Karnataka II PUC Geology (KSEAB) key terms

A relative ranking from 1 (talc) to 10 (diamond) showing which minerals scratch which. A harder mineral scratches a softer one, so the scale tests resistance to scratching rather than absolute hardness.

The tendency of a mineral to break along flat planes fixed by its internal atomic layering. It is described by the number of planes and their angles, and differs from fracture, which is irregular breakage.

The way a mineral surface reflects light, described by words such as metallic, vitreous, pearly, silky or earthy. It is judged on a fresh surface and helps identify ore minerals quickly in hand specimen.

The colour of the powder left when a mineral is rubbed on unglazed porcelain. Streak is often more constant than the colour of the bulk specimen and is especially useful for identifying oxide and sulphide minerals.

The ratio of a mineral's mass to the mass of an equal volume of water. Values near 2.7 suit quartz and feldspar, while above about 4 suggests dense metallic minerals such as haematite or galena.

The basic building block of silicate minerals: one silicon atom surrounded by four oxygen atoms. Linking these units in chains, sheets or frameworks produces the main rock-forming mineral groups.

A group of framework silicate minerals containing aluminium and either potassium, sodium or calcium. Feldspars are the most abundant minerals in the crust and are common in igneous, metamorphic and many sedimentary rocks.

Silicon dioxide crystallising as a hard, glassy mineral with no cleavage and conchoidal fracture. It resists weathering, so it survives transport and becomes concentrated in sandstones and river sands.

The order in which silicate minerals crystallise from cooling magma, from olivine and calcium-rich plagioclase at high temperature to quartz and potassium feldspar at low temperature. It explains mineral associations in igneous rocks.

The process in which early-formed crystals are separated from the remaining melt, so the residual liquid becomes progressively richer in silica, sodium and potassium. It drives the chemical diversification of magmas.

Any process that changes the composition of a magma body so that different parts solidify into different rocks. It includes crystal settling, assimilation of wall rock and mixing of separate magma batches.

Rock crystallised from magma below the surface, typically with visible interlocking crystals because cooling was slow. Batholiths, dykes and sills are common intrusive bodies, and granite is a familiar example.

Rock formed when lava or pyroclastic material reaches the surface and cools quickly. Rapid cooling usually gives fine-grained or glassy textures, as in basalt flows and volcanic ash deposits.

An igneous texture in which large crystals sit in a finer-grained groundmass. It records two cooling stages: slow growth underground followed by faster solidification after the magma moved or erupted.

The texture of a sedimentary rock made of fragments of older rocks or minerals cemented together. Grain size, sorting, roundness and matrix content are described to infer transport history.

Two measures of sediment maturity: sorting describes how uniform grain sizes are, and roundness describes how much grain corners have been worn. Both increase with longer transport and reworking.

A planar layering in metamorphic rock produced by aligned platy minerals or by compositional segregation under directed pressure. Slaty cleavage, schistosity and gneissic banding are forms of it.

The intensity of metamorphism a rock has experienced, usually expressed by temperature and pressure. Low grade gives slate or phyllite, medium grade schist, and high grade gneiss or granulite.

The growth of new mineral grains at the expense of older ones without melting the rock. It changes grain size, shape and mineral assemblage so the rock better fits the new pressure and temperature.

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 of that plane from horizontal.

A fracture along which rock masses have moved relative to each other. Faults are classified as normal, reverse or strike-slip according to the sense of slip and the stress that produced them.

The line drawn along the crest or trough of a folded surface, following its maximum curvature. The axis and axial plane together describe the geometry and orientation of a fold.

A surface representing a gap in the rock record, where erosion removed material before younger strata were deposited. Angular, nonconformity and disconformity types record different histories.

The branch of stratigraphy that divides rocks into named units based on observable rock character rather than age. Formations, members and groups are the formal lithostratigraphic units.

A fossil species with a short geological range and wide geographic spread, used to correlate strata and assign relative ages. Ammonites, graptolites and certain trilobites are classic examples.

Correlation of rock sequences using their contained fossils and the ranges of species. It allows strata in different regions to be matched even where the rock types differ completely.

A body of sedimentary rock distinguished by features reflecting a particular depositional environment. Lateral changes from one facies to another record shifting environments such as beach, lagoon and offshore mud.

A mineral from which a useful element or compound can be extracted at a profit. Ore minerals are usually sulphides or oxides, and their concentration depends on the deposit type and grade.

The unwanted minerals associated with ore in a deposit, commonly quartz, calcite or pyrite. Gangue must be separated from ore minerals during processing, which affects the economic viability of a mine.

A mineral deposit formed from hot aqueous fluids circulating through fractures in rock. Cooling, boiling or reaction with wall rock causes metals to precipitate, producing veins and replacement bodies.

A rock unit that can store and transmit groundwater in usable quantities. Sandstone and fractured granite or basalt can act as aquifers, while unfractured clay-rich units do not.

Porosity is the fraction of rock volume that is open space; permeability is the ease with which fluid flows through connected spaces. High porosity without connection gives storage but little yield.

The upper surface of the saturated zone in an unconfined aquifer, below which pore spaces are filled with water. It rises and falls with recharge and discharge and defines the level reached by wells.

The breakdown of rock in place by physical, chemical or biological processes without transport. It prepares material for erosion and produces soils and secondary minerals such as clay.

Erosion is the removal and transport of weathered material by water, wind or ice; deposition is its laying down when energy falls. Together they shape landforms and build sedimentary sequences.

A residual soil or rock rich in iron and aluminium oxides, formed by intense leaching in tropical climates. Laterite caps and bauxite deposits are widespread on the Karnataka plateau.

The ancient, stable block of Archaean and Proterozoic rocks forming most of Karnataka. It contains greenstone belts with gold, iron and manganese, and is a key reference for regional geology.

An elongated Archaean terrain of metamorphosed volcanic and sedimentary rocks, typically low grade and greenish. Karnataka's belts host important gold, iron and manganese mineralisation.