Forces Affecting the Crust of the Earth | Geography

The forces, which affect the crust of the earth, are divided into two broad categories on the basis of their sources of origin. The forces are: 1. Endogenetic Forces 2. Exogenetic Forces.

1. Endogenetic Forces:

The forces coming from within the earth are called as endogenetic forces which cause two types of movements in the earth viz.:

(I) Horizontal movements, and

(ii) Vertical movements.

These movements motored by the endogenetic forces introduce various types of vertical irregularities which give birth to numerous varieties of relief features on the earth’s surface (e.g., mountains, plateaux, plains, lakes, faults, folds etc.).

Volcanic eruptions and seismic events are also the expressions of endogenetic forces. Such movements are called sudden movements and the forces responsi­ble for their origin are called sudden forces. We do not know precisely the mode of origin of the endogenetic forces and movement because these are related to the interior of the earth about which our scientific knowl­edge is still limited.

On an average the origin of endogenetic forces is related to thermal conditions of the interior of the earth. Generally, the endogenetic forces and related horizontal and vertical movements are caused due to contraction and expansion of rocks because of varying thermal conditions and tempera­ture changes inside the earth.

The displacement and readjustment of geomaterials sometimes take place so rapidly that earth movements are caused below the crust.

The endogenetic forces and movements are divided, on the basis of intensity, into two major categories viz.:

(1) Sudden forces and

(2) Diastrophic forces.

1. Sudden Forces and Movements:

Sudden movements, caused by sudden endogenetic forces coming from deep within the earth, cause such sudden and rapid events that these cause massive destructions at and below the earth’s surface. 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 deep within the earth. 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 plateaux (e.g., Deccan plateau of India, Columbia pla­teau of the USA etc.) and lava plains. Earthquakes create faults, fractures, lakes etc.

2. 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, also termed as constructive forces, affect larger areas of the globe and produce meso-level reliefs (e.g., mountains, plateaux, plains, lakes, big faults etc.).

These diastrophic forces and movements are further subdivided into two groups viz.:

(i) Epeirogenetic movements, and

(ii) Orogenetic movements.

1. Epeirogenetic movements:

The epeirogenetic word consists of two words viz. ‘epiros’ (meaning thereby continent) and ‘genesis’ (meaning thereby ori­gin). Epeirogenetic movements cause upliftment and subsidence of continental masses through upward and downward movements respectively. Both the move­ments are infact, vertical movements. These forces and resultant movements affect larger parts of the continents.

These are further divided into two types viz.:

(i) Upward movement, and

(ii) Downward move­ment.

Upward movement causes upliftment of conti­nental masses in two ways e.g.:

(a) Upliftment of whole continent or part thereof, and

(b) Upliftment of coastal land of the continents. Such type of upliftment is called emergence.

Downward movement causes subsidence of continental masses in two ways viz.:

(a) subsidence of land area. Such type of downward movement is called as subsidence.

(b) Alternatively, the land area near the sea coast is moved downward or is subsided below the sea level and is thus submerged under sea water. Such type of downward movements is called as submer­gence.

(2) Orogenetic movement:

The word orogenetic has been derived from two Greek words, ‘oros’ (mean­ing thereby mountain) and ‘genesis’ (meaning thereby origin or formation). Orogenetic movement is caused due to endogenetic forces working in horizontal man­ner. Horizontal forces and movements are also called as tangential forces.

Orogenetic or horizontal forces work in two ways viz.:

(i) In opposite directions, and

(ii) Towards each other.

This is called tensional force when it operates in opposite directions. Such types of force and movement are also called as divergent forces and movements.

Thus, tensional forces create rupture, cracks, fracture and faults in the crustal parts of the earth. The force, when operates face to face, is called compressional force or convergent force. Compression al force causes crustal bending leading to the formation of folds or crustal warping leading to local rise or subsid­ence of crustal parts.

Crustal bending:

When horizontal forces work face to face the crustal rocks are bent due to resultant compressional and tangential force.

In other words, when crustal parts move towards each other under the influence of horizontal or convergent forces and move­ments, the crustal rocks undergo the process of ‘crustal bending’ in two ways e.g.:

(i) Warping, and

(ii) Folding.

The process of crustal warping affects larger areas of the crust wherein the crustal parts are either warped (raised) upward or downward. The upward rise of the crustal part due to compressive force resulting from convergent horizontal movement is called up-warping while the bending of the crustal part downward in the form of a basin or depression is called down-warping.

When the processes of up-warping or down-warping of crustal rocks affect larger areas, the resultant mecha­nism is called broad warping. When the compressive horizontal forces or convergent forces and resultant movements cause buckling and squeezing of crustal rocks, the resultant mechanism is called folding which causes several types of folds.

Folds:

Wave-like bends are formed in the crustal rocks due to tangential compressive force resulting from horizontal movement caused by the endogenetic force originating deep within the earth. Such bends are called folds wherein some parts are bent up and some parts are bent down.

The up-folded rock strata in arch ­like form are called anticlines while the down folded structure forming trough-like feature is called syncline (fig. 7.3). In fact folds are minor forms of broad warping. The two sides of a fold are called limbs of the fold.

The limb which is shared between an anticline and its companion syncline is called middle limb. The plane which bisects the angle between two limbs of the anticline or middle limb of the syncline is called the axis of fold or axial plane (fig. 7.2). On the basis of anticline and syncline these axial planes are called as axis of anticline and axis of syncline respectively.

It is desirable to explain the characteristics of dip and strike as it becomes absolutely necessary to understand them in order to understand the structural form. The inclination of rock beds with respect to horizontal plane is termed as ‘dip’ (fig. 7.4).

It is apparent that we derive two information about the dip e.g.:

(i) The direction of maximum slope down a bedding plane, and

(ii) The angle between the maximum slope and the horizontal plane.

The direction of dip is measured by its true bearing in relation to east or west or north e.g., 60° N.E.; while the angle of dip is measured with an instrument called clinometer. For example, if any rockbed is inclined at the angle of 60° with respect to horizontal plane and the direction of slope is N then the dip would be expressed as 60°N. ‘The strike of an inclined bed is the direction of any horizontal line along a bedding plane’ (A. Holmes and D.L. Holmes). The direction of dip is always at right angle to the strike (fig. 7.4).

Anticlines:

The up-folded 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 sides of an anticline are uniform, the anticline is called as sym­metrical anticline but when the slopes are unequal, the anticline is called as asymmetrical anticline.

Anticlines are divided into two types on the basis of dip angle e.g.:

(i) Gentle anticline when the dip angle is less than 40°, sometimes 1° or 2° and

(ii) Steep anticline when the dip angle ranges between 40° and 90°.

Synclines:

Down-folded rock beds due to compressive forces caused by horizontal tangential forces are called synclines. These are, in fact, trough ­like form in which beds on either side ‘incline to­gether’ towards the middle part. If folded intensely, the syncline assumes the form of a canoe.

Anticlinorium:

Anticlinorium refers to those folded structures in the regions of folded mountains where there are a series of minor anticlines and synclines within one extensive anticline (fig. 7.5). Anticlinorium is formed when the horizontal compressive tangential forces do not work regularly. Consequently, due to difference in the intensity of compressive forces such structures are formed. Such type of folded structure is also called as fan fold.

Synclinorium represents such a folded structure which includes an extensive syncline having numer­ous minor anticlines and synclines. Such structure is formed due to irregular folding consequent upon ir­regular compressive forces (fig. 7.5).

Types of Folds:

The nature of folds depends on several factors e.g., the nature of rocks, the nature and intensity of compressive forces, duration of the operation of compressive forces etc. The elasticity of rocks largely affects the nature and the magnitude of process. The softer and more elastic rocks are less elastic rocks are intense folding while rigid and less elastic rocks are only moderately folded.

The difference in the intensity and magnitude of compressive forces also causes vari­ations in the characteristics of folds. Normally, both the limbs of a simple fold are more or less of equal inclination but in most of the cases of different folds the inclinations of both the limbs are different.

Thus, based on the inclination of the limbs, folds are divided into 5 types (fig. 7.6):

(1) Symmetrical folds are simple folds, the limbs (both) of which incline uniformly. These folds are an example of open fold. Symmetrical folds are formed when compressive forces work regularly but with moderate intensity. In fact, symmetrical folds are very rarely found in the field.

(2) Asymmetrical folds are characterized by un­equal and irregular limbs. Both the limbs 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. Thus, both the limbs are asymmetrical in terms of inclination and length.

(3) Monoclinal folds are those in which one limb inclines moderately with regular slope while the other limb linclines steeply at right angle and the slope is almost vertical. It may be pointed out that vertical force and movement are held responsible for the formation of monoclinal folds.

There is every possibility for the splitting of the limbs of such folds because of intense folding. Splitting of limbs gives birth to the formation of faults. It is also opined that monoclinal folds are also formed due to unequal horizontal compressive forces coming from both the sides.

(4) Isoclinal folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.

(5) Recumbent folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.

(6) Overturned folds are those folds in which one limb of the fold is thrust upon another fold due to intense compressive forces. Limbs are seldom horizon­tal.

(7) Plunge folds are formed 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.

(8) Fan folds represent an extensive and broad fold consisting of several minor anticlines and synclines. Such fold resembles a fan. Such feature is also called as anticlinorium or synclinorium (fig. 7.5).

(9) Open folds are those in which the angle between the two limbs of the fold is more than 90° but less than 180° (i.e. obtuse angle between the two limbs of a fold). Such open folds are formed due to wave-like folding because of moderate nature of compressive force (fig. 7.7).

(10) Closed folds are those folds in which the angle between the two limbs of a fold is acute angle. Such folds are formed because of intense compressive force.

Nappes:

Nappes are the result of complex folding-mechanism caused by intense horizontal movement and re­sultant compressive force. Both the limbs of a 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 up-thrust part of the fold is called ‘over-thrust fold’. When the compressive force becomes so acute that it crosses the limit of the 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 folds is thrown several kilometres away from its original place and overrides the rock beds of the distant place. Such type of structure becomes un-conformal to the original structure of the place where the broken limb of the fold of the other place overrides the rock beds. Such broken limb of the fold is called nappe (fig. 7.8).

Several examples of nappes are traceable in the present folder mountains. The nappes of the Alps have been more systematically studied. Four major nappes have been identified in the Alps mountains. The structure has become very much complex because of super-imposition of one nappe upon another nappe.

The four major groups of Alpine nappes from below upward are:

(i) Helvetic nappe,

(ii) Pennine nappe,

(iii) Austride nappe, and

(iv) Dinaride nappe.

In fact, these nappes are located like a series of earth-waves. In most of the localities the overriding nappes have been eroded away because of dynamic wheels of denudational processes and thus buried basic structure has been exposed. When the portion of lower nappe is seen because of 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 the Himalayas. The existence of nappes has been discovered by Wadia from Kashmir Himalaya, by Pilgrim from Simla Himalaya, by Auden from Garhwal Himalaya and by Heim and Gansser from Kumaun Himalaya.

It is desirable to mention some facts about nappe structure. When the broken limb of a fold overrides the other fold near to 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 (several kilometres away), the resultant nappe is called exotic nappe.

Crustal fracture:

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 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 into the forma­tion of faults.

Generally fractures are divided into:

(i) joints and

(ii) faults.

A joint is defined as a fracture in the crustal rocks wherein no appreciable movement of rock takes place, whereas a fracture becomes fault when there is appreciable displacement of the rocks on both sides of a fracture and parallel to it.

Faults:

A fault is a fracture in the crustal rocks wherein the rocks are displaced along a plane called as fault plane. In other words when the crustal rocks are dis­placed, due to tensional movement caused by the endogenetic forces, along a plane the resultant struc­ture is called a fault. The plane along which the rock blocks are displaced is called fault plane.

In fact, there is real movement along the fault plane due to which a fault is formed (fig. 7.9). A fault plane may be vertical, or 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.

During the formation of a fault the vertical displacement of rock blocks may occur upto several hundred metres and horizontally the rock blocks may be displaced upto several kilometres but it does not mean that the total displacement occurs at a single time. In fact, fault-movement or the displacement of rocks occurs only upto a few metres only at a time. Fault, in fact, represents weaker zones of the earth where crustal movements become operative for longer duration.

A few terms regarding an ideal fault should be understood before going into the details of the mode of formation of various types of faults:

(1) Fault plane is that plane along which the rock blocks are displaced by tensional and compressional forces acting vertically and horizontally to form a fault. A fault plane may be vertical, inclined, horizon­tal, curved or of any other form.

(2) Fault dip is the angle between the fault plane and horizontal plane (fig. 7.9).

(3) Upthrown side represents the upper most block of a fault.

(4) Downthrown side represents the lowermost block of a fault. Sometimes it becomes difficult to find out, which block has really moved along the fault plane?

(5) Hanging wall is the upper wall of a fault.

(6) Foot wall represents the lower wall of a fault.

(7) Fault scarp is the steep wall-like slope caused by faulting of the crustal rocks. Sometimes the fault scrap is so steep that it resembles a cliff. It may be pointed out that scarps are not always formed due to faulting alone, rather these are also formed due to erosion, but whenever these are formed by faulting (tectonic forces), these are called fault-scarpts.

Types of Faults:

The different types of faulting of the crustal rocks are determined by the direction of motion along the fracture plane. Generally, the relative movement or displacement of the rock blocks or the slip of the rock blocks occurs approximately in two directions viz.:

(i) either to the direction of the dip or

(ii) to the direction of the strike of the fault plane.

Thus, the displacement or movement of rock blocks may be distinguished as:

(a) dip slip movements and

(b) strike slip movements.

Thus, on the basis of the direction of slip or displace­ment faults are divided into:

(i) dip-slip faults and

(ii) strike-slip faults.

Again, the displacement of rock block- mainly upper blocks may be either down the direction of the dip (then the resultant fault is called normal fault) or up the dip (the resultant fault becomes reverse or thrust fault).

In the case of strike-slip movement and fault, the relative displacement of the rock blocks may be either to the right (then the resultant fault will be right-lateral or dextral fault) or to the left-side (the resultant fault becomes left-lateral or sinistral fault). Strike slip faults are also called as wrench faults, tear faults or trans-current faults. The combinations of nor­mal and wrench faults or reverse and wrench faults are called as oblique slip faults.

(i) Normal faults are formed due to the displace­ment of both the rock blocks in opposite directions due to fracture consequent upon greatest stress. The fault plane is usually between 45°and the vertical.

The steep scarp resulting from normal faults is called fault-scarp or fault-line scarp the height of which ranges between a few metres to hundreds of metres. It may be men­tioned that it becomes very difficult to find out the exact height of the fault-scarps in the field because the height is remarkably reduced due to continued denuda­tion (fig. 7.10).

(ii) 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 at an angle between 40 degree and the hori­zontal (0 degree).

The vertical stress is minimum while the horizontal stress is maximum. It may be mentioned that in a reverse fault the rock beds on the upper side are displaced up the fault plane relative to the rock beds below.

It is apparent that reverse faulting 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 as thrust faults. Since the reverse fault 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 over-thrust fault wherein the fault plane be­comes almost horizontal.

(iii) Lateral or strike-slip faults are formed when the rock blocks are displaced horizontally along the fault plane due to horizontal movement. These are called left-lateral or sinistral faults when the displace­ment of the rock blocks occur 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 (fig. 7.11). In majority of the cases there are no scarps in such faults, if they occur at all, they are very low in height.

(iv) Step faults:

When a series of faults occur in any area in such a way that the slopes of all the fault planes of all the faults are in the same direction, the resultant faults are called as step faults (fig. 7.12). 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.

Rift Valley and Graben:

Rift valley is a major relief feature resulting from faulting activities. Rift valley represents a trough, depression or basin between two crustal parts. In fact, rift valleys are long and narrow troughs bounded by one or more parallel normal faults caused by horizontal and vertical movement motored by endogenetic forces.

Rift valleys are actually formed due to displacement of crustal parts and subsidence of middle portion between two normal faults. Rift vallyes are generally also called as graben which is a German word which means a trough-like depression. These two terms are synony­mously used in various parts of the world. ‘Tensional crustal forces, literally pulling the crust apart, are responsible for these down-dropped fault blocks’ (fig. 7.13).

A few scientists have attempted to differentiate a graben from a rift valley on the basis of size and dimension. They believe that a graben is relatively smaller in size than a rift valley but this minor difference of size is not accept­able to others. Thus, both the terms, graben and rift valley should always be considered as synonym.

A rift valley may be formed in two ways viz.:

(i) When the middle portion of the crust between two normal faults is dropped downward while the two blocks on either side of the down-dropped block remain stable and

(ii) When the middle portion between two normal faults remains stable and the two side blocks on either side of the middle portion are raised upward.

Normally, a rift valley is long, narrow but very deep. Rhine rift valley is the best example of a well-defined rift valley. It stretches for a distance of 320 km having an average width between the cities of Basel and Bingen. The one side of this great rift valley is bounded by Vosges and Hardt mountains (block mountains-horst) and the other side is bordered by Black Forest and Odewnwald mountains.

The example of the longest rift valley is the valley that runs from the Jordon river valley through Red Sea basin to Zambezi valley for a distance of 4,800 km. A few of the rift valleys are so deep that their bottom/floor is below sea level. Death Valley of the southern California (USA) is a good example of such graben. Dead Sea of Asia presents an ideal example of typical rift valley. The floor of the Dead Sea is about 867 m below sea level.

The floors of the Jordon rift valley and Death Valley are also 433 m below sea level. The Narmada valley, the Damodar valley and some stretches of the Son Valley, the Tapi valley etc. are considered to be exam­ples of rift valleys but this view is still controversial and is not acceptable to all geologists.

It may be mentioned that the rift valleys are not only confined to the continental crustal surfaces but they are also found on sea floor. In fact, the deepest grabens are found in the form of ocean deeps and trenches. The Bortlet Trough located to the south of Cuba is 4.8 km deep while Java Deep is 6.4 km deep from the sea floor. The central plain of Scotland, Spencer Bay of south Australia etc. are examples of rift valleys.

E.C. Bullard, while conducting the gravity survey, postulated his new concept of the origin of the rift valleys in 1933-34. According to him the rift block cannot slip downward under the impact of gravity, like a keystone of an arch of a building. Thus, the rift valley can be formed only due to compression coming from two sides.

According to Bullard the formation of a rift valley is not com­pleted during a single phase but is completed through a series of sequential phases. First stage, there is com­pression in the crustal rock beds of the rigid part of a plateau due to active horizontal movement. The hori­zontal compressive forces work face to face from both the sides of the land.

This lateral compression causes buckling of the crustal rocks. As the compressive forces continue to increase, the buckling and squeez­ing of the crustal rocks also continue to increase. When the compression becomes so enormous that it exceeds the strength of the rocks, a crack is developed at a place (A in fig. 7.14) in the crustal rocks. This crack is gradually enlarged due to continuous increase in the compressive force.

Second stage, due to the formation of a crack (at A place, fig. 7.14), one portion overrides the other portion. This process is called as thrusting. On the other hand, the second part is thrown downward relative to the first part. This process is called down-thrusting. A- C part (fig. 7.14) has gone upward because of over-thrusting.

Due to up-thrusting of the side block (A- C) up to a height of a few thousand metres the down-thrust block (A-D) develops crack at a place (B) due to resultant compressive force. The place of the crack is located at the highest point of down-thrust block. This newly formed crack continues to increase gradually.

Third stage, the crack developed in down-thrust block at B place (fig. 7.14) becomes enlarged due to increased compression with the result B-D part of the down-thrust block overrides its other part (A-B). Thus, the position of down-thrust A-B part between the two up-thrust blocks (A-C and B-D) become a rift valley. A- B in fig. 7.14 denotes the width of the upper portion of the rift valley.

According to E.C. Bullard the width of the rift valley (A-B) depends upon the elasticity of the rocks, depth of the rift valley and the density of the substra­tum. If the density of the substratum is taken to be 3.3. then the width of the rift valley would be 40 km if the depth of the valley is 20 km. Similarly, for a 40 km deep valley the width would be 65 km.

It may be concluded that neither the tensional hypothesis nor the compressional hypothesis could be able to slove many of the intricate problems of the origin of the rift valleys.

2. Exogenetic Forces:

The exogenetic forces or processes, also called as denudational processes, or destructional forces or processes are originated form the atmosphere. These forces are continuously engaged in the destruction of the relief features created by the endogenetic forces through their weathering, erosional and depositional activities.

Exogenetic processes are, therefore, planation processes. 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 viz.:

(i) Physical or mechanical weather­ing,

(ii) Chemical weathering and

(iii) Biological weath­ering.

Weathering is very important for the biospheric ecosystem because weathering of parent rocks results i n 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, groundwater, sea waves, glaciers, periglacial proc­esses and wind. These erosional processes erode the rocks, transport the eroded materials (except periglacial processes) and deposit them in suitable places and thus form several types of erosional and depositional landforms of different magnitudes and dimensions.