Geology 2021 Paper I 50 marks Explain

Q3

(a) What are the fundamental concepts used in Geomorphological studies? Explain the concept "little of the earth's topography is older than Tertiary and most of it no older than Pleistocene"? (20 marks) (b) What is isostacy and what are the different theories put forward to explain this concept? (15 marks) (c) What is a fold domain? Discuss any eight types of folds depending on the closure of fold domain. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) भूआकृतिक अध्ययन में कौन सी मौलिक संकल्पनाओं का उपयोग होता है ? "पृथ्वी की स्थलाकृति में से कुछ ही तृतीय (टर्शरी) से पुरानी हैं और इनमें से अधिकांश प्लेस्टोसीन से पुरानी नहीं हैं", इस संकल्पना की व्याख्या करें । (20 अंक) (b) समस्थिति क्या है और इस संकल्पना की व्याख्या में कौन कौन से विभिन्न सिद्धांत दिए गए हैं ? (15 अंक) (c) वलन क्षेत्र क्या है ? वलन क्षेत्र के बंद होने के आधार पर कोई भी आठ प्रकार के वलन की व्याख्या करें । (15 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.

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How this answer will be evaluated

Approach

The directive 'explain' demands clear exposition with causal reasoning across all three parts. Allocate approximately 40% of time/words to part (a) given its 20 marks, and roughly 30% each to parts (b) and (c). Structure as: brief introduction on geomorphological time scales → systematic treatment of (a) with Davisian and Penckian concepts → (b) with isostatic models and mathematical basis → (c) with fold geometry definitions and classifications → concluding synthesis on dynamic crustal processes.

Key points expected

  • Part (a): Fundamental concepts—Davisian cycle of erosion (youth-maturity-old age), Penck's crustal movement-erosion interplay, climatic geomorphology, and tectonic geomorphology; explanation of the 'Tertiary-Pleistocene' dictum referencing rapid erosion rates (~10-100 mm/kyr) versus mountain building, preservation bias of recent landscapes, and the role of Pleistocene glaciations and sea-level changes in reshaping topography
  • Part (a): Quantitative support—mention denudation rates and the concept of 'geomorphic antiquity' versus 'rock antiquity' with examples like the Appalachian vs. Himalayan topography
  • Part (b): Definition of isostasy as hydrostatic equilibrium of Earth's crust; Airy's theory (roots of mountains, constant density crust) with mathematical expression h₁ρ₁ = h₂ρ₂ for compensation depth; Pratt's theory (varying density, constant depth) with density variations beneath mountains and oceans
  • Part (b): Vening Meinesz regional isostasy, flexural rigidity of lithosphere, and modern seismic/GPS evidence; Hayford-Bowie and Heiskanen compensation models
  • Part (c): Definition of fold domain as the region of folded rock bounded by inflection points or tangent lines on fold limbs; hinge zone and closure concept
  • Part (c): Eight fold types based on closure—anticline, syncline, antiform, synform, dome, basin, monocline, and chevron fold; description of closure direction (upward/downward) and stratigraphic versus geometric classification
  • Part (c): Indian examples—Aravalli fold belt structures, Krol belt folds in Lesser Himalaya, or Gondwana basin folds

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise definitions across all parts: for (a) distinguishes Davisian 'peneplain' from Penck's 'Endrumpf' and correctly interprets the Tertiary-Pleistocene statement as reflecting preservation potential, not absolute age; for (b) mathematically states Airy and Pratt hypotheses with correct variables; for (c) accurately defines fold domain boundaries and distinguishes geometric from stratigraphic closureBasic definitions correct but conflates Davis and Penck models or confuses Airy root depth with Pratt density variation; fold domain definition vague or omits inflection point criterion; minor errors in closure terminologyFundamental misconceptions—treats Tertiary-Pleistocene as absolute dating, confuses isostasy with eustasy, or describes folds without reference to closure; serious geological errors
Diagram / cross-section20%10Three distinct, labeled diagrams: (a) idealized Davisian cycle stages or erosion rate vs. time curve; (b) comparative Airy-Pratt cross-sections showing compensation depths and density variations; (c) fold domain geometry with hinge, inflection points, and closure arrows for multiple fold types; all with proper geological symbols and scaleTwo adequate diagrams with minor labeling errors or missing components (e.g., no compensation depth marked in isostasy diagrams, or fold diagrams lacking inflection points); hand-drawn but decipherableSingle diagram or none; poorly executed sketches without labels; diagrams contradict text description
Field evidence20%10Specific field evidence for each part: (a) Sierra Nevada relict topography vs. active Himalaya; Appalachian erosional history; (b) Scandinavian post-glacial rebound (Fennoscandian uplift rates ~10 mm/yr), or Himalayan gravity anomalies; (c) mapped examples from Krol belt, Aravalli Supergroup refolded structures, or Singhbhum fold belt with vergence indicatorsGeneral mention of field examples without specificity (e.g., 'Himalayan folds' without naming domain); or correct examples but not explicitly linked to concepts; one part lacks field evidenceNo field examples or invented/localized examples without geological basis; confuses laboratory/theoretical with field evidence
Quantitative reasoning15%7.5Numerical treatment: (a) denudation rates (10-1000 mm/kyr for different regimes), crustal uplift rates; (b) Airy equation with typical values (mountain height 3 km, root depth ~20 km at density contrast 0.4 g/cm³), Pratt density variations (2.67 vs. 2.9 g/cm³); flexural rigidity D values; (c) interlimb angles, wavelength-thickness ratios for fold classificationMentions quantitative aspects qualitatively (e.g., 'roots are deeper') without equations or typical values; or correct numbers in one part onlyNo quantitative content where clearly expected; incorrect orders of magnitude (e.g., isostatic compensation at 100 km depth for Airy model)
Indian / economic relevance20%10Strong Indian context: (a) Western Ghats escarpment evolution, Deccan Trap geomorphology, or Siwalik Hills Pleistocene reworking; (b) Ganga basin sediment loading and isostatic response, Indian shield gravity anomalies, or Kerala-Tuticorin geoid studies; (c) Himalayan fold-thrust belt petroleum traps (e.g., Assam-Arakan basin), Krol belt structural controls on groundwater, or iron ore deposits in folded BIFs of Dharwar/SinghbhumSome Indian examples but generic (e.g., 'Himalayan isostasy' without specifics) or economic relevance stated without elaboration; one part lacks Indian contextNo Indian examples; exclusively foreign examples (Rocky Mountains, Alps) where Indian analogues exist; misses economic significance of fold structures for mineral/hydrocarbon exploration

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