Seafriends - Glossary of geological terms

by Dr J Floor Anthoni
What stands between you and the knowledge of the Earth is not as much the understanding of it, but the many new words necessary for describing the many geologic features and processes. This glossary aims to help you to translate and to remember these new words. Use it as a reference or print it out. Just reading it, is interesting.

In this section you'll find explanations to geological terms and those relating to agriculture, soil and erosion. See also the rock and soil classification table and the periodic table of elements. The coloured words are explained elsewhere in this glossary.


-- home -- books, glossaries & tables -- issues -- Revised 20010612,20020703,


A -- top --
Ablation: (L: ab=away, from; lat= past part stem of ferre=to carry) all processes by which snow and ice are lost from a glacier.
Abrasion: (L: ab=away; radere=to scrape)the mechanical wearing or grinding away of rock surfaces by the friction and impact of rock particles.
Acidic: (L: acidus= sour; acidere= to be sour) rock containing more than 60% silica. Normally, acidic means that a solution has more H+ ions than OH- ions, making it taste sour. But in the rocks from a magma melt, the term is used to grade rock by its silica concentration. Rock with little silica (less than 40%) is called basic. See rocktable/magma.
Acidification: the various processes that make a soil more sour and less suitable for most plants. Naturally, carbon dioxide binds with water to form a mild acid. Leaf litter with a high content of resins, is decomposed producing acids. Sulphuric acid is part of natural rainwater but is also found in much higher concentrations in acid rain from industries. Nitric acid is also formed from industrial smoke. Nitrogen fertilisers like ammonium nitrate can break down to nitric acid in the soil.
Accretion: (L: ad=to/onto; crescere= to grow) the growing of separate things into one. The gradual addition of new land to old by sedimentation or plate tectonics.
Accumulation: (L: ad=to/onto; cumulus=heap) growing by heaping up. See also accretion.
Adobe: (Spanish) a mixture of clay and silt, found in the desert basins of the USA and Mexico, extensively used for building in the form of sun-dried bricks.
Aeolian: (L: Aeolus= god of the winds) transported by wind, wind-borne..
Agate: a translucent cryptocrystalline variety of quartz, being a variegated chalcedony or alternating with opal.
Aggrade: (L: ad=to/onto; gradus=step) opposite of degrade. The process of building up a surface by deposition.
Albite: (L: albus=white) a feldspar, usually white, rich in sodium. NaAlSi3O8. A variety of plagioclase common in igneous and metamorphic rock
Alfisols: (?) soils with clay-enriched subsoils. See rocktable/soil.
Allitic weathering: forming 1:1 clays or two-layer clays. Typical of humid, warm areas; formed through rapid leaching of silica.
Alluvium: (L: ad= to/onto; luere=wash) sediment deposited by rivers and streams. It ranges from clays, through silts and sands to gravels.
Alum: a colourless or white mineral with a sweet-sour astringent taste. KAl(SO4)2.12H2O. Used to stem blood flow from wounds after shaving.
Alumina: oxides of aluminium. Al2O3.
Amorphous clays: (L/Gk: amorphus/ amorphos= shapeless, formless) clays consisting of a mix of iron and aluminium sesquioxides (Fe2O3, Al2O3), lacking the layered structure of one- and two-layer clays.
Amphibole: (L/Gk: amphibolia= ambiguous) the horneblende minerals that are confusing because of their many forms.
Andesite: (Andes= mountain chain in S. America) a fine-grained brown or greyish intermediate extrusive volcanic rock.
Andisol/ andosol: (Andes= mountain chain in S.America) soil originating from volcanic ash.
Ando: (Andes= mountain chain in S America) volcanic ash.
Anion: a negatively charged ion such as OH-, SO4--.
Anorthite: plagioclase calcium feldspar. White to medium gray with striations (stripes). CaAl2Si2O8.
Apatite: (Gk: apate= deceit, from its deceptive forms) a naturally occurring crystalline mineral of calcium phosphate and fluoride, used in the manufacture of fertilisers. Ca5(PO4,CO3)3(F,OH,Cl).
Argillic development: (Gk: argos= white) the development of a hard layer of silicate clays in the soil, restricting soil depth root penetration.
Argillite: (Gk: argos=white) a sedimentary rock composed of clay particles which have been hardened and cemented. Argil = pottery clay. Marl and shale smell earthy when breathed upon.
Aridisols: (L: aridus= dry; solum= ground) soils of deserts and semi-arid regions, and related saline or alkaline soils.
Asbestos: (Gk: asbestos= unquenchable; a-=not; sbestos/ sbennumi=quench) a hydrated silicate mineral of the serpentine (chrysotile asbestos) or amphibole group. Hydrated silicates are minerals composed of silica and a metallic element, which are chemically combined with water. Chrysotile, the best known, most abundant, and widely used type of asbestos, consists of alternate sheets of magnesia and silica, coiled into extremely fine tubes called fibrils. Amphibole asbestos has coarser fibres than chrysotile.
Ash: see tephra.
Asthenosphere: (Gk: asthenes=weak) a hot, soft layer of the earth's mantle, on which the lithosphere floats.
Augite: (Gk: auge= lustre) a complex calcium magnesium aluminous silicate occurring in many igneous rocks. One of the most common pyroxenes.
Autochthonous soils: (Gk: auto= self; chthonos= earth) original soils found at the place where they were formed by weathering.
B -- top --
Bank collapse: mass movement of stream banks, once under-cut by the scouring action of floodwater.
Basalt: (Gk: basanos= touchstone) a dark basic extrusive volcanic rock whose strata sometimes form columns. It consists mainly of the minerals plagioclase feldspar and pyroxene. Rich in iron and magnesium-bearing minerals, poor in silica and characteristic of the ocean's crust.
Basic: (L: basis= stepping; bassus= short) rock having less than 50% silica. Normally, basic means a liquid with more OH- ions than H+ ions.
Biotite: (Named after the French physicist J B Biot who died in 1862) a black, dark brown or green micaceous (mica-like) mineral occurring as a constituent of metamorphic and igneous rocks.
Blanking: replacement of any seedling trees that have died and left blanks in a regular planting pattern.
Braided channel: a river channel that has numerous branches, separated by exposed alluvial bars.
Breccia: a volcanic rock, composed of angular mineral fragments embedded in a matrix, the product of explosive eruptions.
C -- top --
Calcification/ calcify:  (L: calx= lime) to become hardened by the depositon of calcium salts.
Caldera: (L: caldus/calidus=warm; caldarius= steam room; Spanish: caldaria= boiling pot) A large volcanic depresssion, often a large crater with other craters or volcanoes inside.
Carbon ignition: conversion of soil organic matter to CO gas, by high temperture reaction, when overlying vegetation is burned.
Catalist: (Gk: kata=down; luo=set free) substances that speed up a chemical reaction without being consumed by the reaction.
Cation: a positively charged ion such as H+, Mg++, Ca++, K+.
Cation Exchange Capacity: the capacity of soil minerals, particularly clays, to attract, retain and exchange positively charge ions (cations). It is measured in units of centimole/kg of dry soil.
CEC: see cation exchange capacity.
Check dam: steel and concrete structure in a mountain torrent, designed to trap sediment and to prevent scour by debris flows.
Chernozem: (Russian: chernyi= black; zemlya=earth) also called black earth, is a fertile black soil rich in humus, found in temperate regions, particularly in southern Russia. High in silicate clay minerals with minimal clay illuviation and a dark coloured A horizon containing at least 4% organic matter.
Chondrite, carbonaceous: (Gk: chondros=granule)  a type of stony meteorite containing small mineral nodules, rich in water and light elements such as carbon. Assumed to resemble the original material of the mantle.
Clastic: (Gk: klastos= broken in pieces) composed of broken pieces of older rocks. Clastic rocks are conglomerates, sandstone, mudstone, etc.
Close planting: plantation of trees sufficiently dense to form a continuous or close canopy. Seedling trees planted at densities of 1000 stems a hectare or more, form a close canopy within 5 to 10 years, depending on species. Mature trees maintain a close canopy even if thinned to 200-400 stems per hectare.
Collapse hole: circular hole in topsoil, caused by its collapse into a tunnel (under-runner) in the subsoil.
Colloid: (Gk: kolla=glue) ultra small solid particles dispersed in liquid, not settling out. See also emulsion. Whereas emulsions are liquid 'particles' dispersed into another liquid, into which it won't dissolve, colloids are solid particles dispersed into a liquid.
Colluvium: (L: collis= hill; luere= to wash; hill-washed)  gravity-transported soils. Slips and earth slumps can transport soil.
Condition (law): a formal restriction on the way land is used, attached to a resource consent, issued by a district or regional council, or written as a rule in a district or regional plan.
Coquina: (Spanish: coquina= cockle) a soft limestone of broken shells, often used for road-making.
Corundum: (Tamil: kurundam; Sanskrit kuruvinda=ruby) an extremely hard crystalline alumina (aluminium oxide) used especially as an abrasive, and varieties of which (ruby, sapphire) are used for gemstones.
Craton: (?) a stable portion of a continent, commonly of Precambrian age, and not deformed for a long time.
Crust: (L: crusta= rind, shell) the upper solid shell of the earth, separated from the underlying mantle by the Mohorovicic discontinuity, and of lower density than the mantle.
Cryptocrystalline: (Gk: cryptos= hidden) texture of a rock with crystals that are to small to be recognised under a microscope.
Crystal: a clear, transparent mineral.
Cutan: ?
D -- top --
Dacite: (?) An extrusive igneous rock in composition between rhyolite (thick) and andesite (fluid), consisting mainly of the minerals feldspar and quartz. Dacite flows less easily than andesite, and releases its dissolved gases with explosions, forming pumice in the process. Rhyolite is even more explosive.
Debris dam: (Fr: débriser= to break down) timber and netting structure in a hillside gully, designed to trap sediment and to prevent scour by runoff.
Degrade: (L: de,dis=negation, not; gradus=step) to reduce to a lower form.
Density (vegetation): the thickness of plant cover growing on a given area of land, usually expressed as a  percentage of cover for grass, or as stems per hectare for trees.
Desilicification: the loss of silica and siliceous compounds.
Diatomite: (Gk; dia=two; temmo=to cut; diatomos= two-halved, referring to the pill-box shape of diatoms) a silica deposit originating from the siliceous frustules (shells) of marine and freshwater diatoms. Because of its high surface area, absorptive capacity and chemical stability, it is used as a mild abrasive, in filtration, insulation and cement.
Diorite: (Gk: diorizo= to distinguish) a coarse-grained plutonic igneous rock containing quartz. Consists mainly of feldspar, amphibole and biotite.
Dodecahedron: (Gk: dodeka= twelve; hedron= seat) a solid figure with twelve faces.
Dolomite. (French geologist: D de Dolomieu, died in 1801) a mineral of calcium magnesium carbonate.
Downs: undulating, low hills, usually with slopes gentle enough to be cultivated.
Dystrophic: (Gk: dys= bad; trepho= to feed, nourish) lacking nutrients. Nutrient-deficient. Opposite of eutrophic (overfed).
E -- top --
Effervescence: (L: ex=out of; fervere= to be hot) foaming of soil or rock in reaction to hydrochloric acid (HCl); the 'acid test'. Particularly because of its carbonate (CaCO3, MgCO3) content. In arid climates, effervescence is found in the B horizon.
Eluviate: (L: ex= out of; luere= to wash) to leach outward. The loss of nutrients and minerals upward and out of the soil profile.
Emulsion: (L: ex=out/from; mulgere= to milk) a fine dispersion of one liquid in another but not dissolving, often looking milky..
Entisol: (?) soils without layering, except after ploughing. See rocktable/soils.
Environmental effect: any impact which a land use has on soil, water, air, plants or animals.
Enrichment: planting timber trees (either exotic or indigenous) in regenerating scrub.
Enstatite: a mineral of MgSiO3 with colours ranging from white through brown to grey.
Ephemeral (stream): (Gk: epi=upon  hemeros=lasting one day; hemera=day) a water course which flows after heavy rain, and dries up during fine weather.
Epidote: a calcium/aluminium/iron silicate Ca2(Al,Fe)3[OH/(SiO4)]3
Erosion: (L: ex=out of; rodere= to gnaw) the stripping of soil and weathered rock from landforms; creating sediment for transport by water, wind or ice, and enabling the formation of new sedimentary deposits.
Eutrophic: (Gk: eu= well; trepho= to feed/ nourish) rich in nutrients.
Evaporite: (L: ex=out of; vaporare= steaming; vapor=steam) a sedimentary rock produced by the evaporation of seawater and precipitation of its salts. It contains a wide range of components, among which rock salt and gypsum.
Extrusion/extrusive: (L: ex= out of; trudere= to thrust) molten volcanic rock deposited on the surface and cooled rapidly.
F -- top --
Facies: (L: facies=face) the character of rock etc, expressed by its composition, fossil content etc, reflecting the environment in which it was formed.
Farm improvements: buildings, yards, fences, tracks, drains, ponds, water reticulation.
Fault: a fracture in a rock body along which one side has been displaced, relative to the other.
Fayalite: Fe2SiO4. An olivine silicate, dark-green or black.
Feldspar: (German: felspat) any of a group of aluminium silicates of potassium, sodium, or calcium, which are the most abundant minerals in the earth's crust.
Ferrugination: (L: ferro= iron; ferruginis= rust) iron sesquioxides adhere firmly to sand and gravel, giving them a red colour, and may cement them to form a subsurface iron pan.
Floodplain: low-lying alluvium immediately adjacent to a river or stream, inundated as soon as it overflows its banks.
Fluorite: (L: fluere= to flow; fluor= melting flux) a mineral form of calcium fluoride (fluorspar).
Foresterite: ?
Formation: (L: forma= form) a suite of rocks with common characteristics distinguishing it from other suites nearby and traceable or mappable over a reasonably large area. An assemblage of strata having some common characteristic.
Fracture zone: a dislocation at right angles to a mid-ocean ridge, usually marked by a set of transverse ridges and troughs.
Fragipan: (?) A soil horizon consisting of an irregular, very hard accumulation of acid clay, thought to have formed by clay seeping down cracks in the profile.
G -- top --
Gabbro: (It: Gabbro in Tuscany) a dark, granular plutonic rock of crystalline structure.
Gabion: (It: gabbia= basket) a wire basket filled with rocks, used to stabilise a riverbank or foot of an unstable slope. Also used in fortification.
Garnet: (L: granatum= pomegranate, from its resemblance to the pulp of the fruit) a vitreous (glassy) silicate mineral, especially a transparent deep-red kind used as a gem.
Geology: (Gk: ge=earth; logike= art of reason) the science of the  earth, including the composition, structure, and origin of its rocks.
Geography: (Gk: ge=earth; graphe=writing) the study of the earth's physical features, resources and climate, and the physical aspects of its population.
Geosyncline: (Gk: ge=earth; syn= with; klino=to slope) a large, elongated depression in earth's crust in which thick sediments accumulate, followed by deformation into a mountain range.
Gibbsite: Aluminium hydroxide. Al(OH)3. Found in aluminium ore. The most common hydrated aluminium oxide found in soils, consisting of a double layer of hydroxyl molecules enclosing aluminium in the octahedral position.
Glacial: (L: glacies=ice) characterised or produced in the presence of ice. Glacial epoch = an interval of cold climate leading to great expansion of continental icecaps.
Gleisation: (gley = Ukrainian word for clay) under waterlogging conditions, iron becomes ferrous (Fe++). Ferrous soils consist of sticky clay with a grey-blue colour, and are called gley. Because ferrous iron is more mobile than ferric iron (Fe+++), it may get eluviated from the soil, resulting in a leached upper horizon.
Gneiss: (German: Gneiss) a coarse-grained metamorphic rock foliated by mineral layers, principally of feldspar, quartz and ferromagnesian minerals.
Granite: (L: granon= grain) a granular crystalline igneous rock of quartz, mica, feldspar, etc., used for building.
Granodiorite: an intrusive crystalline igneous rock with properties in between granite and diorite.
Greenstone belt: an elongated, folded complex of slightly altered sediments and volcanic rocks of oceanic and intermediate character, usually of Precambrian age.
Greenstone: 1) a greenish igneous rock containing feldspar and hornblende. 2) a variety of jade found in New Zealand.
Greywacke: a sedimentary rock composed of silt or sand particles which have been hardened and cemented.
Groundwater: water which flows through pores and fissures in soil or rock, beneath the water table.
Gullying: erosion of soil or rock by ephemeral streams, downcutting through hill slopes after heavy rain.
Gypsum: (Gk: gypsos= gypsum) a hydrated form of calcium sulphate, occurring naturally and used in the building industry and to make plaster of Paris.
H -- top --
Halite: (Gk: hali/hals= sea; hals= salt) rock-salt, not obtained by evaporating seawater.
Heavy soil: soil with a high ratio of clay to sand or silt; sticky when wet, hard when dry, and difficult to cultivate.
Hill country: steep land with slopes greater than 20 degrees, but low relief, typically to 100-300 m difference in elevation. Valley bottoms are usually narrow.
High country: mountainous land with steep slopes and a high relief; typically more than 300 m difference in elevation. Slopes are usually separated by broad, flat valley bottoms, infilled by alluvium.
Histosols: (Gk: histos=web; solum=ground) ?
Horizon: functionally different layers within the soil profile. O, A, B, C, R horizons.
Hornblende/ horneblende: (German) a dark-brown, black or green mineral occurring in many igneous and metamorphic rocks, and composed of calcium, magnesium and iron silicates.
Hotspot: an anomalously hot area in the mantle below the lithosphere, source of volcanic activity in a plate as it passes overhead.
Humification: (L: humus= soil) the process of forming humus from organic matter. Soil organic mater can be doubled in 5-50 years.
Humus: (L: humus= soil) residues from organic matter like leaves, roots, algae and soil organisms, transferred by microorganisms into a mixture with soil minerals. Humus is a persistent form of organic matter that is not easily broken down.
Hydrargillite: (Gk hydor= water; argos= clay) aluminium hydroxide (Al(OH)3), a colourless white sediment.
Hydrothermal: (Gk: hydor=water; therme=heat) the activity of hot waters in the crust.
Hygroscopic: (Gk: hygros=moist,wet; skopeo=to look at) the ability to take in water from the atmosphere.
Hypersthene: (Gk hyper= over/beyond; sthenos= strength, being harder than hornblende) a rock forming greenish mineral of magnesium iron silicate, harder than hornblende.
I -- top --
Igneous: (Lat: ignis = fire) arising from hot liquid magma deep inside the earth. Volcanic.
Ignimbrite:  sheets of coalesced fine particles which once flowed at high speed as extremely hot fluid (pyroclastic flow) avalanches down a volcano. The high heat of over 1000ºC coalesces all particles to form rock, looking like concrete (welded ignimbrite).
Illuvial horizon: a horizon formed by the deposition of materials leached out of an overlaying layer.
Illuviate: (L: in=in/into; luere= to wash) similar to leaching. Loss of minerals and nutrients by water, moving down in the soil profile.
Inceptisols:  (?) very young soils with weakly developed soil layers, and not much leaching or mineral alteration.
Intermediate: (L: inter= between, among; medius= middle) rock consisting of about 50% silica.
Intrusion/ intrusive: (L: intrudere; intrus= into; trudere= to thrust) an influx of molten rock between or through strata, but not reaching the surface; cooling slowly.
Isostasy: (Gk: isos=equal; stasis= station) the general state of equilibrium with the rise and fall of the earth's crust as a result in changes in its buoyancy.
J -- top --
Jade: (Spanish: piedra de ijeda= stone of the flank, thought to cure colic, a severe spasmodic abdominal pain) a collective term for jadeite NaAlSi2O6 and nephrite Ca2(Mg,Fe)5(OH,F)2Si8O22.
K --top --
Kaolin/kaolinite: (Chinese: gaoling; gao=high; ling= hill) a fine soft white clay produced by the decomposition of other clays of feldspar, used especially for making porcelain, and in medicines. Also called china clay.
Kaolisol: (from kaolin and soil) soils of the humid tropics that contain more bases in their upper horizon, than below, which is opposite to the norm.
Karst: (= a limestone region in YugoSlavia) a limestone region with underground drainage and many passages and cavities caused by the dissolution of the rock.
Krasnozem: (?) a deep, red soil formed under free drainage in the humid tropics. Its clayey texture is dominated by sesquioxides.
Kyanite: (Gk: kyanos= dark blue) a blue crystalline mineral of aluminium silicate.
L -- top --
Lahar: (Javanese) a mud flow formed through interaction of pyroclastic flows with water from melting ice, crater lakes or rain. Having high density (weight), mud flows are fast, and they scour the river beds through which they flow, accumulating more mass as they go, and causing serious damage and hazard to life.
Laterite/ic: (L: later= brick) Laterite is a red or yellow ferruginous clay, friable and hardening in air, used for making roads in the tropics. Lateritic soil is sesquioxide-dominated, deeply weatherd, containing sequences of horizons of laterite.
Lava: (L: lavare= to wash) molten volcanic rock and the rock formed after cooling.
Leaching: (Old-English: leccan= to water; Dutch: lekken= to leak) the loss of water-soluble minerals and nutrients from the soil by water.
Light soil: soil with a high ratio of sand or silt to clay; aggregated when wet, but powdery and difficult to cultivate when dry.
Lithosphere: (Gk lithos= stone) the hard and solid, outermost layer of the Earth.
Lithosol: (Gk: lithos= stone) stony soil with shallow profiles as found on steep mountains.
Loam: (Old-English Lam) a soil with substantial proportions of clay, silt and sand, with an aggregated structure enabling easy cultivation, whether wet or dry.
Loess: (German: loesen= to loosen) silt deposited by winds blowing across sparsely vegetated ground during the last ice age. It mantles many plains, terraces and downs in areas near ancient glaciers. Very fertile when irrigated.

M -- top --
Magma: (Gk: magma/ masso= to knead) fluid or semifluid material deep inside the earth from which igneous rock is formed.
Magnetite: (Magnesia= an island in Asia Minor, from which magnetic rock was obtained) black, unhydrated magnetic iron oxide.
Marble: (L: marmor; Gk: marmaros= shining stone) limestone in a metamorphic crystalline (or granular) state, and capable of taking a polish. Used in sculpture and architecture.
Mass movement: erosion of soil or rock by gravity-induced collapse. Usually triggered by groundwater pressure after heavy rain, but can also have other causes, notably streams undercutting the base of a slope, or earthquakes. Movement can either be rapid and near-instantaneous (landslides, avalanches, debris flows) or slow and intermittent (earthflows, slumps).
Metamorphic/ metamorphism:  (Gk: meta= with/after; morphe= form) rock that has undergone transformation by natural agencies such as heat and pressure.
Mica: (L: mica=crumb) any of a group of silicate minerals with a layered structure, especially muscovite.
Mica-schist: a fissile rock containing quartz and mica.
Mineral: (L: minera=ore) a solid, inorganic substance, found in nature, which has  an orderly internal structure and a specific chemical composition.
Mohorovicic discontinuity (Moho): (named after Mohorovic) the boundary between crust and mantle, marked by a sharp decrease in earthquake wave speed.
Mollisols: (L: mollis=soft; solum=ground) grassland soils, mostly rich in calcium; also forest soils developed on calcium-rich parent materials. They are characterised by a thick surface layer, rich in organic material.
Montmorillonite: (Montmorillon= a place in the south of France) a three-layer clay (smectite group), capable of high cation exchange and holding large amounts of water. Sticky when wet.
Mudstone: a sedimentary rock composed of silt and clay particles, compacted and weakly cemented together by a small quantity of lime.
Muscovite: (L: Muscovia= moscow) a silver-grey form of mica with a sheetlike crystalline structure that is used in the manufacture of electrical equipment, etc.
N -- top --
Nitrogen volatilisation: (volatile= evaporative. L: volare= to fly) conversion of soil nitrogenous matter to NO and NO2 gas, by high-temperature reaction when overlying vegetation is burned, or by bacteria in the soil.
Nontronite: (?) an amorphous smectite clay formed from weathered volcanic ashes, basalt. Na(0.33)Fe2(Al(0.33)Si(3.67))O10(OH)2.nH2O.
O -- top --
Octahedron: (Gk: okto=eight; hedra= seat) a body (crystal) contained within eight triangular planes of equilateral triangles.
Olivine: (Say: "olivene") (olive-green colour) a naturally occurring form of magnesium-iron silicate, usually olive-green and found in igneous rocks. Other names: peridot, chrysolite. (Mg,Fe)2SiO4.
Organic load: the quantity of organic matter passing a stream reach in the course of a year. Usually expressed in terms of biological oxygen demand, i.e the amount of oxygen consumed by micro-organisms as they break it down.
Orogeny: (Gk: oros=mountain; genes=born, of a specified kind; gignomai=be born, become) mountain building, involving deformation, metamorphism, uplift and intrusion of igneous rock. It happens when continents collide.
Orthoclase: (Gk ortho=straight; klasis=cleavage) a common alkali feldspar usually occurring as variously coloured crystals, used in ceramics and glassmaking.
Orthorhombic: (Gk: orthos= straight; rhombos= a parallelogram with oblique angles and equal sides, a rhombus ) a crystal, characterised by three mutually perpendicular axes, which are unequal in length, such as in topaz and talc.
Outwash: alluvial sand and gravel deposited by rivers, draining glaciers during the last ice age. Widespread on terraces and floodplains in areas near ancient glaciers.
Oxysols: (Gk oxus=sharp) Soils that are more weathered than the ultisols, including most laterites.
P -- top --
Pair planting: plantation of trees in pairs on either side of a hill slope watercourse, to prevent gully incision.
Parent material: the bedrock underneath the soil, from which the soil originated.
Ped: (Gk: pedon=ground) a crumb of soil.
Pedalfer: () soil of humid climate.
Pedocal: () soil of arid climate.
Pedology: (Russian: pedologiya: soil study; Gk: pedon= ground) the scientific study of soils, especially its formation, nature and classification.
Peridotite: (Old-French: peritot) A heavy intrusive igneous rock mainly consisting of olivine and 5-15% pyroxene. Peridot: A green variety of olivine, used especially as a precious stone.
Perlite: (French: perle=pearl) a vermiculite volcanic glass with the composition of rhyolite. When heated, it pops, to form a light-weight aggregate for construction.
Permafrost: (from permanent + frost) subsoil which remains below freezing point throughout the year, as in the polar regions.
Phreatic: (Gk: phreatos= well) driven by (ground) water. Phreatic eruptions throw out old, previously cooled rock as a result of sudden violent interaction between hot rock or gas and groundwater. This kind of eruption is also called pyroclastic surge or base surge, resulting in base surge deposits.
Plagioclase: (Gk: plageos= oblique; klasis=cleavage) a series of feldspar minerals forming glassy crystals.
Planosol (or solod): (L: planus= flat; solum= ground) calcium- or hydrogen-saturated soils with considerable difference in clay content between the eluviated surface (top) and the clay illuvial horizon (below).
Plutonic: (L/Gk: Plouton= god of the underworld) formed as igneous rock by solidification below the surface of the earth.
Podsol/isation: also podzol/isation (Russian: pod=under; zola= ashes) weathering in a strongly acid medium eluviatesthe iron and aluminium oxides, causing the formation of amorphous clays with high CEC. The soil gets an upper horizon rich in sand and an illuvial horizon rich in amorphous clays (spodic horizon). These soils are called podsols. Best developed under pine forests.
Precipitate: (L: prae=before; caput=head; praecipitare= to head before) causing to be deposited onto a surface; moisture condensed from vapour.
Profile: (L: pro= for, on behalf of, infavour of; filare=to spin; filum= thread) an outline, a cross-section. A soil profile is a cross- section through the soil.
Pumice: (L: pumex/ pumicis= pumice stone) a light-weight and light-coloured, cellular glassy rock with the composition of rhyolite. Pumice forms when lava from a volcano flows onto the earth's surface or erupts violently into the air.  The hot, gas-filled lava then cools quickly to form glass.  Many tiny holes remain after the volcanic gases escape from the cooling lava.  Pumice floats on water because it contains many air bubbles. Used as light-weight aggregate and for scouring. Also see tephra.
Pyrite (Fool's Gold). (Gk: pyr=fire) A yellow lustrus (shiny) form of irondisulphide.
Pyroclastic: (Gk: pyr=fire; klastos= broken in pieces) debris ejected from volcanoes.
Pyroclastic flow: also called glowing cloud or burning cloud. An explosive volcanic eruption accompanied by rapidly expanding gas and particles, travelling as a fluid gas down hill. Such flows are hot (up to 1000ºC) and move at speeds of several hundred km per hour. They are extremely dangerous.
Pyroxene: (Gk: pyr= fire; xenos= stranger) any of a group of minerals commonly found as componenets of igneous rocks, composed of silicates of calcium, magnesium and iron.
Q -- top --
Quartz: (Gk: Quarz) a mineral form of silica that crystallises as hexagonal prisms.
R -- top --
Regolith: (Gk: rhegos= rug, blanket) Unconsolidated solid material (residue) covering the bedrock of a planet. The basal layer of soil, composed of material that is still recognisable as weathered rock or other geological deposits (such as alluvium). It grades upwards into overlying subsoil.
Regression: (L: re/red=back/again; gradi=step; regressio=backward movement) a seaward shift of the shore, due to a fall in sealevel, a rise of the land or sedimentation.
Resource consent: permit issued by a district or regional council, allowing a change in the use of land (or other resources). Consents are only required if a district or regional plan identifies the proposed use as capable of causing adverse environmental effects.
Retirement: (Fr: rétirer= to withdraw) fencing of land to exclude grazing by stock. It may entail rank grass and weed, or reversion to scrub, or plantation of timber trees on the land that is no longer grazed.
Reversion:  spread of indigenous scrub across land that is lightly grazed or retired from grazing.
Rhyolite: (Gk: rhyax= lava stream; lithos= stone) a fine-grained volcanic rock of granitic composition.
Rift: (of Scandinavian origin) a cleft or fissure; a trough between two fault zones, with a down-dropped central block. It is often the beginning of continental breakup and also occurs on mid-ocean ridge axes.
Rilling: erosion of surface soil particles by rainwater running off across the ground as distinct rills after heavy rain.
Riparian: (L: ripa= bank) pertaining to river banks.
Rock: rock  is an aggregate of one or more minerals.
Rubification: (L: rubidus=red) when soil is thoroughly dried from time to time, precipitates of iron and organic matter cannot accumulate and the organic matter disappears by decay, causing irreversibly dehydrated iron sesquioxides to form. Soils are called cinnamonic (red).
Runoff: water flowing across or through soil after rain. The term is sometimes also applied for water flowing in ephemeral channels.
S -- top --
Sandstone: a sedimentary rock composed of sand grains, compacted and weakly cemented by a small quantity of lime.
Saturation: (L: satur= full) penetration of all the pores and fissures in soil by water, so that air is excluded.
Schist: (L/Gk: schistos=split) a metamorphic rock that tends to split into sheets or slabs, showing glistening or speckled surfaces. It often has white bands of quartz alternating with bands of fine-grained minerals, coloured silvery grey, greenish or pinkish.
Scour: erosion of channel beds and banks by stream flow.
Sediment: (L: sedere=to sit) matter that settles to the bottom of a liquid.
Sediment load: the quantity of sediment passing a stream reach in the course of a year.
Sedimentary rock: (L: sedere=to sit) rock laid down by erosion. Matter that is carried by water or wind and deposited on the surface of the land, and may in time be consolidated into rock. Under heat and pressure, sedimentary rock may metamorphose.
Segregation of iron in soil: (L: segregare= to set apart) in soils rich in iron but poor in two-layer clays that retain it, iron tends to segregate, forming irregular concretions and nodules that become hard as stone. See also ferrugination.
Serpentine: (L: serpentinus= serpent) a soft rock mainly of hydrated magnesium silicate (Mg,Fe)3Si2O5(OH)4. Antigorite, a flaky variety, is found in massive rocks. It is usually dark green and sometimes mottled or spotted like a serpent's skin, taking a high polish (greasy/silky lustre and a slightly soapy feel) and used as a decorative material. Such serpentine is used as ornamental stone called verd antique or serpentine marble.  Chrysotile, a fibrous variety of serpentine, is the most important type of asbestos. Serpentine is also used as an additive in making magnesium superphosphate fertiliser.
Sesquioxides: (L: semi=half; qui=and; one and half unit, 1.5) the hydrated oxides of iron, aluminium, manganese and titanium. Three oxygen atoms for two metal atoms, like Fe2O3.
Shale: (German: schale=scale, plate) a fine-grained sedimentary rock formed from silt and clay, tending to separate in thin sheets along depositional bedding planes. Often contains organic matter like oil.
Sheetwash: erosion of surface soil particles by rainwater running across the ground as thin sheets after heavy rain.
Shield: A large rigid area of the earth's crust, usually of Precambrian rock (basement rock), which has been unaffected by later orogenic (mountain building) episodes.
Shield basalt: A basaltic lava flow that erupted from numerous small closely spaced vents, and coalesced to form a single unit. It is generally of smaller extent than a plateau basalt (flood basalt).
Shield volcano: A broad, gently sloping volcanic cone of flat domical shape, built chiefly of overlapping and interfingering basaltic lava flows.
Silica: (L: silex=flint) silicon dioxide (SiO2) occurring as quartz, etc. and as a principal constituent of sandstone and other rocks.
Silicate: (L: silex=flint) any of the many insoluble compounds of a metal combined with silicon and oxygen, occurring widely in the earth's crust.
Silviculture: (L: silva= a wood) management of timber trees to improve their quality and yield, principally by pruning, and thinning. It also entails seedling selection, fertilising, spraying or disease & weed control.
Sillimanite: (Named after B Silliman, an American chemist, who died in 1864) an aluminium silicate occurring in orthorhombic crystals or fibrous masses.
Smectite: an amorphous clay formed from easily weatherable volcanic ashes, lava or basalt.
Soil conservation: measures which protect soil from excessive erosion, structural breakdown, nutrient loss or pollution, thereby minimising soil decline in its capacity of supporting plant and animal life.
Soil nutrients: minerals in the soil that are beneficial for plant growth, principally the elements N, P, S, Ca, Mg, K and their derived compounds.
Soil structure: the arrangement of soil particles and inter-particle spaces (pores and fissures). Good soil structure is beneficial for aeration, percolation of water, penetration by plant roots, resistance to stock treading and for load-bearing (tractor) when cultivated.
Solid solution: composed of various components such that the chemical formula of the rock is not unique and any combination is possible. Two or more elements can substitute for each other completely. For example, the anions Mg++ and Ca++ , which are similar in size and function, can combine CaSiO3 and MgSiO3 to (Ca,Mg)SiO3 as if the rock components were dissolved into one another. The fact that silicate rocks allow for substitution makes them easy to take apart through weathering.
Solonisation: (?) sodium-saturated soils (solonetz) having a richer clay horizon at depth than at the surface.
Solum: (?) the horizons of the soil which have been changed from the parent material by the soil forming process. Subsoil+top soil.
Solute load: (solute= a dissolved substance) the quantity of dissolved chemicals passing a stream reach in the course of a year.
Spaced planting: plantation of trees, sufficiently apart that a close canopy cannot form. Depending on species, mature trees at a density of 50 to 100 stems a hectare are widely enough spaced for pasture or other vegetation to grow in between.
Spar: (Old-English: sparen= plaster; sparstan=gypsum) any crystalline, easily cleavable and non-lustrous mineral, e.g. calcite or fluorspar.
Spinel: (?) MgAl2O4 a magnesium-aluminium oxide.
Spodosols: (?) see gleisation.  Soils with a light, ashy gray A horizon, and a B horizon containing inorganic matter and clay leached from the A horizon.
Stadial: (Gk: stadion=stage) a colder phase within a glacial interval.
Staurolite: (?) an iron-aluminium silicate Al4Fe[O/OH/SiO4]2.
Stone line: ? the layer of stones found underneath the A-horizon, caused by worms and other soil organisms rotating the soil above it.
Strata/stratum: (L: sternere= to strew; stratum= something laid down) a layer or set of successive layers of any deposited substance.
Stratovolcano: (L: stratum=layer) These are steep volcanic cones built by both pyroclastic (thrown by fire) and lava-flow eruptions. They are composed of volcanic rock types that vary from basalt to rhyolite, but their composition is generally andesite. Stratovolcanoes may erupt many thousands of times over life spans of millions of years. A typical eruption would begin with ash explosions and end with extrusion of thick viscous lava flows. The cone-shaped form begins gradually and becomes steeper (up to 35°) toward the summit, which generally contains a crater. At the end of its activity, lava may gently pur out to form a shield of rock (shield volcano).
Streamflow: water flowing through premanent channels.
Strike-slip fault: a fault along which the displacement of rock bodies is horizontal.
Subduction: (L: sub=under; ducere= to lead, bring) the process by which an oceanic plate slips underneath another plate.
Subsaturite: ?
Subsoil: (L: sub=under; solum= ground) the mineral-rich layer of the soil, lying underneath the topsoil and above the regolith, from which it is derived. It is weathered to a stage where the parent material (rock or sediment) is no longer recognisable.
Superphosphate: the most common form of phosphate fertiliser. Ca(H2PO4)2
Suture: (L: sutura=a stitch; suere= to sew) a join or weld left from a former collision of two continents.
T -- top --
Talc: (L: talcum; Arabic: talk) any crystalline form of magnesium silicate that occurs in soft flat plates, usually white or pale green in colour and used as a lubricator.
Tectonics: (Gk: tektonos= carpenter) originally, the art of producing pleasing buildings. The study of large-scale structural features. The study of the movement and deformation of the earth's crust and mantle.
Tephra: (Gk: tephra= ash) loose material, e.g. ash and pumice, deposited by volcanic eruptions. Tephra is subdivided into three main particle sizes: less than 2mm = ash; 2-64mm= lapilli; over 64mm= blocks or bombs. Consolidated ash is also known as tuff. Scoria is dark-coloured tephra, which has been erupted but not in a molten condition. It is permeated by holes from which gas has escaped, and in colour resembles cinders or slag from a furnace. Pumice is a light coloured equivalent. Tephra is usually dispersed widely during an eruption, depending on particle size. Tephra can be accompanied by emissions of poisonous chlorine and fluorine gases.
Terrace: (L: terra= earth) a flat to gently sloping alluvial surface, elevated above floodplain level.
Tetrahedron/tetrahydral: (Gk: tetra=four; hedron= seat) substance (crystal) contained within four equilateral triangles.
Tholeiite basalt: A basalt resembling that found in Tholey, Germany. It is characterised by the presence of orthopyroxene and/ or pigeonite in addition to clinopyroxene and calcic plagioclase. Olivine may be present.
Till: (origin unknown) chaotic mix of stiff clay, sand and boulders deposited by glaciers during the last ice age.
Topaz: (L: topazus; Gk: topazos) a transparent or translucent aluminium silicate mineral, usually yellow, used as a gem.
Topsoil: the uppermost layer of soil, rich in organic matter, incorporated from the decay of plants and the activities of micro-organisms, intermixed with mineral material derived from the underlying subsoil.
Tor: (Old-English: torr= broken hill) a broken hill or rocky peak.
Torrent: (L: torrens= scorching, boiling, roaring) a steep, permanently flowing channel, incised on a mountain slope.
Transform fault: a fault with horizontal movement connecting two segments of a mid-ocean ridge. More broadly, a plate boundary along which plates move horizontally past one another without either convergence (coming together) or divergence (moving apart).
Transgression: a landward move of the shore, produced by a sea level rise or sinking of the land. Erosion of the shore does not normally cause significant transgression.
Translocation: movement from one place to another.
Tuff: (L: tophus; Italian: tufo) a volcanic sediment, made of compacted debris from old volcanic ash showers. See tephra.
Tundra: (of Lappish origin) a vast, level, treeless Arctic region  with a marshy surface and underlying permafrost.
Tunnel (Under-runner): a natural sub-surface cavity formed by runoff flowing downslope through sub soil in heavy rain.
U -- top --
Ultisols: (?) soils similar to alfisols, but with weathering more advanced. They include some lateritic (highly leached) soils. See rocktable/soils.
Unconformity: a surface separating two beds of sedimentary rock and representing a gap in time during which sedimentation ceased or erosion removed the rock record.
Underthrusting: a process pushing one rock body underneath another along an inclined fault plane. It is usually accompanied by intense deformation and some metamorphism, and is common at the collision edge of two plates.
V -- top --
Vermiculite: (L: vermiculari= be full of worms; vermis=worm) a hydrous silicate mineral occurring as layered flakes.  It resembles the mineral mica.  Vermiculite consists mainly of the chemical elements aluminium, iron, magnesium, oxygen, and silicon.  It may be brown or yellow. When expanded by heat, vermiculite becomes a lightweight, fireproof substance with good heat insulating properties.  It is used chiefly in building materials for insulation and soundproofing.  It is also used as a soil conditioner and potting material.
Vertisols: (?) soils with upper layers mixed or inverted because they contain expandable clays (that swell when wet and crack when dry). See rocktable/soils.
Volcanic: (L: Volcanus= Vulcan, Roman god of fire) produced by a volcano.
W -- top --
Weathering: breakdown of rock or other geological materials (e.g. alluvium) into soil, by physical disintegration and chemical reaction.
Windblow: erosion of surface soil particles by strong wind blowing across sparsely vegetated or bare ground.
Wollastonite: a calcium silicate CaSiO3.
X -- top --
Xerophytic: (Gk: xeros=dry; phyton=plant) drought resisting plants, able to survive water shortage.
Y -- top --
Zircon: a zirconium silicate ZrSiO4 of brown to grey colour. Some translucent varieties are used as gemstones.
Z -- top --
Zeolite: (Gk: zeo= boil; lithos= stone) is any of a group of crystalline mineral compounds whose framework of atoms forms microscopic tunnels and rooms.  The internal structure of zeolites, consisting of equal-sized channels, makes them useful as filters (molecular sieves) and catalysts. Zeolites, one of the most abundant kinds of minerals, are made of silicon, oxygen, and aluminium and other metals. Used for oil refining, adsorbents, water softening, cosmetic additives.

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