Geology 2024 Paper I 50 marks Describe

Q4

(a) Describe fold geometry. Illustrate various types of folds on the basis of their symmetry, orientation of axial plane and the trend of the fold axis. (20 marks) (b) Describe and illustrate different types of plate boundaries, and explain the mechanism of plate motion. (15 marks) (c) "At depth of compensation, the pressure generated by all overlying landmass substances on the earth is everywhere equal." Describe the hypotheses which support this statement. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) वलन ज्यामिति पर प्रकाश डालिए। वलन सममिति, अक्षीय तल विन्यास एवं वलन अक्ष की प्रवृत्ति के आधार पर विभिन्न प्रकार के वलनों का सचित्र वर्णन कीजिए। (20 अंक) (b) विभिन्न प्रकार की प्लेट सीमाओं का सचित्र वर्णन कीजिए तथा प्लेट गतिकीय क्रियाविधि को समझाइए। (15 अंक) (c) "पृथ्वी पर सभी भूद्रव्यमान पदार्थ द्वारा उत्पन्न दाब प्रतिकार गभीरता पर समग्र समान होता है।" इस कथन की पृष्ठकारक परिकल्पनाओं का वर्णन कीजिए। (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, and roughly 30% each to parts (b) and (c). Structure: brief integrated introduction on crustal deformation → body addressing each sub-part sequentially with labeled diagrams → conclusion linking fold geometry, plate tectonics and isostasy as complementary explanations of Earth's dynamic equilibrium.

Key points expected

Part (a): Definition of fold elements (hinge, limbs, axial plane, axis); classification by symmetry (symmetrical/asymmetrical, overturned, recumbent); by axial plane orientation (upright, inclined, recumbent); by fold axis trend (plunging vs. non-plunging); geometric descriptors (wavelength, amplitude, interlimb angle)
Part (b): Three plate boundary types with characteristics—divergent (spreading ridges, rift valleys), convergent (subduction zones, collision orogens), transform (strike-slip faults); driving mechanisms including slab pull, ridge push, mantle drag; role of asthenosphere convection
Part (c): Airy and Pratt hypotheses of isostasy; depth of compensation concept; density vs. root-crust thickness relationships; geodetic and gravimetric evidence supporting equal pressure at compensation depth
Integration: How fold geometry (a) manifests at convergent boundaries (b) and how isostatic adjustment (c) compensates crustal thickening from folding and collision
Indian examples: Aravalli fold belt for part (a); Himalayan orogeny and Indo-Australian plate boundary for part (b); Ganga basin sediment load and isostatic depression for part (c)

Evaluation rubric

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Precise definitions of fold geometry elements (hinge, limbs, axial plane, axis, interlimb angle) in (a); accurate distinction between divergent, convergent and transform boundaries with correct genetic processes in (b); correct statement of both Airy (uniform density, variable crustal thickness) and Pratt (variable density, uniform depth) hypotheses with proper depth of compensation concept in (c); no conflation of fold axis with axial plane	Generally correct definitions but imprecise terminology (e.g., confusing fold axis with axial trace); plate boundary descriptions lacking genetic mechanisms; partial treatment of isostasy mentioning only one hypothesis or conflating Airy and Pratt mechanisms	Fundamental errors such as describing folds as faults; incorrect plate boundary associations (e.g., subduction at divergent margins); complete misunderstanding of isostasy as floating ice analogy without geological application; omission of any hypothesis
Diagram / cross-section	25%	12.5	Minimum 4-5 labeled diagrams: for (a) geometric elements of folds, symmetrical/asymmetrical/recumbent fold profiles, plunging fold map pattern; for (b) cross-sections of all three boundary types with lithosphere-asthenosphere relationships; for (c) comparative Airy and Pratt models with compensation depth marked; clean line work with proper annotation	2-3 diagrams with basic labeling; some missing elements (e.g., fold geometry without plunge indication; plate boundaries without arrow vectors); hand-drawn quality acceptable but incomplete annotation; isostasy diagrams showing concept without mathematical or depth precision	Single generic diagram or none; unlabeled sketches; incorrect orientations (e.g., vertical axial plane labeled recumbent); plate boundaries drawn without distinguishing oceanic/continental crust; absence of isostatic models
Field evidence	15%	7.5	Specific field criteria for fold identification in (a): V-pattern of outcrops on topography, dip isogons for fold classification (Ramsay's method), vergence indicators; for (b): seismicity distribution, ophiolite sequences, blueschist metamorphism as subduction evidence, transform fault offset features; for (c): gravity anomalies, post-glacial rebound, geodetic measurements of Himalayan uplift/subsidence	General mention of field observation without specific criteria; vague references to earthquake distribution or mountain belts without linking to specific boundary processes; general statement that mountains have roots without gravity data	No field evidence cited; purely theoretical treatment; incorrect field relationships (e.g., claiming folds always indicate compressive stress without noting shear folds)
Quantitative reasoning	20%	10	Numerical treatment where applicable: interlimb angle ranges for fold classification (0-30° tight, 30-60° close, etc.); plate velocities (cm/year ranges); isostatic calculations—Airy: h₁ρ₁ + h₂ρ₂ = constant, or root depth estimation; Pratt: ρ₁h₁ = ρ₂h₂ = constant; compensation depth ~100 km; mention of geoid anomalies and gravity values	Qualitative understanding with occasional numbers (e.g., 'plates move few cm per year'); awareness that isostasy involves density-thickness trade-off without equations; fold classification by relative tightness without angle ranges	No quantitative content; incorrect orders of magnitude; claims of plate movement in m/year or km/year; complete absence of numerical relationships in isostasy discussion
Indian / economic relevance	15%	7.5	For (a): Aravalli fold belt (Precambrian), Krol belt folds, or Himalayan fold-thrust belt; for (b): Indo-Australian plate boundary complexity, Himalayan collision, Andaman-Sumatra subduction zone, Deccan flood basalt relation to Réunion hotspot; for (c): Ganga basin isostatic depression, crustal thickness variations under Himalaya vs. Peninsular India, implications for hydrocarbon exploration in sedimentary basins and geothermal gradient	Mention of Himalaya or Indian plate without specific structural detail; general reference to Indian geology without linking to question concepts; economic relevance limited to vague 'mineral exploration' without fold/structure connection	No Indian examples; irrelevant examples from other continents without Indian context; complete absence of economic or applied geological significance

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