(a) Discuss two widely accepted theories of origin of the earth. Elucidate the position of all planets within the solar system…

(a) The origin of Earth is best explained by accretion theories tied to the early Solar System. The Nebular Hypothesis, associated with Kant and Laplace, holds that the Sun and planets condensed from a rotating cloud of gas and dust; as it cooled and contracted, material flattened into a disc and coalesced into planets. The Planetesimal Hypothesis of Chamberlin and Moulton (1905) proposed that a passing star pulled material from the Sun, which cooled into small solid bodies, planetesimals, that collided and accreted into planets. The passing-star idea has been abandoned, but its planetesimal accretion survives in the modern solar-nebula model (Safronov), in which dust in the disc grew into planetesimals and then protoplanets. The nebular model emphasises a continuous disc, whereas the planetesimal model emphasises discrete solid bodies that grew by collisions. The modern consensus combines the two: a collapsing nebula formed the disc, and planetesimal accretion within it built the planets about 4.57 billion years ago, as meteorite ages show. The planets, in order from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; the first four are small rocky terrestrial planets, and the last four are large gas or ice giants, separated from the inner planets by the asteroid belt. Earth is the third planet, about 1 AU (150 million km) from the Sun, about 4.54 billion years old, with a mean radius of about 6,371 km. It is unique in density (about 5.51 g/cm³, the highest of all planets), a thick nitrogen-oxygen atmosphere, a stable hydrosphere with 71 per cent surface water, a strong magnetic field, active plate tectonics, and life.

(b) Planar structures are surfaces in rocks, including bedding, foliation, cleavage, joints and faults. Planar structures may be primary, inherited from deposition or crystallisation, or secondary, produced by deformation. Bedding records sedimentation, foliation reflects directed pressure in metamorphic rocks, cleavage is a repeated parting, joints are fractures without slip, and faults are displacement surfaces. Linear structures are elongated features such as lineation, fold axes and mineral streaks; lineation may be formed by stretched minerals, flow in igneous rocks, or shearing. Boudinage is a ductile extensional structure. Boudins form when a competent layer is stretched parallel to its length in a weaker matrix. As extension continues, the layer necks, thins and finally breaks into isolated blocks, while the matrix fills the gaps. The process is common in extensional terrains, fold hinges and shear zones, and the boudin spacing and shape help estimate strain. Strong competence contrast gives rectangular, sharply separated boudins; weak contrast gives lenticular pinch-and-swell structures; extension in two directions gives chocolate-tablet boudinage.

(c) Remote sensing uses sensors on satellites, aircraft and drones to collect electromagnetic data. In geology it supports lithological mapping by distinguishing rock types through spectral signatures, structural analysis by mapping lineaments, faults and folds, and mineral exploration through alteration minerals such as clays, iron oxides and sulphates. ASTER shortwave infrared data are useful for hydrothermal alteration, while lineament maps guide drilling. Radar (SAR) data are useful under cloud cover, especially for monsoon-affected regions. In India, the Geological Survey of India and ISRO use Landsat, Sentinel, ASTER and thermal data for mineral prospecting, groundwater mapping, landslide monitoring in the Himalaya and Western Ghats, and coastal zone management. It also aids hazard assessment, urban geology and resource inventory.

(d) Soil formation is a slow pedogenic process in which parent material is transformed into a layered profile. Soil formation begins with weathering of parent rock. Mechanical weathering breaks rock into fragments by temperature change, frost, root action and unloading; chemical weathering alters minerals by hydrolysis, oxidation and carbonation. Organic matter accumulates as humus, and biological activity mixes, transports, leaches and enriches the profile. With time, horizons develop: O is organic litter, A is humus-rich topsoil, B is subsoil with accumulation of clays or iron, C is weathered parent material, and R is bedrock. Climate, parent rock, topography, time and organisms control rate and type; for example, hot humid Indian conditions favour laterite, while flat alluvial plains develop thick soils.

(e) Dip is the maximum angle of inclination of a rock bed measured perpendicular to strike. True dip is this maximum angle; apparent dip is the angle measured in any other direction, usually on a map, and is always less than true dip unless it coincides with true dip. It is smaller because the measured section is oblique to the true dip line: tan (apparent dip) = tan (true dip) × sin β, where β is the angle between the strike and the section. The strike line is horizontal and defines the trend of the bed on the map. If a bed dips 30° towards North 30° East, the dip direction is 30° azimuth. Strike is perpendicular to dip direction, so add and subtract 90°: 30° + 90° = 120°, and 30° - 90° = -60°, equivalent to 300°. Thus the strike is N60°W - S60°E, with azimuths 300° and 120°.