UPSC Mains Answer PYQ 2023: Geology Paper 2 (Section- A) | Geology Optional Notes for UPSC PDF Download

Note: These sample answers provide a brief overview of the topic. You may add or reduce information as you see fit, depending on your understanding.

Section - A

Q1: Answer the following questions in about 150 words each :   (10x5=50)
(a) How are the symmetry elements in a crystal of class 6/m 2/m 2/m oriented with respect to its crystallographic axes?
Ans: 
Introduction: 
Crystal classes define the symmetry elements present in a crystal lattice. The notation "6/m 2/m 2/m" indicates specific symmetry elements.

Symmetry Elements: In a crystal of class 6/m 2/m 2/m:
1. 6-fold Rotational Axis (C6):

• Oriented perpendicular to the crystallographic c-axis.
• The crystal exhibits rotational symmetry about this axis. Every 60 degrees, the crystal's appearance repeats.

2. Mirror Planes (m):

• There are three mutually perpendicular mirror planes.
• These planes divide the crystal into mirror-image halves.

3. 2-fold Rotational Axis (C2):

• There are three mutually perpendicular 2-fold rotational axes.
• These axes bisect the crystal into symmetrical halves.

4. Orientation with Respect to Axes:

• The 6-fold rotational axis is oriented along the c-axis, perpendicular to the mirror planes.
• The mirror planes are perpendicular to each other and parallel to the crystallographic axes.

Example:

• Quartz (hexagonal crystal system) often exhibits 6/m 2/m 2/m symmetry due to its crystal lattice structure.

Conclusion: 
The crystal class 6/m 2/m 2/m indicates specific symmetry elements and their orientation with respect to the crystallographic axes, providing insight into the crystal's geometric properties.

(b) Why do some minerals show pleochroism? Explain.
Ans: 
Introduction: 
Pleochroism is the phenomenon in minerals where they exhibit different colors when viewed from different crystallographic directions.

Causes of Pleochroism:
1. Anisotropy of Absorption:

• Minerals are composed of anisotropic crystal structures, meaning their properties vary with direction.
• Different wavelengths of light are absorbed differently along different axes.

2. Orientation of Absorbing Ions:

• Certain minerals contain ions that absorb light selectively depending on their orientation in the crystal lattice.

3. Transition Metal Ions:

• Minerals with transition metal ions, like chromium or iron, often exhibit pleochroism due to electronic transitions.

Example:

• Cordierite, a mineral found in metamorphic rocks, shows strong pleochroism ranging from blue to yellow to brown, depending on crystallographic orientation.

Significance:

• Pleochroism is a valuable tool in mineral identification and can provide insights into the mineral's crystal structure and composition.

Conclusion: 
Pleochroism is a fascinating optical property of minerals that arises from their anisotropic crystal structures, impacting their appearance and aiding in identification.

(c) Describe the compositional changes in the magnesium-rich magma due to progressive removal of olivine.
Ans: 
Introduction: 
Progressive removal of minerals from a magma can lead to significant changes in its composition.

Compositional Changes:

• Enrichment in Silica (SiO₂): As olivine (Mg₂SiO₄) is removed, the remaining magma becomes relatively richer in silica.

• Enrichment in Magnesium (Mg): Initially, the magma is rich in magnesium due to the presence of olivine. As olivine is extracted, the magnesium content decreases.

• Decrease in Ferromagnesian Minerals: Minerals like olivine, pyroxenes, and amphiboles are ferromagnesian minerals rich in iron and magnesium. Their removal leads to a decrease in these elements.

Example:

• In a magnesium-rich magma, the removal of olivine can result in a shift towards a more silica-rich composition.

Conclusion: 
The progressive removal of minerals, such as olivine, from a magnesium-rich magma can lead to significant changes in its chemical composition, affecting the types of minerals that crystallize.

(d) Given a mafic protolith, what would be the characteristic mineral assemblages in (i) greenschist facies, (ii) amphibolite facies and (iii) granulite facies metamorphism?
Ans: 
Introduction: 
Metamorphism involves changes in mineralogy and texture of rocks due to heat, pressure, and chemical activity.

(i) Greenschist Facies:

• Minerals: Chlorite, actinolite, albite, epidote.
• Conditions: Low to moderate temperatures and pressures. Typically associated with regional metamorphism in areas of low-grade metamorphism.

(ii) Amphibolite Facies:

• Minerals: Amphibole (e.g., hornblende), plagioclase feldspar, biotite, garnet (in higher grade).
• Conditions: Moderate to high temperatures and pressures. Associated with intermediate-grade metamorphism.

(iii) Granulite Facies:

• Minerals: Feldspar (potassium feldspar), pyroxenes (e.g., augite), quartz, garnet, amphibole (in lower grade).
• Conditions: High temperatures and pressures. Associated with high-grade metamorphism.

Example:

• A mafic protolith subjected to metamorphism can yield a greenschist facies rock if it experiences low-grade metamorphism, an amphibolite facies rock under intermediate-grade conditions, and a granulite facies rock under high-grade conditions.

Conclusion: The characteristic mineral assemblages in different metamorphic facies provide important information about the conditions under which rocks have undergone metamorphism.

(e) Explain the diagenetic changes in carbonate rocks.
Ans: 
Introduction: 
Diagenesis refers to the physical and chemical changes that occur in sedimentary rocks after deposition but before metamorphism.

Diagenetic Changes in Carbonate Rocks:

• Cementation: Carbonate sediments are often cemented together by minerals like calcite, forming a solid rock.

• Recrystallization: Original carbonate minerals may recrystallize, altering the texture of the rock.

• Dolomitization: Some limestone can undergo dolomitization, where calcium carbonate is partially or entirely replaced by dolomite (MgCa(CO₃)₂).

• Compaction: Overburden pressure compacts sedimentary layers, reducing pore space.

• Replacement: Minerals can be replaced by others, such as calcite being replaced by dolomite.

Example:

• A limestone bed may undergo diagenetic changes, resulting in a more compact, recrystallized, and possibly dolomitized rock.

Conclusion: 
Diagenesis plays a crucial role in the transformation of carbonate sediments into solid rocks, influencing their texture, composition, and overall characteristics.

Q2:
(a) How does the Bragg equation explain X-ray diffraction from a crystal?    (20 Marks)
Ans: 
Introduction: 
The Bragg equation is fundamental in understanding X-ray diffraction, a technique used to study the crystal structure of materials.

Explanation of Bragg Equation:
1. Definition:

• The Bragg equation relates the angle of diffraction (θ), wavelength of X-rays (λ), and the interplanar spacing (d) in a crystal lattice.
• It is given by: nλ = 2d sinθ

2. Diffraction Peaks:

• When X-rays strike a crystal lattice, they interact with the planes of atoms. Some of the X-rays are scattered, creating diffraction peaks.

3. Constructive Interference:

• The Bragg equation demonstrates that diffraction occurs when the path difference of X-rays scattered by adjacent planes is an integer multiple of the wavelength.

4. Determining Crystal Structure:

• By measuring diffraction angles and using the Bragg equation, one can calculate the interplanar spacing and infer the crystal lattice structure.

Example:

• In a crystal of sodium chloride (NaCl), X-rays with a wavelength of 1.54 Å (angstroms) are diffracted at an angle of 32 degrees. By applying the Bragg equation, one can determine the interplanar spacing between the NaCl crystal lattice planes.

Conclusion: 
The Bragg equation is a crucial tool in X-ray diffraction analysis, allowing scientists to probe the internal structure of crystalline materials and determine the arrangement of atoms in a crystal lattice.

(b) How does Si-O polymerism help to classify silicate minerals? Give one example for each of these silicate subclasses.      (15 Marks)
Ans:
Introduction: 
Silicate minerals are characterized by their fundamental building blocks, SiO₄ tetrahedra, which form various structural arrangements.

Si-O Polymerism: Silicon-oxygen (Si-O) polymerism refers to the linking of SiO₄ tetrahedra to form more complex structures in silicate minerals.

Classification of Silicate Minerals Based on Si-O Polymerism:
1. Isolated Tetrahedra (Nesosilicates):

• Structure: SiO₄ tetrahedra are isolated and not linked to each other.
• Example: Olivine (forsterite, Mg₂SiO₄).

2. Single Chain (Inosilicates):

• Structure: SiO₄ tetrahedra form single chains by sharing oxygen atoms.
• Example: Pyroxenes (augite, (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆).

3. Double Chain (Inosilicates):

• Structure: SiO₄ tetrahedra form double chains by sharing oxygen atoms.
• Example: Amphiboles (hornblende, Ca₂(Mg,Fe)₄Al(Al,Si)₃O₂2(OH)₂).

4. Sheet Structure (Phyllosilicates):

• Structure: SiO4 tetrahedra form continuous sheets.
• Example: Micas (biotite, K(Mg,Fe)₃(AlSi₃O₁₀)(F,OH)₂).

5. Three-Dimensional Framework (Tectosilicates):

• Structure: SiO₄ tetrahedra are linked in a 3D network.
• Example: Quartz (SiO₂), feldspars (orthoclase, KAlSi₃O₈).

Example:

• In the mineral olivine, SiO₄ tetrahedra are isolated, giving rise to a simple structure.

Conclusion: 
Si-O polymerism plays a crucial role in the classification of silicate minerals, determining their structural complexity and physical properties.

(c) List all major differences among kaolinite, smectite and illite groups of clay minerals.      (15 Marks)
Ans: 
Introduction: 
Kaolinite, smectite, and illite are important clay minerals with distinct characteristics and applications.

Differences:
1. Composition:

• Kaolinite: Pure kaolinite consists of repeating Si2O5 units.
• Smectite: Smectites have a layered structure with expandable interlayer spaces, allowing for cations and water molecules.
• Illite: Illite is a non-expandable clay mineral, rich in potassium and aluminum.

2. Crystal Structure:

• Kaolinite: Kaolinite has a 1:1 layered structure with a single tetrahedral sheet and a single octahedral sheet.
• Smectite: Smectites have a 2:1 layered structure with two tetrahedral sheets and one octahedral sheet.
• Illite: Illite is a non-expanding clay mineral with a 2:1 layered structure similar to smectite.

3. Water Absorption:

• Kaolinite: Kaolinite has a low water absorption capacity due to its limited interlayer space.
• Smectite: Smectites can absorb and retain large amounts of water due to their expandable interlayer spaces.
• Illite: Like kaolinite, illite has limited water absorption capacity.

4. Cation Exchange Capacity (CEC):

• Kaolinite: Kaolinite has a low CEC due to its non-expandable structure.
• Smectite: Smectites have a high CEC, allowing them to exchange and retain cations effectively.
• Illite: Illite has a moderate CEC compared to kaolinite and smectite.

5. Applications:

• Kaolinite: Used in ceramics, paper industry, and as a filler in paints.
• Smectite: Widely used in drilling muds, as a binder in foundry molds, and in cosmetics.
• Illite: Used in the production of bricks, as a filler in paints, and in the oil and gas industry.

Example:

• Kaolinite is commonly used in the production of porcelain and ceramics due to its fine particle size and high plasticity.

Conclusion: 
Kaolinite, smectite, and illite are distinct clay minerals with varying compositions, structures, and applications. Understanding their differences is crucial in various industrial and geological contexts.

Q3:
(a) Describe the changes in crystallized solid composition in albite-anorthite system at 1 atm pressure during cooling of a liquid of An₅₀ composition from 1500 °C temperature.       (20 Marks)
Ans: 
Introduction: 
The Albite-Anorthite system represents a series of minerals within the plagioclase feldspar group. It is important in understanding the behavior of solid solutions during cooling from a high-temperature liquid.

Changes during Cooling:

• Initial Composition (An₅₀): At 1500°C, the liquid composition is An₅₀, indicating an equal mixture of albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈).

• Solidification Process: As the liquid cools, the first minerals to crystallize are those with compositions closest to the original liquid composition.

• Early Crystallization (Ca-Rich Phase): Anorthite (CaAl₂Si₂O₈) is the first mineral to crystallize from the liquid because it is the Ca-rich end-member.

• Progressive Enrichment in Albite (Na-Rich Phase): As the temperature decreases further, albite (NaAlSi₃O₈) begins to crystallize, leading to an increase in the albite content of the solid phase.

• Final Solid Composition: As cooling continues, the solid composition gradually shifts towards the albite end-member.

Example:

• If the final composition is plotted on an Albite-Anorthite diagram, it would show a trend from An₅₀ towards Albite (An₁₀₀) as the temperature decreases.

Conclusion: 
The crystallization process in the Albite-Anorthite system during cooling from a liquid of An₅₀ composition involves the sequential formation of anorthite followed by albite, resulting in a shift towards the albite end-member in the solid phase.

(b) How is a granite defined? Discuss petrogenesis of a calc-alkaline peraluminous granite.       (15 Marks)
Ans: 
Granite Definition: Granite is a coarse-grained, intrusive igneous rock composed primarily of quartz, feldspar (orthoclase and plagioclase), and mica (biotite or muscovite). It is classified as a felsic rock due to its high silica content.

Petrogenesis of Calc-Alkaline Peraluminous Granite:

• Composition: Calc-alkaline granites are characterized by their high content of aluminum and alkali metals (potassium and sodium). Peraluminous granites have an aluminum content greater than that required to form the mineral cordierite.

• Source Rock: They typically originate from the partial melting of continental crustal rocks, such as sedimentary rocks rich in aluminum, or older granitic rocks.

• Partial Melting Process: Peraluminous granites form through the partial melting of source rocks under conditions of high pressure and temperature. The melt generated is enriched in aluminum.

• Fractional Crystallization: After partial melting, the melt rises and undergoes fractional crystallization. Minerals rich in aluminum, such as biotite and muscovite, crystallize early, leading to an enrichment of aluminum in the residual melt.

• Ascent and Emplacement: The granite melt eventually ascends and is emplaced into the crust, where it cools and solidifies to form a granite intrusion.

Example:

• The Erzgebirge granite in Germany is an example of a calc-alkaline peraluminous granite.

Conclusion: 
Calc-alkaline peraluminous granites are a specific type of granite with distinctive mineralogical and chemical characteristics. They form through partial melting of source rocks enriched in aluminum, followed by fractional crystallization and emplacement into the crust.

(c) What are the assumptions involved for plotting quartz-bearing metamorphic rocks of basaltic composition in an ACF triangular diagram?      (15 Marks)
Ans: 
Introduction:
The ACF triangular diagram is a powerful tool in metamorphic petrology for classifying and understanding mineral assemblages in metamorphic rocks. When plotting quartz-bearing metamorphic rocks of basaltic composition on this diagram, certain assumptions are made to simplify the representation and interpretation of the mineral assemblages.

Assumptions:

• Isochemical Metamorphism: The diagram assumes that the chemical composition of the rock remains relatively constant during metamorphism. This means that there is no significant gain or loss of major elements.

• Steady-State Conditions: The diagram assumes that the rocks have reached a state of equilibrium, where the mineral assemblage reflects the prevailing pressure and temperature conditions.

• Ideal Mineral Assemblages: It assumes that the minerals present in the rocks are in their ideal end-member compositions. In reality, solid solutions and variations in mineral composition can occur.

• Simplified Mineral System: The diagram simplifies the complex mineralogical variations that can occur in natural rocks, allowing for a more manageable classification system.

• Limited to Basaltic Composition: The assumptions are tailored for rocks of basaltic composition. Different assumptions and diagrams would be used for rocks of other compositions.

Example:

• Using the ACF diagram, a metamorphic rock of basaltic composition can be classified into specific facies based on the presence and absence of certain minerals, providing valuable information about its metamorphic history.

Conclusion: 
The assumptions for plotting quartz-bearing metamorphic rocks on the ACF diagram help simplify the classification process and aid in interpreting the mineral assemblages in metamorphic rocks of basaltic composition. These assumptions are based on the understanding of metamorphic processes and the behavior of minerals under specific conditions.

Q4:
(a) Classify the conglomerate rocks on the basis of clast composition and grain-matrix ratio and discuss their genetic importance.      (20 Marks)
Ans: 
Introduction: 
Conglomerate rocks are sedimentary rocks composed of rounded to sub-rounded clasts (pebbles, cobbles, and boulders) embedded in a finer-grained matrix. They are classified based on clast composition and grain-matrix ratio, revealing information about their formation and depositional environment.

Classification Based on Clast Composition:
1. Monomict Conglomerate:

• Clast Composition: Contains clasts of a single rock type.
• Genetic Importance: Indicates a uniform source rock that experienced minimal transport. Common in proximal alluvial fans.

2. Oligomict Conglomerate:

• Clast Composition: Contains clasts of a few rock types.
• Genetic Importance: Suggests multiple sources or selective transport of clasts. Common in mixed sedimentary environments.

3. Polymict Conglomerate:

• Clast Composition: Contains clasts of numerous rock types.
• Genetic Importance: Implies extensive transport and mixing. Common in high-energy settings like river channels.

Classification Based on Grain-Matrix Ratio:
1. Matrix-Supported Conglomerate:

• Matrix Content: Matrix makes up a significant portion of the rock.
• Genetic Importance: Indicates deposition in a high-energy, turbulent setting with clasts held together by matrix material.

2. Clast-Supported Conglomerate:

• Matrix Content: Clasts are closely packed, with limited matrix.
• Genetic Importance: Suggests deposition in a lower-energy environment with clasts touching or nearly touching each other.

3. Clay-Clast Conglomerate:

• Matrix Content: Predominantly composed of clay-sized particles.
• Genetic Importance: May indicate a post-depositional clay infill, which can have environmental and diagenetic significance.

Examples: A monomict conglomerate with well-rounded quartz pebbles suggests a source rock of quartzite. An oligomict conglomerate with a mix of quartz and granite clasts indicates multiple sources, possibly from mountainous terrain.

Conclusion: 
Classifying conglomerate rocks based on clast composition and grain-matrix ratio helps geologists understand the rock's origin, source terrains, and depositional history, providing insights into past geological conditions.

(b) Briefly describe the mechanisms of gravity-controlled sediment flows and write about their characteristic features in the rocks.      (15 Marks)
Ans: 
Introduction: 
Gravity-controlled sediment flows are common geological phenomena involving the downslope movement of sediment materials under the influence of gravity. They include processes like landslides, debris flows, and turbidity currents.

Mechanisms:
1. Landslides:

• Mechanism: Landslides involve the abrupt failure and downslope movement of rock, soil, or sediment due to various triggers, such as heavy rainfall, earthquakes, or human activities.
• Characteristic Features: Landslides often leave a distinctive scar on the landscape, with disrupted layers of sediment and a debris apron at the base.

2. Debris Flows:

• Mechanism: Debris flows are rapid, water-saturated flows of sediments, typically initiated by intense rainfall or snowmelt, leading to loose material mobilization.
• Characteristic Features: They exhibit a distinctive fan-shaped deposit at the flow's terminus, often carrying a mixture of sediment sizes.

3. Turbidity Currents:

• Mechanism: Turbidity currents are underwater sediment flows triggered by sediment-laden water, often associated with submarine canyons or delta environments.
• Characteristic Features: They can deposit well-sorted graded beds on the seafloor, reflecting variations in sediment settling velocity.

Examples:

• The Oso landslide in Washington state is an example of a catastrophic landslide event with a significant impact on the landscape.
• Debris flows are common in mountainous regions, such as the Italian Dolomites.

Conclusion: 
Gravity-controlled sediment flows result from various triggers and exhibit characteristic features in the geological record. Understanding these mechanisms is crucial for assessing geological hazards and interpreting sedimentary deposits.

(c) Explain mineral-based techniques to decipher the source terrains and transport history of sediments. Give a list of minerals diagnostic of igneous and metamorphic provenances.      (15 Marks)
Ans: 
Introduction: 
Mineral-based techniques are employed in sedimentary geology to determine the source terrains and transport history of sediments. Minerals from specific source regions have distinct characteristics that can be used for identification.

Diagnostic Minerals for Igneous Provenance:
1. Zircon:

• Zircons are highly resistant minerals often found in igneous rocks.
• Their uranium-lead dating can reveal the age of the source terrain.

2. Feldspars:

• Plagioclase and alkali feldspars are common in igneous rocks.
• Compositional variations can provide insights into the specific igneous rock type.

3. Pyroxenes and Amphiboles:

• These minerals are abundant in mafic and ultramafic igneous rocks.
• Their presence can indicate a mafic source terrain.

Diagnostic Minerals for Metamorphic Provenance:
1. Garnet:

• Garnets are commonly found in high-grade metamorphic rocks.
• They can reflect the pressure-temperature conditions of the source terrain.

2. Staurolite and Kyanite:

• These minerals are associated with specific metamorphic facies and can indicate the metamorphic grade.

3. Sillimanite:

• Sillimanite is indicative of high-temperature, low-pressure metamorphism.

Techniques:

• Mineralogical Analysis: Petrographic microscopy and X-ray diffraction are used to identify mineral compositions and assess their provenance.

• Heavy Mineral Analysis: Heavy minerals, which are more resistant to weathering and erosion, can be separated from sediment samples and analyzed to determine their source.

Example: The presence of zircons with specific ages in sedimentary rocks can provide insights into the geological history of the source terrain and the sediment transport history.

Conclusion: 
Mineral-based techniques play a crucial role in deciphering sediment source terrains and transport histories, helping geologists reconstruct the Earth's geological past and sedimentary processes.