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BGYCT-131: Physical and Structural Geology

BGYCT-131: Physical and Structural Geology

IGNOU Solved Assignment Solution for 2023

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Assignment Code: BGYCT-131/TMA/2023

Course Code: BGYCT-131

Assignment Name: Physical and Structural Geology

Year: 2023

Verification Status: Verified by Professor


Part A


1. Write short notes on the following:


a) Applied and allied branches of geology (5)

Ans) The Applied and allied branches of geology are:

  1. Mining Geology: The study of how geology can be applied to mining engineering in order to choose locations that are ideal for quarrying and mining is the focus of this field.

  2. Engineering Geology: The construction of dams, tunnels, mountain roads, building stones, and other road materials are some examples of the geological studies that are covered in this field of study, along with the appropriate solutions for any problems that are discovered during the process.

  3. Hydrogeology or Geohydrology: It is a field that lies between geology and hydrology and is concerned with matters pertaining to ground water.

  4. Rock Mechanics: It is connected to geology since it examines the behaviour of rocks when they are subjected to static and dynamic loads.

  5. Geophysics: The field of study known as geophysics examines how geology and physics interact with one another when applied to surveying and prospecting. The materials for this field of research include the make-up of the Earth itself as well as the nature of the physical forces that are at work on and within the planet.

  6. Geochemistry: It is concerned with the quantity, distribution, and movement of different elements found throughout the Earth.

  7. Biogeochemistry: It deals with the study of geological, chemical, physical and biological processes and reactions that regulate the composition of the natural environment. The atmosphere, the biosphere, the hydrosphere, the lithosphere, and the pedosphere are all involved.

  8. Marine Geology or Geological Oceanography: Investigations of a geophysical, geochemical, sedimentological, and palaeontological nature are carried out within ocean basins and along coastal margins as part of this process.

  9. Geoinformatics: It does this by developing a spatial data and informatics framework, which it then uses, in order to tackle geological issues such as geomorphology, mineral prospecting, structural geology, and other similar issues.


b) Formation of the solar system (5)

Ans) It has several suggestions. Many were modified and discarded. Any explanation about the formation and evolution of the Solar System must account for its Sun, eight planets, one dwarf planet (Pluto), known number of satellites, asteroid belt between Mars and Jupiter, millions of comets and meteorites, and interplanetary dust and gases. The Solar nebula theory explains the Solar System's evolutionary history, most of the planets' and moons' properties, compositional variations between the inner and outer planets, and the asteroid belt. Time altered the universe's chemistry. The universe was 100% hydrogen and helium before it became 98 percent and 2 percent of all other elements by weight. Meteorites reveal the abundances of these heavier elements.


The German philosopher's idea. Kant believes nebulae, revolving clouds of gas and dust, created the Solar System. The Sun's gases and dust-sized particles are chemically identical to Earth's. Particles rotated faster as the diffused cloud shrunk under gravity. When ice skaters pull in their arms, their spin rises. The Universe expanded and cooled, forming stars and galaxies. The vast swirling cloud of gas condensed and flattened to form a disc of gas and dust like the Milky Way Galaxy, where the Sun formed in the centre and eddies generated planets. Gravity pulled matter toward the centre. This drifting caused materials to accumulate in the disc's centre to form the proto-Sun, from which the modern Sun formed. The proto-Sun compressed and heated. Compression raised the proto-interior Sun's temperature to millions of degrees, causing nuclear fusion. This nuclear fusion resembles a hydrogen bomb.


The Solar System's nebula collapsed 5-5.6 billion years ago, forming the core and planets 4.6 billion years later. Mercury, Venus, Earth, and Mars are primarily stony and metallic with little gaseous stuff. Jupiter, Saturn, Uranus, and Neptune are larger and less dense than the inner planets. The outer planets form most of the mass in the planetary system. Planets originated differently depending on their distance from the Sun. Thus, their composition altered with Sun distance. The outer planets are primarily hydrogen, helium, and other light constituents of the original nebula, while the inner planets are dense, stony masses.


2. Give an account of the layering and composition of the Earth. (10)

Ans) Earth’s interior has been identified as comprising three layers on the basis of seismic wave behaviour, physical and chemical properties. The three major layers of the Earth are crust, mantle and core. Crust is the outermost solid layer on the surface of the Earth. It is composed of brittle rock that fractures easily. It varies in thickness and chemical composition depending on whether it is oceanic or continental. They are formed by entirely different geological processes. The Earth’s crust is in the form of a very thin layer of heterogeneous solidified rocks. The crust is about 35 km thick and is composed of heterogeneous rocks. Earth’s crust can be divided into oceanic and continental types.  Seismic waves indicate that the crust is thinner and denser beneath the oceans than on the continents. Earth’s crust is subdivided into two sublayers based on their composition as follows:


SiAl: It is also known as Upper continental crust and consists of all types of rocks exposed at the terrestrial surface. Its density is about 2.7 gm/cm3. Sial comprises felsic rocks of granitic to granodioritic composition. Felsic refers to those igneous rocks having more than 65% of silica content. Silica (Si) is about 70%. This is followed by aluminium (Al) as second abundant constituent and hence named as SiAl.


SiMa: It is 22 km thick and is also known as Lower continental crust. It is a layer that envelopes the entire Earth. Basalt is its typical rock type. Its density is about 3 to 3.4 g/cm3. Silica ranges from 40 to 50 % and magnesium takes the second place. Hence, it is called as SiMa (Si-Silica, Ma-Magnesium). On the basis of its composition, SiMa has been divided into:

  1. Outer SiMa: It extends up to the depth of 19 km and comprises rocks of intermediate composition. Intermediate refers to rocks between 55-65% silica.

  2. Inner SiMa: It is located at the depth beyond 19 km up to the depth of 33 km and comprises rocks of mafic to ultramafic composition. Mafic rocks have silica content between 44-55%. Ultramafic rocks contain less than 44%silica content.


Conrad Discontinuity is located at the depth of 11 km which separates the SiAl layer from underlying SiMa layer.


Mantle lies between crust and core. Its upper surface is about 5 to 10 km below the oceanic crust and about 20 to 80 km below the continental crust. The imaginary line that separates the lithosphere from the mantle is known as ‘Moho’ or Mohorovicic discontinuity.


There are two discontinuities namely, Mohorovicic and Gutenberg. Mohorovicic or Moho discontinuity forms outer limit of mantle. Gutenberg discontinuity forms the inner limit of mantle. Mantle comprises 83% by volume of the Earth and 68% by mass. Its average thickness is about 2865 km and extends from 35 km to 2890 km. The mantle is also heterogeneous in nature, which is indicated by many discontinuities of lower order. It is the source region of most forces responsible for ocean floor spreading, continental drift, orogeny and major earthquakes. Higher seismic wave velocity (8 km/sec) of mantle versus crustal rocks is indicative of denser, ultramafic composition.


Crust and upper mantle together form the lithosphere, extending down to average depth of about 100 km, forming tectonic plates. It averages 70 km thickness beneath oceans and 125 to 250 km in thickness below the continents. Beneath the lithosphere, the speed of seismic wave abruptly decreases in a plastic low velocity zone termed asthenosphere. Asthenosphere is a weak ductile layer of rock that constitutes the lower part of the upper mantle over which the lithospheric plates slide.


Core is the innermost part of the Earth below the mantle. On the basis of the study of earthquake waves, the core has been further divided into outer and inner cores. Core is separated from the mantle by Gutenberg discontinuity.


The core extends from 2890 km to 6371 km. Its upper boundary is marked by Gutenberg discontinuity. Astronomical data, laboratory experiments and seismology contribute to our understanding about core. The composition of the core probably is nickel and iron and hence, it is called as NiFe (Ni = Nickel, Fe = Ferrous i.e. Iron). Core composition is inferred from its calculated density, physical and electromagnetic properties, and composition of meteorites.


The behaviour of P waves that the inner core is solid. S waves which cannot penetrate through liquids indicate that the outer core up to its reach to the mantle, which make the shadow zone. Outer core is in liquid state and Inner Core is in solid state.


Discontinuities: Analysis of deep and shallow earthquakes has shown that at certain depth there is an abrupt change or break in the velocity and other characteristics of earthquake waves, which indicates discontinuity. They may be placed in two groups:


a)Major Discontinuities: They are designated as first order discontinuities.


  1. Mohorovicic or simply, Moho discontinuity separates crust from the mantle. Its depth is variable and ranges from 2 to 10 km beneath the ocean to an average of 35 km beneath the continents. Note here that the Moho may reach up to 70 km beneath mountains, e.g., Tibetan plateau.

  2. Gutenberg discontinuity separates mantle from the core.


b) Minor Discontinuities: They are also known as second order discontinuities.

  1.  Lehmann-Bullen discontinuity is between inner and outer core.

  2. Conrad discontinuity is between upper and lower crust which is among the eight such discontinuities.


3. Explain depositional landforms resulting from aeolian activities with the help of neat well labelled diagrams. (10)

Ans) All things that work because of the wind are called "aeolian." It comes from the Greek word, which means "God of the Wind." Eolian or aeolian erosion is another name for wind erosion. Deflation, wind abrasion, and wind attrition are the processes that do it.

  1. Deflation: Deflation comes from a Latin word that means "to blow away." It happens when loose rock pieces are picked up, moved, and blown from one place to another by the wind.

  2. Abrasion: Rub, grind, polish, and abrade rock surfaces. Grind off their angularity, individual grains may lose weight and shape, becoming rounder. Sand blasting and abrasion are similar. Sand, the most prevalent abrasive, rarely rises above a metre, therefore its impacts are usually negligible.

  3. Attrition: Load sediments wear out from transportation stresses. This shrinks further.


Important landforms developed by aeolian processes.


Deflation Basins: Strong winds deflate sand, creating small depressions of various shapes. These evacuated blowouts or depressions are 1.5 km long and a few metres deep. Some depressions form swamps or oasis. Tree roots can reach groundwater in oases.

Desert Pavement: After deflation of fine-grained particles, larger fragments like pebbles (4–64 mm) or boulders (256–256 mm) that are too massive for the wind to move remain for thousands of years. Deflation eliminates finer particles, leaving lag deposits or desert pavement of pebbles and boulders with a rather smooth surface. Deflation limits aeolian activity. Desert pavement protects soil and sediments against erosion with a coarse gravelly surface


Mushroom Table or Zeugens: Wind abrasion cuts soft rocks faster than hard rocks, creating mushroomtables and ridge and furrow structure. Nearly level ridges with hard rock tops, low dips, and joints parallel the prevailing winds. Zeugens are stretched in the wind's direction.

Yardang: Wind abrasion, dust and sand, and deflation created a stream-lined hill from bedrock or any consolidated or semi-consolidated material. Yardangs are long and narrow, like boat hulls from above. Central Asian deserts have hard rocks over 20 metres tall. They have prolonged leeward tails and are asymmetrical, possibly due to wind direction unpredictability


Pedestal Rocks: They form when heavier particles stay near the ground and only lighter particles rise with the breeze. When they hit homogenous rocks, they variously abrade or undercut the upstanding rock mass so that the rock head or cap balances over a relatively slender neck that can be attacked by moving particles, resulting in a pedestal fan-like landform. Pedestal rocks.

Blow Outs: These are wide, shallow caves in hills that wind erosion has made..

Ventifacts: These structures are made when boulders and rock surfaces are worn down by the wind. Ventifacts are rocks that have been smoothed, etched, grooved, or pitted by sand that has been moved by the wind.


4. What is weathering? Describe its types and he factors affecting weathering. (10)

Ans) Weathering is the natural process of rocks breaking apart and breaking down. It includes things that break, decay, or crumble rocks at or near the surface. When things change in the environment, the process of weathering starts. For instance, when a basin where rocks were deposited is lifted up, the rocks are exposed to a different kind of environment. Because of this change, the rocks may break apart, break up, or break down in order to survive in the new environment.


 Weathering is one of the most important parts of the rock cycle. It is the first step in the process of flattening mountains that were raised by processes that come from the inside. The rock cycle is the repeated movement of rock materials. During the rock cycle, rocks are made, destroyed, and changed by both inside and outside processes of the Earth. Weathering changes the shape and topography of the Earth's surface and changes rocks into sediments and soils.


Weathering is also a set of physical, chemical, and biological processes that break down rocks and minerals in the crust to make sediments, new minerals, soil, and dissolved ions and compounds.


  1. Physical weathering happens when solid rock breaks up into small pieces by physical or mechanical means that do not change the rock's chemical make-up. This process is also called "mechanical weathering."

  2. Chemical weathering happens when the minerals in the rock are changed or broken down by chemicals.

  3. Biological weathering is when living things break up or break down rock through physical and/or chemical processes.


Physical weathering, chemical weathering, and biological weathering may all work together to break down crustal rocks. Chemical weathering happens when fluids that are chemically active interact with the surface. Physical and biological weathering break up rocks into smaller pieces. This increases the amount of vulnerable surface area, which makes chemical weathering work better.


Erosion is the process of weathering, which is followed by the gradual removal of the material that has weathered. Erosion is tied to transportation. Sediment is made by weathering, and erosion moves it around the landscape. So, weathering and the process of getting rid of the weathered material are both included in the word "erosion." Erosion can be thought of as a complex set of interconnected processes in which water, wind, and ice break down rock and then remove the broken pieces. In the next unit, you would learn about these groups. The ground gets ready for erosion when it weathers. Material that has been worn away and carried away from the site is eventually dumped. Landforms can be made on the surface of the Earth by these processes of erosion and deposition.


5 Describe various types and stages of rivers with the help of neat diagrams. (10)

Ans) The types of River Channels are:

  1. Straight Channel: They are rare and are usually found where the hard and soft rocks on the ground have the same structure and are in the same place, making the channel straight. When the stream is moving slowly, the sediment can build up to make a point bar.

  2. Meandering Channel: A valley-floor river has it. They thrive on low gradient slopes with easily eroded sediments. In mature rivers, the outer curve is significant due to relatively open water flow, while the inner side sheds some weight, called slip-off-slope. The transverse section shows a slope from cut-off to slip-off.

  3. Braided Channel: It happens when a stream has more dirt in it than it can easily move. It is the stream that splits into smaller channels that branch out and connect with each other. These smaller channels are separated by islands or sandbars. It is also typical of the mature stage of a river's growth, when both erosion and deposition are happening.


Stages of River

The cycle of erosion caused by rivers is usually called the "fluvial cycle of erosion." The cycle starts on land areas that have just risen above sea level. As the land rises, rivers start to flow and start doing their geological work. There are four different stages in how river systems grow and change.



The stages are like the stages of a person's life:

  1. Initial Stage: It's the baby stage. In this stage, river water follows the Earth's linked depressions like a newborn following its mother. Uplift initially outpaced erosion. High gradients and turbulent streams.

  2. Youth Stage: This stage establishes the river and tributaries. The river's strong gradient and extensive bottom erosion allow it to hit/erode any obstacle. V-shaped valleys form. Rills and gullies dominate the slopes, not major streams. Importantly, headward erosion of tributaries lengthens longitudinal profiles. Canyons, gorges, and waterfalls are common. This stage resembles a youth trying to overcome obstacles. As valley headwaters erode, stream channels lengthen upslope.

  3. Mature Stage: Lateral cutting and valley widening erode this stage. Instead of valley deepening, it dominates. Lateral erosion widens the river channel. Rivers meander, creating ox-bow lakes and floodplains.

  4. Old Stage: The old stage is characterised by further decrease in channel gradient, almost total absence of valley deepening, decrease in the number of tributary streams and flattening of valleys. This stage sees no downcutting or lateral erosion. The eroded material that river was carrying is dropped, in the same way as an old person likes to continue further whatever he has gathered and does not venture for any new activity. The valley becomes nearly flat forming plains and ultimately the river meets lake or sea sluggishly forming a delta. Frequently, the river is over flooded and builds up flood plains on both sides.


Part B


6. Describe elements of a fold. Also discuss classification of folds based on their special properties giving neat well labelled diagrams. (10)

Ans) Fold morphology comprises of components/elements like:


a) Wavelength of Fold: The layered rocks have bumps that look like waves. These bumps are called folds. The length of the wave is the shortest distance between two points that are in the same phase. It can also be thought of as the distance between two points where the curve changes direction. For practical purposes, the distance between two consecutive inflection points is considered to be half the wavelength of the fold.

b) Amplitude of Fold: It is the length of the perpendicular drawn from the fold's hinge point to the line that connects the fold's two next inflection points.

c) Hinge Point: It is the point where the profile of a fold curves the most. The fold's profile is a cross-section or transverse section that goes through the fold's hinge line.

d) Hinge Zone: The most curved part of a fold is not always at a point. Sometimes it is in a place called the hinge zone. A hinge zone is an area where the slope of a folded surface changes over a short distance. A fold closure is another name for this kind of area.

e) Hinge Line: It is where the hinges of a certain bedding plane are. The fold's hinge line is the line where the bed is most curved. In other words, we can say that the hinge line is the line along which the direction of the dip changes. On many folds, the hinge line is also the line along which the maximum amount of curvature occurs.

f) Fold Axis: The fold is made by an imaginary line that moves parallel to itself. "Fold axis" is the same as the fold's hinge line if it is straight and the fold is cylindrical.

g) Inflexion Point: It is a point where the shape of the fold changes but there is no curve. That is, it is the point where a cross-section of a fold shows that an antiform changes into a synform or vice versa.

h) Inflexion Line: This line is made by connecting the points where a folded layer bends. In other words, the inflexion line is the point where an antiform changes into a synform or vice versa.

i) Limb: It is the part of a fold between the point where it bends and where it opens.

j) Axial Surface/ Plane: It is a surface made by joining the fold hinge lines of several beds together. It is called an axial plane when the axial surface is flat.

k) Crest: It is the highest point in the profile part of the fold.

l) Trough: It is the lowest point in the profile of a fold.

m) Crestal Line: This is at the top of the fold and is made by joining the layer's crestal points.

n) Trough Line: This is at the bottom of the fold and is made by joining the trough points of the layer that has been folded.


o) The highest point on the crest line is the point of culmination.

p) On the trough line, depression is the point that is the lowest.


q) Fold closure shows the direction that the limbs come together, which is also called arching or the nose of the fold. How the folded surface curves around the hinge affects the shape of the fold closure. The hinge can be very sharp, and the arms can be mostly straight or have a smoother curve near the fold.


7. Differentiate the following:


a) Contraction and expansion hypotheses of mountain building (5)

Ans) Mountain building is a geological process that occurs when large masses of rock are pushed upwards to form elevated landforms. The Earth's crust is composed of tectonic plates that move and interact with one another, leading to the formation of mountain ranges. The contraction and expansion hypotheses are two theories that attempt to explain the mechanisms behind mountain building.


The contraction hypothesis suggests that mountain building is the result of the cooling and contraction of the Earth's interior. As the Earth cools, the interior contracts, which causes the crust to buckle and fold, leading to the formation of mountain ranges. This theory is supported by observations of folded and faulted rocks in mountainous regions and the fact that many mountain ranges are located near areas of tectonic convergence. The tectonic convergence occurs when two plates collide, and the denser oceanic plate subducts beneath the less dense continental plate, leading to compression and folding of the crust. The Himalayas are an example of mountain building caused by the contraction hypothesis. The collision of the Indian Plate with the Eurasian Plate led to the formation of the Himalayas.


In contrast, the expansion hypothesis suggests that mountain building is the result of the expansion of the Earth's interior. This theory is based on the idea that the Earth's mantle is convicting, and the upwelling of hot material from the mantle causes the crust to rise and form mountains. This theory is supported by observations of volcanic activity and the fact that many mountain ranges are associated with areas of active volcanism. The East African Rift Valley is an example of mountain building caused by the expansion hypothesis. The rift valley is a divergent boundary where the Earth's crust is pulling apart, causing the mantle to rise and form mountains.


Overall, the contraction and expansion hypotheses are two competing theories that offer different explanations for the processes that lead to mountain building. While there is evidence to support both theories, the exact mechanisms that drive mountain building remain an active area of research in geology. Understanding the processes behind mountain building is important for predicting geological hazards such as earthquakes and landslides and for understanding the formation and evolution of the Earth's crust.


b) Measurement of strike and dip of an inclines bed (5)

Ans) In geology, strike and dip are measurements used to describe the orientation of an inclined rock bed relative to the Earth's surface. Strike refers to the compass direction of a horizontal line on the inclined bed, while dip refers to the angle of inclination of the bed relative to a horizontal plane. The measurement of strike and dip is essential for interpreting the geological history and structural features of a region.


To measure the strike and dip of an inclined bed, geologists use a compass and a clinometer. The compass is used to measure the strike, while the clinometer is used to measure the dip. To measure the strike, the geologist places the compass on a horizontal surface such as a rock outcrop or the ground and aligns the compass needle with the direction of the inclined bed. The direction of the needle is then recorded as the strike of the inclined bed.


To measure the dip, the geologist places the clinometer on the inclined bed and aligns it perpendicular to the strike. The clinometer measures the angle of inclination of the bed relative to a horizontal plane. The dip angle is recorded as the angle of inclination of the bed.


In addition to measuring the strike and dip of an individual bed, geologists also use these measurements to determine the orientation of multiple beds and their relationships to one another. By examining the strike and dip of several beds in an area, geologists can construct a geological map that shows the distribution and orientation of the rock layers. This information is essential for understanding the geological history and structural features of a region, such as the formation of mountain ranges, faults, and folds.


In summary, strike and dip are measurements used to describe the orientation of an inclined bed relative to the Earth's surface. To measure the strike and dip, geologists use a compass and a clinometer. The measurement of strike and dip is essential for interpreting the geological history and structural features of a region and constructing geological maps.


8. What is a joint? Explain genetic classification of joints giving suitable diagrams. (10)

Ans) Joints connect two fractured bones and do not move. Blocks may slide millimetres to centimetres perpendicular to the fracture surface. In geology, joints are common. Joints are found in all rocks, whether lava, silt, or heat and pressure. Coal joints are cleat. Joints show how and when fractures occur. Pure and applied science. Fractures and joints are crucial geological features for hydrology, engineering, mining, etc.


Genetic Classification of Joints


  1. Reasons for Origins of Joints

Tectonic stresses produce most fractures.

The tectonic event long before the cracking left some stresses.

It shrank as it cooled.

Downhill rocks or glaciers.


So, the forces that cause development can be either tectonic or not tectonic.


Tectonic Joints: It has been seen that most joints form when the rock is subjected to tectonic stresses that cause deformation at the upper levels of the rock surface where the rock is more brittle. The tectonic joints can also be identified by the following:

  1. Tension Joints: These joints are made by forces that pull them apart or pull them together. Plumose marks are small circular grooves or ridges that can be seen on tension joints.

  2. Shear Joints: They are made by forces that push together. In this case, joints form in pairs that dip in the opposite direction.

3. Extension Joints: Extension joints strike perpendicular to the fold. People used to think "folds" made the building bigger.

4. Release Joints: Release joints are jointing that form in the same direction as the folds. These joints form when the weight on top of the joint is taken away.

5. Feather Joints: Feather joints are a group of small, partially overlapping joints that form near fault zones due to brittle deformation. These joints are a type of extension joint called a pinnate joint.

6. Hybrid Joints: When tension, compression, and rotation are present, hybrid joints are formed. Despite their appearance, these joints can be rotated. En echelon and tensile gash veins are hybrid joints. Deformation rotates pinnate or feather joints into "S" or "Z" shapes.


Non-Tectonic Joints: Joints that are not affected by tectonic forces are easy to spot. The nontectonic causes could be hillside creep, landslides, or even human-made events like impacts, explosions, and so on. They only affect a small area or only a few people.


Columnar Joints: These joints are hexagonal at the surface and columns at depth. Most columns are a few to many metres in diameter and several metres long. non-tectonic columnar joints may follow a pattern. Cracks occur when lava cools on Earth. They are called "primary" or "shrinkage" joints. Cooling shrinks’ joints. Columnar joints make basaltic lava polygonal. Some columns have six sides, others four or five.


Mural Joints: Mural joints are formed when three perpendicular joints meet at the same distance. Mural joints assist quarry granite into cubes.


Sheet Joints: Granite and other plutonic igneous intrusions occur in groups parallel to the ground. Rocks that expand and shrink with the sun may cause them.


9. Give an account of classification of mountains with the help of suitable diagrams. (10)


On the Basis of Location:

  1. Continental Mountains: They're continental. Continental mountains include India's Himalaya, Aravalli, Satpura, Western and Eastern Ghats, Asia's Kunlun, Tien Shan, Altai, Rockies, Appalachians, Alpine Mountain systems, and Russia's Urals.

  2. Oceanic Mountains: They inhabit continental shelves, ocean floors, and mid-ocean ridges. Example: Mid Atlantic Ridge.


On the Basis of Period of Origin:

  1. Precambrian Mountains: Precambrian mountains originated between 3800 and 550 million years ago. These mountains were upheaved, denuded, and metamorphosed. "Residual mountains" remain. Indian Anamalai and Nilgiri mountains and Canadian Laurentian mountains are examples.

  2. Caledonian Mountains: The Caledonian orogeny created the Aravallis and Mahadeo of India and the Appalachians of North America 430–380 million years ago.

  3. Hercynian Mountains: They appeared 350–250 million years ago. Hercynian orogeny formed the Caucasus, Altai, Tien Shan, and Ural Mountains.

  4. Tertairy Or Alpine Mountain: The Alpine orogeny developed the Rockies in North America, the Alpine mountains of Europe, the Atlas mountains of north-western Africa, and the Himalayas of India from 65 million years ago to the present. These rough mountains are the tallest. India's Himalayas are young.


On the Basis of Mode of Origin:


Volcanic Mountains: They are called mountains of accumulation because volcanic material piled up around the eruption zone. Rock materials are molten magma due to high Earth temperatures. Volcanic eruptions release lava, ash, tuffaceous debris, and volcanic gases. The material around the vent rises, becoming a mountain.


Erosional Mountains: They are the last relics of mountain formation. These mountains emerge when external forces and isostatic readjustment erode magmatic intrusion-formed mountains to their current altitudes. Upwarping of the exposed surface creates Dome Mountains. Dome Mountains form when a lot of lava rises from below the Earth's crust but cools and hardens below the surface. Thus, upwarped mountains are domes. Intruded magma generates upwarping and exposes the Dome Mountains as the overlaying material erodes. Known as Erosional Mountains,

Fold Mountains: The most prevalent type of mountains on Earth, they cover thousands of km. Uplifted folded sedimentary strata form Fold Mountains. Horizontal compressional pressures on the massive pile of sedimentary rocks in the oceanic basin for millions of years fold rocks (Fig. 14.6). Earth movements elevate rocks to great heights, creating Fold Mountains. Mountains form from continental plates colliding and folding crust. Diastrophic mountains are folded mountains. Fold Mountains include India's Himalayas and Europe's Alps.


Rift Valley and Block Mountains: Rift valley mountains are fault-block or block mountains. Before discussing these mountains, what is a rift valley? Rift valleys are linear, narrow depressions or faults on Earth caused by crustal extension. Block Mountains and graben are the rift valleys raised and lowered blocks. Block Mountains form when a landmass between two roughly parallel faults rises over its surroundings. Thus, fissures or cracks in the Earth's crust push intervening blocks up or down, creating Block Mountains. Fault-block mountains have sharp fronts and sloping backs. Narmada rift valley, Central Asia.


Residual or Relict Mountains: Weathering and erosion created them. Winds, glaciers, rivers, and oceans erode old mountains and plateaus to a certain height, becoming Residual or Relict Mountains. Due of rock erodibility, differential erosion creates them. Weathering and erosion continue for thousands of years, eroding soft rocks into sand and leaving hard rocks at a much lower height. Monadnocks can resist geological lowering. Rivers erode and level plateaus like Peninsular India's Deccan Plateau more than relict mountains. The Indian remnant mountains include the Anamalai, Nilgiris, Aravallis, and Rajmahal traps.


10. Write short notes on the following:


a) Criteria for recognition of faults in field (5)

Ans) When found in the field, several indicators indicate faulting:


  1. Visible Displacement: Field displacements of veins, dikes, strata, etc. are best evidence of faulting. Small flaws occur. Larger faults have indirect evidence.

  2. Slickensides: Slickensides are polished and striated surfaces along a fault plane caused by friction between two faulted blocks.

  3. Fault Breccia: It is usually found near the fault plane and contains many angular country rock fragments.

  4. Dragging of Strata: In the field, when beds bend along a plane. This plane may be a problem.

  5. Crushing, Shearing and Pulverisation: Due to friction between blocks, faulting fragments or pulverises parent rocks.

  6. Presence of Mylonites: Mylonites are minuscule fault breccias. "Ultramylonites" and "pseudotachylites" are sometimes glassy.

  7. Silicification and Mineralisation: Faults can be good places for solutions and other liquids to move through and build up. If the solution has a lot of quartz in it, the fine-grained quartz may replace the country rock. This is called silicification. Mineralization happens when mineral-rich solutions are used to replace water.

  8. Difference in Sedimentary or Metamorphic Conditions: When sedimentary or metamorphic conditions that are usually found in different places are close to each other, it may be a sign of a fault.

  9. Abrupt Change in Topography: Because of faulting, the land often changes quickly. This can be seen as a valley, a cliff, or triangle-shaped facets on mountains. Large faulting may also be shown by sudden changes in the heights of the mountains.

  10. Abrupt Change in River Profile: A river can change the direction of its flow quickly or make a waterfall. This could be a sign of a mistake!

  11. Spring Line: Often, you can find springs on mountains. If the springs are all in a row, there may be a problem.


But it is important to remember that even though topographic or physiographic criteria are signs of faulting, they are not proof that faulting has happened. There are also other things that could cause these landforms to form.


b) Principles of plate tectonics (5)

Ans) The working principles of plate tectonics:


Lithospheric plates float horizontally above the asthenosphere due to mantle convection currents. Convection currents loop clockwise or counter clockwise from the mantle-core barrier.


Two neighbouring convection current cycles drive the plates toward each other. Plates atop convection currents move in opposite directions.


Mid Oceanic Ridges (MORs) form when two plates separate. New magma fills the area between the two sliding plates along the MOR. The MOR's sides get new ocean floor. Submarine volcanism is constant along MOR. Plate tectonics holds that new lava hardens and expands both plates' crusts. Divergent plate borders, sometimes called constructive or accreting plate barriers, create new crust.


Deep ocean trenches form at plate boundaries. When two oceanic plates collide, one submerges. Subduction is the process through which one plate travels down and the other up. Subduction occurs in the Benioff zone.


The subducting plate melts when oceanic plates move closer together. The crust melts when two plates contact. This makes the convergent plate boundary the destructive or consuming plate boundary. Rocks melt, increasing their volume, making the resultant melt less dense than the surrounding rocks. Because lava is lighter than water, it rises and sometimes reaches the plate above it, creating oceanic volcanic islands.


Continental plates collide without subduction. They collide, fold, and become mountains. Plate tectonics states that the continental component of the plate is the lightest material that cannot sink, thus it stays on top of the Earth. The continent-oceanic plate can sink, but the continental plate cannot.


Plate tectonics states that plates moving away from one other create new crust while plates moving toward each other destroy it. Since Earth is not growing or shrinking, its total surface area remains the same.


Strike-slip faults control plate movement on the spherical Earth. This maintains Earth's sphere. Transform faults cut the Mid Oceanic Ridges (MORs). It disrupts multiple divergent plate boundaries (MORs), but no crust is being produced or destroyed here. Thus, the conservative plate boundary is the transform fault boundary.


Subduction zones and Mid Ridges produce volcanic rock. The plate can also erupt. This is intraplate volcanism. Plates have volcanic hotspots. Most hotspots have a big, heated mantle plume. Convection currents help lower mantle plumes rise slowly. Intraplate volcanism occurred when the Indian Plate crossed the Reunion Hotspot in the Cretaceous. This generated Deccan Volcanism, which affects many Indian states.

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