Maharashtra HSC Geology (MSBSHSE) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Nebular hypothesis

Model in which the solar system formed from a rotating cloud of gas and dust that condensed and accreted into planetesimals; Chamberlin-Moulton added the planetesimal stage.

Moho discontinuity

Boundary between the crust and mantle, marked by an abrupt increase in seismic wave velocity; it lies about 5-10 km below oceans and 30-70 km below continents.

S-wave shadow zone

Region beyond about 103 degrees from an earthquake epicentre where S waves are not recorded, proving that the outer core is liquid since shear waves cannot pass through liquids.

Airy isostasy

Model in which crustal blocks of equal density float with variable thickness, so a mountain range is supported by a deep low-density root projecting into the mantle.

Half-life formula

Radioactive decay follows N = N0 e to the power minus lambda t, and half-life equals 0.693 divided by the decay constant lambda; used for radiometric dating of rocks.

Principle of superposition

In an undisturbed sequence of sedimentary rocks, each layer is younger than the one below it and older than the one above it; a basic relative dating rule.

Hexagonal crystal system

Crystal system with four axes: three equal horizontal axes at 120 degrees and one vertical c axis at 90 degrees; includes quartz and beryl, with sixfold or threefold symmetry.

Monoclinic system

Crystal system with three unequal axes, two at 90 degrees and one inclined; it has one twofold axis or a mirror plane, as in gypsum, orthoclase, and augite.

Mohs hardness scale

Relative scratch hardness from talc 1 to diamond 10; quartz is 7, calcite 3, and a steel knife about 5.5, allowing quick field identification of minerals.

Silicate tetrahedron

Fundamental SiO4 unit of all silicates, with one silicon surrounded by four oxygens; tetrahedra link by sharing oxygens to form chains, sheets, or frameworks.

Phyllosilicate structure

Silicate class with continuous sheets of linked tetrahedra, giving perfect basal cleavage and flaky habit; includes mica, chlorite, talc, and clay minerals.

Bowen's reaction series

Sequence of crystallisation from cooling magma: olivine, pyroxene, amphibole, biotite on the discontinuous branch, and calcium to sodium plagioclase on the continuous branch.

QAPF classification

Streckeisen diagram for plutonic igneous rocks using modal quartz, alkali feldspar, plagioclase, and feldspathoid percentages to assign rock names such as granite or gabbro.

Porphyritic texture

Igneous texture with large crystals set in a finer groundmass, recording two stages of cooling: slow crystallisation at depth followed by rapid cooling near the surface.

Ophitic texture

Texture typical of dolerite in which plagioclase laths are enclosed by larger pyroxene crystals; it indicates simultaneous crystallisation of the two minerals.

Deccan Traps

Flood basalt province of western India erupted about 66 million years ago near the Cretaceous-Palaeogene boundary, covering much of Maharashtra with tholeiitic lava flows.

Graded bedding

Sedimentary structure in which grain size decreases upward within a bed, formed by waning currents; characteristic of turbidites and used to identify way-up direction.

Dunham classification

Carbonate rock classification based on depositional texture: mudstone, wackestone, packstone, grainstone, boundstone, and crystalline limestone, reflecting mud content and grain support.

Isograd

Line on a map joining points of equal metamorphic grade, marked by the first appearance of an index mineral such as garnet or kyanite in rocks of suitable composition.

Slate to gneiss series

Progressive foliated metamorphic sequence with increasing grade: slate, phyllite, schist, and gneiss, reflecting increasing grain size and segregation of minerals.

Strike and dip

Strike is the compass direction of a horizontal line on a bedding plane; dip is the angle of steepest descent perpendicular to strike, giving the plane's orientation.

Normal fault

Fault caused by extension in which the hanging wall moves down relative to the footwall; common in rift valleys and associated with crustal stretching.

Recumbent fold

Fold with a nearly horizontal axial plane, often produced by intense compression; one limb may be overturned so that older rocks are displaced over younger ones.

Sea-floor magnetic stripes

Symmetrical bands of alternating normal and reversed magnetisation on either side of mid-ocean ridges, recording sea-floor spreading and supporting plate tectonics.

Koyna-Warna seismicity

Reservoir-triggered earthquake activity in western Maharashtra near the Koyna dam, one of the best-documented examples of human-induced seismicity in the world.

Index fossil

Fossil of a species with a short stratigraphic range and wide geographic distribution, used to identify and correlate rock layers, such as ammonites and graptolites.

Gondwana Supergroup

Permian to Jurassic continental sequence of peninsular India with glacial Talchir beds, Barakar coal measures, and plant fossils such as Glossopteris.

Siwalik Group

Neogene foreland basin sediments along the Himalayan foothills, famous for Miocene-Pliocene vertebrate fossils including Sivapithecus and Ramapithecus.

K-Pg iridium anomaly

Thin clay layer at the Cretaceous-Palaeogene boundary with high iridium, shocked quartz, and microtektites, evidence for the Chicxulub impact and mass extinction.

Wardha valley coalfield

Gondwana coalfield in eastern Maharashtra with Permian Barakar coal seams worked at Umrer, Kamptee, and Wani; coal is high-ash but important for thermal power.

Lateritic bauxite

Aluminium ore formed by intense tropical weathering of basalt, leaving residual alumina and iron oxides; occurs on plateaus of Ratnagiri, Sindhudurg, and Kolhapur.

Darcy's law

Groundwater flow rate equals hydraulic conductivity times hydraulic gradient times cross-sectional area; the basis of aquifer yield and well hydraulics calculations.