Geology 2021 Paper II 50 marks Discuss

Q3

(a) Discuss briefly the processes of magma generation in the Earth's interior. How is grain size of an igneous rock related to the rate of cooling of magma? Discuss the role of fractional crystallization and assimilation in magmatic differentiation. (20 marks) (b) Describe with suitable sketches four different types of structures/textures found in metamorphic rocks and add brief notes on their origin. (20 marks) (c) Define 'migmatite'. How does the process of migmatization help to understand the origin of granites? (10 marks)

हिंदी में प्रश्न पढ़ें

(a) पृथ्वी के आन्तरिक भाग में मैग्मा उत्पत्ति की प्रक्रियाओं की संक्षिप्त चर्चा कीजिए। आग्नेय शैलों के कणों का आकार मैग्मा के ठंडे होने की दर से किस प्रकार सम्बन्धित है? मैग्मीय विभेदन में भिन्नात्मक क्रिस्टलीभवन और स्वांगीकरण की भूमिका की विवेचना कीजिए। (20 अंक) (b) उपयुक्त रेखाचित्रों द्वारा कायान्तरित शैलों में पायी जाने वाली चार संरचनाओं/गठनों का वर्णन कीजिए तथा उनकी उत्पत्ति पर संक्षिप्ट टिप्पणी कीजिए। (20 अंक) (c) 'मिग्मेटाइट' को परिभाषित कीजिए। मिग्मेटाइजेशन की प्रक्रिया किस प्रकार ग्रेनाइट की उत्पत्ति को समझने में मददगार होती है? (10 अंक)

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Directive word: Discuss

This question asks you to discuss. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'discuss' requires a critical examination with balanced coverage across all three sub-parts. Allocate approximately 40% effort to part (a) given its 20 marks and multi-component nature (magma generation, grain size-cooling relationship, and magmatic differentiation processes), 40% to part (b) for four metamorphic textures with sketches, and 20% to part (c) for migmatite definition and granite origin. Structure with brief introductions for each part, systematic development of concepts, and integrated conclusions showing linkages between igneous and metamorphic processes.

Key points expected

  • Part (a): Magma generation mechanisms—decompression melting, flux melting, and heat transfer; relationship between cooling rate and grain size (coarse phaneritic vs. fine aphanitic textures); fractional crystallization (Bowen's reaction series) and assimilation as differentiation processes
  • Part (a): Clear explanation of how rapid surface cooling produces glassy/vitreous textures while slow intrusive cooling yields coarse-grained rocks; mention of porphyritic textures indicating two-stage cooling
  • Part (b): Four metamorphic structures/textures with sketches—foliation (slaty cleavage, schistosity, gneissic banding), lineation, porphyroblasts, and granoblastic texture; origin linked to directed pressure, recrystallization, and metamorphic grade
  • Part (b): Additional textures like augen, mylonitic, or hornfelsic with appropriate genetic contexts; sketches showing 3D orientation of platy minerals in foliation and rotated porphyroblasts
  • Part (c): Definition of migmatite as mixed rock with melanosome (dark) and leucosome (light) components; migmatization as evidence for in-situ granite formation through anatexis and melt segregation
  • Part (c): Connection between migmatites and granites via the 'granite problem'—demonstrating transition from metamorphic to igneous realms; reference to metatexis vs. diatexis and field evidence from high-grade terranes

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise understanding of magma generation thermodynamics (solidus-liquidus relationships), accurately applies Bowen's reaction series to explain fractional crystallization, correctly distinguishes between assimilation and magma mixing, and presents scientifically accurate definitions of metamorphic textures and migmatite componentsCovers basic concepts with minor errors—may confuse partial melting mechanisms, oversimplify grain size relationships, or provide generic definitions without genetic context for metamorphic texturesFundamental misconceptions about magma generation (e.g., confusing melting with crystallization), incorrect grain size-cooling relationships, or confused definitions between migmatite and composite gneiss
Diagram / cross-section20%10Provides four clear, labeled sketches for part (b) showing 3D block diagrams or photomicrograph-style representations; includes optional schematic for Bowen's reaction series in (a) or migmatite structure in (c); diagrams show mineral alignment, porphyroblast rotation, and compositional layeringSketches present but inadequately labeled or showing only 2D sections; missing scale indicators or mineral symbols; diagrams generic rather than specific to requested texturesAbsent or irrelevant diagrams; sketches confused with igneous textures; no attempt to illustrate metamorphic structures despite explicit directive
Field evidence20%10Cites specific Indian examples—Charnockite series of Tamil Nadu for high-grade metamorphism and migmatites, Eastern Ghats granulite terrane for metamorphic textures, or Deccan Traps for cooling rate-texture relationships; references classic field studies from Indian Geological Survey literatureMentions generic field contexts without specific localities; may cite international examples (Scottish Highlands, Adirondacks) but misses Indian occurrencesNo field evidence cited; purely theoretical treatment; confuses laboratory experimental evidence with natural field relationships
Quantitative reasoning15%7.5Incorporates quantitative constraints—melting temperatures for different magma types (basaltic ~1200°C vs. granitic ~700°C), approximate cooling rates for different environments (intrusive: 10^-4 to 10^-6 °C/year; extrusive: 10^2 to 10^4 °C/hour), or P-T conditions for granulite facies metamorphismQualitative mention of temperature/pressure without numerical values; vague references to 'high temperature' or 'deep burial' without quantitative boundsAbsent quantitative treatment; confused temperature scales; incorrect orders of magnitude for geological processes
Indian / economic relevance20%10Links migmatites and granites to India's economic mineralization—rare earth elements in Kerala's khondalite belt, gemstone-bearing pegmatites associated with migmatites in Rajasthan, or dimension stone potential of Charnockites; connects metamorphic grade to mineral exploration strategies in Archaean cratonsBrief mention of granite as building stone or generic reference to mineral deposits without specific Indian linkagesNo economic or applied relevance discussed; misses opportunity to demonstrate practical importance of petrological concepts for India's resource base

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