Karnataka II PUC Geology (KSEAB) flashcards

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

Mohorovičić discontinuity

The boundary between the Earth's crust and mantle, marked by an abrupt increase in seismic P-wave velocity to about 8 km/s. It lies roughly 35 km deep beneath continents and 5–10 km beneath oceans.

Angle of repose

The steepest slope angle at which loose, unconsolidated granular material remains stable without sliding. It depends on grain size, shape, sorting and moisture, and marks the threshold for mass wasting on a slope.

Dendritic drainage pattern

A branching, tree-like drainage network that develops on rocks of uniform resistance, such as granite or flat-lying sediments. Tributaries join main streams at acute angles, following the regional slope.

Mohs hardness scale

A relative ten-point scale of mineral scratch hardness from talc (1) to diamond (10). Each mineral scratches those below it; the intervals are not equal, so it indicates order rather than absolute hardness.

Silicate tetrahedron

The fundamental SiO4 structural unit of silicate minerals, in which one silicon atom is surrounded by four oxygen atoms. The way these tetrahedra link determines the silicate class and cleavage.

Rhombohedral cleavage

Cleavage shown by minerals such as calcite, breaking along three planes that are not mutually perpendicular, giving rhombohedron fragments. It reflects the internal atomic arrangement of the mineral.

Pleochroism

A change in colour of an anisotropic mineral when viewed in different crystallographic directions under plane-polarised light. It is caused by selective absorption of light and helps identify amphiboles and biotite.

Bowen's reaction series

The sequence in which minerals crystallise from a cooling silicate melt: olivine, pyroxene, amphibole and biotite in the discontinuous branch, with Ca- to Na-plagioclase in the continuous branch, ending with quartz.

Porphyritic texture

An igneous texture with large crystals (phenocrysts) set in a finer-grained groundmass. It records two stages of cooling, with slow crystallisation at depth followed by rapid cooling nearer the surface.

Graded bedding

A sedimentary structure in which grain size decreases upward within a single bed. It forms from a waning current, such as a turbidity current, and helps identify the tops of beds in deformed sequences.

Index minerals

Minerals such as chlorite, biotite, garnet, staurolite, kyanite and sillimanite whose first appearance marks a specific metamorphic grade. Their zones map increasing temperature and pressure in the field.

Quartzite

A non-foliated metamorphic rock formed from quartz-rich sandstone by recrystallisation. Quartz grains fuse into a hard, compact mosaic, giving the rock high resistance to erosion and little cleavage.

Anticline

A fold in which strata arch upward, with the oldest rocks exposed in the core and limbs dipping away from the axis. Its hinge line may plunge, and it forms under compressional stress.

Normal fault

A dip-slip fault in which the hanging wall moves down relative to the footwall, caused by tensional or extensional stress. It produces horst and graben structures and rift valleys.

Angular unconformity

An unconformity where tilted or folded older strata are overlain by younger, nearly horizontal beds, with an erosion surface between them. It records deformation followed by erosion and renewed deposition.

Ophiolite sequence

A distinctive assemblage of peridotite, gabbro, sheeted dykes, pillow basalt and deep-sea sediments, interpreted as obducted oceanic crust. Its presence marks a former ocean basin and a suture zone.

Index fossil

A fossil species with a wide geographic range, short stratigraphic duration and easy recognition, used to correlate rock sequences. Trilobites, graptolites and ammonites are classic examples.

Principle of superposition

In an undisturbed sequence of sedimentary rocks, the oldest layer lies at the bottom and the youngest at the top. This principle underpins relative dating and the reconstruction of geological history.

Glossopteris flora

A Permian seed-fern flora of the Gondwana continents, characterised by tongue-shaped leaves. Its distribution supports continental drift and Gondwana glaciation, and it occurs in Indian coal measures.

Stromatolite

A layered, domed or columnar structure built by mats of blue-green algae trapping sediment. Stromatolites are common in Precambrian carbonate rocks and record shallow marine environments.

Banded iron formation

A sedimentary rock with alternating thin layers of iron oxide and chert, mostly of Precambrian age. It is the principal source of iron ore, including deposits in the Dharwar schist belts of Karnataka.

Placer deposit

A concentration of heavy, resistant minerals such as gold, diamond or monazite formed by water or wind sorting during weathering and transport. Placers occur in stream gravels and beach sands.

Kolar Gold Fields

Historic gold mines in the Kolar schist belt of Karnataka, where gold occurs in quartz veins within Archaean greenstone rocks. They illustrate the gold-bearing potential of Dharwar schist belts.

Cut-off grade

The minimum metal concentration at which ore can be mined and processed at a profit. Material below this grade is treated as waste, and the value depends on metal price and processing cost.

Darcy's law

The flow rate through a porous medium equals hydraulic conductivity times cross-sectional area times the hydraulic gradient. It is the basic equation for groundwater flow and well yield estimation.

Confined aquifer

An aquifer overlain by an impermeable layer, so water is under pressure and rises in a well to the potentiometric surface. If that surface is above ground level, the well flows artesian.

Specific yield

The volume of water an aquifer releases by gravity drainage per unit volume of rock, expressed as a fraction. It is always less than porosity because some water is retained by surface tension.

Rock mass rating

A numerical classification of rock mass quality based on strength, RQD, joint spacing, joint condition, groundwater and orientation. It guides tunnel support and foundation design in engineering geology.

Electrical resistivity survey

A geophysical method that measures subsurface resistance to electrical current to detect layers with contrasting conductivity. It is widely used for groundwater, ore and shallow engineering investigations.

Factor of safety

The ratio of resisting forces to driving forces on a potential landslide surface. A value above one indicates stability; drainage and benching increase it by reducing driving forces.

Closepet Granite

A late Archaean, north–south trending belt of granite and migmatite in Karnataka, about 2.5 billion years old. It marks a major thermal boundary within the Dharwar Craton.

Dharwar Supergroup

An Archaean sequence of metasedimentary and metavolcanic rocks in Karnataka, divided into the Bababudan and Chitradurga groups. It hosts important iron, manganese and gold deposits.