(a) What are the different layers in the Earth's interior ? How is the layered structure of the Earth determined ? Name two most…

(a) Earth’s layered interior The Earth is differentiated into crust, mantle, outer core and inner core. The crust is a thin silicate shell; the mantle is a silicate layer; the outer core is a liquid Fe-Ni alloy; the inner core is solid Fe-Ni. The layered structure is determined chiefly by seismic wave studies — travel times, refraction and reflection at discontinuities (modern tomography adds three-dimensional detail): P and S waves change velocity at the Moho, the 410/660 km discontinuities, the Gutenberg (core–mantle) discontinuity and the Lehmann (inner-core) discontinuity; S waves do not travel through the liquid outer core, creating shadow zones. Meteorite analogies, high-pressure experiments, geothermal gradients and geomagnetic field data refine density, temperature and state. Density increases with depth (crust about 2.7–3.0, mantle 3.3–5.7, outer core 9.9–12.2, inner core 12.8–13.1 g/cm³); the inner core is solid because pressure raises the melting temperature faster than temperature rises, and convection in the outer core sustains the geodynamo. The two most abundant elements are oxygen and silicon in the crust, oxygen and magnesium in the mantle (with silicon a very close third), and iron and nickel in both the outer and inner cores, with light elements such as sulphur and oxygen in the outer core.

(b) Elemental classes and trace elements Major elements exceed 1 wt% in a rock; minor elements are 0.1–1 wt%; trace elements are below 0.1 wt%. Goldschmidt’s classification links chemistry to mineral hosts, and his rules of diadochy (camouflage, capture and admission) govern how trace elements substitute in host minerals: lithophile (oxygen-loving) elements such as Al, Na, Ca, Mg and Si enter silicates; chalcophile (sulphur-loving) elements such as Cu, Zn, Pb, Ag, Hg and As (nickel is also chalcophile) enter sulphides; siderophile (iron-loving) elements such as Fe, Ni, Co, Au, Pt, Ir and Os concentrate in metal phases (and hence in the core); atmophile (gas-loving) elements such as H, N and noble gases remain volatile. Trace elements are more efficient than major elements for interpreting Earth processes because their concentrations respond strongly to small changes in temperature, pressure, oxygen fugacity and melt composition. Modern analytical methods detect them at low concentrations, so small geochemical signals are not masked by major-element abundance; partition coefficients make them sensitive tracers of partial melting, crystal-liquid fractionation, metasomatism, fluid transport, ore-forming processes and tectonic settings, and give discriminatory power in petrogenetic modelling. Compatible elements (e.g., Ni in olivine) stay in the solid, whereas incompatible elements (Rb, Ba, U, REE) concentrate in melts; REE patterns, spider diagrams and discrimination diagrams are the working tools.

(c) Mining and water pollution Mining disturbs rock, exposes sulphides, and creates waste that mobilises contaminants. The issue has chemical, hydrological and socio-economic dimensions. Surface water is polluted by acid mine drainage: pyrite oxidation (2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4) produces sulphuric acid, and microbial catalysis (Acidithiobacillus ferrooxidans) oxidises Fe2+ to Fe3+, whose hydrolysis (Fe3+ + 3H2O → Fe(OH)3 + 3H+) drops the pH to 3–4 and gives the yellow-ochre precipitate; Fe, Mn, As, Cd, Pb and Hg are leached in the process; by sediment loading from overburden and tailings; and by tailings-dam failures that release slurries. Groundwater is contaminated through seepage from waste-rock dumps, tailings ponds and pit lakes, and by dewatering that lowers water tables, concentrates salts and metals, and induces subsidence. Open pits and shafts intercept aquifers, while abandoned workings store contaminated water. In India, coal mining in Jharia and the Damodar basin has degraded rivers, soils and aquifers; iron-ore mining in Goa and Odisha has increased turbidity and sedimentation in the Mandovi and coastal waters; uranium mining at Jaduguda raises radionuclide and heavy-metal concerns in groundwater; copper mining at Khetri and bauxite mining in the Eastern Ghats have affected local water quality and recharge, and acid drainage from coal mining in the Jaintia Hills (Meghalaya) has made streams such as the Lukha acidic. The impacts are economic and social: reduced crop yields, damaged fisheries, higher public-health costs, and loss of drinking-water sources.

Mitigation. Geochemical controls decide how serious the pollution is: Eh-pH, redox and mineral stability determine whether metals remain solid or become mobile. Sustainable practice therefore requires mine-water treatment, neutralisation of acidity with lime, passive treatment (constructed wetlands, sulphate-reducing bioreactors), lined tailings with dry stacking or paste disposal, controlled dewatering, monitoring of aquifers and rivers, and closure planning. It also requires long-term monitoring after closure, because abandoned mines can continue to generate acid water for decades. In India, Coal India and other miners now treat and supply surplus mine water for drinking, irrigation and industry, while SDG 6 and SDG 12 link clean water and responsible consumption to mining governance. A balanced approach should combine stricter environmental clearances, community participation, remediation funds and circular use of mine water, so that mineral development does not transfer pollution to surface and groundwater.