Maharashtra HSC Geology (MSBSHSE) key terms

The upper continental crust, rich in silica and alumina, made largely of granite and granitic gneiss. It is lighter than the crust beneath the oceans and floats above the denser sima.

The lower crust and upper mantle zone dominated by silica and magnesium minerals such as olivine and pyroxene. It is denser than sial and underlies the ocean basins directly.

The Mohorovicic discontinuity, a seismic boundary where P-wave velocity jumps to about 8 km/s, marking the base of the crust. It lies near 5-10 km depth under oceans and 30-70 km under continents.

The state of gravitational balance in which lighter crustal blocks float on the denser mantle, adjusting until pressure at a compensating depth is equal. Loading by ice or sediment causes slow vertical adjustment.

The standard division of Earth history into eons, eras, periods and epochs, based on fossils and radiometric ages. Boundaries mark major changes in life or tectonics, such as the Permian-Triassic extinction.

The principle that in an undisturbed sequence of sedimentary rocks, each layer is younger than the one below it and older than the one above. It is the basis for relative dating.

One of seven geometric classes defined by the symmetry and axial relationships of a crystal: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and trigonal. Each permits a limited set of forms.

A geometric operation that leaves a crystal pattern unchanged: a plane of reflection, an axis of rotation, a centre of inversion or a rotoinversion axis. Their combination determines the crystal class.

Three integers (hkl) that describe the orientation of a crystal face or lattice plane, obtained from the reciprocal of its intercepts on the crystallographic axes. They are used to identify forms and cleavage.

The tendency of a mineral to break along planar surfaces determined by weak bonding or close-packed atomic planes. Its number and angle distinguish minerals such as mica, calcite and galena.

A relative scale from 1 (talc) to 10 (diamond) ranking resistance to scratching. A mineral scratches those below it and is scratched by those above, giving a quick field identification test.

The ratio of a mineral's mass to the mass of an equal volume of water, reflecting atomic packing and cation content. Most silicates lie near 2.6-3.0 while oxides and sulphides exceed 4.

The arrangement of silicon-oxygen tetrahedra in a mineral, from isolated units to chains, sheets and frameworks. The degree of sharing of oxygen atoms controls cleavage, hardness and stability.

The order in which silicate minerals crystallise from a cooling basaltic magma, from olivine and calcium plagioclase at high temperature to quartz and potassium feldspar at low temperature. It also predicts weathering resistance.

The process by which a single parent magma produces compositionally different rocks through crystal settling, filter pressing or mixing. Fractional crystallisation of mafic minerals leaves residual melt richer in silica.

The size, shape and mutual relations of crystals in an igneous rock, recording its cooling history. Coarse grains indicate slow, deep cooling; fine grains or glass indicate rapid surface chilling.

An igneous texture in which large crystals (phenocrysts) sit in a finer groundmass. It records two stages of cooling, typically slow crystallisation at depth followed by rapid cooling on ascent.

The average diameter of particles in a clastic rock, used to name it as gravel, sand, silt or clay. Size reflects transport energy and is measured by sieving or settling methods.

The degree to which sediment grains are of similar size. Well-sorted sediment has been reworked by wind or water; poorly sorted sediment records rapid deposition without winnowing.

The set of processes, including compaction and cementation, that turn loose sediment into a hard sedimentary rock. Cement of silica, calcite or iron oxide fills pore spaces between grains.

A planar layering in metamorphic rock produced by realignment of platy minerals or segregation of light and dark bands under directed stress. Slaty cleavage and gneissic banding are examples.

The intensity of metamorphism a rock has experienced, expressed by its mineral assemblage from low to high temperature and pressure. Index minerals such as chlorite, biotite and garnet mark successive grades.

Two angles that define the orientation of a planar geological surface. Strike is the compass direction of the horizontal line on the plane; dip is the steepest angle and direction of tilt.

A fracture along which rock bodies have moved relative to each other, classified as normal, reverse or strike-slip by the sense of displacement. Movement is driven by tectonic stress.

A bend in layered rock produced by compression, described as anticline when layers arch upward and syncline when they sag downward. Its axis and limbs are mapped to reconstruct structure.

The theory that the lithosphere is divided into rigid plates that move over the asthenosphere, creating earthquakes, volcanoes and mountain belts at their boundaries. Sea-floor spreading and subduction drive the motion.

A convergent margin where an oceanic plate sinks beneath another plate into the mantle. It generates deep earthquakes, arc volcanoes and trenches, and is the site of high-pressure metamorphism.

A fossil species with wide geographic range and short stratigraphic duration, used to correlate rocks and fix ages. Ammonites, graptolites and trilobites are classic examples.

The ancient southern supercontinent uniting India, Africa, South America, Australia and Antarctica, assembled in the late Precambrian and broken up in the Mesozoic. Its sedimentary sequences record glaciation and coal formation.

A vast flood basalt province covering much of Maharashtra and neighbouring states, erupted around the Cretaceous-Palaeogene boundary. Its lava flows and intertrappean beds record rapid volcanism and fossil life.

A mineral from which a metal or valuable substance can be extracted economically. Common examples are galena for lead, sphalerite for zinc and bauxite for aluminium.

The worthless minerals associated with ore in a deposit, such as quartz, calcite or pyrite, which must be separated before smelting. Its proportion affects the economic viability of mining.

A natural concentration of minerals in the crust that is large enough and rich enough to be worked at a profit. Deposits form by magmatic, hydrothermal, sedimentary or residual processes.

A residual ore of aluminium formed by intense tropical weathering of aluminous rock, rich in gibbsite and boehmite. Maharashtra and other Indian states hold significant lateritic deposits.

A geological formation that stores and transmits groundwater in usable quantities, such as sandstone, fractured basalt or gravel. Its yield depends on porosity and permeability.

The fraction of a rock's volume occupied by open spaces or pores. It determines how much water a rock can hold but not how easily the water moves.

The ability of a porous material to transmit fluid through connected pore spaces. High permeability allows wells to yield water readily; clay may be porous yet nearly impermeable.