Assam Higher Secondary Geology cheat sheet

Earth's internal layers by composition: crust (oceanic ~5-10 km, continental ~30-70 km), mantle (to ~2900 km), core (Fe-Ni); by rheology: lithosphere (rigid, ~100 km), asthenosphere (plastic, low-velocity zone), mesosphere, outer core (liquid, S-wave shadow), inner core (solid).

Seismic evidence: P-waves (compressional, fastest, travel through solids and liquids) refract and are absent in the 103°-143° shadow zone; S-waves (shear, slower) cannot pass the liquid outer core, creating the S-wave shadow beyond 103°.

Moho = crust-mantle boundary (increase in P-wave velocity); Conrad discontinuity = upper-lower crust; Gutenberg discontinuity (~2900 km) = mantle-core boundary; Lehmann discontinuity marks inner-core boundary.

Isostasy: Airy model assumes uniform density with varying root thickness; Pratt model assumes varying density at constant depth. Continental crust floats higher because it is thicker and less dense (~2.7 g/cm³ vs ~3.0 g/cm³ oceanic).

Wegener's continental drift evidence: jigsaw fit of continents, matching fossils (Glossopteris), glacial striations, matching rock belts; rejected for lack of a driving mechanism, later supplied by sea-floor spreading.

Sea-floor spreading (Hess, Dietz): new oceanic crust forms at mid-ocean ridges, magnetic polarity stripes are symmetrical about ridges, crust age increases away from ridges, and ocean trenches mark subduction.

Plate boundaries: divergent (ridges, rifts, shallow quakes, basaltic volcanism), convergent (trenches, Benioff zone deep quakes, andesitic volcanism, fold mountains), transform (strike-slip, shallow quakes, no volcanism).

Plate motion drivers: slab pull (dominant), ridge push, and mantle convection; rates typically 1-10 cm/year, measurable by GPS and magnetic anomaly dating.

Indian plate context: the Indian plate moves NNE at roughly 5 cm/year; collision with Eurasia formed the Himalaya and continues to cause shallow-to-intermediate seismicity in Northeast India and the Assam region.

Mineral definition: naturally occurring, inorganic, solid, definite chemical composition, ordered atomic structure. Key physical properties: hardness (Mohs 1 talc to 10 diamond), cleavage vs fracture, lustre, streak, specific gravity, colour.

Silicate structures: isolated tetrahedra (olivine), single chain (pyroxene), double chain (amphibole), sheet (mica, clay), framework (quartz, feldspar). Silicates dominate the crust because Si-O bonds are strong and abundant.

Bowen's reaction series: discontinuous branch olivine → pyroxene → amphibole → biotite; continuous branch Ca-plagioclase → Na-plagioclase; then orthoclase → muscovite → quartz (lowest temperature, most felsic).

Igneous classification by SiO2: ultramafic (<45%, peridotite), mafic (45-52%, gabbro/basalt), intermediate (52-65%, diorite/andesite), felsic (>65%, granite/rhyolite). Texture reflects cooling rate: phaneritic = slow/intrusive, aphanitic = fast/extrusive, porphyritic = two-stage, glassy = very rapid.

Sedimentary rocks: clastic (conglomerate, sandstone, shale) classified by grain size; chemical/biochemical (limestone, dolomite, chert, evaporites); organic (coal). Grain rounding and sorting indicate transport distance and energy.

Metamorphic rocks: foliated (slate → phyllite → schist → gneiss, increasing grade) and non-foliated (quartzite from sandstone, marble from limestone). Index minerals (chlorite, biotite, garnet, staurolite, kyanite, sillimanite) map metamorphic grade.

Rock cycle: weathering and erosion → sediment → compaction and cementation (lithification) → sedimentary rock; heat and pressure → metamorphic; melting → magma → igneous. Any rock type can transform into any other; the cycle is not a fixed sequence.

Assam-relevant geology: the Shillong Plateau and Mikir Hills expose Precambrian granites and gneisses; the Assam-Arakan basin contains Tertiary coal, limestone, and oil-bearing sandstones; the Barail and Tipam formations are key hydrocarbon reservoirs.