Compressional Joint: Formed due to pushing together forces (compression).
Shear Joint: Formed due to sliding/tearing forces (shear).

notes·notes p.20

LIST

Geometric Classification of Joints

Strike Joint: Joint runs parallel to the strike of the inclined bedding plane.
Dip Joint: Joint runs parallel to the dip direction of the inclined bedding plane.
Diagonal/Oblique Joint: Joint runs at an angle oblique to both strike and dip of the bedding plane.
Bedding Joint: Joint is parallel to the bedding plane.

notes·notes p.23-24

DEF

Columnar Joints

Sets of intersecting closely spaced fractures formed as rock cools and contracts. Cracks develop vertically, dividing rock into hexagonal or pentagonal columns. Most commonly observed in basalt. Columns can vary from a few centimetres to few metres in diameter, and can be several hundred metres in height. Number of sides can vary from 3 to 8.

notes·notes p.22

EXAMPLE

Columnar Joint Examples

Naldurga Fort (Osmanabad), Panhala Fort (Kolhapur), Gilbert Hill (Mumbai), St. Mary's Island (Malpe, Udupi, Karnataka). Columnar joints in rhyolite at Utan in Thane district.

textbook·p.52

DEF

Mural (Cuboidal) Joint

A set of three mutually perpendicular joints (two vertical and one horizontal) that divide rock into cuboidal blocks. Commonly seen in granites.

notes·notes p.21

DEF

Sheet Joint

Fractures that are essentially horizontal and concentric, cutting rock into sheet or slab-like forms. Commonly seen in plutonic igneous rocks (e.g., granite domes).

notes·notes p.22

LIST

Significance of Joints

Split rocks into blocks, reducing competence of rock mass.
Increase porosity and permeability, making rocks susceptible to quick decay and weathering.
Increase the groundwater potential in any area.
Minerals are exposed on the surface through joint planes.
Groundwater is exposed as springs through joint planes.
Different shapes of rock weathering (columnar, cuboidal) are due to joint planes.
Joints can be improved by suitable methods such as cement grouting or plugging.

notes·notes p.24

Unconformities

~8m

DEF

Unconformity

A surface of erosion or non-deposition separating younger strata from older rocks, representing a gap in the geologic record. A buried erosional surface where an expected age of rock layers is missing due to erosion. This gap in time is called a hiatus.

notes·notes p.24

EXPLAIN

Conformable vs Unconformable Series

Conformable series: An unbroken sequence of sedimentary beds deposited one above another without any tectonic disturbance, weathering, or erosion. All beds have the same strike, dip direction, and dip amount. Unconformable series: A sequence where older beds have undergone tectonic movement, weathering, or erosion, followed by deposition of a younger series.

notes·notes p.25

DEF

Angular Unconformity

An unconformity where older sedimentary beds are inclined/tilted at an angle, while younger beds above are horizontal or less tilted. The older and younger beds are not mutually parallel.

notes·notes p.25

STEPS

Formation of Angular Unconformity

1Deposition and stratification of older sediments to form the older series.
2Upliftment and tilting/folding of the older beds.
3Weathering and erosion of the tilted beds to form an eroded surface.
4Deposition of a younger series of sediments on top of the eroded surface.

notes·notes p.26

DIAGRAMAngular Unconformity

Older beds are tilted while younger beds above are horizontal. U marks the unconformity.

Must label:older tilted bedsyounger horizontal bedsunconformity surface

EXAMPLE

Hutton's Angular Unconformity

At Siccar Point, 345-million-year-old Devonian Old Red Sandstone overlies 425-million-year-old Silurian greywacke. James Hutton first recognized angular unconformity.

textbook·p.53

DEF

Disconformity (Parallel Unconformity)

An unconformity where the older and younger series are parallel to each other, both in horizontal position (dip angle = 0°). It is an erosional contact that separates parallel bedding planes. Hard to recognize — often discovered through fossil study. May result from erosion or non-deposition.

notes·notes p.26

STEPS

Formation of Disconformity

1Deposition of sediments to form the older series.
2Upliftment of the older series without tilting.
3Erosion of the upper/youngest layers of the older series.
4Deposition of beds of the younger series.

notes·notes p.27

DIAGRAMDisconformity

In a disconformity, both older and younger beds are parallel but separated by an erosion surface.

Must label:older bedsyounger bedsunconformity surface

DEF

Nonconformity

An unconformity in which the underlying older formation is igneous or metamorphic rock and the overlying younger formation is sedimentary rock. Suggests a period of long-term uplift, weathering, and erosion to expose the deeper crystalline rock before burial by younger sediments.

notes·notes p.26

STEPS

Formation of Nonconformity

1Formation of plutonic igneous or metamorphic rock.
2Upliftment of this crystalline basement.
3Erosion of the upper layer of the older rock.
4Deposition of sedimentary beds of the younger series.

notes·notes p.27

DIAGRAMNonconformity

In a nonconformity, sedimentary rocks overlie igneous or metamorphic basement.

Must label:younger sedimentary bedsigneous or metamorphic rockunconformity surface

EXAMPLE

Disconformity Example

Disconformity between sedimentary rocks in California, with conglomerate deposited upon an erosion surface on the underlying rocks.

textbook·p.53

EXAMPLE

Nonconformity Example

Nonconformity at Yellowstone Highway, Wyoming.

textbook·p.54

LIST

Significance of Unconformities

Represents a gap/break in deposition of beds and a record of a gap in time.
Useful for subdividing stratigraphic units, determining timing of tectonic activity, correlating stratigraphic boundaries, interpreting sea-level changes.
Produces oil traps and aquifers in certain situations.
Helps in visualizing and reconstructing palaeogeography of a region.
Favourable sites for mineralization (uranium, aluminium, phosphates, gold).
Forms a weak zone affecting site conditions for engineering works.
Tells the relative age of rock layers.

notes·notes p.27

Applications of Structural Geology

~3m

LIST

Applications of Structural Geology

Engineering geology and geotechnology: Construction of bridges, dams, power plants, highways, airports, building foundations.
Environmental geology: Land use planning, earthquake hazard, volcanic hazard, distribution of groundwater.
Petroleum and mining geology: Geometric techniques, projection of faults, geologic contacts, trends of regional processes controlling mineral and hydrocarbon concentration.

textbook·p.54

Distinguish Between

Anticline vs Syncline

Aspect	Anticline	Syncline
Shape	Convex upward (arch)	Convex downward (trough)
Limbs	Dip away from each other	Dip towards each other
Beds at core	Oldest beds at core	Youngest beds at core
Greek meaning	Opposite inclined	Together inclined

textbook·p.47

Symmetrical Fold vs Asymmetrical Fold

Aspect	Symmetrical Fold	Asymmetrical Fold
Axial plane	Vertical	Inclined
Limb dip	Equal angles on both sides	Unequal angles on both sides
Compressive forces	Equal (P1 = P2)	Unequal (P1 > P2)
Mirror image	One half is mirror image of other	Halves are not mirror images

notes·notes p.4

Normal Fault vs Reverse Fault

Aspect	Normal Fault	Reverse Fault
Hanging wall movement	Moves downward	Moves upward
Stress type	Tensional forces (extension)	Compressional forces (shortening)
Crustal effect	Lengthening/extension of crust	Shortening of crust
Also called	Gravity fault	Compressional fault / Thrust fault (per notes)

notes·notes p.13

True Dip vs Apparent Dip

Aspect	True Dip	Apparent Dip
Direction of measurement	Perpendicular to strike	Any other direction
Value	Maximum angle of inclination	Always less than true dip
Parallel to strike	Not measured in this direction	Zero when parallel to strike

notes·notes p.2

Joint vs Fault

Aspect	Joint	Fault
Displacement	No displacement along fracture	Significant displacement along fracture
Nature	Crack or fracture only	Fracture with block movement
Scale	Usually smaller in scale	Can be very large scale

notes·notes p.20

Disconformity vs Nonconformity

Aspect	Disconformity	Nonconformity
Older formation	Sedimentary rocks (parallel to younger)	Igneous or metamorphic rocks
Younger formation	Sedimentary rocks (parallel to older)	Sedimentary rocks
Recognition	Hard to recognize; found through fossils	Easier to recognize (different rock types)
Bed orientation	Both sets horizontal/parallel	Crystalline basement below sedimentary

notes·notes p.26

Horst vs Graben

Aspect	Horst	Graben
Block movement	Central block displaced upward	Central block displaced downward
Landform	Block mountain	Rift valley
Relative position	Elevated between fault planes	Depressed between fault planes

notes·notes p.17

Glossary(29 terms)

Practice Questions (35)