Cameroon GCE A-Level Geology key terms

A ranking of ten reference minerals from talc (1) to diamond (10) that orders them by how easily one is scratched by another. It is a relative scale, not a measure of absolute hardness.

The tendency of a mineral to break along flat planes that follow zones of weak bonding in its crystal lattice. Described by number of directions, angle between them and quality, for example perfect basal cleavage in mica.

One of seven classes (cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, trigonal) defined by the symmetry and axial relationships of a mineral's unit cell. It predicts crystal shape and helps identify unknown specimens.

The arrangement of silicon-oxygen tetrahedra in a mineral, ranging from isolated tetrahedra through chains and sheets to three-dimensional frameworks. The type of linkage controls cleavage, hardness and resistance to weathering.

An experimental sequence showing the order in which minerals crystallise from a cooling basaltic melt, from olivine and calcium-rich plagioclase first to quartz and potassium feldspar last. It also predicts the reverse order of melting and weathering resistance.

The process by which crystals forming in a cooling magma are separated from the remaining liquid, so the residual melt becomes progressively richer in silica, alkalis and volatiles. It drives compositional variation within igneous suites.

The general set of processes, including fractional crystallisation, assimilation of wall rock and liquid immiscibility, that make related magmas diverge in composition from a common parent. It explains the diversity of igneous rocks in one province.

Incomplete melting of mantle or crustal rock, producing a melt enriched in silica and incompatible elements relative to the source. The degree of melting and source composition control the chemistry of the magma generated.

The texture of a sedimentary rock made of broken grains, described by grain size, sorting, roundness and the nature of the matrix or cement. It records transport energy and distance from the source area.

The degree to which grains in a sediment or sedimentary rock are of similar size. Well-sorted sediment indicates prolonged transport or winnowing by wind or water; poorly sorted sediment suggests rapid deposition close to source.

All the low-temperature physical and chemical changes that convert loose sediment into solid rock after deposition, including compaction, cementation, recrystallisation and mineral replacement. It excludes metamorphism and weathering.

A set of mineral assemblages that characteristically develop in rocks of a given bulk composition over a limited range of pressure and temperature. Named facies such as greenschist and amphibolite allow metamorphic grade to be mapped.

A mineral whose first appearance or disappearance in a rock marks a specific pressure-temperature boundary, allowing metamorphic grade to be mapped in the field. Common examples are chlorite, biotite, garnet and sillimanite.

A planar fabric in metamorphic rock produced by the parallel alignment of platy or elongate minerals, or by compositional layering. Its development and orientation record the direction of directed stress during metamorphism.

Two angles that define the orientation of a planar geological feature. Strike is the compass direction of the line where the plane meets horizontal; dip is the angle of steepest descent measured perpendicular to strike.

The line that, when moved parallel to itself, generates the folded surface of a bed. Its trend and plunge, together with the axial plane, describe the geometry and orientation of a fold.

A fracture along which the two sides have moved relative to each other. Classified as normal, reverse, strike-slip or oblique according to the direction of slip, which in turn reflects the stress regime.

A surface representing a break in sedimentation, where younger strata overlie older rocks after erosion or a pause in deposition. Angular, nonconformity and disconformity types each imply a different history.

The branch of stratigraphy that divides rock successions into named units based on observable lithology and boundaries, such as formations, members and groups. It is independent of fossil content and geological age.

The correlation of rock successions using the fossil content they contain, especially short-ranging index species. It provides relative ages and allows correlation between areas with different lithologies.

A fossil species with a wide geographical range but a short stratigraphic range, making it useful for identifying and correlating a narrow time interval. Ammonites and graptolites are classic examples.

The sum of lithological and palaeontological characters of a sedimentary rock that reflect its environment of deposition. Lateral changes in facies record how environments shifted across a basin.

A transgression is a landward shift of the shoreline as sea level rises or land subsides; a regression is the seaward shift as sea level falls or sediment progrades. Each produces a characteristic vertical facies succession.

The work of rivers in eroding, transporting and depositing sediment, producing features such as channels, point bars, floodplains and terraces. Discharge and sediment load control the channel pattern and deposit character.

A deeply weathered, iron- and aluminium-rich residual soil formed under hot, humid tropical conditions with strong leaching of silica and bases. In Cameroon it hosts bauxite and nickel-cobalt deposits and hardens on exposure.

The process by which descending meteoric water dissolves metals from the upper part of an ore body and redeposits them at the water table, raising grades. Common in sulphide deposits of copper and other metals.

A mineral from which a metal or other valuable commodity can be extracted economically. Its identity, abundance and texture, together with gangue minerals, determine the ore's grade and processing behaviour.

A concentration of dense, resistant minerals formed by the winnowing action of water or wind, which removes lighter material and leaves the heavy grains behind. Alluvial gold and diamond placers are economically important.

A Cretaceous sedimentary basin trending northeast from the Gulf of Guinea across Nigeria and into northern Cameroon. It records rifting linked to the opening of the South Atlantic and hosts lead-zinc and other mineralisation.

A roughly 1,600 km chain of Cenozoic volcanic centres and plutonic complexes running from the Gulf of Guinea through Cameroon to Lake Chad. Its origin is debated, with plume and lithospheric crack models proposed.

A late Neoproterozoic to early Palaeozoic episode of collision and crustal thickening that welded much of Africa, including the Central African Orogenic Belt. Its granitoids and migmatites form much of Cameroon's basement.

A map showing the distribution, boundaries and orientation of rock units at the surface, with a legend, scale and north direction. Structural symbols convey strike, dip, faults and folds.

A vertical profile drawn along a line across a geological map to show the subsurface arrangement of rock units and structures. It must be consistent with surface dips and map relationships.

A scaled vertical log showing the sequence and thickness of rock units in an area, from oldest at the base to youngest at the top. It summarises lithology, fossils and boundaries observed in the field.

Establishing the order of geological events without assigning numerical ages, using principles such as superposition, cross-cutting relationships, inclusions and faunal succession. It is the first step before radiometric dating.

Determining the numerical age of a rock or mineral from the ratio of a radioactive parent isotope to its decay product, given the decay constant. Useful systems include potassium-argon, rubidium-strontium and uranium-lead.

A gravity value corrected for latitude, elevation and the mass of rock between the station and the reference level, revealing subsurface density variations. It is used to detect basins, intrusions and crustal thickness changes.

A geophysical method that images subsurface boundaries by recording the travel time of sound waves reflected from layers where acoustic impedance changes. It is a standard tool in petroleum and basin exploration.