Unit - 3
Structural Geology, Plate Tectonics
Structure geology is the study of origin, types, occurance, classification and effect of various structure like folds, joints, fault, rock cleavage and which are different from bedding.
Rocks are not exposed everywhere on earth, but they are covered mostly by layer of soil which are thin called as alluvium or called as soil.When weathering and erosion expose part of a rock layer or formation, an outcrop appears.in some are of earth rocks are not visible anywhere because it is covered by soil up to 1000Km by covering its all are of rocks.Outcropping bedrock is great stuff, because it allows geologist to determine the local geology, and helps them put together the geologic history of the earth. An outcrop is simply defined as an exposure of a solid rock on the surface of earth.
The angle made by rock bed with horizontal plane is termed as dip. The true dip is the steepest line that can be drawn on an inclined plane or maximum angle of dip on rock bed.an example of recording dip is 45o, N 60oE.this former one means that bed dip at an angle of 45o in a north-easterly direction.Angle of Dip, also known magnetic dip or magnetic Inclination is the angle made by the Earth's magnetic field lines with the horizontal. The dip angle is measured by clinometer and for direction compass is used.
The direction of line formed by the intersection of bedding plane and horizontal plane is called as strike. Or direction of the line formed by the intersection of a fault, bed, or other planar feature and a horizontal plane is also termed as strike. The strike is right angles to the true dip direction (The strike of a rock is the orientation of a horizontal line drawn perpendicular to the dip)
On a large scale, conformable series is the study of the three-dimensional interaction and relationships of stratigraphic units within terraces of rock or geological regions.
Overlap can be understand by taking an example of sea. The sea is advancing slowly across a land surfaces then rate of depression not much exceed the rate of deposition. Overlap can also be defined by relation with successive layers of a conformable series.
The process of erosion in the surface within a sequence of rocks This is occur when sedimentary layers are deposited on top of a surface of erosion. There are three kinds of unconformities:
When 2 strata either of get displaced or shifted in any direction w.r.t to each other and now 2 strata lies on different level is termed as fault .In an active fault, the pieces of the Earth's crust along a fault move over time. The moving rocks can cause earthquakes. The development of fracture and displacement of strata w.r.t to each other the whole process is termed as faulting.
Various types of faults:-
A] On the basis of apparent movement:-
1} Normal Fault- Normal faults form when the hanging wall drops down. In this case the fault plane dips towards the down side. Generally faults are formed by gravity fault and result into lengthening of the earth crust.
2} Reverse Fault- Reverse faults form when the hanging wall appears to moves up. These fault represent shortening of faults.
B] On the basis of relationship between the strike of fault and attitude of stars, the fault are classified into 5 group:-
1} Dip fault:-when a fault start parallel to dip direction of beds is called the dip fault.
2} Tear or Wrench fault:-when the faulting get displaced in the horizontal direction then the fault is known as Tear fault.
3} Oblique fault:-when the strike and dip direction of strata is in oblique or run oblique is called as oblique fault.
4} Bedding fault:-The plane of movement is the bedding plane of the strata is termed as bedding fault. These fault are difficult to recognise.
5} Strike fault:-When a fault run parallel to strike of strata is termed as strike fault, it is also called as longitudinal fault.
C] Other types of fault according to their pattern:-
1} Garben or rift fault:-Graben are usually represented by low-lying areas such as rifts and river valleys. These fault are in long and relative narrow in shape and are bounded by parallel high angle fault. Graben often occur side-by-side with horsts
2} Step fault:-when the fault are seems to be in parallel down throw in same direction.
3} Horst fault:-horsts represent the ridges between or on either side of these valleys.
4} Parallel fault:-when fault which contain same strike and dip is termed as parallel fault.
Folds are defined as the horizontal layers are subjected to compressive forces they bend either upward or downward the bend of rocks is called as fold. The shape of fold depend upon number of factors like nature, magnitude duration and direction of forces acting on it. Folds may occur as single local bend or may occur repeatedly. The size of fold is from Km to Meter and Centimetre. The process of development of folds in rocks is called as folding. As compare to fold it is very slow in process. Folding is ductile type of deformation experienced by rocks compared to brittle deformation where the rocks actually get broken and displaced when stressed.
Types of folds:-
A] Classification on basis of nature of bending:-
1. Anticline:-when beds are bent upward the resulting fold is anticline. The shape is of arched up. The limbs of anticline slope is in opposite direction or away from each other.
2] Syncline:-It is just opposite to anticline. When bed are bent downward or down flexure. The two limbs of such fold dip in opposite direction but towards each other.
B] Classification on basis of position of axial plane and limbs:-
1. Symmetrical Fold- when the axial plane is vertical position then fold iss asymmetrical or upright fold. They may be Anticlinal and synclinal in nature.
2. Asymmetrical Fold- when both the limbs dip at different angles and the axial plane cannot divide the fold into two symmetrical halves.
3. Over turned Fold– when the axial plane is inclined and both the limbs dip in the same direction usually at different angles.
4. Recumbent Fold- In this type of fold the axial plane is horizontal. In it one limb lies exactly under the other limb. The overlying limb is right side up and is called as normal limb and underlying limb is upside down called inverted .these folds are also called as arch, shell, core and root zone.
5. Isoclinal Fold- the corresponding both limbs are dipping at equal amounts. Both the limbs have the same amount of dip, towards the same direction.
C] Classification on basis of depths:-
1. Parallel Fold–folds are here concentric semicircles have the constant centre and regularly increasing radius. In it the anticlines become sharper with depth but broader and more pointed upward. Similarly the synclines become broader and more pointed upwards.
2. Similar Fold-when the shape of fold remain same with depth. In this the shape of the fold may vary along the axial plane and at right angles to the fold axis.
3. Suprataneous Fold– the raising anticlines show thin beds over them because of differential accumulation of sediments. When the sediments got deposited over a ridge and may also develop through differential compaction of sediments around such ridges.
D] Classification on basis of intensity of deformation:-
Type of Fold | Inter limb angle |
| Greater than 70° |
| 30° to 70° |
E] Special types of folds:-
1. Anticlinorium- It is a large anticline with smaller fold thrown on it.
Synclinorium- It is a large syncline with secondary folds of smaller size developed on it.
2. Monocline- This is a fold in which only one limb is bent. This is a case when a rock-bed bends abruptly and resumes the original attitude at the lower level.
3. Homocline:-when set of dipping beds are considered as one.
4. Chevron:-folds are rounded along their axial parts. This is a fold having a sharp angular crest or trough.
5. Basin and Domes:-when beds are dip away from central points in all direction is termed as dome. Basin fold consists of a set of rock beds which are sunk down centrally giving the feature of a basin.
D] On the basis of degree of dip:-
1} High angle fault:-those fault whose dip is greater than 45o.example are normal fault.
2} Low angle fault:-those fault whose dip is less than 45o.exapmles are thrust fault.
When older rocks surround the younger rocks of outcrop is termed as outlier. Outlier is formed when erosion of surrounding rocks has taken place to sever the younger Rock's with a larger mass of the same younger rocks nearby in the area. Such features result into formation of garben faulting.
In case folded rocks, a synclinal hill makes an outlier, due to outcrop of overlying younger bed is surrounded by under laying older bed.
An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the erosion of overlying younger rocks to reveal a limited exposure of the older underlying rocks
Selection of sites affected by the various engineering structures are very large presence of faults and folds, as they affect the safety of facilities, and pose a significant risk it.
The presence of faults or breaks, reduces the resistance of the rocks of the jurisprudence, and introduced the facilities provided by them to collapse. The folds also a problem, since the existence of structures concave folds it with a great ability to collect ground water, causing water problems at large Construction of buildings or tunnels located in areas where this kind of folds So we justify the importance of geological studies and their role in avoiding collapse of structures and buildings in the regions illustrated by the geological maps based on these studies, as well as to avoid the high costs that may be borne by the concerned authorities In the event that set up these projects on the areas of joints and faults and folds, as well as the lives that may lose by default in the implementation of geological studies
A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton.Intrusive Structures are as follows:-
Batholiths are the largest of all igneous intrusions. They are formed by the massive invasion of hot magma into country rock.Igneous intrusive which are also called as underground surfaceactivity is the force of magma underground into spaces that exist in the interior of rock strata. When this magma cools and it hardens, results in occurrence of intrusive volcanic landforms. Which is converted into igneous activities.
Joints are fracture that separate the rocks into different parts. Joints and faults are types of fractures. A joint is a crack along which no movement has taken place, they are generally caused by tensional forces and compression stress. A fault is a fracture in the rock along which movement has taken place Joints have very little or no movement at all because they do not completely separate rock formations. It occur in all types of rocks.
Formation of Joints:-
They are occur due to brittle fracture in the body of the rocks due the 2 different stresses which are:-1] Tensile stresses 2] Compression stresses.
Generally due to expansion and contraction of rocks, contraction during formation and crustal disturbances are the reason for the formation of joints.
Types of Joints:-
A] On the basis of Spatial Relationship:-
1} Systematic joints or Regular joints:-
These joints show the uniformity in their occurrence. It can be measured easily. They generally occur in parallel or sub parallel joints that are reacted in the rocks at proper defined interval. It is further divided into 2 types- Conjugate (when dihedral angles are from 30o to 60o) and orthogonal joint sets (when dihedral angles are approximately equal to 900).
Columnar and mural joints are the example of Regular joints.
2} Non-systematic or Irregular joints:-
These joints show irregular in the orientation and forms. They can appear in the random rocks. They cannot be readily grouped into distinctive, through going joint sets.
B] On the basis of their Genes:-
It is very difficult to find the exact reason of joints so to identify the joints genetic types of subjected is considered. The various types of genetic joints are as follows:-
1} Tension Joints- Tension joints are those, which are formed as a result of tension forces. They are also produced in igneous rocks during cooling. These joints are relatively open and irregular surfaces. Longitudinal joints in anticline that runs parallel to axis of fold are examples of these joints.
2} Compression joints- compression joints are those, which are formed as a result of compression forces. In these joints compressive forces are dominant in nature.
3}Shear joints:-Shear joints are occur due to shearing stresses acting on folding and faulting of rocks. Shear joints occur in 2sets and form conjugate joint. These joints are tightly closed packed.
C] On the basis of its geometry:-
1} Strike Joints: Joints that are parallel to the strike of rocks are called Strike Joints.
2} Dip Joints: Joints that are parallel to the dip of rocks are called Dip Joints.
3} Oblique Joints: Joints which run in a direction that lies between the strike and dip direction of the rock beds are called Oblique Joint.
Stratification. -Most sedimentary rocks contain internal layering called bedding. This may be developed very prominently and can be seen from distance of miles or in other cases may have to be ascertained after close examination of the rocks. Stratification may range from a bed thickness of many meters down to fine millimetre size laminations. Bedding is generally horizontal or nearly so.
Planes of weakness –the bedding planes separate the bed from each other. Large scale cross-bedding in sandstone, within horizontal layers a few to many feet thick, indicates deposition in sand dunes.
Lamination-layered structure similar to bedding as found in sedimentary rocks. The individual layer are called as lamina and are distinguished commonly on the basis of difference in colour.
A laccolith represent magma that pushes overlying rocks layer upward to form a Condordant, mushroom like structure, whereas A Batholiths the largest bodies of igneous rocks, irregular in shape and cover major area. Batholith is a Discondordant magma body with exposed surface area of more than 1002Km, mainly consist of multiple intrusion.
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