Landform, whether large or small result from the interaction of certain forces, they accomplish their work by various means of processes, which may be described as geologic, Climatic and biologic and these processes bring about the changes in the Earth’s surface which may be Classified as (I) Long Period Changes (Man Is unable to notice this change) and (II) short period Changes. The forces which affect the crust of the earth are broadly classified in two types
- ENDOGENETIC FORCE-ORIGINATES FROM WITHIN THE EARTH
- EXOGENETIC FORCE-ORGINATES FROM WITHOUT,OR BEYOND THE EARTH
The forces coming from within the earth are called as Endogenetic forces which causes two types of movement in the earth viz (I) Horizontal movements (II) Vertical movements. These forces derive their energy from changes such as radioactivity, chemical recombination, expansion or contraction or displacement of molten materials which Occurs in the interior of the earth. This group of forces may be called as Tectonic forces which manifests through Processes Called Diastrophism and Volcanism. The origin of Endogenetic forces and related horizontal and vertical movements are caused due to contraction and expansion of rocks because of varying thermal conditions and temperatures inside the earth. Volcanic eruptions and seismic events are also expressions of Endogenetic forces. The displacement and readjustment of geomaterials sometimes take place so rapidly that earth’s movements are caused below the crust. On the basis of intensity the Endogenetic forces and movements are divided in to two major Categories
- SUDDEN FORCES
- DIASTROPHIC FORCES
SUDDEN FORCES AND MOVEMENT
Sudden and become disastrous When they occur in densely populated localities. These forces work very quickly and their results are seen within minutes. Movements, caused by sudden-Endogenetic forces coming from deep within the earth cause sudden and rapid events that cause massive destructions at and below the earth’s surfaces. Such events like volcanic eruptions and earthquakes, are called ‘EXTREME EVENTS’ and become disastrous hazards when they occur in densely populated localities. These forces work very quickly and their results are seen within minutes. ‘It is important to note that these forces are the result of long period Preparation. Only their cumulative effects on the earth’s surface are quick and sudden.’ Geologically, these sudden forces are termed as ‘constructive forces’ because these create certain relief features on the earth’s surface. For example, volcanic eruptions result in the formation of volcanic cones and mountains while fissure flows of lavas form extensive lava plateau and lava plains Earthquakes create faults, fractures, lakes etc.
Figure 1 Deccan Plateau in India
Figure 2 Columbia Plateau in USA
Diastrophic Forces and Movements
Diastrophic forces include both vertical and horizontal movements which are caused due to forces deep within the earth. These diastrophic forces operate very slowly and their effects become discernible after thousands and millions of years. These forces are also termed as constructive forces, affect large areas of earth and produce meso-level reliefs such as mountains, Plains, Plateaus, lakes, big faults etc. These diastrophic forces and movements further subdivided in to
- EPEIROGENETIC MOVEMENTS
- OROGENETIC MOVEMENTS
Epeirogenetic movement causes upliftment and subsidence of continents masses through upward and downward movements respectively. Both, the movements are in fact vertical movements. These are further divided in to
- Upward Movement
- Downward Movement
Upward movement causes upliftment of continental masses in two ways
- The upliftment of whole continent or part there of
- The upliftment of Coastal land of the continents and such type of upliftment is called EMERGENCE
Downward movement causes subsidence of continent in two ways
- The subsidence of the land area
- Alternatively, the land area near the sea coast is moved downwards or subsided below sea level and is thus submerged under seawater and such type of downward movement is called SUBMERGENCE
Orogenetic movements results in the formation of mountains which are caused due to the Endogenetic forces working in a horizontal manner. The horizontal forces and the resultant movements are also called as ‘tangential forces’. These forces work in two ways and they are
These forces operate in opposite direction and thus create ruptures, cracks, fractures and faults in the crustal parts of the earth. Such type of forces and movements are also Called as DIVERGENT forces and movements.
These forces operate towards each other or face to face and they cause crustal bending leading to the formation of folds
or crustal warping leading to the local rise or subsidence of crustal Parts. They are also called as CONVERGENT forces.
When horizontal forces work face to face the crustal rocks are bent due to the resultant Compressional and tangential force, the crustal rocks undergo the process of ‘crustal bending’ in two ways
The process of crustal warping affects larger areas of the crust where in crustal parts are warped upward or downward. The upward rise of the crustal part due to the compressive force resulting in the Convergent horizontal movement is called UPWARPING while bending of the Crustal parts downward in the form of a basin or depression is called DOWNWARPING. When the process of upwarping and downwarping affects larger areas, the resultant mechanism is called BROADWARPING. When the compressive horizontal forces and the resulting convergent movements cause buckling and Squeezing of crustal rocks, the resultant mechanisms is called folding which causes several types of folds.
Wave like bends are formed in crustal rocks due to the tangential compressive force resulting from horizontal movement caused by the Endogenetic force originating deep within the earth. Such bends are Called ‘folds’ Where in Some parts are bent up and some parts are bent down.
Figure 3 Different Components of fold
The up folded rock strata in arch-like form are called ‘anticlines’ while the down folded feature forming trough-like feature is called ‘synclines’. The two sides of the fold are called limbs of the fold. The limb which is shared between an anticline and its companion syncline is called middle limb. The Planes which bisects the angle between the two limbs of the anticline or middle limb of like syncline is called the axis of fold or axial plane. On the basis of anticline and syncline these axial planes are called as axis of anticline and axis of syncline respectively.
Figure 4 Anticlines and Synclines
The inclination of the rock beds with respect to the horizontal Plane is termed as ‘dip’. Two information are derived from the dip
- The direction of maximum slope down a bedding Plane
- The angle between the maximum slope and the horizontal plane
The direction of the dip is measured by its true bearing in relation to east or west of north e.g. 60°NE; while the angle of dip is measured with an instrument called clinometer.
The Strike of an inclined bed is the direction of any horizontal line along the bedding plane. The direction of the dip is always at right angle to the strike.
Figure 5 Dip and Strike
The unfolded rock beds are called anticlines. In simple fold the rock strata of both the limbs dip in opposite directions. Sometimes, folding becomes so acute that the dip angle of the anticline is accentuated and the fold becomes almost vertical. When the slopes of both the (limbs or the sides) of the anticline are uniform, the anticline is called ‘symmetrical anticline’ but when the slopes are unequal, the anticline is called as ‘asymmetrical anticline’. Anticlines are divided in to two types on the basis of dip angle
- Gentle anticline when the dip angle is less than 40° , sometimes 1°or2°
- Steep anticline when the dip angle ranges between 40° and 90°
The Down folded rock beds due to compressive forces caused by horizontal tangential forces are called synclines. These are trough like form in which beds on either side ‘incline together’ towards the middle part. If folded intensely, the synclines assume the form of a canoe.
Anticlinorium refers to those folded structures in the regions of folded mountains where there are series of minor anticlines and synclines with one extensive anticline. They are formed when the horizontal compressive tangential forces do not wok regularly. Such type of fold is also called as fan fold.
Figure 6 Illustration of Anticlinorium and synclinorium
Synclinorium represents such folded structure which includes an extensive syncline having numerous minor anticlines and synclines which formed due to irregular folding of irregular compressive forces.
Types of folds
The Nature of the folds depends on various factors such as the nature of rocks, the nature and the intensity of compressive forces, duration of the operation of the compressive forces etc. Based on the inclinations of the limbs the folds are classified in to 5 types.
Figure 7 Types of folds
- Symmetrical folds
- Asymmetrical folds
- Monoclinal folds
- Isoclinal folds
- Recumbent folds
If both the limbs incline uniformly then they are called as symmetrical folds. These folds are an example of open folds and are formed when Compressive forces work regularly but with moderate intensity.
These are characterized by unequal irregular limbs which incline at different angles. One limb is relatively larger and the inclination is moderate and regular while the other limb is relatively shorter with Steep inclination.
These are the folds in which one limb inclines moderately with regular slope while the other limb inclines steeply at right angle and the slope is almost vertical.
These folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.
These folds are formed when compressive forces are so strong that both the limbs of the folds become parallel as well as horizontal.
The folds in which one limb of the fold is thrust upon the another fold due to intense compressive forces. Limbs are season horizontal.
When the axis of the fold instead of being parallel to the horizontal plane becomes tilted and forms plunge angle which is the angle between the axis and the horizontal plane.
They represent an extensive broad fold consisting of several minor anticlines and synclines which resembles a fan. Such feature also called as Anticlinorium and synclinorium.
The folds in which the angle between the two limbs of the fold is more than 90° but less than 180° (obtuse angle). These open folds are formed due to wave like folding because of moderate nature compressive forces.
Figure 8 Open Folds
The folds in which the angle between the limbs is acute then they are called as closed folds and are formed due to intense Compressive forces.
Figure 9 Closed Folds
Nappes are the result of complex folding mechanism caused by intense horizontal movement and resultant compressive forces. Both the limbs of the recumbent fold are parallel and horizontal. Due to further increase in the continued compressive force one limb of the recumbent folds slides forward and overrides the Other fold This process is called ‘thrust‘ and the Plane along which one Part of the fold is thrust is called ‘thrust plane’. The upthrust part of the fold is called ‘Overthrust fold’. When the compressive forces become so acute that it crosses the limit of elasticity of the rock beds, the limbs of the fold are so acutely folded that these break at the axis of the fold and the lower rock beds come upward. Thus the resultant structure becomes reverse to the normal structure. Due to continued horizontal movement and compressive force the broken limb of the fold is thrown several kilometers away from the original structure and overrides the rock beds of the distant Place. Such type of structure becomes unconformal to the original structure of the place where the broken limb of the fold of other place overrides the rock beds. Such broken limb of the fold is called ‘napple‘.
Figure 10 Formation of Nappes
Several examples of nappes are traceable in the present folded mountains. The nappes of the Alps have been systematically studies and divided in to four major groups from below upward they are
- Helvetic nappe
- Pennine nappe
- Austride nappe
- Dinaride nappe
In most of the localities the overriding nappes have been eroded away because of dynamic wheels of denudation processes and thus buried basic structure has been exposed. When the Portion of lower nappe is seen because of the denudation of overriding nappe, the resultant open Structure is called ‘structural window’. Several examples of ‘complete window’ have been discovered in eastern Alps. A few examples of nappes have also been traced out in Himalayas. When the broken limb of a fold overrides the other fold near the broken fold, the resultant nappe is called ‘Autochthonous Nappe’. On the other hand, when the limb of a fold, after being broken, overrides the other fold at a distant place, the resultant nappe is called ‘exotic nappe’.
Crustal fracture refers to displacement of rocks along a plane due to tensional and Compressional forces acting either horizontally or vertically or sometimes even in both ways. Crustal fracture depends on the strength of the rocks and intensity of tensional forces. The crustal rocks suffer only cracks when the tensional force is moderate but when the rocks are subjected to intense tensional force, the rock beds are subjected to dislocation and displacement resulting in to the formation of faults. Generally fractures are divided in to 2 types
A joint is defined as a fracture in the crustal rocks wherein no appreciable movement of rocks takes place.
When the crustal rocks are displaced, due to tensional movement caused by the Endogenetic forces, along a plane, the resultant structure is called fault. The Plane along which the rock blocks are displaced is called fault plane. A fault Plane may be vertical, 0r inclined, or horizontal, or curved or of any type and form. The movement responsible for the formation of a fault may operate in vertical or horizontal or in any direction.
Figure 11 Different Components of a Fault
The plane along which the rock blocks are displaced by tensional and Compressional forces acting vertically and horizontally to form a fault, fault plane may be vertical, inclined, horizontal, curved or of any other form.
The angle between the fault plane and the horizontal plane
Represents the uppermost block of a fault
Represents the lowermost block of a fault
The upper wall of the fault
The lower wall of the fault
The steep wall like slope caused by faulting of the crustal rocks, Sometimes the fault scarp is so steep that is resembles a cliff. Scarps are not formed by faulting alone and it may form due to erosion also, but whenever these are formed by faulting they are called ‘fault scraps‘.
Types of Faults
The different types of faulting of the crustal rocks are determined by the direction of the motion along the fracture plane. Generally, the relative movement or displacement of the rock blocks occurs approximately in two directions
- To the direction of the dip-Dip slip movements
- To the direction of the strike-Strike slip movements
Thus on the basis of the direction of slip or displacement faults are divided into
- Dip-slip faults
- Strike –slip faults
If the displacement of the rock blocks is down to the direction of the dip then the resultant fault is called Normal fault. Normal faults are formed due to the displacement of both the rock blocks in opposite directions due to tensional force. The fault plane is usually between 45° and vertical. The Steep scrap resulting from the normal fault is called fault-scarp or fault-line scarp the height of which ranges between a few metres to hundreds of metres.
Reverse faults are formed due to the movement of both the fractured rock blocks towards each other. The fault Plane, in a reverse fault is usually inclined of an angle between 40° and horizontal 0° .The vertical Stress is minimum while the horizontal stress is maximum. In reverse faults the rock beds on the upper side are displaced up the fault plane relatively to the rock beds below .It is apparent that reverse faults results in the shortening of the faulted area while normal faults cause extension of the faulted area. It is thus, also obvious that some sort of compression is also involved in the formation of reverse faults. Reverse faults are also called of thrust faults. Since reverse faults is formed due to compressive force resulting from horizontal movement and hence this is also called as Compressional fault. When the compressive force exceeds the strength of the rocks , one block of the fault overrides the other block and the resultant fault is called as Overthrust fault where in the fault plane becomes almost horizontal.
Figure 12 (A) Normal fault and (B) Reverse Fault
Lateral or Strike Strip Faults
This type of faults are formed When the rock beds are displaced horizontally along the fault plane due to horizontal movement These are called left-lateral or sinistral faults when the displacement of rocks occurs to the left on the far side of the fault and right lateral or dextral faults when the displacement of rock blocks takes place to the right on the far side of the fault.
Figure 13 Formation of Strike -slip or transcurrent faults
When a series of faults occur in any area in such a way that the slopes of all fault planes are in the same direction the resultant faults are called step faults. It is a prerequisite condition for the formation of step faults that the downward displacement of all the downthrown blocks must occur in the same direction.
Figure 14 Illustration of Step faults
Rift Valley and Graben
Rift valley is a major relief feature resulting from faulting activities. It represents a trough, depression or basin between two crustal Parts. Rift valleys are formed due to displacement of crustal Parts and subsidence of middle portion between two normal faults by horizontal and vertical movements motored by Endogenetic forces Rift valley are generally also called as ‘graben’ which is a German word which means trough-like depression. A rift valley may be formed in two ways viz
- When the middle portion of the crust between two normal faults is dropped downward while the two blocks on the either side of the down dropped block remain stable
- When the middle portion between two normal remains stable and the two side blocks on the either side of the middle position are raised upward
Figure 15 Illustration of rift valleys and graben
Rhine rift valley is the best example rift valley. The one side of the rift valley is bounded by Vosges and Hardt mountains (block mountains-horst) while the other side is bordered by Black forest and odenwald mountains. Some of the other rift valleys are Jordan River valley, Death Valley of southern Californian and Dead Sea in Asia. The rift valleys are not only confined to continental crustal surfaces but they are also found on the sea floor. The deepest grabens are found in the form of ‘ocean deeps’ and trenches.
Figure 16 Rhine Rift Valley
Figure 17 Rhine and Death Valley of California
Origin of rift valleys
The hypothesis regarding the origin of rift valleys are generally grouped in to two categories
- Tensional hypothesis-based on tensional forces
- Compressional hypothesis-based on compressional forces
Both these hypothesis have lot of limitations and was not able to solve many of the intricate problems of the origin of rift valley.
Hypothesis of E.C Bullard
E.C. Bullard, while conducting the gravity survey, postulated his new concept of the origin of the rift valleys and according to him the formation of rift valley is completed through a series of sequential phases of compressional forces coming from both the sides of the land. The horizontal compressive forces work face to face from both the sides of the land. This lateral compression becomes so enormous that it exceeds the strength of the rocks; a crack is developed at a place in the crustal rocks. This crack is gradually enlarged due to continuous increase in the compressive force.
Figure 18 Stage 1
Second stage, due to the formation of crack, one portion overrides the other Portion and this portion is called thrusting. On the other hand, the second part is thrown downward relative to the first part. This process is called down thrusting.
Figure 19 Second Stage
A-C part has gone upward because of over thrusting .Due to the Upthrusting of the side block (A-C) up to a height of few thousand metres the downthrust block(A-D) develops crack at a place (B) due to the resultant compressive force. The place of the crack is located at the highest point of the downthrust block. This newly formed crack continues to increase gradually.
Figure 20 Third Stage
Third stage,the crack developed at downthrust block B place becomes enlarged due to increased compression with the result B-D part of the downthrust block overrides its other part (A-B).Thus, the position of downthrust A-B part between the two upthrust blocks (A-C and B-D) becomes rift valley.
The exogenetic forces or Processes, also called as denudational processes, or ‘destructional forces or processes’ are originated from the atmosphere. These forces are continuously engaged in the destruction of the relief features created by Endogenetic forces through their weathering, erosional, depositional activities. Denudation includes both weathering and erosion where weathering being a static process includes the disintegration and decomposition of rocks in situ whereas erosion is dynamic process which includes both, removal of materials and their transportation to different destinations.
Weathering is basically of three types
- Physical or Mechanical weathering
- Chemical weathering
- Biological weathering
These processes are very important for the biosphere ecosystem because weathering of parent rocks results in the formation of soils which are very essential for the sustenance of the biotic lives in the biosphere.
The erosional processes include running water or river, ground water, sea –waves, glaciers, periglacial processes and wind. These erosional processes erode the rocks, transport the eroded materials and deposit them in suitable places and thus form several types of erosional and depositional landforms of different magnitudes and dimensions.