Geology 2025 Paper I 50 marks Describe

Q4

(a) On the basis of dip isogons describe the classification of folds with neat diagrams. (20 marks) (b) What are volcanoes ? Describe various causes and products of volcanism. (15 marks) (c) How does strain analysis help in understanding the nature of rock deformation. Discuss centre to centre method of estimation of two dimensional strain. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) समनति रेखा के आधार पर वलन वर्गीकरण का सचित्र वर्णन कीजिए । (20 अंक) (b) ज्वालामुखी क्या है ? ज्वालामुखी उद्भव के विभिन्न कारणों तथा उत्पादों का वर्णन कीजिये । (15 अंक) (c) विकृति विश्लेषण किस प्रकार शैल विक्षेपण की प्रकृति को समझने में मदद करता है । द्विआयामी विकृति का आकलन करने के लिये केन्द्र से केन्द्र विधि की विवेचना कीजिये । (15 अंक)

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Directive word: Describe

This question asks you to describe. 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 'describe' demands systematic, detailed exposition with visual support. Allocate approximately 40% of time/words to part (a) given its 20 marks weightage, with ~30% each to parts (b) and (c). Structure: brief introduction defining folds, volcanoes and strain; detailed body addressing each sub-part sequentially with diagrams for (a) and (c); concluding synthesis on how these tools decode Earth's deformation history.

Key points expected

  • Part (a): Definition of dip isogons; Ramsay's classification into Class 1A (strongly convergent), 1B (parallel/thickening), 1C (weakly convergent), 2 (similar), 3 (strongly divergent) with layer thickness variations
  • Part (a): Tangent diagram method and relationship between dip isogon geometry and fold mechanism (flexural slip vs. pure shear)
  • Part (b): Definition of volcano; causes—mantle plumes, subduction, rifting, hotspot volcanism; products—lava flows, pyroclastics, volcanic gases, intrusions
  • Part (c): Strain analysis significance—determining deformation regime, strain ellipsoid orientation, kinematic vorticity number; centre-to-centre method procedure using deformed objects (ooliths, reduction spots)
  • Part (c): Calculation of Rs (strain ratio) from centre-to-centre distances and Fry plot construction; limitations of method

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5For (a) correctly defines dip isogons and distinguishes all five fold classes with precise thickness ratios; for (b) accurately links volcanic causes to plate tectonic settings; for (c) correctly explains strain tensor components and centre-to-centre statistical basisIdentifies basic fold classes and volcanic types but confuses Class 1B/1C boundaries or omits mantle plume dynamics; describes strain ellipsoid but misstates Fry plot procedureFundamental errors like equating dip isogons with fold axes; conflates extrusive/intrusive products; treats strain as simple distortion without tensor basis
Diagram / cross-section25%12.5For (a) draws five fold profiles with correctly oriented dip isogons and annotated thickness variations; for (c) illustrates Fry plot construction with deformed ooliths and derived strain ellipse; neat, labelled, scale-appropriateSketches 3-4 fold types with partially correct isogons; basic strain ellipse shown but Fry plot mechanics unclear; diagrams present but lack annotationMissing diagrams for high-weight part (a); incorrect isogon orientations; no visual representation of centre-to-centre method; untidy or unlabelled sketches
Field evidence15%7.5Cites specific Indian examples: Dharamkot folds (Simla Himalaya) for Ramsay classes; Deccan Traps flood basalts for volcanism; quartzites with deformed ooliths from Chitradurga or Cuddapah for strain analysisMentions generic field contexts (Himalayan folds, volcanic islands) without specific locations; acknowledges strain markers exist but names no formationsNo field examples; entirely theoretical treatment; invents non-existent localities
Quantitative reasoning20%10For (a) states thickness ratio formulae (t'/t = cosα/cosα'); for (c) explains Fry plot statistics, derives Rs from centre distances, discusses confidence intervals; mentions strain magnitude (e.g., Rs = 2.5 for Chitradurga quartzites)Qualitative description of quantitative methods; mentions strain ratio without derivation; no numerical values citedAvoids all mathematics; treats strain analysis as purely descriptive; no understanding of statistical basis of centre-to-centre method
Indian / economic relevance15%7.5Links fold classification to hydrocarbon traps in Assam-Arakan basin; volcanic products to Deccan bauxite/China clay deposits and geothermal potential; strain analysis to orogenic gold mineralization controls in Dharwar cratonMentions Deccan Traps as volcanic example and mining generally without specific economic linkages; notes strain affects ore shoots but lacks detailNo Indian examples; no economic significance discussed; misses opportunity to connect structural geology to resource exploration

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