Geology 2024 Paper II 50 marks 150 words Compulsory Explain

Q1

Answer the following questions in about 150 words each. (a) What are the different types of rotational axes of symmetry present in a crystal ? What are the different types of twinning observed in quartz ? (10 marks) (b) How does one define "Double refraction" and "Birefringence" of an anisotropic mineral ? Write with the help of suitable sketches. (10 marks) (c) Show diagrammatically the characteristics of binary eutectic system under 1 atmosphere (1 atm pressure). How does one explain the formation of porphyritic basic rock with phenocryst of plagioclase in a groundmass with plagioclase and clinopyroxene with the help of a suitable binary eutectic system ? (10 marks) (d) Explain the processes involved in magmatic differentiation. (10 marks) (e) With the help of suitable diagrams, describe Folk's graphic classification of carbonate rocks. (10 marks)

हिंदी में प्रश्न पढ़ें

निम्नलिखित में से प्रत्येक प्रश्न का उत्तर लगभग 150 शब्दों में दीजिए। (a) किसी क्रिस्टल में मौजूद विभिन्न प्रकार के घूर्णी सममिति अक्ष कौन-से हैं ? स्फटिक (क्वार्ट्ज) में प्रेक्षित विभिन्न प्रकार के यमलन कौन-से हैं ? (10 अंक) (b) एक विषमदैशिक खनिज में "दोहरा अपवर्तन" और "द्विअपवर्तन" को कैसे परिभाषित करते हैं ? समुचित चित्रों की सहायता से लिखिए। (10 अंक) (c) 1 वायुमंडलीय दाब (1 atm प्रेशर) पर द्विअंगी गलनांकांतिक तंत्र की विशेषताओं को आरेखीय रूप से दर्शाइए । प्लेजियोक्लेस तथा क्लाइनोपाइरॉक्सीन युक्त आधात्रिका में, लक्ष्यक्रिस्टलीय प्लेजियोक्लेस धारित पोरफिराइटी अल्प-सिलिका शैल निर्माण को उचित द्विअंगी गलनांकांतिक तंत्र की सहायता से कैसे समझाया जा सकता है ? (10 अंक) (d) मैग्मीय विभेदन में सम्मिलित प्रक्रियाओं की व्याख्या कीजिए। (10 अंक) (e) समुचित चित्रों की सहायता से फोक के कार्बोनेट शैलों के आरेखमय वर्गीकरण का वर्णन कीजिए। (10 अंक)

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Directive word: Explain

This question asks you to explain. 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 'explain' demands clear, logical exposition of processes and concepts with supporting evidence. Allocate approximately 30 words per mark: ~30 words for (a) on symmetry axes and quartz twinning, ~30 words for (b) on double refraction with sketches, ~30 words for (c) on binary eutectic diagrams and porphyritic texture, ~30 words for (d) on magmatic differentiation processes, and ~30 words for (e) on Folk's classification with diagrams. Structure each part as: definition → types/processes → examples → diagrams where required.

Key points expected

  • (a) Rotational axes: 1-fold, 2-fold, 3-fold, 4-fold, 6-fold; no 5-fold or >6-fold in crystals; Quartz twinning: Dauphiné (electrical), Brazil (optical), Japan (rare, contact)
  • (b) Double refraction: splitting of light into two rays; Birefringence: difference in refractive indices (δ = |nε - nω|); sketches showing uniaxial indicatrix and ordinary/extraordinary rays
  • (c) Binary eutectic diagram: liquidus curves, eutectic point, lever rule; Porphyritic texture: early plagioclase crystallization above liquidus, quenched groundmass at eutectic (plagioclase-pyroxene)
  • (d) Magmatic differentiation: fractional crystallization, crystal settling, filter pressing, gas streaming, liquid immiscibility, assimilation; Bowen's reaction series application
  • (e) Folk's classification: allochem types (intraclasts, oolites, fossils, peloids), matrix (micrite vs sparite), textural maturity; triangular diagram with 8 major rock types

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precisely defines all crystallographic axes (1,2,3,4,6-fold only), distinguishes birefringence from double refraction quantitatively, correctly applies lever rule to eutectic systems, accurately describes all 6 differentiation mechanisms, and correctly identifies Folk's allochem categoriesIdentifies most symmetry axes but misses restriction on 5-fold; conflates double refraction with birefringence; basic eutectic understanding without lever rule application; lists 3-4 differentiation processes; recognizes Folk's triangle but misallocates some rock typesIncludes impossible 5-fold or 7-fold axes; fails to distinguish O-ray from E-ray; confuses eutectic with peritectic; omits crystal settling or Bowen's series; fundamentally misunderstands allochem vs orthochem distinction
Diagram / cross-section25%12.5Clean uniaxial indicatrix with nε and nω labeled for (b); temperature-composition eutectic diagram with liquidus, solidus, and tie-lines for (c); complete Folk's triangle with all 8 fields labeled for (e); quartz twin diagrams showing penetration for (a)Rough but recognizable indicatrix; basic eutectic 'V' shape without labeled axes; simplified Folk's triangle missing minor fields; textual description substitutes for some diagramsMissing required diagrams for (b), (c), or (e); incorrect geometry (e.g., peritectic instead of eutectic); unlabeled or illegible sketches; diagrams contradict written explanation
Field evidence15%7.5Cites specific Indian occurrences: Deccan Trap porphyritic basalts for (c); Rajasthan carbonate deposits for Folk's classification (e); mentions optical properties of calcite/dolomite from Himalayan carbonate rocks for (b)Generic references to basaltic provinces or carbonate platforms without Indian specificity; mentions common rock types without locality dataNo field examples; invented or incorrect localities; confuses igneous with sedimentary examples in inappropriate contexts
Quantitative reasoning20%10States birefringence values (e.g., quartz 0.009, calcite 0.172); applies lever rule to determine phase proportions in eutectic system; mentions specific temperature values (~1200°C for basaltic systems); calculates approximate modes from phase diagramsQualitative mention of 'high' vs 'low' birefringence; understands eutectic composition conceptually without numerical application; omits quantitative aspects entirely in some partsIncorrect numerical values; misapplies lever rule (wrong phase proportions); confuses weight percent with mole percent in phase diagrams; no quantitative treatment where required
Indian / economic relevance15%7.5Links quartz twinning to piezoelectric applications in electronics industry; connects Deccan differentiation to Deccan stratigraphy and groundwater aquifers; references cement industry reliance on Folk's classification for limestone quality assessment; mentions optical calcite from Andhra PradeshGeneral mention of economic importance without specific Indian industry links; standard references to construction materials without quality assessment contextNo economic or Indian relevance discussed; irrelevant economic examples (e.g., gold mining in carbonate classification); misses obvious connections to Indian mineral industry

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