RBSE SS Geology key terms

A relative ranking of ten reference minerals from talc (1) to diamond (10) that expresses how easily one mineral scratches another. It is ordinal, not proportional, so the gap between 9 and 10 is far larger than between 1 and 2.

The tendency of a mineral to break along flat planes determined by planes of weak atomic bonding in its crystal lattice. It is described by the number of directions, their angles, and quality, and it differs from fracture, which is irregular breakage.

One of seven classes (cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, trigonal) defined by the lengths of the unit-cell axes and the angles between them. The system constrains the symmetry elements and forms a mineral may display.

The difference between the highest and lowest refractive indices of an anisotropic mineral, seen under crossed polars as interference colours. It is a key diagnostic property in thin-section identification and is reported as a numerical value.

A change in colour or shade of a mineral as the microscope stage is rotated under plane-polarised light, caused by selective absorption of light along different crystallographic directions. Strong pleochroism helps identify minerals such as biotite and hornblende.

An experimentally derived sequence in which minerals crystallise from a cooling basaltic magma, from olivine and calcium-rich plagioclase at high temperature to quartz, muscovite and potassium feldspar at low temperature. It explains mineral associations and weathering resistance in igneous rocks.

The process by which crystals formed early are separated from the remaining melt, so the residual liquid becomes progressively enriched in silica, alkalis and incompatible elements. It is a principal cause of compositional variation within igneous suites.

Any process that changes the bulk composition of a magma body, including fractional crystallisation, assimilation of wall rock, magma mixing and volatile loss. It produces the range of rock types seen in a single igneous province.

The incorporation and melting of country rock into a magma, which shifts the melt composition toward the material consumed. It is recognised by xenoliths and by isotopic and trace-element signatures that cannot be explained by crystal fractionation alone.

The size, shape and mutual arrangement of grains in a rock, reflecting its cooling history. Phaneritic textures indicate slow cooling at depth, aphanitic textures rapid cooling at the surface, and porphyritic textures two-stage cooling.

The framework of broken detrital grains, matrix and cement in a sedimentary rock. Grain size, sorting, roundness and matrix content record transport distance and depositional energy.

The degree to which grains in a sediment or sedimentary rock are of similar size. Well-sorted sediment implies sustained transport by wind or water; poorly sorted sediment suggests rapid deposition or mass flow.

How far a sediment has evolved toward a stable end-product, judged by textural features (sorting, roundness) and mineralogical features (loss of feldspar and unstable heavy minerals). High maturity indicates prolonged weathering and transport or repeated recycling.

A planar fabric in a metamorphic rock produced by the preferred orientation of platy or elongate minerals, or by compositional layering. Its style and orientation record the direction of differential stress during deformation.

The intensity of metamorphism a rock has experienced, expressed as a range of temperature and pressure between diagenesis and melting. It is estimated from mineral assemblages and index minerals rather than from depth alone.

A mineral whose first appearance in a rock of suitable bulk composition marks a specific temperature-pressure condition, allowing metamorphic zones to be mapped. Examples include chlorite, biotite, garnet, staurolite, kyanite and sillimanite.

The sequence of metamorphic facies developed across a belt as pressure and temperature change, reflecting the geothermal gradient of the tectonic setting. Low-pressure series indicate contact or rift settings, whereas high-pressure series indicate subduction.

The standard measurement of an inclined planar surface: strike is the compass direction of the line formed where the plane meets horizontal, and dip is the angle of steepest descent measured perpendicular to strike. Together they fully define the attitude of bedding, foliation or a fault.

The line that, when moved parallel to itself, generates the folded surface; its orientation is described by trend and plunge. The axis is used to classify folds and to reconstruct the direction of shortening.

The three-dimensional orientation in space of a geological surface, usually given as strike and dip or as dip direction and dip amount. Correct attitude measurements underpin all structural interpretation and cross-section construction.

A fracture along which the two sides have moved relative to each other, classified as normal, reverse, strike-slip or oblique from the sense of slip. Slickensides, drag folds, offset markers and fault breccia are common field evidence.

A surface representing a break in sedimentation, where younger strata overlie older rocks after erosion or non-deposition. Angular, disconformity and nonconformity types record different histories of tilting, uplift and erosion.

The branch of stratigraphy that divides rocks into named units based on observable lithology and boundaries, arranged in a hierarchy of group, formation, member and bed. The formation is the fundamental mappable unit.

A designated reference section and point that defines the base or boundary of a stratigraphic unit, providing a fixed standard for correlation. A Global Boundary Stratotype Section and Point formally defines a stage boundary.

A body of strata characterised by a distinctive natural association of three or more fossil taxa, used for biostratigraphic correlation. It is defined by the total assemblage rather than by the range of a single species.

A species with a wide geographic range and a short stratigraphic duration, making it useful for correlating rocks between distant sections. Good examples include graptolites, ammonites and certain foraminifera.

The study of what happens to an organism between death and discovery, including decay, transport, burial and diagenetic alteration. It explains why fossil assemblages rarely mirror the original living community.

The ancient southern supercontinent assembled from South America, Africa, Antarctica, Australia, India and Madagascar, which began to break up in the Mesozoic. Its former unity is supported by matching glacial deposits, fossil floras and faunas, and structural trends.

A large igneous province of flood basalts covering much of western and central India, erupted mainly around the Cretaceous-Palaeogene boundary. Its lava flows and intertrappean beds are classic field localities for Indian geology.

A mineral from which a metal or other valuable constituent can be extracted economically. Ore minerals are distinguished from gangue minerals, which are the unwanted associated minerals in the same deposit.

The non-valuable minerals associated with ore in a deposit, such as quartz, calcite or pyrite. Gangue content affects processing costs and therefore the economic cut-off grade of the deposit.

The concentration of the valuable element or mineral in an ore body, expressed as weight percent, grams per tonne or parts per million. Grade combined with tonnage and recovery determines whether a deposit is economic.

Mineralogical and chemical change of the host rock adjacent to a vein or intrusion, caused by hydrothermal fluids. Zonation of alteration minerals such as sericite, chlorite and epidote is a widely used exploration guide.

A scale representation of rock units, structures and boundaries projected onto a horizontal plane, with a legend, scale and orientation. It is the primary synthesis product of field mapping and the basis for cross-sections.

A vertical profile drawn along a chosen line across a geological map, showing the interpreted arrangement of units and structures at depth. It must remain consistent with map patterns, dip measurements and stratigraphic thicknesses.

A system such as RMR or Q that scores a rock mass for engineering purposes using intact strength, joint spacing, joint condition, groundwater and orientation. The resulting index guides tunnel support and slope design.

Downslope movement of rock, debris or soil under gravity when driving forces exceed resisting forces. Slope angle, rock structure, weathering, water pressure and vegetation loss are controlling factors assessed in engineering geological mapping.

The upper surface of the saturated zone in an unconfined aquifer, below which pore spaces are filled with water. Its depth and fluctuation affect slope stability, foundation design and borehole yields.