(a) Describe the tectonic features and evolution of the 'continental crust'. (10 marks) (b) Describe the physical and chemical…

(a) Tectonic features and evolution of the continental crust

The continental crust is the thick, buoyant, silicic to intermediate upper layer of the plates, 30 to 70 km thick, with a density of about 2.7 g/cm³, against 5 to 10 km and about 3.0 g/cm³ for oceanic crust.

(i) Tectonic features. Stable Archaean cratons and shields, such as the Dharwar and Singhbhum cratons of India, with surrounding mobile belts (orogens); layered upper and lower crust separated by the Conrad discontinuity; rifts, passive margins and sedimentary basins; and large igneous provinces such as the Deccan Traps.

(ii) Evolution. The first crust was differentiated from mafic protocrust by partial melting, giving Archaean tonalite-trondhjemite-granodiorite (TTG) suites. It then grew by subduction-related arc magmatism and accretion of arcs, plateaus and terranes, and was reworked and stabilised (cratonised) by collision, partial melting and granite intrusion, in cycles of supercontinent assembly and breakup (Wilson cycles). Erosion and delamination recycle part of it back.

Because it is light, it resists subduction, so it preserves rocks up to about 4 billion years old.

(b) Physical and chemical weathering

Weathering is the in-place breakdown of rock at the Earth's surface by physical, chemical and biological processes, leaving regolith.

(i) Physical (mechanical) weathering. Rock is broken without change in composition. Processes are frost wedging, where water freezes in joints and expands, as in the Himalaya; exfoliation by pressure release in granite domes; thermal expansion and contraction in deserts; salt crystallisation in pores; and root wedging.

(ii) Chemical weathering. The minerals themselves change, and the processes are speeded by warmth and water. Hydrolysis turns feldspar into clay (kaolinite); oxidation turns iron minerals and pyrite to iron oxides and hydroxides; carbonation and solution dissolve limestone, forming karst, and hydration turns anhydrite into gypsum. Laterite forms on Deccan basalt under a humid tropical climate.

Physical weathering increases surface area, which speeds chemical weathering. The products form soils, supply sediments and solutes to rivers and the sea, and concentrate residual ores such as bauxite.

(c) Spatial and spectral resolution of a remote sensor

(i) Spatial resolution. This is the size of the smallest ground area recorded in one pixel, in metres, and it determines how much detail can be seen. Landsat-8 OLI has 30 m, Sentinel-2 has 10 m bands, IRS LISS-III 23.5 m, and Cartosat-1 panchromatic about 2.5 m, which can show individual buildings.

(ii) Spectral resolution. This is the ability to separate narrow wavelength intervals, by the number and width of bands. Multispectral sensors, such as Landsat-8 and LISS-III, have a few broad bands in the visible, near-infrared and shortwave infrared, good for vegetation, water and broad rock mapping. Hyperspectral sensors, such as AVIRIS and PRISMA, and India's HySIS, record hundreds of narrow contiguous bands, which can identify minerals such as clay and carbonate by their absorption features.

Finer spatial resolution shows the shape and size of objects, and finer spectral resolution shows their composition, but the two trade off against each other, and finer resolution in either lowers the signal-to-noise ratio.

(d) Cataclasite and pseudotachylite

Both are fault rocks formed in shear zones.

(i) Cataclasite. It forms by cataclasis, the brittle fracturing, grinding and rotation of grains in the upper crust, at shallow to medium depth. The rock is cohesive and non-foliated, with angular fragments (porphyroclasts) set in a fine-grained crushed matrix. It records brittle deformation and strain localisation along faults, and occurs along thrusts such as the Himalayan Main Boundary Thrust.

(ii) Pseudotachylite. This is a dark, glassy or very fine-grained rock formed by frictional melting of the wall rock during rapid, seismic slip on a fault. It occurs as thin veins and injection veins, with quenched melt and relict clasts, and sometimes spherulites and glass that later devitrifies. It is direct evidence of ancient earthquakes, and differs from ultramylonite, which forms by ductile shear.

(e) Symmetrical and asymmetrical folds

A fold is a bend in layered rock, with a hinge, two limbs and an axial plane joining the hinges.

(i) Symmetrical fold. The two limbs dip at equal angles in opposite directions, and the axial plane is vertical. The limbs are mirror images. It forms under uniform, balanced compression, as in the gentle folds of foreland basins.

(ii) Asymmetrical fold. The limbs dip at unequal angles, one steeper than the other, and the axial plane is inclined and does not bisect the angle between the limbs. The steep limb points in the direction of tectonic transport, as in the thrust belt of the Himalayan front.

(iii) Related forms. Further tilting gives overturned (both limbs dipping the same way) and recumbent folds, and the plunge of the hinge gives plunging folds.

The difference rests on the dip of the limbs and the orientation of the axial plane, which record the stress and shear that produced them.