IGNOU BGYCT 135 Solved Free Assignment 2023-Helpfirst

BGYCT 135

PETROLOGY

BGYCT 135 Solved Free Assignment 2023

BGYCT 135 Solved Free Assignment

Write short notes on the following:

a) Rock Cycle

Ans. The rock cycle is a continuous process that describes the transformation of different types of rocks through various geological processes. The three main types of rocks are igneous, sedimentary, and metamorphic.

Igneous rocks are formed from cooled and solidified magma or lava. Sedimentary rocks are formed from the accumulation and compression of sediment, such as sand, gravel, and mud. BGYCT 135 Solved Free Assignment

Metamorphic rocks are formed from the alteration of existing rocks due to heat, pressure, and chemical processes.

The rock cycle also includes the erosion and weathering of existing rocks, which can lead to the formation of new sedimentary rocks.

The cycle is constantly ongoing, with rocks being transformed from one type to another through natural processes over time.

b) CIPW classification

Ans. The CIPW (Cross-Indexed Point-Counting) classification is a system used to identify and classify igneous rocks based on their mineral content and texture.

The classification is based on the relative proportions of the rock’s minerals, including quartz, feldspar, and mafic minerals.

The system uses point counting, a method of counting the number of points of each mineral on a thin section of the rock, to determine the relative proportions of the minerals. BGYCT 135 Solved Free Assignment

The CIPW classification is based on the principle that the minerals present in a rock are indicative of the conditions under which the rock formed.

For example, rocks containing high proportions of feldspar are thought to have formed at shallower depths, while rocks containing high proportions of mafic minerals are thought to have formed at deeper depths.

The CIPW classification system is widely used in petrography and geology, it helps to understand the petrogenetic processes and geologic history of an area by analyzing the mineralogy and textures of rocks.

BGYCT 135 Solved Free Assignment
BGYCT 135 Solved Free Assignment 2023

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

Ans. Igneous rocks can have several different types of structures based on the way they cooled and solidified. The most common types of structures found in igneous rocks are: BGYCT 135 Solved Free Assignment

Phaneritic: This type of structure is characterized by large visible crystals that can be easily seen with the naked eye. The crystals form as the magma or lava cools slowly, allowing ample time for large crystals to grow.

This type of structure is common in intrusive igneous rocks, such as granite.

Aphanitic: This type of structure is characterized by small, fine-grained crystals that cannot be easily seen with the naked eye.

The crystals form as the magma or lava cools quickly, not giving enough time for large crystals to grow. This type of structure is common in extrusive igneous rocks, such as basalt. BGYCT 135 Solved Free Assignment

Porphyritic: This type of structure is characterized by a combination of large visible crystals (phenocrysts) and small, fine-grained crystals (groundmass).

The phenocrysts form as the magma or lava cools slowly, while the groundmass forms as the magma or lava cools quickly. This type of structure is common in intermediate rocks, such as andesite.

Glassy: This type of structure is characterized by a smooth, shiny surface and the absence of visible crystals.

The magma or lava cools so quickly that no crystals have time to form. This type of structure is common in extrusive volcanic rocks, such as obsidian.

Vesicular: This type of structure is characterized by the presence of small air pockets or vesicles within the rock. BGYCT 135 Solved Free Assignment

The vesicles form as trapped gases escape from the magma or lava as it solidifies. This type of structure is common in extrusive volcanic rocks, such as scoria.

Q 3. Explain the chemical and mineralogical composition of magma in detail.

Ans. Magma is a molten or partially molten mixture of rock-forming minerals, crystals, and gases. BGYCT 135 Solved Free Assignment

It is composed mainly of silicon dioxide (SiO2), aluminum oxide (Al2O3), and various metal oxides such as iron oxide (FeO), magnesium oxide (MgO), and calcium oxide (CaO).

Silicon dioxide (SiO2) is the most abundant mineral in magma, typically making up between 45 and 75 percent of the total. It is a key component in the formation of minerals such as quartz and feldspar.

Aluminum oxide (Al2O3) is typically present in magma in smaller quantities, typically making up between 2 and 10 percent of the total. It is a key component in the formation of minerals such as corundum and bauxite.

Magnesium oxide (MgO) and iron oxide (FeO) are also typically present in magma in small quantities, typically making up between 1 and 10 percent of the total.

These elements are important in the formation of minerals such as olivine and pyroxene. BGYCT 135 Solved Free Assignment

Calcium oxide (CaO) is also typically present in magma in small quantities, typically making up between 0.5 and 5 percent of the total. It is a key component in the formation of minerals such as calcite and limestone.

Magma also contains various dissolved gases such as water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). These gases can play an important role in the formation of minerals and the eruption of magma.

In addition to these major elements, magma also contains smaller amounts of other elements such as potassium (K), sodium (Na), and titanium (Ti).

These elements are important in the formation of minerals such as orthoclase feldspar and ilmenite. BGYCT 135 Solved Free Assignment

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

Ans. Granodiorite is a type of intrusive igneous rock that is characterized by a medium- to coarse-grained texture and a composition that is intermediate between granite and diorite.

It typically contains a high percentage of quartz and feldspar, with lesser amounts of mafic minerals such as hornblende and biotite.

Megascopic characters refer to the visible features of the rock that can be observed with the naked eye. These include:

Texture: Granodiorite has a medium- to coarse-grained texture, with visible crystals that can range in size from a few millimeters to several centimeters.

The crystals are typically equigranular, meaning that they are roughly the same size and shape. BGYCT 135 Solved Free Assignment

Color: Granodiorite can have a wide range of colors, including gray, pink, or black. The color of the rock can be used as an indicator of the mineral content and the conditions under which the rock formed.

Luster: Granodiorite typically has a dull to waxy luster.

Fracture: Granodiorite has a conchoidal fracture, which means that it breaks in smooth curved surfaces.

Microscopic characters refer to the features of the rock that can only be observed with the aid of a microscope. These include:

Mineralogy: Under a microscope, granodiorite is composed mainly of quartz and feldspar, with lesser amounts of mafic minerals such as hornblende and biotite.

The relative proportions of these minerals can vary depending on the specific rock.

Crystal size: The size of crystals in granodiorite can be observed under microscope, they are typically between 0.1 mm and 1 mm.

Zoning: Some granodiorite samples may show zoning, which refers to variations in mineral composition and/or crystal size within a single sample.

Q 5 Explain Bowen’s reaction series.

Ans. Bowen’s reaction series is a theoretical model developed by the American geologist N. L. Bowen in the early 20th century, which describes the sequence of mineral crystallization that occurs as magma cools.

The model proposes that as magma cools, different minerals will crystallize out in a specific order, based on the temperature, pressure, and chemical composition of the magma. BGYCT 135 Solved Free Assignment

The reaction series is divided into two main categories: the continuous series, which includes minerals such as olivine, pyroxene, and amphibole, and the discontinuous series, which includes minerals such as plagioclase feldspar, quartz, and biotite.

The continuous series describes the sequence of mineral crystallization that occurs as magma cools and solidifies. It starts with the formation of the high-temperature mineral olivine, which is rich in magnesium and iron.

As the temperature decreases, minerals such as pyroxene (which is rich in magnesium, iron, and silica) and amphibole (which is rich in magnesium, iron, silica, and aluminum) will form.

The discontinuous series describes the sequence of mineral crystallization that occurs as magma cools and solidifies.

It starts with the formation of the low-temperature mineral plagioclase feldspar, which is rich in sodium and calcium. As the temperature decreases further, minerals such as quartz and biotite will form.

It’s important to note that Bowen’s reaction series is a theoretical model, and in nature the cooling of magma and the formation of minerals is a complex process that may not always occur in the exact order proposed by the model.

Additionally, it’s influenced by the chemical composition of magma, pressure, and the presence of other minerals. BGYCT 135 Solved Free Assignment

The Bowen’s reaction series is widely used in geology to understand the petrogenetic process of rocks and minerals, it helps to understand the conditions under which a rock was formed, and how it has evolved over time.

BGYCT 135 Solved Free Assignment 2023
BGYCT 135 Solved Free Assignment 2023

Q 6 Explain the processes that are involved in the formation of sedimentary rocks.

Ans. Sedimentary rocks form through a process known as sedimentation, which involves the accumulation and compression of sediment, such as sand, gravel, mud, and other materials.

The process of sedimentation can be divided into several main stages:

Weathering and erosion: The first stage in the formation of sedimentary rocks is the weathering and erosion of existing rocks.

Weathering is the physical and chemical breakdown of rocks due to exposure to the elements, such as rain, wind, and temperature changes.

Erosion is the movement of rock and soil by the action of water, wind, or ice. Together, weathering and erosion break down existing rocks into smaller pieces, known as sediment. BGYCT 135 Solved Free Assignment

Transport: The second stage in the formation of sedimentary rocks is the transport of sediment from its source to a new location.

Sediment can be transported by a variety of agents, including water, wind, and ice. The mode of transport will determine the characteristics of the sediment.

Deposition: The third stage in the formation of sedimentary rocks is the deposition of sediment in a new location.

Deposition occurs when the energy of the transporting agent decreases, allowing the sediment to settle out and accumulate in a new location.

This can happen in a variety of environments, such as river channels, deltas, beaches, and ocean floors.

Compaction and cementation: The fourth and final stage in the formation of sedimentary rocks is the compaction and cementation of the deposited sediment.

Compaction occurs as the weight of overlying sediment causes the sediment grains to become closer together.

Cementation occurs as mineral-rich fluids fill the spaces between the sediment grains, binding them together and forming a solid rock.

Sedimentary rocks formed by these processes include clastic rocks (formed from broken fragments of rocks) such as sandstone, siltstone and shale, chemical rocks (formed from precipitation of mineral crystals) such as limestone, dolomite, and evaporites, and organic rocks (formed from accumulation of plant or animal debris) such as coal and limestone.

It’s important to note that the formation of sedimentary rocks is a gradual process that can take millions of years to complete.

The process is also influenced by various factors such as the climate, tectonics and the erosion of the source rocks. BGYCT 135 Solved Free Assignment

Q 7 Discuss the effects of packing of grains on the porosity and permeability of rocks.

Ans. Porosity and permeability are two important properties of rocks that determine their ability to store and transmit fluids.

Porosity is a measure of the amount of void space within a rock, and is typically expressed as a percentage of the total volume of the rock.

Porosity is affected by the packing of grains within a rock. Closely packed grains will result in a lower porosity, while loosely packed grains will result in a higher porosity. BGYCT 135 Solved Free Assignment

Permeability is a measure of the ability of a rock to transmit fluids, and is typically measured in units of flow rate per unit area.

Permeability is also affected by the packing of grains within a rock. Closely packed grains will result in a lower permeability, while loosely packed grains will result in a higher permeability.

In general, sedimentary rocks tend to have higher porosity and permeability than igneous or metamorphic rocks because their grains are more loosely packed.

The porosity and permeability of a rock can also be affected by the presence of fractures, which can provide additional pathways for fluids to flow through.

However, the porosity and permeability of a rock can be affected by different factors, for example, a rock that is composed of large grains with a lot of space between them (high porosity) may not necessarily have a high permeability if the space between grains are not interconnected, the fluid will not be able to flow through.

On the other hand, a rock composed of small grains that are closely packed together (low porosity) may have high permeability if the grains are well interconnected.

It’s important to note that rocks with high porosity and high permeability are considered as good reservoirs for oil and gas, while rocks with low porosity and low permeability are considered as poor reservoirs.

Understanding the effects of packing of grains on the porosity and permeability of rocks can help geologists to evaluate the potential of a rock unit as a reservoir.

Q 8 Discuss in brief the factors affecting metamorphism.

Ans Metamorphism is the process by which existing rocks are altered by heat, pressure, and chemical processes. There are several factors that can affect the rate and style of metamorphism, including:

Temperature: Temperature is one of the most important factors affecting metamorphism. As the temperature of a rock increases, the rate of chemical reactions within the rock also increases, leading to faster rates of metamorphism.

Temperature can be caused by heat generated by the Earth’s interior, by intrusion of magma, or by tectonic activity. BGYCT 135 Solved Free Assignment

Pressure: Pressure is another important factor affecting metamorphism. As the pressure on a rock increases, the rate of chemical reactions within the rock also increases, leading to faster rates of metamorphism.

Pressure can be caused by tectonic activity, by the weight of overlying rocks, or by the intrusion of magma.

Composition: The composition of the rock also plays a role in metamorphism, different minerals react differently under different conditions.

For example, rocks that are rich in silica tend to be more resistant to metamorphism, while those that are rich in calcium tend to be more susceptible to metamorphism.

Fluids: Fluids, such as water and gases, that are present in the rock can also affect metamorphism.

Fluids can carry dissolved minerals and chemical elements, which can react with the rock and change its mineralogy and texture.

Tectonics: Tectonics plays a big role in metamorphism, as it is related to the movement of the Earth’s crust. BGYCT 135 Solved Free Assignment

The forces generated by tectonic activity can cause rocks to be buried to great depths, resulting in high pressure and high temperature metamorphism.

Time: Time is also a factor that affects metamorphism, as certain processes need more time to happen than others. In general, the longer a rock is exposed to metamorphic conditions, the more it will change.

Q 9 Write short notes on the following:

a) Grain size parameters

Ans. Grain size refers to the size of the individual crystals or particles that make up a rock or a sediment. There are several parameters that can be used to describe the grain size of a rock or sediment, including:

Mean grain size: This is the average size of the grains within a rock or sediment. It is typically determined by measuring the size of a large number of grains and taking the average.

Grain size distribution: This is the range of grain sizes present within a rock or sediment. It can be described using various statistical measures, such as the standard deviation or the coefficient of variation.

Mode: This is the most common grain size within a rock or sediment. It is the grain size that appears most frequently in the grain size distribution.

Skewness: This is a measure of the asymmetry of the grain size distribution. A distribution that is skewed to the left, for example, would have a larger proportion of smaller grains than a symmetrical distribution.

Kurtosis: This is a measure of the peakedness of the grain size distribution. A distribution that is more peaked than a normal distribution is said to be leptokurtic, while a distribution that is less peaked than a normal distribution is said to be platykurtic. BGYCT 135 Solved Free Assignment

b) Graded Bedding

Ans Graded bedding, also known as graded layering or sorting, is a type of layering that is commonly found in sedimentary rocks.

It is characterized by a change in grain size from one layer to the next, with the largest grains at the bottom and the smallest grains at the top.

This type of layering can be caused by a variety of processes, including:

Depositional processes: Graded bedding can form as a result of the depositional processes that occur in environments such as rivers, deltas, and beaches.

In these environments, the grain size of the sediment can change due to variations in the energy of the water or wind that is transporting it.

For example, in a river, the water will be more energetic in the middle and less energetic near the banks, leading to the deposition of larger grains in the middle and smaller grains near the banks.

Turbulent flows: Graded bedding can also form as a result of turbulent flows such as turbidity currents and debris flows.

These flows can transport a wide range of grain sizes, and the larger grains will tend to settle out first, leading to a change in grain size from one layer to the next.

Biogenic processes: Graded bedding can also form as a result of biogenic processes such as the transport of shells and other fossils by waves and currents.

The shells and fossils are usually larger than the surrounding sediment and so they will settle out first, leading to a change in grain size from one layer to the next.

c) Palimpsest Texture

Ans Palimpsest texture is a type of texture found in metamorphic rocks, characterized by the presence of multiple generations of minerals that have formed at different stages of metamorphism.

The term “palimpsest” refers to the idea that the original rock has been “erased” and replaced by new minerals, similar to how a palimpsest is a parchment that has been written on multiple times.BGYCT 135 Solved Free Assignment

Palimpsest texture can form as a result of progressive metamorphism, in which a rock is repeatedly subjected to changing temperatures and pressures over time.

Each time the rock is subjected to new metamorphic conditions, new minerals will form and replace the existing minerals.

A common example of palimpsest texture can be observed in rocks that have been subjected to contact metamorphism, in which a rock is heated by an intrusion of magma.

The first minerals to form will be those that are stable at the highest temperatures, such as biotite and hornblende.

As the temperature decreases, new minerals such as garnet and staurolite will form, replacing the earlier-formed minerals.

Palimpsest texture can also be observed in regional metamorphism, where a rock is subjected to high-pressure and high-temperature over a wide area.

In this case, the minerals that form early will be those that are stable under high-pressure conditions, such as kyanite and sillimanite, and then replaced by others that are stable under lower-pressure conditions like muscovite and biotite.

Palimpsest texture can provide important information about the history of metamorphism of a rock and the conditions under which it formed.

It also can be used to infer the tectonic setting of the rock and the nature of the fluids that were present during metamorphism.

d) Foliation and lineation

Ans Foliation and lineation are two important textural features that are commonly found in metamorphic rocks.BGYCT 135 Solved Free Assignment

Foliation refers to the planar alignment of mineral grains within a rock. It is typically caused by the alignment of elongate minerals such as mica, which align parallel to the direction of maximum compression. Foliation can take several forms, including:

Schistosity: This is the most common form of foliation, characterized by the parallel alignment of elongate minerals such as mica and amphibole.

Gneissosity: This form of foliation is characterized by the alternating bands of light and dark minerals, such as quartz and feldspar, which are parallel to the foliation.

Slaty cleavage: This form of foliation is characterized by the parallel alignment of flat, platy minerals such as micas and chlorite.

Lineation refers to the linear arrangement of mineral grains or structural features within a rock.

It is typically caused by the alignment of mineral grains along a direction of maximum compression. Lineation can take several forms, including:

Mineral lineation: This is characterized by the alignment of mineral grains along a specific direction, such as the alignment of elongate minerals such as mica and amphibole.BGYCT 135 Solved Free Assignment

Foliation lineation: This is characterized by the alignment of foliation planes along a specific direction, such as the alignment of schistosity or gneissosity planes.

Axial planar lineation: This is characterized by the alignment of foliation planes that are perpendicular to the lineation, such as the alignment of axial planes in folds.

Both Foliation and Lineation are important features in metamorphic rocks because they can provide important information about the history of metamorphism and the conditions under which the rock formed.

They can also be used to infer the tectonic setting of the rock and the nature of the fluids that were present during metamorphism.

BGYCT 131 Solved free assignment 2023

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