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BGYCT-135: Petrology

BGYCT-135: Petrology

IGNOU Solved Assignment Solution for 2023

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Assignment Solution

Assignment Code: BGYCT-135/TMA/2023

Course Code: BGYCT-135

Assignment Name: Petrology

Year: 2023

Verification Status: Verified by Professor


Part A


1. Write short notes on the following:


a) Rock cycle (5)

Ans) The rock cycle is the continuous process by which rocks are created, destroyed, and transformed into new types of rocks over millions of years. It is a fundamental concept in geology and describes the dynamic nature of the Earth's crust. The rock cycle is driven by a combination of geological processes, including weathering, erosion, deposition, burial, heat, and pressure. The rock cycle begins with the formation of igneous rocks through the solidification of molten magma or lava. These rocks can be classified as intrusive or extrusive, depending on whether they cool slowly beneath the Earth's surface or rapidly at the surface. Intrusive rocks, such as granite, cool slowly and have large crystals, while extrusive rocks, such as basalt, cool rapidly and have small crystals. Over time, igneous rocks can be weathered and eroded by physical and chemical processes, such as wind, water, and acid rain. The resulting sediment can be transported and deposited in new locations, forming sedimentary rocks. Sedimentary rocks are classified based on their grain size, texture, and composition. Examples include sandstone, shale, and limestone.


Sedimentary rocks can be transformed into metamorphic rocks through heat and pressure. This process, known as metamorphism, can occur when rocks are buried deep within the Earth's crust or when they come into contact with hot magma. Metamorphic rocks can have a range of textures, from fine-grained to coarse-grained, and can be composed of various minerals, including mica, quartz, and feldspar. Examples include marble, slate, and gneiss. Metamorphic rocks can be melted and transformed into new igneous rocks through the process of melting and solidification. Alternatively, rocks can be uplifted and exposed to weathering and erosion, beginning the rock cycle anew. The rock cycle is driven by a complex interplay of geological processes, and it can take millions of years for a rock to complete a full cycle. Understanding the rock cycle is important for geologists, as it provides insight into the history and evolution of the Earth's crust. The rock cycle also has practical applications in fields such as mineral exploration, as different types of rocks can be indicative of different mineral deposits. Overall, the rock cycle is a fundamental concept in geology and plays a critical role in shaping the Earth's surface and subsurface.


b) CIPW classification (5)

Ans) The CIPW (Cation, Ion, and Proportionate Weight) classification is a method used in petrology to calculate the mineral composition of a rock based on its chemical analysis. The method was developed in the early 1900s by Joseph Iddings, a prominent American petrologist, and later refined by several other geologists.


The CIPW classification is based on the principle of chemical stoichiometry, which states that the number of atoms of each element in a chemical compound must be in proportion to the compound's molecular formula. Using this principle, the CIPW method calculates the mineral composition of a rock by balancing the cation and anion charges of the rock's chemical constituents. The CIPW method uses a set of mineral equations to calculate the proportions of minerals present in a rock. These equations are based on the mineralogy of common igneous rock types and take into account the chemical composition of the rock. The equations use oxide weight percentages, which are derived from the chemical analysis of the rock. The CIPW classification produces a list of minerals and their corresponding weight percentages for a given rock. These minerals are classified into different groups, based on their crystal structure and chemical composition. The groups include feldspars, pyroxenes, amphiboles, micas, and oxide minerals.


The CIPW classification has several advantages over other methods of rock classification. One of its primary advantages is its ability to calculate the mineral composition of a rock based solely on its chemical analysis, without the need for petrographic examination. This makes it a valuable tool for petrologists working with poorly preserved or altered rocks. Another advantage of the CIPW classification is its ability to provide insights into the genesis and evolution of igneous rocks. By identifying the minerals present in a rock and their proportions, petrologists can infer the conditions under which the rock was formed and the processes that led to its formation. In conclusion, the CIPW classification is a valuable tool in petrology for calculating the mineral composition of igneous rocks. Its ability to provide insights into the genesis and evolution of rocks makes it a valuable tool for understanding the Earth's geologic history. The CIPW method is widely used in the field of petrology and continues to be refined and improved by geologists around the world.


2. Discuss the different types of structures found in igneous rocks with the help of neat well labelled diagrams. (10)

Ans) Forms of the igneous as follows:


Sill: Sill is an igneous body that matches the bedding and foliation planes of the country rock and lies in the same direction. These sheets of igneous rock broke through the country rock and spread out in the same direction as the bedding or foliation planes.

Dyke: Dike is a piece of igneous rock that is different from the country rock and has a shape that is more or less rectangular. The word "dike" comes from Scotland, where it means "a stone wall." These are big blocks of igneous rock that look like walls and cut through other rocks, which may be igneous, sedimentary, or metamorphic. When magma cools and hardens, it forms dykes that go through cracks or fissures in the rocks.

Laccolith: Laccoliths are intrusive dome-shaped masses of igneous rock that arch up and have a floor that is more or less flat. They are lens-shaped substructures that are made when igneous rocks push up the rocks above them. They only become visible after long, steady erosion. In a typical laccolith, the diameter is only a few times bigger than the thickness..


Bysmalith: Bysmalith is a special kind of laccolith. It is a roughly vertical, cylinder-shaped igneous intrusion that is surrounded by faults that are very steep. It makes a mass that looks like a cylindrical plug. Bysmalith is the name for a body whose roof rises along a circular or curved fault..


Lopolith: The Greek word lopas, which means basins, is where the word lopolith comes from. Lopoliths are roughly the shape of a saucer or a basin and are made of igneous rock that has been broken down and sunk.


Phacolith: Phacoliths are concavo-convex concordant igneous rocks that are found along the crest and trough of folds in the country rocks.


Chonolith: Chonolith is an igneous intrusion whose shape is so strange that it can't be put in the same category as a laccolith, dike, sill, or any other known body..


Volcanic Neck: It is a tall, discordant, igneous monolith mass (body) that looks like a pipe and where lava solidified. Volcanic landforms are made up of cinder cones, which are easy to wear away because they are made of loose volcanic materials. So, the land volcanoes are always getting smaller and smaller because of weathering. Even after most of the cones have been worn down, the rock in a volcanic pipe is usually made of a strong material that can stay standing above the surrounding land. South Africa's diamond-bearing structures are the best-known volcanic pipes. The rocks in these pipes must have come from at least 150 kilometres below the surface, where the pressure is high enough to make diamonds and other high-pressure minerals.

Batholith: The largest intrusive igneous body is the batholith (bathos means depth, lithos means stone). They are long, thin structures that can be up to 100 kilometres wide and hundreds of kilometres long. Batholiths are big chunks of igneous rock that make up the core of a mountain. For example, the Ladakh batholith and the Mt. Abu batholith are both examples of this. Batholiths are always made of granitic (felsic) and intermediate rocks, so they are often called granite batholiths. They are made deep below the surface of the Earth and are only seen when the rocks on top of them are taken away. They do not have a clear floor, but their walls are high.


Stock: It is an influx of plutonic igneous rocks, but it is smaller than a batholith and usually has a cross section that is more or less round or elliptical.


Boss: A boss is a group of plutonic igneous rocks with a round shape on the surface.


3. Explain the chemical and mineralogical composition of magma in detail. (10)

Ans) The chemical and mineralogical composition of magma are:


Chemical Composition of Magma

Magmas are not always made of the same things. Petrologists talk about the chemical make-up of magma in terms of how much of the major oxides it has. Magma is made up of 10 elements: silicon (Si), titanium (Ti), aluminium (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), potassium (K), hydrogen (H), and oxygen (O) (O). Together, they make up more than 99% of the fixed constituent.


All of the elements in magma are in the form of electrically charged ions. The first eight elements listed above are cations, which have positive charges from +1 to +4. Oxygen, on the other hand, is an anion, which has a negative charge of -2. As there are both cations and anions, there is a natural tendency for the different charged ions to bond together to make molecules that are not charged. The most important parts of any magma are SiO2 and Al2O3.


All magmas have silicon and oxygen in them, which come together to form a bond called the silicon-oxygen tetrahedron. In a silicon-oxygen tetrahedron, an atom of silicon is surrounded by four atoms of oxygen, one in each corner of the shape of a tetrahedron. Siliconoxygen tetrahedron is a key part of most silicates in the crust of the Earth. But magmas also have different amounts of other elements like aluminium (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg). Each of these ions also forms bonds with oxygen. The magma is either liquid or almost liquid, so its molecules don't line up in a crystalline lattice but instead are grouped together in clusters or short chains.


Mineralogical Composition of Magma

As magma cools and hardens, the elements (SiO2 and Al2O3)  come together to form two main groups of silicate minerals. The dark silicate minerals, called mafic, have a lot of iron and/or magnesium and not much silica. Dark silicate minerals are usually made up of olivine, pyroxene, amphibole, and biotite mica. The light silicates (felsic) have more silica and fewer ferromagnesium minerals. These minerals include quartz, muscovite mica, and the feldspar group (orthoclase, microcline, and plagioclase). At least 40% of most igneous rocks are made up of feldspars. Felsic minerals have the most silica, while mafic minerals have the least.


4. Discuss the megascopic and microscopic characters of granodiorite with the help of neat well labelled diagrams. (10)

Ans) The megascopic and microscopic characters of granodiorite are:


Megascopic Characters

Granodiorite is a light-colored intrusive felsic rock with medium to coarse grains. Slow cooling created this rock's enormous phaneritic crystal. It contains quartz (10-30%), feldspar, biotite, and amphibole. Na-rich plagioclase makes up more than Kfeldspar in granodiorite. Amphibole, pyroxene, and biotite are mafic minerals.


a) Colour: Granodiorite comes in many different colours, but most of the time it is light colours like grey, orange, pink, and white. Some types of darker granodiorite can also be greyish black.

b) Appearance: It looks like it is holocrystalline, has medium to coarse grains, and is a light colour.

c) Mineral Content: The essential, accessory and secondary minerals are described below:

1) Essential minerals: Na-rich plagioclase, K-feldspar, quartz, biotite, amphibole. Quartz is glassy, hard, cleavage-free, and smokey. Feldspar's colour, cleavages, and appearance dominate granodiorite. Hornblende is prismatic and black/dark-green, while biotite is flaky and black/silver.

2) Accessory minerals: Muscovite, apatite, magnetite, zircon.

3) Secondary minerals: Sericite, chlorite, epidote.

d) Texture: The different mineral parts of the rock have the following appearance, size, and arrangement:

1) Physical Appearance: Granodiorite is made up of crystals, so it has a holocrystalline texture and has big phaneritic crystals.

2) Grain Size: Most of the time, it is a rock with medium to large grains. They can have both equigranular and porphyritic (not-equal-sized grains) textures. The way the parts or fabric are put together: The way the parts are put together is both random and tight.

e) Structure: A lot of granodiorite plutons have this layered structure. Some granodiorites do not have foliations very often.


Microscopic Characters

The granodiorite has phaneritic texture in places where it is thin. Under polarised light, it has many colourless minerals like plagioclase, alkali feldspar, quartz, etc., and a few coloured minerals like biotite and hornblende. Here are some details about the rock at the microscopic level:


Mineralogy: It is mostly plagioclase, alkali, quartz, biotite, and hornblende in varying amounts. The rock's feldspar is about 2/3 plagioclase. Its light-colored felsic minerals predominate over mafic and black minerals including biotite, hornblende, and pyroxene. Thin portion shows some muscovite and pyroxene. Quartz is anhedral and fills mineral interstices, hiding its crystal form. Quartz exhibits undulose/wavy extinction. Lamellar twining under cross nicols characterises plagioclase. These rocks have green hornblende with two sets of cleavages and pleochroism and brown biotite with one set. Muscovite, zircon, titanite (sphene), apatite, tourmaline, magnetite, etc., are accessory minerals. Feldspars have altered to sericite, whereas hornblende and biotite may become secondary chlorite. Granodiorite occasionally contains epidote.


Texture: The shape and size of the crystals are as follows:

  1. Crystallinity: Granitic granodiorite. Quartz, biotite, and feldspar are anhedral. Some thin granodiorites have euhedral grains of hornblende, zircon, sphene, and plagioclase. Granodiorite is hypomorphic.

  2. Granularity: Medium-to-coarse rock dominates. It is normally granular with granules of roughly identical size, but porphyritic texture is also common. Feldspar phenocrysts surrounded by smaller quartz, feldspar, and biotite in porphyritic structure. Some granodiorite has zoned feldspar.

  3. Mutual Arrangement of the Constituents/Fabric: Granodiorite often has intergrowth. Granodiorite has graphic and myrmekitic textures like granite.


Rock Types: The following rock type is observed:

  1. Tonalite: It is coarse-grained with fewer quartz and feldspars. They are mostly Na-rich plagioclase with minimal alkali feldspar.


Classification: Granodiorite is a type of igneous rock called a felsic plutonic rock.


Occurrence: Intrusive granodiorite. Subduction zones and collisional settings compose it. Small, shallow intrusive bodies of andesitic to dacitic volcanoes also have it. Hard, crystalline, and homogeneous rock.


5 Explain Bowen’s reaction series. (10)

Ans) Petrographic studies have identified large-scale igneous rock types. Field studies supported petrography's critical approach. Petrogenesis examines igneous rocks' genetics. Petrologists grew interested in genetic features of rocks, which led them to consider "evolution of igneous rocks". Petrogenetic petrology replaced petrography. In 1922, Dr. N. L. Bowen introduced the Bowen reaction series or principle, which established essential notions about magmas, their evolution, and genesis. Bowen's reaction series or principle describes this crystallisation sequence. In this series, minerals are organised to react with magmatic fluid to form the mineral below. Bowen's reaction series shows how magma can form one or more rock kinds.


Parallel Bowen reaction series. Discontinuous sequence on the left shows how ferromagnesian minerals change with temperature. The continuous sequence on the right show’s plagioclase minerals cooling and crystallising with decreasing temperature. Discontinuous and continuous series mix to generate a discontinuous reaction series. The continuous reaction series forms the next plagioclase mineral by reacting with magma. Instead, the discontinuous reaction series forms new minerals when magma combines with previously created minerals at a certain temperature. Bowen's reaction series minerals crystallise in a generalised order from cooling basaltic magma. The melt's composition can alter unless early-formed crystals are removed. After early crystals are removed, magma reacts with them to make new minerals. Bowen's reaction series operates at 1100–573°C. At 1100° C, spinel minerals crystallise, followed by silicate minerals. Spinel (MgAl2O4) is a cubic mineral having octahedral crystals.

Discontinuous Reaction Series

Mg-rich olivine crystallises first in the discontinuous reaction series, followed by Fe-rich. Olivine reacts with magma to make the next mineral unless it is removed from the crystallisation site. As temperature drops, Mg-rich olivine becomes Fe-olivine (mineral fayalite). This continues until all olivine is transformed into Mg-pyroxene under optimal physico-chemical conditions. As magma crystallises, solid and liquid phases maintain equilibrium. Thus, early-formed crystals react with the liquid and change composition to preserve balance. Reaction pairs are the series' two minerals. With dropping temperature, Mg-pyroxene reacts with the residual magma to generate Fe-pyroxene, hornblende, and biotite.


Continuous Reaction Series

Bowen's reaction series continues. Plagioclase minerals crystallise with olivine or shortly after. The earliest plagioclase crystals are calcium-rich, but as reaction continues and temperature drops, they become more sodic. Zoned plagioclases with a calcic core and soda-rich zones demonstrate this. Olivine, pyroxene, and quartz may crystallise from olivine, melt/magma, and silica-enriched liquid. Released minerals include quartz. Doliomorphic rocks release minerals. Thus, doliomorphic rocks contain released minerals from incomplete reaction or early minerals that evaded reactivity.


Importance of Bowen’s Reaction Series

The Bowen reaction series explains its importance.

  1. It differentiates and diversifies magma.

  2. It shows that lava can harden into one or many rock kinds. Depending on fractionation and removal of early produced minerals from further reactivity with the melt, initial basaltic magma may crystallise as gabbro, diorite, granodiorite, or granite.

  3. From olivine to quartz, silicate structures get increasingly complex.

  4. The Bowen's reaction series' early crystals are denser (rich in Mg and Fe) than the late ones.


Part B


6. Explain the processes that are involved in the formation of sedimentary rocks. (10)

Ans) The processes that are involved in the formation of sedimentary rocks are:


Weathering: Rocks that are exposed to the air and water are always changing because of the different ways they break down physically and chemically. This is called "weathering." It happens when it rains, when the temperature changes, when it freezes, when plants grow, when animals and people do things, and when chemicals break down minerals. During weathering, rocks change in both how they look and how they behave. The physical and chemical processes of weathering are different, but they work together to break rocks and minerals apart and turn them into smaller pieces. Rocks can be broken down by physical, chemical, and biological forces.


Erosion: Erosion is the process by which natural forces like moving water, wind, and glaciers break up rocks and move them away at the same time. Streams use their water flow to break up and pick up pieces of rock and sediment from solid rock beds. Depending on how fast it moves, wind erodes and moves loose sand, silt, and clay-sized particles over short or long distances. In the same way, glaciers move pieces of rock and sediment to lower slopes.


Transportation: Sediment transport happens in natural systems where the particles are clastic rocks, like sand, gravel, boulders, mud, or clay. Transportation is the movement of solid particles or dissolved ions from where they come from or where they are being eroded to where they will settle. Moving water, wind, and glacial ice can all be used to move things. Most of the time, water is used to move sediments. Streams, rivers, and the wind are better at moving things that are lighter and smaller than they are at moving things that are heavier and bigger. The particles move along the sloped surface where they are resting because of the force of gravity.


Deposition: Sediments fall to the ground when there is no longer enough energy to move the particles. If the medium being moved has too slow of a speed, the deposition will happen. Mineral grains slowly settle, which causes sediments to fall to the ground. These sediments build up in layers in a basin, with the oldest layers at the bottom and the newest layers on top of the older ones. Some layers or beds can be horizontal and flat, while others can be cross-bedded or have different heights. So, the energy level of the environment where the deposit was made shows up in the deposit. Changes in the basin of deposition can also cause chemical precipitation and evaporation, which can lead to deposition. So, the type of sedimentary rock depends not only on the amount of sediment, but also on how it was deposited in the basin where it formed.


7. Discuss the effects of packing of grains on the porosity and permeability of rocks. (10)

Ans) Packing is how the grains in a sedimentary rock are arranged, spread out, and packed together. It depends on the size and shape of the grains, how they are sorted, and how packed the sediment is. How the rocks' grains are packed together affects how dense, porous, and permeable they are. Permeability is the way that a fluid can get through something. Porosity is the amount of empty space between the grains. Depending on the history of sedimentation and diagenetics, there are two types of packing: primary and secondary. Primary packing happens when sediments settle, while secondary packing is caused by things that happen after sediments have settled, like when sediments on top of them press down on them.


Depending on how the rock's grains and matrix fit together, packing can be:

  1. Grain or Clast Supported: The main thing that makes up the rock is grains. There is not much matrix between the grains. Beach sand deposits and flood sediments from a stream.

  2. Matrix Supported: The main part of the rock is the matrix, and the grains float in the matrix. Example: mudflow deposits. Most rocks that are held together by a matrix are not well organised.


During lithification, the forces of compaction bring the grains closer together and cause changes in how the grains touch each other. The way the grains touch can be:

  1. Tangential: grains that are close to each other.

  2. Concavo-convex: grains going through each other.

  3. Sutured: Interpenetration of grains by grains.

  4. Long: right to the point.


Under a petrological microscope, we can see these contacts in sandstone that has hardened a lot. Packing grains can be done by order or by chance. Sediments can be packed loosely or tightly. When sediments are well-sorted and not too packed together, the packing is as loose as when spheres are packed in a cube. When sediments are packed down, they are packed as tightly as spheres are when they are in a rhombohedral shape. Increasing the packing density makes rocks less porous and less permeable.


Importance of Packing

  1. Studying how rocks are packed helps us figure out how they were made. For example, clast-supported gravels show a typical stream bed or beach deposit, while matrix-supported gravels are usually made by mudflow.

  2. In imbricated conglomerates, the long axes of the clasts often point upstream and are not parallel to each other. Pebbles, sand grains, mica flakes, some fossils, and other things can be used to figure out what a fabric is made of.

  3. This can be used to figure out the direction of paleocurrents in rocks from long ago. Palaeocurrent means the current that was there when sediment was being laid down at some point in geological history.

  4. The way the rock is packed affects how porous and permeable it is, which is important to know when studying reservoir rock for oil, gas, and groundwater.

  5. It changes how strong an aggregate is when it is sheared or put under a vertical load.


8. Discuss in brief the factors affecting metamorphism. (10)

Ans) The factors affecting metamorphism as follows:


Temperature: Temperature usually accelerates metamorphism. It heats chemical reactions that re-crystallize minerals or produce new minerals. Heat for metamorphism originates from:

  1. The heat comes from the Earth's deep interior and is a leftover from when it grew more than 4.5 billion years ago.

  2. When radioactive elements break down, they give off heat.

  3. Frictional heat is made along fault lines or shear zones. This heat is local and only affects parts of the Earth close to the surface.

  4. Latent heat from igneous intrusions and magma crystallising.


Geothermal: Gradient is the Earth's temperature rise as you travel deeper. The temperature rises 20–30°C each kilometre as you descend into the Earth's crust. Deeper beneath the Earth, rocks formed at the surface get hotter. Clay minerals will become unstable in a rock buried 8 km deep at 200 °C. They will re-crystallize into stable minerals like chlorite and muscovite. Iron- and magnesium-rich silicate minerals alter structure to form mica-like chlorite. Continental geotherm is substantially lower than oceanic crust at 10–40 km deep, where the intermediate to lower continental crust is located.

Load Pressure: Pressure or tension on the rock induces metamorphism, which alters its chemical composition, mineralogy, and texture. The type of metamorphic rock formed relies on temperature, parent rock chemistry, chemically active fluids, and pressure. Rock pressure fluctuates along the vertical space coordinate (z) from the Earth's surface to its centre (it is assumed that pressure surfaces are parallel to the surface of the Earth). Thus, the pressure on a volume of rock at a depth (h) depends on the acceleration due to gravity (g) and the average density of the material above it. The load pressure (Pload), lithostatic pressure (Plitho), total pressure (Ptotal), or solid pressure (Psolid) is the load on a volume of rock or the lithostratigraphic column. At high depths, load pressure makes minerals denser and anhydrous (without water).


Fluid Pressure: Intruding magma usually adds metallic cations, dissolved chemicals, ions, and volatiles to meteoric, ground, or seawater. These fluids containing compounds help metamorphism. Water, gases, and ions accelerate chemical reactions by dissolving ions and forming new minerals. The geothermal gradient suggests that T and Pload rise simultaneously as you go deeper. Pressing sediments along continent margins and at subduction zones releases massive amounts of watery fluids. Pressing sediments releases most of the water in the pores and hydrous minerals such kaolinite, illite, and montmorillonite.


Shear Stress: When a solid rock at depth is under a load pressure (Pload), it is said to be in hydrostatic equilibrium, which means that it is being affected by the same amount of force in every direction. This stress, which is caused by an outside force, is isotropic, which means it is the same in all directions. It is shown by a sphere with a rock system in the middle. But if the force acts in a certain direction, like when plate tectonics is at work and a lithospheric plate is coming closer together, the stress is no longer isotropic. This system of stress is now shown by a triaxial stress ellipsoid with the maximum stress at σ1, the minimum stress at σ3, and the middle stress at σ2. This shearing stress, also called deviatoric stress, is at its highest in planes that are 45 degrees from either the main stress σ1 or the minimum stress σ3.


9. Write short notes on the following:


a) Grain size parameters (5)

Ans) Sediment grain-size distribution is often defined by the following factors.

  1. Mode: It is the particle size or size class that shows up most often in the size distribution. It is the highest point on the frequency curve, also called the peak. If a curve has only one dominant peak, it is said to be unimodal. If the curve has two peaks, it is said to be bimodal, and if it has more than two peaks, it is said to be polymodal.

  2. Median: Median is the size of the grain that is in the middle of the range of sizes. This means that, by weight, half of the grains are bigger, and the other half are smaller. The median is the point on the cumulative curve where the 50 percent line meets it.

  3. Mean: It is the sum of all the sizes of particles in a sample. The true arithmetic mean of most samples cannot be found because each small grain can't be counted or measured to find out how many are in the sample. But the approximate arithmetic mean can be found by picking the 16th, 50th, and 84th percentile values from the cumulative curve.

  4. Sorting: Sorting is the most useful part of how the grain sizes are spread out. It is a way to measure how different the sizes of the rocks' grains are. Sorting basically shows the sedimentary process and the effects of what happened after the sediment was laid down. When the grains are mostly the same size and shape, the rock is said to be well sorted. When the rocks have grains of different sizes, this is seen as a sign that they were not sorted well. There are three different ways to sort sediments:

a) Well Sorted: In the rock, all of the clasts are about the same size. Sand from dunes and beaches is often well-sorted.

b) Moderately sorted: As in river and tidal current deposits, the sizes of the grains vary, but not by a small amount.

c) Poorly Sorted: Grain sizes are all over the place. Most deposits of glacial till, debris flow, and mudflow are not well organised.


b) Graded Bedding (5)

Ans) Graded bedding is a sedimentary rock structure that forms as a result of the layering of sediments of different sizes and densities. This geological feature is commonly found in environments where there are large volumes of sediment being deposited in water, such as river deltas, alluvial fans, and deep-sea fans. The process of graded bedding begins when a large amount of sediment is transported by water and deposited in layers. The largest and heaviest sediment particles, such as gravel and sand, settle to the bottom first, forming a layer of coarse sediment. As the water slows down, smaller particles such as silt and clay settle on top of the coarser sediment layer. This results in a layering effect where the sediment is sorted by size and density, with the largest and heaviest particles at the bottom and the smallest and lightest particles at the top.


Graded bedding has important implications for interpreting the history of the Earth's surface. The layers of sediment can reveal the type of environment in which they were deposited, the intensity of the currents and waves that transported them, and the source of the sediments. For example, a layer of sandstone with graded bedding may indicate that it was deposited by a river delta or a beach environment, whereas a layer of shale with graded bedding may indicate a deep-sea fan or a turbidite deposit.  Graded bedding is also important for the construction industry, as it can affect the stability and durability of buildings and infrastructure. For example, buildings constructed on top of graded bedding may experience differential settling, which can lead to structural damage over time.


c) Palimpsest Texture (5)

Ans) It is sometimes possible to see the original texture of the rock. This is called palimpsest texture. We use the word "blastic" or "blast" as a suffix to describe metamorphic textures that look like igneous ones. Palimpsest texture is also called "relict texture" because it is a texture that has survived metamorphism and still shows the original textures of protolith rocks. Low-grade metamorphic rocks keep relict textures well because they do not change too much. Textures like basto-ophitic, blastoporphyritic, and blasto-intergranular are examples of palimpsest (relict) textures.

Blasto-ophitic Texture: Laths of plagioclase can be seen in a pyroxene matrix (sometimes olivine). Plagioclase laths can be completely surrounded by the matrix (ophitic), or they can be partially surrounded (sub-ophitic). If this type of microscopic texture is still there in a metamorphic rock after it has been changed, it is called a relict texture.


Blasto-intergranular Texture: The igneous relict textures in metamorphic rocks that show that ferromagnesium minerals have moved into the spaces between plagioclase crystals (such as pyroxene, olivine etc.) Most of the time, the spaces between crystals are angular and form between two large crystals.


Blasto-porphyritic Texture: The fine-grained matrix or glassy groundmass holds the larger grains or crystals in place. They are more likely to be found in extrusive igneous rocks, but they can also be found in rocks with medium to coarse grains. When they are found in a metamorphic rock, it is thought that they are left over from the texture of the parent rock.


Blasto-cumulate Texture: High-density crystals that formed early on and settled down give cumulate texture its name. This texture is still there in metamorphic rocks, where it is called relict texture.


d) Foliation and lineation (5)

Ans) Foliation is a type of texture found in metamorphic rocks in which mineral grains are arranged in a certain way. Foliated metamorphic rocks have aligned platy minerals or layers of light (felsic) and dark (mafic) minerals that change places. Foliation is the way that metamorphic rocks are layered. It comes from the Latin word for "leaves," which is "folia." Penetrative surfaces that are nearly or completely parallel and make up planar fabric elements are called foliation, and penetrative sets made up of parallel or nearly parallel lines that make up linear fabric elements are called lineation. Penetrative structures can be found almost everywhere in the rock, even at the microscopic level. However, structures penetrative in one domain may not be penetrative in other domain/s.


Foliation is a feature that is flat, while lineation is a feature that is long and thin. Foliations could be seen as lines on all sides of the rocks, and lineations could be seen as round or irregular dots or specks on at least one side of the rock. Foliations show that planar and linear features form in a plane that is perpendicular to the direction of the maximum principal stress on flaky minerals (mica and chlorite) that are lined up in parallel. The tendency of a rock to break along certain surfaces is called cleavage. Foliations are the same thing as cleavages, and the two words are often used to talk about the same structure. Foliation is a broader term than cleavage because it includes flat geometric features that might not cause a cleavage.

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