STRUCTURAL GEOLOGY
This blog will help you to learn about structural geology and few more topics related to geology in simplest and presentable way.
Table of Content: -
- DEFINITION
- IMPORTANCE
- OUTCROPS
- STRIKE
- DIP
- SLOPE
- BRUNT ON COMPASS
- CLINOMETER
- BED
- BEDDING
- FORMATION OF BED
- TYPES OF BED
- IDENTIFICATION OF BEDS
- FOLDS
- FOLDS TERMINOLOGY
- CLASSIFICATION OF FOLDS
- CAUSES OF FOLDS
- JOINTS
- JOINT SET
- JOINT SYSTEM
- CLASSIFICATION OF JOINTS
- FAULTS
- FAULTS TERMINOLOGY
- FAULTS CLASSIFICATION
- CAUSES OF FAULTS
- TYPES OF ROCKS
- TYPES OF IGNEOUS ROCKS
- TEXTURE OF IGNEOUS ROCKS
- CRYSTALLINITY
- LEVEL OF CRYSTALLINITY
- DEVITRIFICATION
- CRYSTAL FORMATION PROCESS
- GLOBULITE
- LONGULITE
- MARGARITE
- TRICHITE
- MICROLITE
- SHAPE OF CRYSTALS
- DEGREE OF CRYSTALLISATION
- GRANULARITY
- TEXTURE
- PARAGENESIS
- XENOLITH
DEFINITION-
It is a critical part of engineering geology which is interested in the physical and mechanical properties of rocks OR it is a scientific discipline that is concerned with rock deformation on both large and small scale.
Read Also -INTRODUCTION TO GEOLOGY
IMPORTANCE:
The study of structural geology tries to connect between current geometries such as folds and faults with its deformation histories.
Read Also - Bowen reaction series
OUTCROPS:
Outcrops allow direct observation and sampling of the bedrock in situ (original location) for geologic analysis and creating geologic maps.
They are very important for understanding fossil assemblages, paleo - environment and evolution as they provided record of relative changes within geologic strata (sedimentary rock layer bounded by 2 stratification planes).
STRIKE:
It is the direction of the line formed by the intersection of a fault, bed or other planar feature and a horizontal plane.
It indicates position of linear structural features such as faults, beds, joints and folds.
DIP:
It is an angle at which a planar feature is inclined to the horizontal plane . It is measured in a vertical plane perpendicular to the strike of the feature.
SLOPE:
A slope is an inclined ground surface which can be either natural / human made.
BRUNT ON COMPASS:
A brunt on compass properly known as the brunt on pocket transit, is a precision com pass made by BRUNTON.
It is specialised instrument used widely by those needing to make accurate navigational and slope angle measurements in the field.
CLINOMETER:
It is an instrument used for measuring angles of slope, elevation or depression of an object with respect to gravity's direction.
BED:
A bed is a layer of sedimentary rock or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".
BEDDING:
Bedding (also called stratification) is one of the most prominent features of sedimentary rocks, which are usually made up of 'piles' of layers (called 'strata') of sediments deposited one on top of another.
FORMATION OF BED:
They are formed from sediments that is deposited on the Earth's solid surface over long periods of time.
TYPES OF BED:
1. CROSS BEDS:
They are not layered horizontally are formed by a combination of local deposition on inclined surface of ripples or dunes.
2. GRADED BEDS:
They show a gradual change in grain sizes from one side of the bed to other.
IDENTIFICATION OF BEDS:
Bedding refers to sedimentary layers that can be distinguished from one another on the basis of characteristics such as texture, composition, colour, or weathering characteristics.
FOLDS:
The undulation or waves in the stratified rocks of Earth's crust is called folds or a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation.
FOLDS TERMINOLOGY:
LIMBS:
A fold consists of two portions dipping in opposite directions, known as limbs of the fold. The two limbs of the fold may have the same or different angle of dip and dip direction.
AXIAL PLANE:
A plane bisecting the angle between the 2 limbs of a fold is called its axial plane. Thus, an axial plane divides a fold into 2 equal and similar halves.
CREST:
The highest part of a fold arched upwards is called crest.
TROUGH:
The lowest part of a down flexure fold is termed as trough.
FOLD AXIS:
The line of intersection of the axial plane with any bedding plane is called fold axis. The fold axis may be horizontal or it may be inclined.
PLUNGE(PITCH):
In the case of inclined fold axis, the angel which it makes with the horizontal is known as plunge of the fold. Such a fold is called plunging fold.
CLASSIFICATION OF FOLDS:
1.On the basis of nature of bending:
(a) ANTICLINE:
An anticline is that fold in which the beds are arched up. The two limbs of such a fold dip in opposite directions or away from each other.
(b) SYNCLINE:
A syncline is that fold in which the beds are down flexure. The two limbs of such a fold dip in opposite directions but towards each other.
2. On the basis of attitude of the fold axis :
(a) NON - PLUNGING FOLDS:
In these folds the fold axis is in horizontal position. The limbs are revealed over the ground surface that extend parallel to each other.
(b) PLUNGING FOLDS:
In these folds the fold axis is in an inclined position. The beds of limbs converge in the direction of inclination in anticline while they diverge in case of syncline.
3.On the basis of position of axial planes and the limbs:
(a) SYMMETRICAL FOLDS:
Those folds in which axial plane is in vertical position so the two limbs show the same angle of dip in opposite directions.
(b) ASYMMETRICAL FOLDS:
Those folds in which axial plane is inclined so that the two limbs show different angles of dip in opposite directions.
(c) OVER - FOLDS:
Those in which the axial plane is so much inclined that the two limbs dip in the same direction commonly at different angles.
(d) RECUMBENT - FOLDS:
Those folds in which axial plane is in horizontal position, so that one limb rest over the other.
(e) ISOCLINAL FOLDS:
Those folds in which two limbs are parallel to each other.
4.On the basis of intensity of deformation:
(a) OPEN FOLDS:
These are the gentle and widespread folds produced due to mild intensity deformation.
(b) CLOSED FOLDS:
When the deformation is of high intensity, the resulting folds are closely packed, showing thinning and thickening of the limbs.
5.On the basis of behaviour with depth:
(a) SIMILAR FOLDS:
Those in which the shape of folds remains the same with depth. However, the thickening and thinning effect is seen.
(b) PARALLEL FOLDS:
Those in which shape of fold varies with depth but the thickness of the folded beds remains more or less constant.
(c) SUPRATENUOUS FOLDS:
These folds develop during an environment when the process of folding takes while sedimentation is going on.
CAUSES OF FOLDS:
TECTONIC CAUSES:
(a) LATERAL COMPRESSION:
The rocks of the earth's crust are often subjected to lateral compression developed due to numerous stresses operating within the body of the earth and is the chief cause of folding. The flatly laid when subjected to lateral compression thrown into folds.
(b) IGNEOUS INTRUSIONS:
The igneous intrusions into the surroundings rock may also result folding. The accumulations of highly viscous magma results arching up of the overlying beds making an anticlinal fold (dome). Similarly, the mass of magma of flow viscosity depresses the underlying beds, giving rise to a synclinal fold.
(c) SALT DOMES:
The upward movement of salt domes may result in lifting up of the overlying sediments ,making domes.
NON - TECTONIC CAUSES:
(a) LANDSLIDING:
Landsliding may result in folding of the rocks. Large parts of the rock beds sliding down the slopes may buckled up or folded because of the compression produced as they come to rest.
(b) CREEPING:
Creeping (slow movement) along hill slopes comprising weak, incompetent, soft rocks results in bending of the beds. Rocks like shale, which are highly weak and plastic in nature are subjected to this kind of folding.
(c) GLACIATION:
The glaciers, which are huge moving masses of ice, exert enormous pressures on the rock formations coming across their way. The dragging effect of ice sometimes causes folding in weak and incompetent rocks.
JOINTS:
Joints are defined as cracks or fractures present in the rocks. These may be small sized extending for a few centimetres in length or extremely extensive. Joints in rocks develop on a more or less regular pattern.
JOINT SET:
A no. of parallel joints constitute a joint set.
JOINT SYSTEM:
No. of joint sets comprise a joint system.
CLASSIFICATION OF JOINTS:
They are of 2 types:
(a) GEOMETRICAL CLASSIFICATION.
(b) GENETIC CLASSIFICATION.
(a) GEOMETRICAL CLASSIFICATION:
This classification is based on the attitude of the joints with respect to that of the beds. The following 3 main types are:
1. DIP JOINT:
Those in which the strike of the joints is along the dip of the beds.
2. STRIKE JOINT:
Those in which the strike of the joints is along the strike direction of the beds.
3. OBLIQUE JOINT:
Those in which the strike of the joints is neither along strike nor along dip direction of beds.
(b) GENETIC CLASSIFICATION:
This classification is made on basis of origin of joints. There are 2 main types:
1. TENSION JOINTS:
These joints are formed due to tensional force or stress. The direction of such joints is always perpendicular to that of the force which tends to pull the rocks apart.
Some of the tension joints are:
COLUMNAR JOINTS:
It is a structure where sets of intersecting closely spaced fractures referred to as joints.
. MURAL JOINTS:
There are three sets of joints mutually perpendicular. Two sets are in vertical position and one in horizontal position. The spacing between the joints is more or less equal so that rock splits into cubical blocks. These joints are typically developed in granites.
TENSION JOINTS IN FOLDED ROCKS:
Tension joints are formed during folding of the rocks. The axial part of the folds along which beds are bent, are subjected to tension. Thus folded develop tension joints along the bends.
2.SHEAR JOINTS:
These joints are formed due to shearing stresses which tend to slide one part of the rock against the other.
SHEAR JOINTS IN FOLDED ROCKS:
Shear joints may well develop along the limbs of the folds. When the beds are subjected to folding, the layers within the limbs tends to slide past one another. The resulting shearing effect may cause rupturing of the rock.
FAULTS:
Fault is a discontinuity that is formed by fracture in the surface rocks of the earth or is a fracture between two blocks of rock.
FAULTS TERMINOLOGY:
FAULT PLANE:
The plane splitting the rocks into 2 blocks, along which displacement occurs is known as fault plane.
FAULT TRACE:
It is the line of intersection of a fault plane with the ground surface.
DIP AND STRIKE OF THE FAULT:
The inclination of a fault plane with the horizontal is called dip of fault.
The direction perpendicular to the dip direction is the strike of the fault.
HADE:
It is the angle which a fault plane makes with the vertical.
WALL:
Each of the 2 blocks of the rocks, separated by a fault plane is termed as wall.
HANGINGWALL:
When rocks slip past each other in faulting the upper block along the fault plane is called hanging wall.
FOOTWALL:
When rocks slip past each other in faulting the block below is called the footwall.
UPTHROW SIDE:
Out of 2 blocks of a fault, the one which moves up with respect to the other is called upthrow side.
DOWNTHROW SIDE:
Out of 2 blocks of a fault, the one which moves down with respect to the other is called downthrow side.
THROW:
The total vertical displacement in a fault is known as throw of the fault.
HEAVE:
The total horizontal displacement in a Fault is known as heave of the fault.
SLIP:
Slip of a fault is defined as relative displacement of 2 points which were initially against each other.
FAULT ZONE:
When a fault plane is not distinctly marked, rather it is replaced by a zone of crushing, it is termed as fault zone.
FAULT BRECCIA:
Sometimes, during intense faulting the rocks get badly crushed. The angular fragments thus formed along the fault zone constitute fault breccia.
GAUGE:
The crushing of rock along the fault plane may at times result in the formation of finely pulverised clay known as gauge .
FAULTS CLASSIFICATION:
1.On the basis of apparent movement of blocks:
(a) NORMAL FAULT:
A normal fault is that in which the hanging wall has apparently moved down with respect to the footwall.
(b) REVERSE FAULT:
A reverse fault is that in which the hanging wall has apparently moved up with respect to the footwall.
2.On the basis of dip of the fault :
(a) HIGH - ANGLE FAULTS :
Those faults in which the fault plane dips steeply at angles more than 45 degrees.
(b) LOW - ANGLE FAULTS:
Those faults in which the fault plane dips gently at angles less than 45 degrees.
3.On the basis of attitude of the faults :
(a) DIP FAULT:
It is the fault in which the strike of the fault is along dip direction of the beds.
(b) STRIKE FAULT:
It is that fault in which the strike of the fault is along the strike of the beds.
(b) OBLIQUE FAULT:
It is that fault in which strike of the fault is neither along dip nor along strike of the beds.
4.On the basis of general pattern of the faults:
(a) RADIAL FAULTS:
It is combination of faults arranged in a radial manner.
(b) ENECHELON FAULTS:
It is combination of faults which overlap one another.
(c) PARALLEL FAULTS:
A sets of fault having the same dip and strike.
(d) STEP FAULTS :
It is a combination of parallel faults in which the beds are thrown into a step like arrangement.
(e) PERIPHERAL FAULTS :
It is a set of arcuate faults which are arranged in a peripheral manner , enclosing more or less circular area.
(f) HORST:
A Horst is a combination of 2 normal faults occurring in such a manner that the side blocks moved down with respect to the central block.
(g) GRABEN:
A graben is a combination of 2 normal faults but occurring in such a way that the central block has moved down with respect to the side blocks, resulting in formation of trough.
CAUSES OF FAULTS:
It occurs when rocks break due to the forces acting on them.
TYPES OF ROCKS:
1. IGNEOUS ROCKS:
It forms when molten rock (magma or lava) cools and solidifies.
2. SEDIMENTARY ROCKS:
They are formed from deposited and lithified mineral material.
3. METAMORPHIC ROCKS:
They are formed when chemical and physical changes occur to igneous, sedimentary or other metamorphic rocks.
NOTE: Among Above given rocks only igneous rocks types are explained.
1. IGNEOUS ROCKS:
A. TYPES OF IGNEOUS ROCKS:
1) INTRUSIVE IGNEOUS ROCKS:
If magma cools slowly at great depths, mineral grains formed in the rocks may be very large.
Such rocks are called intrusive or plutonic rocks.
These rocks appear on the surface only after being uplifted.
2) EXTRUSIVE IGNEOUS ROCKS:
Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks. (Reason: Rapid cooling prevents crystallisation)
3) HYPABYSSAL ROCKS:
These rocks occupy an intermediate position between the deep -seated plutonic bodies and the surface lava flows.
These are semi -crystalline in nature.
B. TEXTURE OF IGNEOUS ROCKS:
1. PHANERITIC:
(Phaner - visible) These textures are typical of intrusive igneous rocks. The minerals in a phaneritic are large to see with naked eyes.
2. APHANITIC:
(A- not, phaner - visible) rocks in contrast to phaneritic rocks. Crystals in these types of textures are not distinguishable to naked eye.
3. PORPHYRITIC:
This texture occurs when magma crystallizes below a volcano but erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with smaller crystals.
4. GLASSY:
This texture occurs during some volcanic eruptions when lava is quenched so rapidly that crystallization not occur.
5. PYROCLASTIC:
(Pyro - igneous, clastic -fragment)
These textures occur when explosive eruptions blast the lava into air resulting in fragmental, typically glassy material.
6. PEGMATITIC:
This texture occurs during magma cooling when some minerals may grow so large that they become massive.
CRYSTALLINITY:
It refers to the degree of structural order in a solid. It is usually specified as a percentage of the volume of material that is crystalline.
LEVEL OF CRYSTALLINITY:
1. HOLOCRYSTALLINE:
They are completely crystalline.
2.HYPOCRYSTALLINE:
These rocks are partially crystalline with crystals embedded in an glassy matrix.
3.HYPOHYALINE:
They are partially glassy.
4.HOLOHYALINE:
These rocks are completely glassy.
DEVITRIFICATION:
The transformation of glassy substance into crystalline matter is called devitrification.
CAUSE OF DEVITRIFICATION:
It can be caused from firing too slowly through devitrification range of temperatures.
It can also caused from firing contaminants like oils and fingerprints.
CRYSTAL FORMATION PROCESS:
Crystallisation is the process by which a solid forms, where the atoms/molecules are highly organized into a structure known as crystals.
CRYSTALLISATION OCCUR IN 2 MAJOR STEP:
1. FIRST STEP IS NUCLEATION:
The appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent.
2.SECOND STEP IS CRYSTAL GROWTH:
In this step there is increase in size of particles and leads to a crystal state.
GLOBULITE:
It is a tiny globular body of mineral crystallite.
LONGULITE:
A crystallite of elongated form.
MARGARITE:
A gray, pink or yellow mica occurring in brittle monoclinic crystals.
TRICHITE:
A minute acicular body such as a hair like crystallite occurring singly or in clusters.
MICROLITE:
Microlite are minute crystals in amorphous matrix.
SHAPE OF CRYSTALS:
1.Shape of crystal based on texture:
(A) EUHEDRAL -They are having well developed crystals.
(B) SUBHEDRAL - when a half faces of crystals are developed.
(C) ANHEDRAL - when crystal -faces are absent (non -crystalline).
2.Shape of crystal based on dimension :
(A) EQUIDIMENSIONAL - Crystals found to have developed equally along all direction in space ex garnet, Lucite.
(B) TABULAR- When crystals are developed better in 2 directions.
(C) PRISMATIC-When crystals are developed better in 1 directions.
DEGREE OF CRYSTALLISATION:
The process of liquid rock freezing into solid rock is called degree of crystallisation.
GRANULARITY:
Granularity (also called graininess), the condition of existing in granules/grains refers to the extent to which a material /system is composed of distinguishable pieces.
TEXTURE:
Texture has already been defined as the intimate mutual relations of the mineral constituents and glassy matter in a rock made up of a uniform aggregate.
PARAGENESIS:
It is the sequence in which the minerals are formed in an ore deposit. OR it is the sequence of formation of associated minerals in rocks and rock suites.
XENOLITH:
During cooling of magma foreign rock fragment which enters is called xenolith.
