(a) Describe the geomorphic landforms produced by structural, weathering, erosional and depositional processes. Give four examples of each process. (20 marks)

(b) Illustrate the discontinuities in the Earth's interior and discuss the mechanical and

Question

(a) Describe the geomorphic landforms produced by structural, weathering, erosional and depositional processes. Give four examples of each process. (20 marks)

(b) Illustrate the discontinuities in the Earth's interior and discuss the mechanical and compositional layering of the Earth. (15 marks)

(c) Illustrate the principles of stereographic projection. How are the 'pi' and 'beta' diagrams useful to analyze fold structure? (15 marks)

UPSC Answer Check · Accepted Answer

Geomorphic landforms record tectonics, climate, rock resistance and time; a block diagram would show uplifted strata, residual forms, valleys and ridges, and deposits.

Structural landforms arise where deformation or faulting changes relief. Fold mountains form by compressional folding, e.g. Himalayas and Alps. Fault scarps form by displacement along normal or strike-slip faults, e.g. Satpura fault scarp and Dead Sea scarp. Rift valleys form by extensional down-dropping between normal faults, e.g. Narmada-Son rift and East African Rift. Block mountains form by uplift of fault-bounded horsts, e.g. Aravalli and Black Forest.

Weathering landforms are residual forms produced by chemical, mechanical and biological breakdown in situ. Tors are isolated granite blocks left after surrounding rock is removed, e.g. Dartmoor and Aravalli granite tors. Inselbergs are isolated hills rising from plains, e.g. Socotra and Machu Picchu. Exfoliation domes form by sheet-like peeling of granitic rock, e.g. Half Dome and Uluru. Tafoni are honeycomb cavities in resistant rock, e.g. Himalayan quartzite and desert sandstone.

Erosional landforms are carved by rivers, glaciers, wind and solution. River valleys are V-shaped or entrenched channels, e.g. Ganga and Indus valleys. Cirques are amphitheatre-shaped glacier headwalls, e.g. Himalayan and Alpine cirques. Yardangs are wind-sculpted ridges in arid plains, e.g. Badain Jaran and Thar margin. Zeugens are residual limestone masses left after surrounding strata are dissolved, e.g. Black Forest zeugen.

Depositional landforms are built by transported material. Deltas form where rivers enter standing water, e.g. Ganga-Brahmaputra and Nile deltas. Alluvial fans form where streams spread on gentle slopes, e.g. Indus piedmont and Thar fan. Moraines are ridges of glacial till, e.g. Himalayan and Alpine moraines. Dunes are wind-built sand forms, e.g. Thar and Sahara dunes.

Discontinuities and layering. A labelled cross-section shows seismic discontinuities where P- and S-wave velocities change. The Mohorovičić discontinuity, at about 5–10 km beneath oceans and 35 km beneath continents, separates crust from mantle. The Gutenberg discontinuity at 2,900 km separates mantle from liquid outer core; S-waves are absent below it. The Lehmann discontinuity at about 5,150 km marks the solid inner core. The 410–660 km transition zone records mineral phase changes in the upper mantle. Compositionally, the crust is silica-alumina, the mantle is ultramafic Mg-Fe silicate, and the core is Fe-Ni. Mechanically, the rigid lithosphere comprises crust and uppermost mantle; the asthenosphere is dominantly solid, ductile, with only minor melt in the low-velocity zone; the mesosphere is the solid but plastic lower mantle; the outer core is liquid and convecting; the inner core is solid. They explain seismic wave behaviour and relate to Indian geothermal provinces, e.g. Himalayan and Eastern Ghats, where heat flow and crustal structure control geothermal gradients.

Stereographic projection and fold analysis. Stereographic projection transfers planes and lines from a sphere to a flat net, usually lower-hemisphere. The primitive circle represents the horizontal plane; N–S great circles are meridians; other planes are great circles, and their poles are plotted 90° from the plane. A stereonet with primitive, meridians and small circles is used to plot bedding, foliation and lineations. In a pi diagram, poles to bedding planes of a cylindrical fold are plotted; if the fold is cylindrical, they lie on a small circle, and the pole to that small circle gives the fold axis and its plunge. In a beta diagram, intersection lineations between bedding and foliation are plotted; their common great circle or pole gives the fold axis orientation. The number of lineations from n planes grows as n(n−1)/2, a binomial increase, not exponential. Such analysis aids structural mapping in fold belts like the Himalayas by determining axial planes, plunge and sense of folding from field data.

Thus, landforms record surface processes, discontinuities reveal Earth's mechanical state, and stereographic methods quantify the structures that control both.

(a) Describe the geomorphic landforms produced by structural, weathering, erosional and depositional processes. Give four…

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