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

(a) Landforms of structural, weathering, erosional and depositional processes

(i) 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. the Wasatch scarp in Utah and the escarpments of the Narmada-Son fault zone. 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. Vosges and Black Forest.

(ii) 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 the granite tors around Hyderabad and in Bundelkhand. Inselbergs are isolated hills rising from plains, e.g. Uluru and Sugarloaf Mountain. Exfoliation domes form by sheet-like peeling of granitic rock, e.g. Half Dome and Stone Mountain. Tafoni are honeycomb cavities in resistant rock, e.g. the coastal granites of Corsica and desert sandstone cliffs.

(iii) 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. the Lut desert of Iran and the Qaidam basin. Zeugens are tabular ridges of resistant cap rock over softer rock, undercut by wind abrasion, e.g. zeugens of the Sahara and Thar deserts.

(iv) 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. the Kosi megafan and the bhabar fans of the Siwalik piedmont. Moraines are ridges of glacial till, e.g. Himalayan and Alpine moraines. Dunes are wind-built sand forms, e.g. Thar and Sahara dunes.

(b) Discontinuities and layering of the Earth

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. Beneath India the Moho lies at about 35–40 km under the Dharwar craton and deepens to about 70 km beneath the Himalaya and Tibet, where the crust is doubled by collision.

(c) Stereographic projection and pi and beta diagrams

Stereographic projection transfers planes and lines from a sphere to a flat net, usually lower-hemisphere, on an equal-angle (Wulff) net or, for statistical plots, an equal-area (Schmidt) net. 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 great circle (the π-circle), and the pole to that great circle gives the fold axis and its plunge. In a beta diagram, bedding planes are plotted as great circles; their mutual intersections (β-points) cluster at 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.