Earth’s Movements/Geomorphic Processes – ENDOGENETIC FORCES

Studying forces impacting the Earth’s crust and geological processes is crucial due to their role in shaping and altering geological materials and various relief features. These forces constantly modify the Earth’s surface through long-term and short-term changes. Long-term changes occur slowly, while short-term changes like earthquakes and volcanic eruptions happen suddenly. Earth’s forces are categorized into endogenetic and exogenetic forces based on their origins.

Forces originating within the Earth are referred to as endogenetic forces. These forces bring about two types of movements within the Earth: horizontal movements and vertical movements. These movements, driven by endogenetic forces, lead to various vertical irregularities that give rise to a wide range of relief features on the Earth’s surface, such as mountains, plateaus, plains, lakes, faults, and folds. Volcanic eruptions and seismic events also stem from endogenetic forces and are considered sudden movements, with the forces responsible termed sudden forces.

The precise origin and nature of endogenetic forces and movements remain unclear due to limited knowledge about the Earth’s interior. Generally, these forces and associated movements are linked to the thermal conditions of the Earth’s interior. They are often caused by the contraction and expansion of rocks due to varying temperatures within the Earth. Sometimes, the displacement and adjustment of geological materials happen so swiftly that they lead to movements beneath the crust.

Based on their intensity, endogenetic forces and movements are categorized into two main groups: diastrophic forces and sudden forces.

SUDDEN FORCES AND MOVEMENTS

Sudden movements caused by deep-seated endogenetic forces within the Earth lead to rapid and destructive events, causing massive disruptions on and below the Earth’s surface. These events, such as volcanic eruptions and earthquakes, are known as ‘extreme events’ and become disastrous hazards when they impact densely populated areas. These forces work swiftly, with visible effects emerging within minutes. Notably, these forces result from long-term preparations deep within the Earth, and their cumulative effects on the surface are quick and abrupt.

Geologically termed ‘constructive forces,’ these sudden forces shape specific relief features on the Earth’s surface. Volcanic eruptions, for instance, give rise to volcanic cones and mountains, while lava flows from fissures create extensive lava plateaus (e.g., the Deccan Plateau in India, the Columbia Plateau in the USA) and lava plains. Earthquakes lead to the formation of faults, fractures, and lakes.

DIASTROPHIC FORCES AND MOVEMENTS

Diastrophic forces, originating deep within the Earth, encompass both vertical and horizontal movements. These forces operate at a slow pace, with their effects becoming noticeable over thousands to millions of years. Referred to as constructive forces, diastrophic forces impact larger geographical regions, giving rise to meso-level reliefs like mountains, plateaus, plains, lakes, and significant faults. These forces and movements are further categorized into two groups: epeirogenetic movements and orogenetic movements.

Epeirogenetic Movements

Epeirogenetic refers to the origin of continents. These movements cause the uplift and subsidence of continental masses through upward and downward vertical shifts. These movements impact significant portions of continents and are classified into two types: (i) Upward Movement and (ii) Downward Movement.

  • Upward Movement involves two forms: (a) Uplift of entire continents or parts of them, and (b) Uplift of coastal areas. This uplift is known as emergence.
  • Downward Movement occurs in two ways: (i) Subsidence of land areas, referred to as subsidence, and (ii) Submersion of land near coastlines, causing it to sink below sea level and become submerged under seawater. This downward movement is termed submergence.

Orogenetic Movements

The term orogenetic comes from ‘oros’ (mountain) and ‘genesis’ (origin/formation) in Greek. Orogenetic movements result from horizontal endogenetic forces, also known as ‘tangential forces.’ These forces act in two ways: (i) Opposite directions, termed ‘tensional force’ or divergent force, which leads to rupture, cracks, and faults in the Earth’s crust. (ii) Towards each other, termed ‘compressional force’ or convergent force, causing crustal bending that forms folds, or crustal warping causing local elevation or subsidence.

Crustal Bending

When horizontal forces act face-to-face, crustal rocks bend due to the combined effects of compressional and tangential forces. This occurs when crustal sections move toward each other under the influence of convergent horizontal forces. Crustal bending takes two forms: (i) Warping and (ii) Folding.

  • Crustal Warping: This impacts larger crustal areas where parts are either raised (upward warping) or lowered (downward warping). Upward warping occurs due to compressive force from convergent movement, while downward bending forms basin-like depressions.
  • Broad Warping: When upwarping or downward bending affects extensive areas, it’s termed broad warping.
  • Folding: Compressive horizontal or convergent forces cause buckling and compression of crustal rocks, resulting in various fold formations.
Folds

Wave-like bends develop in crustal rocks due to tangential compressive force from horizontal movement driven by deep-seated endogenetic forces. These bends are known as ‘folds,’ with sections bending upwards and downwards. Upward-bent rock layers in an arch-like shape are ‘anticlines,’ while the downward-folded structure that creates a trough-like feature is a ‘syncline’. Folds are minor versions of broad warping. Each fold has two sides known as limbs, with the limb shared between an anticline and its accompanying syncline called the ‘middle limb.’ The plane bisecting the angle between the limbs of an anticline or the middle limb of a syncline is the ‘axis of fold’ or axial plane. These axial planes are referred to as the axis of anticline and axis of syncline based on whether they relate to an anticline or syncline, respectively.

Dip and Strike Explanation: Understanding ‘dip’ and ‘strike’ is crucial for comprehending structural form. ‘Dip’ refers to the inclination of rock beds concerning the horizontal plane. It provides two pieces of information: (i) The direction of maximum downward slope along a bedding plane, and (ii) The angle between this maximum slope and the horizontal plane. Dip direction is measured by its true bearing from north, such as 60°N.E. The angle of dip is gauged with a clinometer. For instance, if a rock bed inclines 60° with respect to the horizontal plane and slopes northward, the dip is expressed as 60°N.

On the other hand, the ‘strike’ of an inclined bed is the direction of any horizontal line along a bedding plane, always at a right angle to the dip (see fig. 9.4), as defined by A. Holmes and D.L. Holmes.

Anticlines: Upwardly folded rock beds are called anticlines. In simple folds, the rock strata of both limbs dip in opposite directions. Sometimes, the folding is so extreme that the anticline’s dip angle becomes pronounced, nearly vertical. Symmetrical anticlines have uniform slopes on both sides, while asymmetrical anticlines have unequal slopes. Anticlines come in two types based on dip angle: gentle anticlines with a dip angle below 40° and steep anticlines ranging between 40° and 90°.

Synclines: Downfolded rock beds due to compressive forces from horizontal tangential forces are known as synclines. These resemble trough-like forms where beds on either side slope towards the middle. In intense folding, a syncline can appear canoe-shaped.

Anticlinorium: Anticlinorium refers to folded structures in mountainous regions with a series of minor anticlines and synclines within an extensive anticline. This occurs when horizontal compressive tangential forces vary irregularly, resulting in differing intensities of compression. Such a structure is also termed a fan fold.

Synclinorium: Synclinorium is a folded structure encompassing a substantial syncline containing numerous minor anticlines and synclines. This structure forms due to irregular folding caused by uneven compressive forces.

Types of Folds

Folds’ characteristics depend on factors like rock type, compressive force nature and intensity, and operation duration. Rock elasticity notably influences folding’s nature and scale. Softer, elastic rocks undergo intense folding, while rigid rocks experience moderate folding. Variations in compressive force intensity lead to differences in fold traits. Typically, simple fold limbs have roughly equal inclinations, but various fold types often exhibit differing limb inclinations. Based on limb inclination, folds are categorized into five types.

Symmetrical Folds: Simple folds with uniformly inclined limbs, often open folds. Rare in the field, they form with regular, moderate compressive forces.

Asymmetrical Folds: Limbs are uneven and irregularly inclined. One limb is larger with moderate, regular slope, while the other has a shorter, steep inclination.

Monoclinal Folds: One limb moderately inclined, other steeply inclined almost vertically due to vertical forces. Can split, leading to faults.

Isoclinal Folds: Compressive forces make both limbs nearly parallel but not horizontal.

Recumbent Folds: Compressive forces make both limbs parallel and horizontal.

Overturned Folds: One limb thrusts over another due to intense compressive forces, rarely with horizontal limbs.

Plunge Folds: Axis of fold tilted, forming a plunge angle between the axis and horizontal plane.

Fan Folds: Extensive folds comprising minor anticlines and synclines, resembling a fan; also called anticlinorium or synclinorium.

Open Folds: Limbs of the fold form an angle between 90° and 180°, due to moderate compressive forces causing wave-like folding.

Closed Folds: Limbs of the fold form an acute angle due to intense compressive forces.

Nappes

Nappes result from complex folding due to intense horizontal movement and compressive force. Both limbs of a recumbent fold are horizontal and parallel. Increased compressive force causes one limb to slide forward over the other in a process known as ‘thrust.’ The plane of this sliding is called the ‘thrust plane.’ The upthrust portion is termed the ‘overthrust fold.’ If compressive force surpasses rock bed elasticity, the limbs break at the fold axis, causing lower rock beds to move upward. The resulting structure becomes reverse to the normal configuration. Due to continued horizontal movement and compressive force, the broken limb of the fold can travel kilometers away, overriding distant rock beds. This creates an unconformity, as the overridden limb alters the structure. The broken fold limb is called a ‘nappe.’

Present folded mountains exhibit several nappe instances, notably studied in the Alps. This complex structure involves one nappe overlaying another. In the Alps, four primary nappe groups have been identified from bottom to top: (i) Helvetic nappe, (ii) Pennine nappe, (iii) Austroalpine nappe, and (iv) Dinaric nappe. These nappes resemble a sequence of earthwaves. Erosion has removed overriding nappes in many areas, revealing the underlying basic structure. When erosion exposes lower nappes due to the removal of the overriding ones, it creates a ‘structural window.’ Several ‘complete windows’ have been observed in the eastern Alps.

Himalayan Nappes: Nappes are also found in the Himalayas. Wadia discovered them in Kashmir Himalaya, Pilgrim in Simla Himalaya, Auden in Garhwal Himalaya, and Heim and Gansser in Kumaun Himalaya. Some key facts about nappe structure are worth noting. When a broken fold limb overrides another fold nearby, it’s an autochthonous nappe. However, if a broken fold limb overrides another fold far away (several kilometers), it’s an exotic nappe.

Crustal Fracture

Rocks displace along a plane due to horizontal or vertical tensional or compressional forces, sometimes both. Fracturing relies on rock strength and tensional force intensity. Rocks crack under moderate tension, while intense tension causes rock beds to dislocate, forming faults. Fractures are generally categorized as (i) joints and (ii) faults. Joints are fractures without significant rock movement, while fractures become faults when substantial displacement occurs parallel to and on both sides of the fracture.

Faults

Faults are fractures in crustal rocks resulting in displacement along a fault plane. Such displacement occurs due to tensional movement from endogenetic forces. The fault plane can be vertical, inclined, horizontal, curved, or any form. Movement-causing faults can be vertical, horizontal, or in any direction. Fault formation involves vertical displacement of rock blocks, sometimes several hundred meters, and horizontal displacement of several kilometers. However, total displacement doesn’t happen at once; it occurs in small increments. Faults are weaker zones where prolonged crustal movements operate. Key terms about ideal faults should be understood before discussing various fault types’ formation modes.

  • (1) Fault Plane: Plane along which rock blocks are displaced due to vertical and horizontal tensional and compressional forces, forming a fault. The fault plane can be of various orientations.
  • (2) Fault Dip: Angle between the fault plane and the horizontal plane.
  • (3) Upthrown Side: Upper block of a fault.
  • (4) Downthrown Side: Lowermost block of a fault. Sometimes, determining which block moved along the fault plane can be challenging.
  • (5) Hanging Wall: Upper wall of a fault.
  • (6) Footwall: Lower wall of a fault.
  • (7) Fault Scarp: Steep slope resulting from faulting of crustal rocks. Sometimes resembles a cliff. When formed due to tectonic forces, they are known as ‘fault-scarps.’ Note that scarps can also form due to erosion.
Types of Faults

Different types of faults result from the direction of motion along the fracture plane. Relative movement or displacement of rock blocks occurs primarily in two directions: either along the dip or along the strike of the fault plane. This leads to categorizing displacement into (a) dip-slip movements and (b) strike-slip movements. Based on displacement direction and movement, faults are classified as (i) dip-slip faults and (ii) strike-slip faults. For dip-slip faults, upper blocks may displace down the dip (normal fault) or up the dip (reverse or thrust fault). In strike-slip faults, relative displacement can be to the right (right-lateral or dextral fault) or left (left-lateral or sinistral fault). Strike-slip faults are also termed as wrench, tear, or transcurrent faults. When normal or reverse faults are combined with wrench faults, they’re referred to as oblique-slip faults.

(i) Normal Faults: Formed by displacement of rock blocks in opposite directions due to fracture caused by greatest stress. The fault plane is typically inclined between 45° and vertical. The resultant steep scarp is called fault-scarp or fault-line scarp and ranges from a few meters to hundreds of meters in height. Actual fault-scarp height is often reduced by denudation.

(ii) Reverse Faults: Result from the movement of fractured rock blocks towards each other. The fault plane is inclined between 0° and 40° to the horizontal. Vertical stress is minimized, and horizontal stress is maximized. Rock beds on the upper side displaced upwards relative to those below, leading to the shortening of faulted area. The compressive force from horizontal movement is involved in their formation. Also known as thrust faults. If the compressive force exceeds rock strength, one block overrides the other, forming an overthrust fault with an almost horizontal fault plane.

(iii) Lateral or Strike-Slip Faults: Occur when rock blocks are horizontally displaced along the fault plane due to horizontal movement. Classified as left-lateral (sinistral) when displacement is to the left on the far side of the fault and right-lateral (dextral) when displacement is to the right on the far side of the fault. Often lack significant scarps, if present, they are usually low in height.

(iv) Step Faults: Arise from a series of faults where the slopes of all fault planes are in the same direction. Displacement of all downthrown blocks must occur downward in the same direction for step fault formation.

Rift Valley and Graben

A rift valley is a major depression resulting from faulting. It’s a trough between crustal parts formed by normal faults due to endogenetic forces. These valleys result from crustal displacement and subsidence between faults. They’re also called ‘graben,’ which means depression. Tensional forces cause the faults, pulling the crust apart. Some differentiate by size, but Graben and rift valley are generally synonymous.

A rift valley can form in two ways: (i) with the middle portion between normal faults dropping downward while the flanking blocks remain stable, or (ii) with the middle portion between normal faults remaining stable while the flanking blocks on either side are raised upward.

Typically, a rift valley is elongated, narrow, and quite deep. The Rhine Rift Valley serves as a prime example, spanning 320 km with an average width between Basal and Bingen. One side is bordered by the Vosges and Hardt mountains, while the other is flanked by the Black Forest and Odenwald mountains. The longest rift valley stretches from the Jordan River through the Red Sea basin to the Zambezi Valley, covering 4,800 km. Some rift valleys are so deep that their floors are below sea level. Death Valley in southern California, USA, exemplifies this, as does the Dead Sea in Asia, with its floor at about 867 m below sea level. The floors of the Jordan Rift Valley and Death Valley are also 433 m below sea level. While the Narmada Valley, the Damodar Valley, and parts of the Son Valley and Tapi Valley are considered rift valleys by some, this perspective remains controversial among geologists.

Rift valleys are not limited to continental crust; they also exist on the ocean floor. The deepest grabens take the form of ocean ridges and trenches. For instance, the Bortlet Trough, south of Cuba, reaches a depth of 4.8 km, while the Java Deep is 6.4 km deep from the sea floor. The central plain of Scotland and Spencer Bay in South Australia are additional examples of rift valleys.

Origin of Rift Valleys

The enigma surrounding the origin of rift valleys and grabens, distinct topographic features resulting from faulting, remains unresolved. While numerous scientists have presented their perspectives on the genesis of rift valleys based on their individual studies, these concepts and theories continue to be contentious. No universally agreed-upon theory has emerged so far. Hypotheses concerning the origin of rift valleys are broadly categorized into two groups: (1) the tensile hypothesis and (2) the compressional hypothesis.

1. Tensional Hypothesis

This hypothesis initially linked rift valleys to the concept of a “dropped keystone of an arch” in building structures. It suggested that rift valleys formed by a process similar to an arch’s keystone falling downward, creating an open space. In geological terms, when tensional forces cause parallel cracks in the crust, pulling apart the side blocks, the middle portion between the cracks drops, forming a rift valley.

However, this “keystone hypothesis” faced criticism due to its flawed ideas. Unlike building arches, the Earth’s crust lacks open space below it, making it improbable for the middle block between normal faults to slip downward. The faulted block would need to displace underlying magma, which contradicts the theory’s volcanic implications. Additionally, rift valleys don’t always correspond with volcanic activity. Observations and experiments indicate that existing volcanic activity often ceases during rift valley formation, suggesting magma pathways might be blocked by faulting, contrary to the theory. The tensional hypothesis is rejected because it cannot explain the cessation of volcanism and the dynamics of magma ascent.

2. Compressional Hypothesis

To address the challenges of the tensional hypothesis, several scientists like Wayland, Baily Willis, Warren D. Smith, and E.C. Bullard proposed the compressional hypothesis. Wayland studied Lake Albert and Ruwenzori, while Baily Willis focused on the Dead Sea, suggesting that rift valleys aren’t created by tension but by compression at greater depths.

In this view, intense compression leads to the uplift of side blocks along thrust faults, forming horsts. These elevated blocks are termed “over thrusting rift blocks.” The middle portion is compelled to move downward due to pressure from the rising side blocks. This descending central portion between faults is called the rift block, narrow at the top and wider below. In essence, the rift block gradually widens downward, creating rift valleys. This formation occurs as the middle block slips downward between rising side blocks, driven by convergent compressional forces causing thrust faulting.

3. Hypothesis of E.C. Bullard

E.C. Bullard introduced a novel hypothesis on rift valley origins in 1933-34 during gravity surveys. He rejected the idea of a rift block slipping downward under gravity like a building’s keystone. Instead, he proposed that rift valleys form solely due to compression from two sides. Bullard’s theory suggests that rift valley formation occurs in sequential phases.

In the First Stage, horizontal compression affects the rigid rock layers of a plateau due to active horizontal movement. Lateral compressive forces come from both sides of the land, leading to crustal rock buckling. As the compression intensifies, so does the buckling and squeezing of the rocks. Eventually, the compression surpasses rock strength, causing a crack to form. This crack widens due to continuous compression.

Second Stage: After the crack forms (at point A in fig. 9.14), one portion overrides the other, a process termed “thrusting.” Simultaneously, the second portion is pushed downward, a phenomenon called “downthrusting.” The upthrusting side block (A-C) elevates by several thousand meters due to overthrusting. This upward movement leads to the development of a new crack (at point B) in the downdropped block (A-D) due to compressive forces, occurring at the highest point of the downdropped block. This newly formed crack continues to expand gradually.

Third Stage: The crack formed at point B in the downdropped block (fig. 9.14) enlarges due to increased compression. As a result, the B-D part of the downdropped block overrides the A-B part, which lies between the two upthrust blocks (A-C and B-D), giving rise to a rift valley. The width of the upper portion of the rift valley is represented by A-B in fig. 9.14.

E.C. Bullard’s theory suggests that the width of the rift valley (A-B) depends on rock elasticity, valley depth, and substrate density. If the substrate density is assumed as 3.3, a 20 km deep valley would have a width of 40 km, while a 40 km deep valley would have a width of 65 km.

In conclusion, neither the tensional hypothesis nor the compressional hypothesis fully addresses the intricate problems related to rift valley origin.

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