Absolute Age: The age of a rock, rock unit, fossil or event given in units of time, usually years. An absolute age is usually determined by radiometric dating but may also be found by other methods such as counting tree rings or varves or using growth rates or similar methods. It also implies a greater degree of accuracy than is usually possible.

Accretionary Prism: A thick wedge of sediment occurring on the landward side of many oceanic trenches. Sediment from a subducting plate descending beneath another plate at the trench is scraped off against the adjacent plate (accreted). Sediments derived from the land are also deposited in fault bounded basis within the wedge itself.

Aeolian Deposit: A deposit of sand or silt derived from windblown particles. Aeolian deposits may show large crossbedding due to the formation and migration of dunes (desert dunes), or they may gently mantle topography (loess).

Aeromagnetic Surveying: Measuring the total intensity of the earth's magnetic field with a magnetometer carried in an aircraft or helicopter. Often used in detecting strongly magnetic rocks which may contain economic mineral deposits.

Amphibolite: A regional metamorphic rock consisting mainly of the amphibole hornblende, together with plagioclase, and possibly epidote, biotite, quartz and titanite. The hornblende crystals are often strongly aligned by the pressure of the metamorphism, and rocks of this type are usually derived from metamorphosed basaltic rocks.

Amygdule: Small cavity or vesicle within a lava or similar volcanic rock which has been filled with secondary minerals such as calcite, quartz (often as agate) or zeolites.

Andesite: A volcanic rock, named after the Andes Mountains of South America. Andesite is often black or grey, fine grained, and contains plagioclase feldspar, pyroxenes and amphiboles. Andesites form viscous, blocky lava flows, and are common in continental subduction zones where stratovolcanoes occur, such as the North Island of New Zealand.

Angle of Repose: The maximum slope or angle that loose, cohesionless material (sediment) will remain stable. It varies depending on sediment particle size but usually ranges between 33° and 37° on natural slopes.

Anorthosite: A type of plutonic rock composed mainly of 90% plagioclase feldspar together with minor iron and magnesium minerals. Anorthosites are known to occur in old (Precambrian) rocks as part of layered plutonic intrusions. Anorthosites are among the rocks found on the moon.

Anoxic: Lack of oxygen at the sediment-water interface (sea bed), caused by lack of water movement and/or water column stratification. Sediments deposited under these conditions often contain a great deal of organic material as the bacteria that normally decompose this material require oxygen. If there is enough organic material the sedimentary rock produced is called a blackshale and can be an oil source.

Anticline: A fold where the oldest rocks are in the centre.

Antiform: Any convex upward fold (arch like).

Aphanitic Texture: A term used for any igneous rock which is so fine grained that the crystals cannot be seen without a microscope. Aphanitic texture is typical of volcanic igneous rocks which have crystallised quickly after losing their heat and dissolved gases very near the surface or after eruption to the surface.

Aquifer: An aquifer is any body of rock, often unconsolidated sands or gravels, which is sufficiently porous and permeable so as to allow water to flow through it. An aquitard is a less porous and permeable body of rock that impedes the flow of water, while an aquiclude is even less porous and therefore almost impermeable to the flow of water. The ability to locate and identify rocks with these qualities is important for finding groundwater for irrigation, industries and cities. arenite A synonym for sandstone, from the Latin for sand: 'arena'.

Artesian Wells: A well bored into a confined aquifer (between aquicludes) and is sufficiently pressurised to force water to the surface without pumping.

Ash: Any volcanic debris or fragments (tephra) which are less than 2mm in size.

Asthenosphere: A region within the upper mantle, approximately 100 to 350 km deep beneath the lithosphere, comprising olivine rich rocks such as peridotite, lherzolite and harzburgite. These rocks are under immense heat and pressure, such that they move in response to temperature and density convection, giving rise to the driving forces of plate tectonics.

Augen Texture: A texture found in some metamorphic rocks, particularly gneisses and mylonites, where porphyroblasts (large metamorphic crystals) of feldspar or garnet form oval or 'eye' shaped structures.

Authigenesis: The development of minerals in a rock during or soon after it is deposited.


Barchan Dune: A crescent shaped sand dune found in deserts with winds blowing predominantly from one direction.

Basalt: A dark grey to black fine grained volcanic igneous rocks. Basalts are one of the most common igneous rocks, containing the minerals plagioclase feldspar, pyroxene and magnetite, and sometimes olivine. They are typically poor in silica (53% or less by weight.) and are divided into alkali basalts (from oceanic islands and continental rifts) and tholeiitic basalts (typical of the ocean floors and continental flood basalt provinces.).

Basin: Any large depression into which sediments and other rocks may be deposited. Basins may range in size from small lakes to continental rifts, or the huge ocean basins between continents. They may be formed by a variety of methods, including erosion, faulting, collapse and continental drift. The term is also used to describe the drainage area of a stream.

Batholith: A very large igneous intrusion, usually granitic in composition and extending over areas of 100 km2 or more. Batholiths are built up of many individual intrusions or plutons clustered together, and are often generated in subduction zones at the root of a volcanic system.

Bed, Bedding: A bed is the main descriptive term for a single layer of sediment or sedimentary rock. A single bed is the smallest identifiable division that a geologist can find in any group of layered or stratified rocks. Beds can be a few centimetres to many metres thick, so they may be described as thin or laminated for beds a few centimetres thick, moderately thickly bedded for beds up to a metre thick or very thickly bedded or massively bedded for beds many metres thick.

Benioff Zone: The Benioff Zone (or Benioff-Wadati Zone, after its discoverers) is the dipping zone of earthquake activity in a subduction zone, where the slab of ocean crust dives into the mantle beneath a subduction zone, generating earthquakes as it deforms. These earthquake focii can be traced from the trench at the surface to as deep as 700km into the mantle, showing that the subducted plate is still rigid and deforming to generate earthquakes for at least that depth.

Bioclastic: Descriptive term for rocks comprised of broken fragments of the shells or skeletons of living creatures, for example, a bioclastic limestone is comprised of broken shell fragments.

Biogenic: A descriptive term for any rock, structures, trace fossils or similar features formed by the activity of living creatures. biostratigraphy Stratigraphy or interpreting the order of deposition of sedimentary rocks based on the fossils present, or determining the age of a rock unit by the fossils present.

Bioturbation: The disturbance of a sediment by burrowing and feeding activities of creatures living in the sediment. Bioturbation may destroy many previous sedimentary structures, but may also leave trace fossils such as burrows and trails which can provide information about the type of creatures which once lived in the environment the sediment was deposited in.

Bitumen: The black, sticky residue of crude oil that has seeped to the surface and lost much of its volatile components. Bitumen (also called asphalt) burns with a smoky flame, smells of pitch or tar, and is used as a sealant and adhesive in road building.

Black Sand: Black mineral sands found on many west coast beaches in New Zealand, composed of the iron minerals ilmenite, hematite, magnetite and titanomagnetite. These heavy minerals are concentrated by the action of waves on the beaches. In the South Island, these minerals are derived from the erosion of the granites of the West Coast Batholiths (Separation Point Batholith and the Karamea Batholith) while in the North Island, they are derived from erosion of old volcanoes from the Taupo and Coromandel volcanic zones. Black sands are an important ore mineral for iron, and are also a good prospecting tool for gold and other heavy minerals.

Blueschist: A type of regional metamorphic rock typical of low temperature but high-pressure metamorphism. Blueschists are found in the base of many accretionary wedges in subduction zones, and are named for the blue amphibole (glaucophane) which is typical of the high-pressure conditions in which they are formed.

Bolide: A large meteorite which explodes into a fireball when it enters the Earth's atmosphere.

Borehole: A hole drilled into the surface of the earth, either for extraction of a fluid mineral resource (water, oil or gas), or for taking samples of the a rock or ore body. Boreholes are usually made with rotary drilling machines, often using a diamond tipped drilling bit.

Borings: A boring is a hole ground or cut into solid material by an organism, either rock or the shell of another creature, for the purpose of creating a living space.

Botryoidal Texture: A mineral texture of clusters of rounded balls or spheres, like a bunch of grapes. Botryoidal texture is typical of minerals that grow as spheres of radiating crystals or as concentric growth layers (azurite, or some varieties of agate).

Boudinage: A structure found in deformed rocks where solid layers of hard, competent rock (usually sandstone) within softer rock, have been pulled or stretched apart into a series of 'sausage' shaped fragments.

Braided River: Type of river channel found in many rivers which carry high sediment loads, particularly those derived from glacial systems. Braided streams have many small channels which weave or 'anastomose' back and forth between many small islands and bars of sediment, giving the appearance that the channels are plaited or braided together when viewed from above.

Breccia: A clastic sedimentary rock comprised of angular, broken fragments or clasts of rock dominantly larger than 2 mm in diameter, set in a finer grained matrix. Breccias are usually very close to the rock from which they were formed, and may be composed of fragments of one rock type (a 'monomictic' breccia, of one source) or many rock types (a 'polymictic breccia', from many sources.) Breccias are often described according to their location or origin.

Burial: The process by which sediments deposited in a subsiding basin are covered by progressively younger sediments, with those at the bottom of the basin changing from soft sediment to hard rock, a process called lithification involving many different processes called diagenesis. For burial to occur, the basin must be subsiding and the sediment supply must continue, to allow sediment to accumulate.


Calcite, aragonite: Calcite (CaCo3) is an exceptionally common mineral, forming many rocktypes, particularly limestones, marbles, chalks and travertines. It occurs in two forms, aragonite, a less stable polymorph found in many seashells and as a secondary mineral in cavities and veins in volcanic rocks, and as calcite, a more stable polymorph, which is the principle constituent of limestones, marbles and chalks, as well as a common secondary mineral in cavities, veins, cements and evaporites. Given time, crystals of aragonite will revert to the more stable crystal structure of calcite.

Calcrete: A hard layer of soil, sand or gravel formed by precipitation of calcite from ground or surface waters into pore spaces. Frequently occurs in arid or semi arid regions where evaporation increases salt concentrations in waters and promotes precipitation. Such hard layers are also called duricrusts or caliche. Nodules form first and then layers develop.

Caldera: A round, steep walled depression (often anywhere from 1 to 100km in diameter) formed in many rhyolitic volcanoes by the post eruption collapse of the overlying rocks into the roof of the magma chamber. Lake Taupo is one of several calderas in the Taupo Volcanic Zone.

Carbon dating: A method of dating relatively young (< 70,000 years) biological material and sub fossil material through measuring the rate of the decay of radioactive carbon-14 to carbon-12.

Cataclasis, cataclasite: Cataclasis is the metamorphic process of shearing in a fault zone whereby rocks are torn apart and minerals are ground down, and reformed into a stretched, granular rock called cataclasite. Cataclasites are associated with major fault lines (like the Alpine Fault) and rocktypes like mylonite.

Cataclastic metamorphism: Metamorphism that involves high shear stress and tearing apart and grinding up rocks, in shear or fault zones. This metamorphism involves high pressure and shear stress, but does not require high temperatures. Rocks produced range from fault breccias and cataclasites to mylonite depending on the amount of shear stress, the depth of the shearing (temperature) and the length of time that shearing has occurred (also total movement across the shear zone).

Catastrophism: An early theory supported by many 19th Century geologists, Catastrophism explained the geological record by postulating a series of immense catastrophic events.

Cementation: A diagenetic process, where the particles that make up a sedimentary rock are cemented together by new minerals after deposition. Cements are precipitated from mineral rich water moving through any cavities or pore spaces between the grains of sediment.

Chalk: A very fine grained and porous variety of limestone composed of the tiny calcite skeletons of foraminifera and coccoliths. Chalk deposits are common in the Upper Cretaceous of Europe.

Channel, channel fill: A channel is a linear route of least resistance along which surface water flows. Channels are usually concave in cross section, and they may follow straight, sinuous, anastomosing or dendritic paths. A narrow sea-way may also be called a channel. Channel fill sediments may be from sediment deposited in the channel by the water flowing through it, or from infilling of an abandoned channel. The type of sediment within a channel fill deposit may provide information on the type of channel environment, such as a floodplain channel, submarine channel, braided stream channel, etc.

Chert: Fine grained or cryptocrystalline silica (sio2) deposits lacking developed crystal form, (also 'chalcedony' or 'agate'). Chert may occur as nodules, lenses or beds within other rock types, and may formed by either biogenic, diagenetic or volcanogenetic processes.

Chilled margin: The fine grained, often glassy margin found at the edge of an intrusive magma body, particularly dikes and sills. The magma at the edge of the intrusion is chilled or 'supercooled' against the country rock, and crystallises quickly, producing fine grained rock with many small crystals or, if chilled enough, glass.

Chondrite, chondrite model: Chrondrites are the most common meteorites, and are of basaltic compositions with olivine, pyroxene and plagioclase feldspar, together with iron and iron-nickel minerals. They are characterised by the presence of small round glassy spheres called chrondules, thought to be derived from sudden shock melting during impacts. The chrondrite model proposes that the bulk composition of the Earth is similar to that of the high volatile carbonaceous chrondrites.

Clast, clastic rocks: A clast is a fragement of rock or mineral produced by the disintegration of a larger rock or rock mass. Clastic rocks are sedimentary rocks composed primarily of clasts derived from erosion of older rocks.

Clay: Clay can either mean any clast of any composition that is smaller than 1/256 mm (4 microns), or a sediment composed of clay sized particles, or a mineral belonging to a complex group of water bearing sheet silicate minerals, for example kaolin, illite, smectite, bentonite, and montmorillionite. Clay minerals typically have the ability to absorb water and ions onto their surface, due to their layered structures. Different groups have different capabilities, and are used as filters, for cleaning up contaminants etc. Some groups absorb so much water that they swell (so-called ‘swelling clays’), and this creates problems for slope and building stability.

Claystone: Sedimentary rock composed of clay-sized particles (clay that has been lithified).

Cleavage: Cleavage can refer to the breaking of a mineral along planes of weakness corresponding to crystallographic planes, or the similar breaking of a rock along planes of weakness caused by the alignment of minerals (usually platy minerals, clays or micas). Slates, phyllites, schists and gneisses all possess cleavage, created during regional metamorphism when increased pressure and temperature causes existing minerals to realign themselves and new micaceous minerals to grow in a particular orientation.

Coal: A rock composed of more than 50% carbonaceous material, formed by compaction and lithification of plant remains. Forming coal requires significant thicknesses of plant debris, so swamp conditions and peat formation are generally required. The coal is classified based on its volatile matter and water composition, and the type of carbonaceous matter present, and the amount of impurities (ash) present. Carbonaceous matter types vary depending on the original flora and the amount of decomposition that occurred prior to burial.

Colluvium: Loose and unconsolidated deposits that have not been transported by wind or water. Usually applied to material lying at the base of a slope that has been transported there by gravity.

Columnar jointing: Joints or fractures that are regular and parallel and create columns with a polygonal cross section in lava flows and other igneous rocks due to contraction of the rock during cooling.

Conchoidal fracture: A smoothly curved fracture surface, characteristic of glassy minerals and rocks, such as quartz, olivine (minerals) and obsidian (rock).

Concretion: An irregular to subsperical shaped mass of cemented material, formed by precipitation of a mineral (usually calcite, also siderite, silica, hematite, pyrite) from solution around a nucleus (usually a shell, bone, grain), in a sedimentary or pyroclastic rock. The grains within the concretion are identical to those outside, but the cement is different. Concretions often occur in layers, where there is a concentration of suitable nuclei, or where water has flowed more easily.

Conformable: Where there are no breaks in sedimentation and beds are deposited one on top of the other without break or changing orientation of the bedding surfaces. Opposite of unconformable.

Conglomerate: A clastic sedimentary rock where the majority of clasts are larger than 2 mm in diameter (granules, pebbles, cobbles, boulders) and rounded or subrounded. Clasts are set in a finer grained matrix, and can be supported by the matrix or by clast to clast contact. Conglomerates are the lithified equivalents of gravel.

Contact: The boundary between two beds, layers, units, rock types, intrusions and country rock, metamorphic zones etc.

Contact metamorphism: Metamorphism caused by the intrusion of a body of hot igneous rock into cooler older sedimentary, igneous or metamorphic rock. The heat from the intrusion causes new minerals to develop in a zone around the intrusion called an aureole. Also called thermal metamorphism.

Continental crust: The crust that underlies the continents and ranges in thickness from about 35 km to as much as 60 km under high mountain ranges. The crust is relatively silica rich compared to mantle and ocean crust compositions and has a density in the upper part of ~2.7g/cm3. It is derived by recycling of older crust into sedimentary rocks, deformation of rocks, and intrusion of silica rich magmas. The resistance of quartz to weathering accounts for much of the dominance of silica in continental crust.

Continental drift: The original theory proposed by Alfred Wegener in 1912 to explain the good fit of African and American coastlines and the distribution of similar flora and fauna across Gondwanan continents. The theory is that large plates of continental crust moved freely across ocean crust like ice drifts through water. The theory has been largely superseded by plate tectonics.

Continental margins: The margins of continents where continental crust grades into oceanic crust. Continental margins come in two sorts, passive continental margins (e.g. The West Coast of NZ) where there is no plate boundary at the margin and the continental crust beneath the continental shelf passes into oceanic crust beyond the continental slope, and active continental margins where the edge of the continent is coincident with a plate boundary, either convergent (e.g. North Island of New Zealand) or divergent (e.g. California).

Convection: Convection is density or heat driven movement of fluids, and occurs in hydrothermal systems (heat driven movement of water adjacent of igneous intrusions), in the atmosphere (density and heat driven), in the ocean (density and heat driven), and possibly in the mantle (density and heat driven movement of mantle material (very slowly) causing movement of the crustal plates above).

Convergent margin: The margin of a tectonic plate where the adjacent plates are converging towards each other. Convergence usually causes subduction of the denser (older, cooler or oceanic) plate beneath the other plate; however where plates are of similar densities convergence can result in uplift and mountain building (e.g. Himalayas, Alps, Southern Alps).

Coprolite: Fossil crap… excrement from large vertebrates (reptiles, dinosaurs, mammals) which has been buried and usually replaced by other minerals such as calcium phosphate.

Core: The centre of the earth, from about 2900 km depth, it is dividable into an inner and outer core. The outer core may be liquid and the inner core solid. It is likely composed of iron-nickel alloy. The word core is also used to refer to cylindrical section of rock obtained by drilling, and as the central part of folds.

Coriolis force: The apparent force caused by the earth’s rotation which causes objects to move to the right in the northern hemisphere and to the left in the southern hemisphere.

Country rock: An older rock that is intruded by an igneous intrusion or mineral deposit.

Crater: Craters are round pits or depressions, rimmed or not, formed either by volcanic eruptions at the top or on the flanks of a volcanic cone, or by meteorite impacts or explosions at the earth’s surface.

Craton: A craton is a part of the earth's surface or crust that has been stable and undeformed for a long time (100’s of millions of years).

Cross-bedding: Also cross-stratification and cross-lamination, where laminae or beds within a bed or layer are formed at an angle to the bedding surfaces. This occurs where ripples or dunes develop, and sediment is deposited on the downstream, lee face of the ripples or dunes at an angle to the stream/sea/land surface. Variations include herringbone cross-bedding where successive beds have different directions of cross-beds, indicating tidal flows; hummocky cross-stratification where the laminae form hummocks and swales caused by wave action during storms; low-angle cross-bedding formed on dunes by aeolian action; and linsen bedding or lenticular bedding where sand ripples are isolated in a muddy sediment (the opposite is flaser bedding where mud ripple trough fills are preserved in sand beds) all formed in shallow tidal flow regimes or deeper episodic currents.

Crust: The outermost layer of the earth above the Mohorovicic discontinuity. The crust is subdivided into continental and oceanic crust, which form the land surfaces and the ocean floors and have different compositions and densities reflecting their different origins.

Cryptocrystalline: Where the crystals are too small too be seen under a microscope.

Crystal: A solid body of a particular chemical element of compound of elements with a regular repeated atomic arrangement and homogenous composition. The atomic arrangement is reflected in crystal faces, and the arrangement is used to classify crystals based on the symmetry.

Crystalline: Said of rocks that are composed of interlocking crystals of the same or different compositions.


Dating methods: Geologists use many methods of dating rocks. Absolute dating involves using radiometric isotopes to determine the absolute age of the rock in terms of years B.P. (before present). See radiometric dating. IF the rock contains fossils then the types of fossil present can be used to determine what age the rock is. Certain types of fossil are diagnostic of certain time periods in the earth’s history. This method is known as biostratigraphy and is good for determining relative ages and correlating strata but is not able to provide absolute ages without reference to a radiometric date obtained somewhere.

Debris Flow: A flow of rock fragments, soil, sands and mud with most of the particles larger than sand size. Debris flows can reach speeds of several hundred kilometres per hour, as the water and mud greases the movement until the flow reaches flat ground and ‘freezes’. Debris slides occur where material slides downslope but does not continue to flow outwards.

Deformation: The generalised term for folding, faulting, shearing or otherwise altering the orientation of rocks as a result of stresses within the earth’s crust.

Delta: A relatively flat area at the mouth of a river system where it enters a lake or the sea, where the sediment carried by the river is deposited in the relatively still water. Deltas take their name from their common triangular shape, like a Greek letter delta (?). Deltas are usually partly submerged and partly emergant, and their shape is governed by the dominant process acting on the delta area. A delta where river processes are dominant is very elongate and often has multiple active lobes – the mouth of the Mississippi or the Selwyn into Lake Ellesmere are good examples. Other deltas are dominated by marine processes, either tidal currents (elongate estuaries and bars like the Waimakariri) or waves (straight shorelines).

Deposit: Material that has accumulated. Used for sedimentary deposits and for igneous rocks and especially for economic minerals.

Deposition: The laying-down of rock-forming material by any process. An example is the settling of particles from water (sedimentation), or the crystallisation of quartz and sulphide minerals in a vein from solution.

Detritus: Loose material produced by disintegration or abrasion and transported, or any fine debris of organic origin.

Diachronous: A rock unit or surface is said to be diachronous if it is not the same age in all the areas it occurs in, if it was deposited or formed at different times in different areas. Also known as time-transgressive.

Diagenesis: All the changes that occur to a sediment after its initial deposition to turn it into a sedimentary rock (lithification). These include compaction (increasing pressure forces grains together and forces water out), cementation and growth of authigenic minerals (new minerals form in pore spaces or fill pore spaces holding the rock together) and replacement (minerals are replaced by different minerals), caused by changing chemical and pressure-temperature conditions. Diagenesis grades into metamorphism at higher pressures and temperatures.

Diapir: Doming caused by the rise of a plastic core material, which ruptures overlying rocks. Salt and shales can cause diapirs, or igneous intrusions can show similar structures.

Diatomite: Organic sedimentary rock composed of siliceous skeletons of microscopic single-celled diatoms, related to algae. It is distinguished by pale colour and very light weight. Diatomite is very porous but has small pore spaces and a high surface area but is chemically stable owing to its siliceous composition and is used commercially for filtering systems. It is also used to mop up petrochemical spills and as a very fine abrasive.

Diatreme: A pipe formed in a volcanic explosion that is filled with breccia from the magma, pipe walls and other country rock.

Dike (or dyke): Either a tabular body of igneous rock (usually fine grained) that cuts across other rocks or through massive rocks at a high angle (see sill); or an embankment around a low-lying area or along waterways to prevent flooding.

Dinosaur: A reptile belonging to the subclass Archosauria, which are distinguished from other reptiles by pelvic bone features, and lived in the Triassic to Cretaceous Periods (see geological timescale).

Diorite: Plutonic igneous rock intermediate in composition between felsic and mafic rocks, usually contains amphibole (hornblende), plagioclase feldspar and pyroxene. The intrusive equivalent of andesite.

Dip: see strike and dip

Disconformity: An unconformity where the beds below and above the surface are parallel, but there is evidence of erosion on the surface.

Divergent Plate Boundary: A boundary between plates where the plates are moving apart from each other. The extension causes fracturing and extension in the crust, and hot mantle material to well up beneath the stretched area and cause volcanic activity. Examples of spreading zones are the Pacific and Atlantic ridges, and the Red Sea, Gulf of Aden and East African Rift system.

Dolerite: An intrusive igneous rock composed of plagioclase feldspar (labradorite) and pyroxene, where the feldspar commonly occurs included within the pyroxene crystals.

Drillholes: Any holes drilled for exploration, water wells, oil wells or other purposes, using some sort of drill rig. Drilling requires lubrication, for which drilling mud, a viscous fluid of clays in water or oil, is commonly used. For exploration purposes recording the rocks drilled through is either as chips of pulverised rocks collected in bags or cores of the rock.

Drumlin: A low hill composed of moraine, till or other material, with a rounded, elongate shape, either accumulated under a glacier or shaped out of older till material by a readvancing glacier. Drumlins usually have a blunt end pointing upstream and a more gentle slope downstream, and is elongate parallel to the ice flow.

Ductile Deformation: Deformation where the rock moves, compresses, flows under the stress applied up to a certain point before fracturing. Ductile deformation is permanent and cannot be recovered from once stress is removed. Brittle deformation is where the rock fractures under stress, and no internal movement occurs.

Dynamothermal Metamorphism: See regional metamorphism.


Earthflows: A mass movement involving soil and weathered rock flowing or creeping downslope on a basal plane with well defined lateral boundaries, ending in a mounded lobe. Slow movements are called soil creep, while extremely fast liquid movements are called mudflows.

Earthquake: An abrupt movement or trembling in the earth, generated by the abrupt release of slowly accumulated strain on faults within the crust. The release of the strain energy results in elastic wave energy radiating out from the focus. Various types of elastic waves radiate out; p-waves are compression waves and travel at 5 km/sec. S-waves are shear waves which move vertically and travel at 3 km/sec. The time between the arrival of p and s-waves tells seismologists the distance to the earthquake's focus Readings from three stations allow the earthquake location to be determined. Slower waves called l-waves produce sideways movement which is what causes most damage. Earthquake magnitude is measured on two different scales. The Mercalli intensity scale measures the effects of the earthquake at the focus, in terms of how much damage and how the earthquake is felt. The Richter scale measures how much energy is release by the earthquake.

Eclogite: Metamorphic rock composed of garnet and pyroxene, with rutile, kyanite and quartz. These rocks are formed from mafic original rocks under high-pressure metamorphism.

Embayment: An embayment is both the formation of a bay or indentation into a coast or surface and the bay or indentation itself. Embayment is used to refer to bays in coastlines and in crystals.

Epicentre: The point on the earth's surface directly above the focus of an earthquake.

Erosion: The removal of material by physical and chemical processes such as weathering, fracturing, and the action of water, wind, waves, glaciers and underground water. Erosion produces distinctive landscapes that vary depending on the rock type beneath, and results in an erosion surface and various scarps and slopes.

Eruption: The process of release of volcanic material, ranging from overflow of lava to the violent explosion of pyroclastic material, onto the earth’s surface and into the atmosphere. Also the explosions that occur in geothermal fields. Eruptions can occur from vents, in previously quiescent areas or from fissures.

Escarpment: A long and fairly continuous face or steep slope between two more gently sloping or flat areas of land. Escarpments can be formed by erosion, faulting or the outcrop of a relatively resistant bed of rock. Commonly abbreviated to ‘scarp’.

Esker: A sinuous ridge of roughly stratified gravel and sand that was deposited by a stream running beneath a glacier, and was left behind when the glacier retreated.

Eustasy: Worldwide changes in sea level, resulting from an increase or decrease in the total volume of water in the oceans, or an increase or decrease in the size of the ocean basins, resulting in concurrent rise or fall of local sea levels worldwide. Eustasy can be caused by climate changes (formation or melting of polar or continental ice sheets) or by major tectonic changes (uplift or downwarping in oceanic basins).

Evaporite: A chemical sedimentary deposit, deposited by precipitation of minerals from a solution that is becoming increasingly saline due to partial or total evaporation. Examples of evaporite minerals are gypsum and halite (salt).

Evolution: Evolution is used to refer to the ‘theory of evolution’, that life on earth has developed from a few simple organisms to many complex organisms, and to the process of successive generations of organisms differing from previous generations until the latest members differ significantly from the earliest (speciation).

Extension: When a rock is being stretched or pulled apart in one direction, resulting in faults and fractures or ductile deformation. Crustal extension results in normal faulting and basin development, and may end in rifting (divergent plate boundary).

Extinction: The disappearance of a species, genus or other taxonomic group, so that it no longer exists anywhere.

Extrusive rock: Igneous rocks that are erupted onto the surface of the earth and cool rapidly in contact with the atmosphere or with water. Such rocks are fine grained and may contain structures indicating flow (flow banding, layering) or jointing from cooling (columnar jointing).


Facies: The characteristics of a rock or rock unit, including appearance, composition and structure, which usually reflects the mode of origin of the rock. The term is used in conjunction with others to indicate what type of facies is being talked about: sedimentary facies, metamorphic facies, biofacies, petrographic facies etc.

Fault: A fracture in a rock mass along which there has been movement of one side with respect to the other. Several different kinds of fault exist: normal faults where the block overlying the fault plane moves (hanging wall) moves downward relative to the block below the fault plane (footwall), reverse faults where the hanging wall moves upwards, thrust faults where the fault plane dips less than 45 degrees and the movement is reverse, and strike-slip faults where the two sides of the fault move past one another with no vertical movement. Strike-slip faults can be dextral (or right-lateral) if the other side of the fault is moving to the right, or sinistral (or left-lateral). Most faults have a combination of movement senses and are called oblique. For example the Alpine fault is a dextral strike-slip fault with a reverse vertical component. The fault dips to the east over most of its length. Faults tend to move episodically, as irregularities on the fault planes prevent easy movement. Stress builds up until the rock gives way causing an earthquake. Not all of a fault may move in an earthquake, often earthquake occur deep underground and the surface part of the fault may not move. If it does, a surface rupture develops. Old surface ruptures help to define how much movement has occurred on the fault in the past, and how often earthquakes happen.

Fauna: The animal population of an area, environment, formation or time span.

Felsic: Derived from feldspar and silica, used to describe igneous rocks abundant in feldspar and quartz (silica), and also to minerals rich in silica: quartz, feldspars, feldspathoids and muscovite. A similar term is acidic, which describes rocks with more than 60% by weight silica.

Ferricrete: Cementation by iron oxides, usually of surface soils, sands and gravels.

Ferruginous: Iron bearing, usually cemented with or containing iron oxide.

Fissure: An extensive crack, break or fracture in rocks, may contain mineral deposits or be the source of a fissure eruption, where lava erupts from a fissure rather than an isolated vent (such as the Tarawera eruption in 1886).

Fjords: A long narrow u-shaped sea-inlet, with steep walls and generally very deep, formed as a glacial valley and drowned when the ice melted.

Floodplain: The part of a valley that is formed by deposition due to the current activities of a stream or river and is covered with water during flood stages.

Flora: The plant population of an area, environment, formation or time span.

Fluorescence: Emission of visible light by a substance or minerals when it is exposed to ultraviolet light.

Fluvial: Of rivers or streams, or the deposits of rivers or streams.

Focus: The point at which a fault plane initially ruptures, usually beneath the ground surface. The epicentre is directly above this point.

Fold: A bend in any layering in rocks. Folds usually indicate deformation, compression perpendicular to the fold axis, which runs along the maximum curvature of the fold. The folds axial plane joins all the fold axes, and the trace of this plane of the surface of the earth is called the axial surface trace. Folds can be concave up (synforms) or convex up (antiforms), and if the order of age of the strata is known then folds with the oldest rocks in the middle are anticlines, and folds with the youngest rocks in the middle are synclines. If the fold axis is not horizontal the fold is said to be plunging.

Foliation: A planar arrangement of grains or textural or structural features in any rock, usually the layered structure produced by the alignment of micaceous minerals and the separation of felsic and mafic minerals into layers in a metamorphic rock.

Formation: A lithologically distinct and mappable body or unit of rock, that is defined by its unique characteristics such as colour, composition, structures. Formations are formally defined and can be subdivided into members, or combined in groups.

Fossil: Any remains, trace or imprint of plant or animal presence that has been preserved. Fossil preservation or fossilisation includes all processes that preserve a plant or animal in sediment. Fossilisation includes burial, compression, replacement, mould and cast formation, and cementation.

Fractionation: The process by which the composition of a residual magma is changed by the progressive crystallisation of minerals from an initial magma, resulting in a final magma composition extremely different to the initial composition.


Gabbro: A group of dark coloured mafic intrusive igneous rocks composed of plagioclase feldspar and pyroxene, with olivine and orthopyroxene. Gabbro is the intrusive equivalent of basalt.

Gemstone: Any precious or semi-precious stone especially when cut and polished for decorative purposes. New Zealand examples include nephrite and other forms of ‘greenstone’, several types of agate (cryptocrystalline quartz) and garnets.

Geochemistry: The study of the distribution of elements, molecules, ions and isotopes in rocks, minerals, water, atmosphere, and soils, and the transport and circulation of elements, molecules, ions, and isotopes. Various methods including x-ray fluorescence, mass spectroscopy and laser ablation are used to analyse material.

Geochronology: Studying time in relation to the history of the earth, using both absolute age (usually radiometric dating) and relative dating (biostratigraphy) systems.

Geode: A more or less spheroidal body, usually hollow, composed of layered chalcedony (cryptocrystalline quartz) and inwardly projecting crystals of quartz, or more rarely calcite or other minerals, within a body of limestone, volcanic rocks or rarely other sedimentary rocks. The geode is easily separable from the parent rock, as it is composed of a different mineral or suite or minerals to the parent rock. Smaller cavities completely or partially filled are called amygdules.

Geological Timescale: A chart of events covering geological time (from the formation of the earth to the beginning of human history). The divisions are initially based on the rock record, but represent time intervals not stratigraphic intervals. Various schemes of names exist around the world, each based on local stratigraphy. Correlations of the timescale to absolute ages are problematic because of inaccurate correlation of lithostratigraphic units worldwide, and lack of opportunities for radiometric dating.

Geomagnetism: The magnetism of the earth and the phenomena associated with that magnetism.

Geophones: A detector placed on or in the ground that reacts to seismic waves at its location. Geophones are used to detect energy released purposefully to investigate the structure of the subsurface. The reflection or refraction of the energy waves from their point source is determined by the type and position of layers beneath the earth’s surface. This is part of seismic exploration or seismology, or geophysics.

Geothermal: The heat of the interior of the earth. The geothermal gradient refers to the rate at which the temperature increases with increasing depth, a high gradient occurs where there are heat sources close to the surface (Taupo for example), a low gradient where there are no heat sources (under the Canterbury Plains). Geothermal energy is energy extracted using the internal heat of the earth, usually using steam driven turbines from bores drilled in a geothermal area (e.g. Wairaki), which is where heat rises to the surface in the form of heated water causing mud pools, steam and hot springs to occur.

Glaciation: The formation and movement of a glacier and of glacial processes, or a period of time in the earth’s history where extensive glaciers developed, advanced, reached a maximum extent, and retreated, also called an ‘ice age’.

Glacial: Pertaining to glaciers, their actions, effects and deposits. Also a short term for ‘glaciations’.

Gneiss: A foliated metamorphic rock formed in high temperature and pressure conditions during regional metamorphism, with distinct separation of felsic and mafic minerals into layers.

Gondwana: A supercontinent that existed in the southern hemisphere in the late Paleozoic. Gondwana resulted from the rifting of the supercontinent Pangaea into Gondwana and Laurasia (parts of China, Russia, Europe, N. America). Gondwana subsequently split up in the Mesozoic into: fragments that drifted north to join Laurasia, India, Australia/New Zealand, Antarctic, South America and Africa. The affinity of the continents is shown by a similarity of fauna and flora, particularly the occurrences of glossopteris fossils, Nothofagus (beach) trees and fossil dinosaurs and other animals.

Gossan: The weathering product of exposed sulfide-bearing ore deposit, formed by the oxidation of the sulphide minerals, removal of sulphur and metal ions and formation of hydrated iron oxides (limonite and goethite). The presence of gossan can be used by prospectors and an indication for sulphide mineralisation at depth.

Graben: A fault-bounded block that has been relatively dropped down, which may or may not result in a surface expression (rift valley). Many of the Tertiary basins in New Zealand were formed in grabens or half-grabens (only one side is fault bounded), during a period of extension across New Zealand relating to the separation of New Zealand from Antarctica and Australia.

Graded Bed: A sedimentary bed in which the grain size changes within the bedding surfaces, either normal grading with coarse grains at the base and fine grains at the top or reverse grading. Graded beds are caused by deposition during waning (normal grading) or waxing flow.

Grainsize: The size of crystals in igneous and metamorphic rocks and grains in sedimentary rocks, expressed as an arbitrary size (fine, medium coarse), as a millimetre size range, in the phi scale, or descriptive size (fine sand size).

Granite: A plutonic igneous rock where quartz is between 10% and 50% of the felsic components and there is more alkali feldspar than plagioclase feldspar. May contain a minor amount of biotite or hornblende.

Granodiorite: Plutonic igneous rock intermediate in composition between a diorite and a granite, containing quartz with more plagioclase feldspar than alkali feldspar and some minor mafic minerals.

Greensand: A sand or sandstone consisting of sand particles and grains of the mineral glauconite. Glauconite is a sheet silicate with potassium, iron and water, and forms in marine environments in quiet water with abundant potassium and iron and slow sediment supply. In New Zealand the glauconite is found in association with high energy sandstones, so it has been transported some distance from its environment of formation. Greensands exist in the Waipara River (north Canterbury), and around Oamaru and all over south Canterbury and north Otago, and in limited areas on the West Coast of the South Island, as wells as the west and east coasts of the North Island.

Greenschist: A schist that has a green colour, due to the presence of green minerals such as chlorite, epidote or actinolite. Greenschists are commonly the metamorphic product of slightly mafic rocks, such as volcanogenic sediments.

Greenstone: Dark green altered or metamorphosed rock, originally a mafic igneous rock, green because of the presence of chlorite, epidote, actinolite, serpentinite, nephrite, jadeite, or other green minerals. In New Zealand greenstone occurs in pockets among the schists formed by the compression under the Southern Alps and uplifted along the Alpine Fault.

Greywacke: A grey coloured hard sandstone, where there is a significant quantity of matrix and feldspar and lithic fragments as well as quartz. Derived from rapid weathering of recently exposed granitic and metamorphic rocks. Usually cemented with silica.

Groundwater: All subsurface water, usually that water that is within the zone of saturation, below the water table, including within aquifers or underground streams.

Guyot: A seamount that has a flat top formed by weathering or erosion (once or now emergent).


Hardground: Hardgrounds form by cementation of the seafloor, usually with calcite, during periods of little or no sediment supply. Animals colonise the harder substrate, further binding the sediment, and boring into the cemented seafloor to form dwellings. Hardgrounds can form during maximum sea-levels, or during sea-level fall where sediment bypasses the area.

Highstand: A time of maximum sea-level, in between sea-level rise and fall. The term is usually applied to eustatic sea-level but can be used for local sea-level changes due to tectonic or other causes. The highstand usually corresponds to the maximum transgression (most landwards shoreline). During a highstand, marine sediment is deposited over areas previously land surfaces, and more basin-wards areas are starved of sediment, so that only very fine, fossil-rich condensed sediments are deposited, or hardgrounds or shellbeds are deposited. All the sediments deposited during a highstand are called the highstand systems tract.

Hornfels: A contact metamorphic rock, composed of fine-grained equigranular crystals with no preferred orientation, sometimes with porphyroblasts (larger crystals).

Horst: A fault-bounded uplifted block that may or may not be expressed as a hill. Mt Grey in North Canterbury is an example of a horst block that is a hill.

Hotspot: A relatively large volcanic centre that is persistent for at least a few 10’s of millions of years and is thought to be the surface result of a rising plume of hot mantle material. Hot spots are therefore volcanoes that not linked with plate boundaries in any way, unlike the volcanoes that occur near subduction zones or around rift zones. They tend to have chemistry similar to mantle chemistry and different from other volcanoes that occur away from spreading ridges.

Hummocky Cross-Stratification: See cross-bedding.

Hydrocarbons: Any organic compound or mixture of compounds that consists entirely of carbon and hydrogen. Hydrocarbons are the result of deep burial of organic matter. The pressure and temperature causes the release of volatile components, and then of gaseous hydrocarbons, followed by liquid hydrocarbons and finally solid carbon (graphite). The liquid and gaseous hydrocarbons migrate up through the rock as they are less dense than water. They either reach the surface to form a seep or they are trapped by an impermeable layer and can then be tapped by drilling.

Hydrogeology: The study of water movement, both at the surface and underground. Finding water underground, measuring flow rates, determining gradients and designing irrigation and drainage systems.

Hypabyssal Intrusion: Minor intrusions such as sills and dikes, which have intruded in conditions between plutonic and extrusive and therefore have intermediate grainsize and textures.

Hypogene: Any geologic process that occurs within the crust of the earth, as opposed to epigene, on the surface. Hypogene deposits are mineral deposits formed by ascending solutions, such as copper hydroxides (malachite, azurite).


Ichnofossils: A synonym of trace fossil, the traces left in sediments by the activities of animals or plants, including burrows, borings, tracks and trails. Ichnofossils represent all modes of activity, from walking, feeding, living spaces, hiding and so forth. Any one animal could produce many different traces through its life, and ichnofossils are classified based on their own morphology, not on the animal that formed them. Often for older rocks, it is not known what animal formed the traces.

Ichnology: The study of trace fossils.

Igneous Rocks: Rocks that solidified from magma, or molten rock.

Ignimbrite: Volcanic rock formed from deposition and consolidation of ash flows, typically from collapse of eruption columns. Ignimbrites typically contain welded ash with fragments of lava, other volcanic rocks, glass and any other material picked up by the collapsing column, including organic matter. Because of the heat and flow, fragments are often elongate and flattened, indicating flow direction and compression by the weight of overlying material. Ignimbrites are typically formed during particularly violent rhyolitic eruptions, such as have frequently occurred in the North Island, and therefore there are fine examples all over the central North Island.

Induration: The measure of the hardness of a sedimentary rock, which is related to the degree of lithification and weathering.

Inlier: An area of rock surrounded by younger rock, for example a fault bounded block (horst) or an eroded anticline crest. See outlier.

Interbedded: Where the contact between two units of rock consists of alternating layers of each lithology. Also intercalated, interfingering, interlayering, intertongued. Interfingering can happen in both vertical section and laterally, due to the fluctuation of the boundary between environments in which sediment is being deposited.

Interglacial: The time between glaciations.

Intergrowth: Interlocking grains of two different minerals that happens when the minerals are crystallising simultaneously.

Intermediate Rocks: Rock intermediate in composition between felsic and mafic or acidic and basic.

Intrusive Rock: Igneous rocks formed by intrusion of magma into previously formed sedimentary, igneous or metamorphic country rock, followed by crystallisation of minerals due to cooling of the melt.

Iron Sand: Sand composed of iron oxide minerals, such as hematite, ilmenite, magnetite, formed by the concentration of heavy iron oxide minerals by waves on beaches or stream action. Also known as black sands.

Isochron: In geochronology, a line on a graph of isotope ratios showing the relationship between the ratios and to the age of the sample.

Isostasy: The conditions under which the lithosphere floats above the asthenosphere at equilibrium, comparable to floating blocks in water. Weighting of the crust causes it to sink, high mountains must have deep roots to balance their height. Areas of Scandinavia are currently rising to regain their equilibrium after being depressed by the weight of ice over them during the last glaciation.

Isotopes: Different types of the same chemical element, where each has the same number of protons in its nucleus, but differing numbers of neutrons. The difference in mass means that the isotopes have slightly different properties, and react at different rates to chemical, physical and biological processes. The amount of any particular isotope in, for example, the ocean depends on the processes that have been recently acting in the ocean, and how much they favour the removal of one isotope over another. Fluctuation in carbon and oxygen isotopes have been linked to climate changes. Isotopes can also be used in dating, where the presence of one isotope over another indicates the action of a decay process, allowing the time elapsed since the growth of the mineral containing the isotopes to be determined.


Jointing: Joints are fractures within a rock or rock mass that do not have significant movement across them. Joints frequently occur in parallel sets. Several sets of joints may occur in a rock mass, each relating to different phases of compression or decompression that the rock mass has undergone. Jointing is a very important control on rock mass stability and also controls the penetration of a rock by water which allows weathering to take place.


Kame: A mound, terrace, or ridge of stratified sand and gravel, which has formed by stream deposition at the margin of a glacier, by deposition onto the surface of a glacier, or by deposition into a depression on or at the margin of a glacier.

Karst: Topography formed by dissolution of limestone and marble by rainwater or underground streams, resulting in sinkholes, caves, underground drainage, closed drainage systems and depressions.

Kettle Lake: Small closed depression filled with water in glacial till or moraine, formed by the melting of a block of ice within the till leaving a depression or kettle hole. The depression becomes filled with swamps or a lake.

Kimberlite: A variation of the igneous rock peridotite with olivine, calcite and other minerals, named after the Kimberley district in Africa where pipes of kimberlite contain diamonds.


Laccolith: An igneous intrusion lying parallel to the country rock strata, that has a domed top surface and has pushed overlying rocks up, but has a flat floor and a dike-like feeder beneath its thickest point.

Lacustrine: To do with lakes, the sediments deposited in lakes, the waters in the lake or the animals within the lake, or an area characterized by lakes.

Lagoon: An embayment partly or completely separated from the ocean by a reef or barrier, or similarly a body of fresh water separated from a lake by a bar or a shallow body of water enclosed within an atoll.

Lahar: A landslide, mudflow or debris flow of volcanic material from the flank of a volcano. Lahars are caused by abrupt increases in water flow and can be triggered by volcanic activity melting snow or ice, or by crater lake overflows as in the Tangiwai disaster in 1953, or by heavy rain and dam bursts.

Lamination: The formation of laminae, or very thin (0.05 to 1 mm thick) layers in a sedimentary rock, or the state of being laminated.

Landslide: General term for processes of downslope movement of soil and rock material and the landforms that result. The different categories are defined by the material involved, and the type of movement: fall, slide or flow. For example; rockfall, mudflow and slump.

Lapilli: Pyroclastic material (volcanic debris) that is between 2 and 64 mm in size. This can include particles of solidified lava, small bombs, clumps of ash, and so on. Smaller size particles are called ash.

Laterite: Highly weathered material formed in wet tropical to warm temperate regions, rich in oxides of aluminium and iron, with quartz (or silica cements) and kaolinite (a kind of clay). Laterites can under certain conditions become ores for aluminium and iron. Laterite formation can result in fluids rich in silica creating extensive silicification and silcrete formation in downstream areas, either above or below ground.

Laurasia: The supercontinent from which the northern hemisphere continents have separated, that itself separated from the supercontinent Pangaea in the Permian.

Lava: Fluid melted rock that comes from a volcanic vent or fissure, or the same material once solidified.

Lenticular bedding: See cross-bedding.

Limestones: Any sedimentary rock consisting primarily of calcium carbonate. Limestones may contain minerals calcite and aragonite, and some amounts of dolomite. Limestones can be formed by chemical precipitation of calcium carbonate, for example to form stalagtites in cave systems or travertine by springs, or they can be formed by the accumulation and lithification of whole and/or broken shells or lime mud (organic limestones). If the shell material has been transported a significant distance and is deposited entirely by current activities, then the limestone may be called a calcarenite or calcirudite (sand sized particles or gravel sized particles) to reflect the dominance of current deposition. Otherwise limestones are classified and named based on their composition (lime mud versus shells versus crystalline calcium carbonate) and texture (are the grains supporting each other versus supported by the matrix?).

Lineation: Any linear structure in a rock, namely flow lines, slickensides, stretched clasts, preferred alignment of fossils or minerals, fold axes, tool marks etc.

Lithification: The process of turning a sediment into a sedimentary rock involving many processes collectively called diagenesis. Processes include compaction, cementation, recrystallization, and replacement. Lithification may take place sometime after the sediment is deposited, or it may begin immediately after deposition, for example cementation of layers or of concretions may occur at or just below the sea-floor.

Lithology: The physical characteristics of a rock, usually in hand specimen and outcrop, including colour, mineralogy, grain size, textures, structures and so on. Describing the lithology of rocks is the basis of lithostratigraphy, or the description of rock successions based on the lithology of rocks, grouping lithologic units into larger groups and families and correlating rock units based on comparative lithology. The same lithology does not always mean that the rocks were deposited at the same time, often lithology is environment-sensitive, not time-sensitive, so that lithologic units are often time transgressive (occur at different times in different places).

Lithosphere: The solid or partially plastic strong layer that includes the crust and part of the upper mantle of the earth. It is usually around 100 km thick and has rather different properties to the underlying asthenosphere.

Loess: A deposit of pale coloured silt, that blankets topography, is generally weakly stratified to massive, easily broken up, porous, weakly calcareous but coherent enough to stand in vertical faces. It occurs worldwide (the word is German) but occurs around the east coast of the South Island on the Port Hills and other hills. It is a wind blown deposit that was derived from glacial flour, or finely ground but relatively unweathered material created during the glacial maxima. Strong westerly winds favoured the transport of the material and explains the skewed distribution of loess on fewer faces. Loess is relatively stable but easily removed by water, and for this reason is an unsafe surface. It is particularly prone to the development of tunnel gullies, where infiltrating water runs along a hard, cemented layer within the loess or along hard rock underneath, and washes material away where it exits in the local surface gullies. The erosion slowly steps back until a tunnel is created, which eventually becomes unstable and collapses. Landslides lubricated by water running along the hard surfaces are also common on loess slopes. Drainage and retaining walls are the main tools to preventing slope erosion in hillside Christchurch suburbs.

Longshore Drift: The movement of material, sand, gravel or silt, along the shore by a current that runs parallel to the shoreline. The effect of longshore drift is to remove sediment from some areas and deposit in others, it also results in elongated bars developing at the mouth of rivers. For example, the South Island has a strong north flowing current up the east coast, which takes gravel from the southern rivers and deposits it in bars south of Banks Peninsular, where the current is deflected. Hence the building of the gravel bar that isolates Lake Ellesmere from the sea from south to north. The deflected current the flows on, but a large eddy occurs just north of the peninsula, and the Brighton Spit at Christchurch and the bar at the Waimakariri River mouth are built north to south. Further north, bars are once again built south to north.

Lustre: The appearance of light reflected off the surface of a mineral, both the quality and intensity, for example metallic or vitreous, bright and dull.


Macrofossil: Fossils that are large enough to be studied without the aid of a microscope.

Mafic: Silicate minerals that are dark and rich in iron and magnesium or the rocks that contain a lot of mafic minerals. The opposite of felsic.

Magma: Molten rock naturally formed within the earth and able to be intruded or extruded, which may or may not contain solid crystals or rock fragments, or gas phases. Igneous rocks are formed from the solidification of various compositions of magma at varying depths or at the surface.

Magma Chamber: A chamber of magma that forms the source or reservoir of a volcano, and occurs on a few km or tens of km below the surface of the earth. The source of the magma is much deeper in the crust, long lived volcanoes may have multiple injections of magma into the magma chamber, and the composition of the eruptions may vary to reflect the changing composition of the magma in the chamber.

Magnetic Anomaly: An area that has unexpected or unexplained magnetic properties. Detecting magnetic properties is a useful prospecting tool, as many rocks and geological structures (like faults) have properties that either change the signal of the earth’s magnetic field or generate their own magnetic field. Magnetic maps are usually obtained by aeromagnetic surveying where the total intensity of the earth's geomagnetic field is measured with a magnetometer carried in an aircraft or helicopter.

Magnetostratigraphy: The magnetic field of the earth has switched polarity in the past many times. Magnetostratigraphy maps the changes of the magnetic field from ‘normal’ (as it is now) to ‘reversed’ through time. Minerals that contain iron tend to crystallise within the earth's magnetic field so that they are orientated in sympathy with that field. Provided no significant alteration or tectonic deformation has occurred since that crystallisation the ‘remnant magnetism’ of a rock can be measured and can be used to determine what orientation the magnetic field was in when the rock was formed. Measuring the magnetism of a succession of rocks can help date the succession (magnetochronology).

Mantle: The mantle is the part of the earth between the crust and the core (at around 2900 km depth). It is divided into the upper and the lower mantle with a transition zone, and other minor discontinuities may be present. The composition of the mantle is similar to olivine, but minor amounts of sulphides and other minerals may be present in the upper part, and iron-nickel alloys may be present in the lower part. It is hypothesised that slow convection cells in the mantle driven by heat from the core and density differences may be one of the driving forces behind the movement of the plates around the surface of the earth. Plumes of hot material rise up through the mantle, and may drive spreading (large plumes, such as exists under Iceland), or create hot spots (smaller plumes like under Hawaii). The plumes are thought to originate from discontinuities within the mantle or from the core-mantle boundary (the Gutenburg Discontinuity).

Marble: Metamorphic rock formed by either thermal (heating only) or regional (heating a pressure) metamorphism. It is the result of metamorphosing limestones, and is composed of fine to coarse grained recrystallized calcite. Often all trace of the original stone is lost, but certain marbles retain the shapes of shell fragments and other textures. Impurities present in the original limestone can result in brightly coloured layers or patches in the marble.

Marl: A sedimentary rock composed of almost equal amounts of clay or silt and calcium carbonate. It is frequently found interbedded with or grading into limestones.

Massive: A description of any rock that has a more or less homogeneous texture, or a sameness about outcrops or hand specimens Massive rocks do not show flow structures, foliation, cleavage, joints and bedding. In mineral deposits, massive is used to describe ores that are concentrated into one place, rather than deposited in a thin vein-like structure, for example massive sulphide deposits.

Mélange: A fancy name for a mappable unit that is composed of blocks in a range of sizes, usually including some house sized blocks, embedded in a sheared and very fine-grained, crushed matrix. The blocks can be of local origin (the surrounding units) or of exotic origin (no correlative rocks nearby). A mélange can be sedimentary in origin, but they are often caused by intense deformation along a major tectonic boundary or fault.

Metamorphic Rock: A rock that has been derived from pre-existing rocks by changes in the mineralogy, structure and fabric and sometimes chemistry, while the rock remains in solid state (no melting). The process of change in solid state other than those changes that occur at the earth’s surface due to weathering or cementation (see diagenesis) is called metamorphism. The changes are in response to changes in temperature (thermal metamorphism), pressure and temperature (regional metamorphism), shearing stress (cataclastic metamorphism) and injections of hot fluids, usually from nearby magmatic bodies (metasomatism). Different grades of metamorphism are linked to particular pressure and temperature conditions, and marked by which minerals grow under these conditions, although the composition of the original rock largely governs what minerals will crystallise. A metamorphic rock may have undergone many such changes under different conditions, for example in regional metamorphism a rock is subjected to increasing pressure with causes different minerals to grow as the pressure and temperature conditions change (prograde metamorphism), and then if the rock is uplifted and exposed at the surface then it goes through a new series of conditions of decreasing temperature and pressure as it rises, causing new reactions to occur (retrograde metamorphism).

Metasomatism: Solid state reactions that result from hot fluids passing though a rock mass and altering the minerals and the rocks overall composition. Metasomatic minerals include tourmaline, topaz and serpentine. The fluids can be derived from igneous intrusions or be heated by passing though zones of high heat flow, and carry minerals dissolved from other areas which are then concentrated in some zones (hydrothermal systems). Some metasomatic mineral deposits include serpentinite (greenstone), clay, and gem stones such as topaz and beryl.

Meteorites: Solid objects from space which have fallen to the earth's surface, without being vaporised in the atmosphere. Meteorites are usually stony or metallic and are though to be fragments of comets and similar to the material from which the earth was originally assembled. Meteors are objects that vaporise high in the earth’s atmosphere, without falling to earth. Late meteorite impacts create large craters and throw large volumes of dust and fragmented material into the atmosphere, if they land in oceans they cause large volumes of water to rise into the atmosphere. Thus large meteorite impacts have global effects on the climate.

Micas: A group of minerals called sheet silicates or phyllosilicates, where the silica tetrahedral form layers that are connected by other ions, such as potassium, sodium, iron, magnesium and aluminium. The structure means that the minerals have one perfect cleavage, and split easily into thin elastic sheets. The minerals range in colour from colourless (muscovite) to black (biotite) including various green (chlorite) and brown (phlogopite) shades. Micas are a common accessory mineral in igneous rocks, are relatively rare in sedimentary rocks, but extremely common in metamorphic rocks, especially regional metamorphic rocks, where the preferred orientation of mica (perpendicular to the maximum pressure) creates cleavage, schistosity and foliation. Large ‘books’ of mica occur in pegmatites, and are used as insulation, and in paint.

Micrite: Very fine grained (less than 4 microns) particles of calcium carbonate, usually as a matrix in a limestone, interpreted to be lithified lime mud, formed by nannofossil skeletons, direct precipitation of minute crystals, or grinding of larger calcium carbonate grains by animal boring activity.

Microfossil: Any fossils too small to be studied without the aid of a microscope, such as foraminifera, diatoms, dinoflagellates and ostracods.

Migmatite: An intimate association of igneous rock and coarsely crystalline metamorphic rock, so that it becomes difficult to distinguish between them. Migmatites may be derived from the intrusion of a magma into the layers of a metamorphic rock, involving some mingling and recrystallization of the metamorphic rock, or they may be derived from partial melting followed by rresolidificationof part of the metamorphic rock.

Milankovitch Cycles: In 1941, Milankovitch described and calculated the fluctuations in the orbit of the earth around the sun. These are the precession cycles (rotation of the earth's axis; cycles have periods between 19 and 24 kyr (thousand years)), obliquity cycles (tilt of the earths axis relative to the orbital plane; periods between 40 and 54 kyr) and eccentricity cycles (how elliptical the earth's orbit is, major periods around 100 and 400 kyr). These cycles affect the climate of the earth by varying the amount of sunlight to reach the atmosphere and the earth’s surface (insolation). This then causes relatively cool or relatively warm periods (there are various complicated feedback and cancelling processes) which can trigger glaciations or interglacials. Studies of the recent glaciations have shown that since about 800 kyr ago, the glaciation-interglaciation fluctuations have been on 100 kyr periodicities, so are being controlled by eccentricity cycles. However, before this period climatic fluctuations were controlled by obliquity cycles. Milankovitch cycles have now been identified through much of the history of the earth, although the periodicities have changed through time as the earth-moon-sun system has evolved.

Mine: N. An underground or open-cast excavation for the extraction of mineral deposits, as distinguished from surface and small-scale excavation for stones or earth which are called quarries. V. To excavate for and extract mineral deposits.

Mineral: An inorganic element or compound which has an ordered, repeated internal structure, and is characterised by specific chemical composition, crystal form, structure and physical properties.

Mineralisation: Used to describe the process by which valuable minerals are deposited or crystallised within a rock, resulting in an ore deposit. There are various kinds of mineralisation, including replacement of pre-existing minerals, filling of veins, cracks or fissures, metasomatism and so on. Alternatively mineralization can refer to the process of replacing organic remains by inorganic materials – a process of fossilisation.

Mineralogy: The study of the formation, occurrence, properties, composition and classification of minerals.

Mohorovicic Discontinuity: The abrupt change in density at the base of the crust, the boundary between the crust and the mantle. The boundary was discovered because it marks a change in the velocity that earthquake waves travel at, and studying earthquakes led to the discovery of this layer. The discontinuity lies between 5 and 15 km beneath ocean floor, and around 35 km beneath continental crust, although up to 60 km beneath mountain ranges.

Mohs Scale: A set of ten minerals that is used to determine the hardness of unknown minerals. The scale, from softest to hardest, is talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, diamond.

Monocline: A local increase in the dip of a surface or an anticline-syncline pair where the direction of dip on all limbs remains the same.

Monzonite: Plutonic igneous rocks which contain equal amounts of alkali (potassium rich) feldspar and plagioclase, with little or no quartz, and augite.

Moonstone: A variety of alkali feldspar (adularia) or fine intergrowths of alkali feldspar and quartz, that has a bluish to milky white colour, pearly to opaline luster, is transparent to translucent and is used as a gemstone.

Moraine: A mound or ridge of glacial debris or till, mainly unstratified and deposited as the direct result of glacial ice. Variations include terminal moraine (deposited at the front end of a glacier) and lateral moraine (deposited at the sides of a glacier). Moraines are frequently reworked by glaciers advancing over them, or by stream action redepositing the material and carrying the very fine sediment away.

Morphology: Literally, shape. Geomorphology is the study of the shape of the earth’s surface. Palaeontologists study the morphology of fossils, animals or plants, and especially looking at the relationships and growth of different parts of animals or plants.

Mud: A wet mixture of silt and clay sized particles, generally containing more than 10% clay and less than 50%.

Mud Cracks: Cracks in the surface of mud, clay or silt, in an irregular or roughly polygonal shape, and caused by shrinkage associated with drying at the surface (dessication cracks), or shrinkage associated with changes in salinity (syneresis cracks).

Mudflow: See earthflows.

Mudstone: Mud that has been lithified.

Mylonite: A rock produced by cataclastic metamorphism, where the original rock has been intensely sheared and ground up by large or long lasting shear stresses. Mylonites are produced in deep, extensive or old fault systems.


Nanofossil: Fossils such as coccoliths that are too small to be studied with conventional reflected ligh microscopy, and are best imaged with electron microscopy. Some pollens and marine algae also come under this category.

Nappe: A sheetlike body of rock that has been moved on a relatively horizontal surface from elsewhere, and is composed of rocks dissimilar or deposited in a different place to those that surround it. The mechanism for nappe formation can be folding, thrust faulting, or gravity sliding.

Neotectonics: The study of current faults, fault movement and earthquakes.

Nodules: Small rounded masses or lumps of any particular mineral or mineral aggregate. Nodules usually lack internal structure and contrast to the surrounding rock or sediment. Nodules can form by several mechanisms, by diffusion of a mineral though pore spaces to a nucleus (see concretions), by direct precipitation or growth on the sea-floor or in the sediment, or concentration processes. Nodules can be valuable economically as they concentrate minerals in one place.


Obduction: The opposite process to subduction, where rocks are pushed up between two colliding plates. Obduction often results in ophiolite complexes which are parts of the sea-floor and oceanic crust caught up when two masses of continental crust come together at a subduction zone and collide. The mafic oceanic crust becomes metamorphosed and squished between the more resistant continental crust. A New Zealand example is the Esk Head Mélange, which divides two parts of the Torlesse Terrane, deposited in different parts of an ocean basin to the east of the Australia/New Zealand/Antarctica land mass between 270 and 70 million years ago. The mélange contains large lumps of limestone from the ocean floor, as well as basalt, Torlesse sandstones and other lithologies.

Obsidian: Volcanic glass, formed by the eruption of rhyolitic lava into a lake or the ocean. It is dark brown to black coloured and characterized by conchoidal fracture.

Ocean: Oceans are the largest bodies of salty water on the planet. They occupy ocean basins, the area between the margins of continental crust. Ocean floors are oceanic crust, created at mid-ocean spreading ridges, and destroyed at continental margins in subduction zones. Sediments mantle the oceanic crust away from the spreading ridges, with increasing thickness further away from the ridge. Deposition in the oceans is one of the ways in which the oceans help to regulate the earth’s climate through regulating the composition of the atmosphere and its circulation. Carbon dioxide, an efficient greenhouse gas, is bound into organic carbon and calcium carbonate and deposited in the ocean basins. Eventually the carbon is release through cycles of uplift and erosion, or through subduction and volcanic activity, but the amount of carbon dioxide bound up in sediments far exceeds the current volume in the atmosphere. Oceanic currents, driven by heat and salinity differences, carry heat around the world keeping some areas free of ice and temperate, while others are cold and icy (compare Scandinavia and Greenland, similar latitude, different conditions).

Oceanic Crust: Oceanic crust has a similar composition to the mantle, and is composed of basalt, pyroxenite, peridotite and dunite. It is created by upwelling magma at mid-ocean spreading ridges, and is relatively thin compared to continental crust, only reaching around 15 km thickness. Some areas of oceanic crust are thicker, and these are called oceanic plateaus. These areas probably result from a faster flow of magma from the spreading ridge. Because oceanic crust is created almost continuously and at a linear feature the crust gets older the further away from the ridge on both sides. Basalt has a high concentration of iron rich minerals, which align themselves to the earth’s magnetic field. The reversals in the magnetic field over time mean that the basalt ocean floor on either side of spreading ridge has stripes of basalt with different magnetic polarity. Tracking these stripes back through time has allowed recent spreading to be accounted for when doing continental reconstructions (how the continents looked in the past), and also allowed the rate of spreading to be calculated for each ridge.

Onshore: Landward from the sea.

Oölite: A limestone composed of oöliths, which are small rounded bodies composed of calcium carbonate, which have formed by accretion of concentric layers of calcium carbonate around a nucleus such as a sand grain or shell fragment.

Ooze: A sediment that has been deposited by settling of particles through the water column, and contains at least 30% skeletal remains of either calcareous or siliceous organisms (living in the water column). Oozes are defined by their composition or their dominant organism, e.g. Calcareous ooze, radiolarian ooze or diatom ooze.

Ophiolite: See obduction.

Ore: The material from which a valuable mineral can be extracted at a reasonable profit, and/or the mineral(s) extracted. Ore is usually designated by the name of the valuable constituent, e.g. Aluminium ore. Ores occur in various bodies or forms, depending on the origin of the ore deposit. Generally, an ore deposit will contain various grades of ore throughout the ore body.

Orogen: An orogenic belt, or an area that has been extensively deformed, usually by folding, during an orogenic cycle. Orogenic belts are commonly mountain chains now, usually due to uplift after the main orogenic episode or orogeny. Various orogenies have occurred in New Zealand's history, the formation of the Otago Schist is one, but we are currently living through the Kaikoura Orogeny, which involves folding, schist formation and mountain uplift in the Kaikoura Mountains and the Southern Alps region.

Outcrop: The part of a geological formation that is exposed at the earth’s surface. Outcrops can also be called exposures.

Outlier: An isolated area of rock surrounded by older rocks, for example a hill, or the centre of a syncline.

Overburden: In mining overburden refers to the rock material, either loose or consolidated, that overlies a valuable mineral deposit, and must be removed prior to opencast mining. Overburden is also used in basin analysis to refer to the thickness of sedimentary deposits which is causing compression and compaction of the underlying sediments.


Pahoehoe: A basaltic lava with low viscosity that flows rapidly and the solidified surfaces appear smooth or ropey. The term comes from Hawaii, as does the term for slightly more viscous lava: aa. Aa flows are characterised by clinkered or rough surfaces. More viscous lavas which do not flow easily are called blocky.

Paleo: A prefix meaning old or ancient, for example, aleozoic meaning ‘ancient life’ (see geological timescale)

Paleocurrent: A current that existed when the rocks were deposited. Generally used in reference to the primary structures in sedimentary rocks, which can be used to determine the direction of water (or wind) flow which was occurring when those sediments were deposited.

Paleontology: The study of life in the past using fossils of plants and animals, and studying the relationships between different species in terms of evolution and the ecology of past communities and environments.

Paleosol: Old, buried soil.

Palynology: The study of pollen and spores and other tough parts of algae and plants for use in stratigraphy and paleo-environment studies.

Pangea: A supercontinent that existed in the late Paleozoic period (between about 300 and 200 million years ago), and comprised almost all the continental crust present at the time. The continent broke up into the present continental masses by first separating into two smaller supercontinents: Laurasia (northern hemisphere continents) and Gondwana (southern hemisphere continents). Pangea can also be spelled Pangaea.

Paralic: Deposits or environments at the margin of the sea, or subject to marine incursions, but not fully marine. Lagoons, beaches, esturies or tidal reaches are all included. A more restricted term is littoral, which refers to the area between the high and low tide levels.

Parasitic Cone: A cone on the side of a previously built volcano that is deriving its material from the same source as the initial volcano, although it may have its own pipe and magma chamber system. Often parasite cones become the main vent of lava for the volcano, or they may represent only a minor part of the volume of erupted material. Ngarahoe is a parasite cone on Tongariro.

Particle Size, Shape: Particles are fragments or grains within a rock or sediment, usually a sedimentary rock. They are described based on their size (usually average or maximum diameter), their roundness (whether corners are worn or angular) and their sphericity. All three parameters can be interpreted to give information about the transport history of the sediment.

See grainsize.

Passive Margin: The margin of a continent that is not the location of active tectonics, either rifting or convergence. The east coast of America, all around Australia and the east coast of the South Island of New Zealand are examples.

Pegmatite: An extremely coarse grained igneous rock that occurs as veins or irregular bodies at the margins of igneous intrusions, and may contain unusual minerals due to a high concentration of elements present only in small amounts in the other parts of the igneous body. This is due to their formation as the crystallisation of the last, water rich part of the magma after most of the rest has crystallized.

Pelagic, Pelagic Ooze: Pelagic animals are those that live in the water column of the ocean, either floating (planktonic) or swimming (nektonic). The word is generally used to describe the water of the ocean as an environment, especially when describing the various depth zones: epipelagic, mesopelagic, bathypelagic. Pelagic ooze is formed by the passive settling of sediment particles and dead organisms or shells to the sea floor.

Pellets: A small rounded aggregate of sedimentary particles, typically clay to silt sized, which are usually the excrement of small animals such as worms and other invertebrates.

Peridotite: Ultramafic plutonic igneous rock which contains olivine and may contain pyroxene and amphiboles. It is found associated with oceanic crust that has been obducted, or uplifted and exposed between two continental plates.

Permeability: How easily fluid flows through a rock or soil, which is related to the spaces between grains (pore spaces), the connections between pore spaces, and the presence or absence of any rock mass defects such as jointing or shearing.

Petrography: The study of rocks by looking at thin slices under a polarising microscope, and the classification of rocks based on microscopic descriptions.

Petroleum: Liquid hydrocarbon, from which fuels and other petrochemicals are derived by distillation.

Petrology: The study of the origin, occurrence and history of rocks, especially igneous and metamorphic rocks, using microscopic, geochemical and field techniques.

Phenocryst: A crystal in an igneous rock that is relatively larger than the majority of crystals present.

Phreatic Eruption: An explosion caused by superheating of water by interaction with magma, and the eruption of steam, mud and pulverised rocks, but not magma.

Phyllite: A metamorphic rock where very fine crystals of mica have begun to grow, resulting in fine schistosity developing.

Phylogeny: The line of descent of organisms from one species to another, or the study of such relationships.

Pillow Lava: Lava which has solidified in discontinuous round to oblong pillow-shaped forms with very chilled or glassy margins, and evidence that the molten lava has continued to be erupted into the centre after the margin has solidified. The structures are usually considered indicative of lava erupting into a body of water, where the outermost layer cools extremely rapidly (frequently developing glassy rims) while the inner part is still molten. Pillow lavas are common in oceanic crust, and also where volcanoes have erupted onto the ocean floor, and the lava is usually basaltic or andesitic in composition. A good example occurs in Boatmans Harbour in Oamaru.

Pitchstone: A type of dark volcanic glass.

Placer Deposit: A mineral deposit formed by concentration of heavy minerals in surface streams or beaches while lighter minerals are transported further. Good examples are the alluvial (river) gold concentrations in Otago and on the West Coast, and also the black sand concentrations on the west coast of both islands.

Plate: Part of the earth’s lithosphere that moves in relation to other plates and in response to convection within the mantle and forces on its boundaries. Plate boundaries are zones of active tectonics. The boundaries between plates can be extensional (where plates are moving apart), translational (moving past) and convergent (moving towards each other). Extensional boundaries result in rifting and eventually the formation of oceanic crust, translational boundaries result in faulting, while convergent boundaries cause subduction or uplift and mountain building (orogenies).

Platform: Near horizontal surface, like terraces, beaches, plateau and so on. In terms of continents, the platform is a region that is underlain by gently tilted or flat-lying sedimentary rocks on older deformed or crystalline basement, and that is generally flat-lying and undeformed at the present day.

Plinian Eruption: An explosive eruption, typically of rhyolitic to andesitic composition material, that is characterized by rapid ejection of fragmented magma, ash, blocks and gas in tall columns and vast pyroclasic flows. The eruption style is named after Pliny the Younger’s description of the eruption of Mt Vesuvius that buried the cities of Pompeii and Herculaneum in 79 A.D.

Pluton: A coarse grained igneous intrusion formed at depth.

Plutonic Rock: Rocks that solidified from magma, or molten rock, at a depth in the earth’s crust sufficient for the crystallization to occur slowly, resulting in relatively large crystal size and interlocking crystal boundaries.

Plutonism: Either the effects associated with intrusion of magma and the formation of plutons, or the theory attributed to James Hutton (18th century geologist) that the earth solidified from a molten mass. The opposing theory neptunism (A.G. Werner) is that the earth is deposited or crystallized out of water.

Poikilitic Texture: The texture of a rock where small grains or crystals are enclosed within a single crystal of another mineral, for example in igneous rocks where crystals of plagioclase are enclosed within pyroxene, or in sedimentary rocks where shells or detrital grains are enclosed within calcite crystals.

Polymictic: Where the grains or clasts within a sedimentary rock are composed of many different rock types.

Polymorphism: The characteristic of a chemical substance to crystallize in more than one form, for example in minerals where kyanite, sillimanite and andalusite all share the same chemical composition yet have different crystal structures depending on the conditions under which the mineral crystallizes. The three minerals are polymorphs.

Pore Water Pressure: Pressure of water contained within the pore spaces of a rock or soil. Pore water pressure can be a major controlling factor in the stability of slopes, so that additional water (rain or seepage) can destabilize a slope.

Porphyritic Texture: Occurring in igneous rocks where there are some crystals (phenocrysts) that are much larger than the majority of crystals. The occurrence of these large crystals indicates that the magma went through more than one stage of cooling, and initial relatively slow cooling stage where the larger crystals formed, and a later faster cooling stage where the rest of the magma crystallized. Porphyritic texture can occur in fine grained (volcanic) and coarse grained (plutonic) igneous rocks.

Porphyry: Any igneous rock that contains conspicuous phenocrysts. A porphyry deposit is a body of porphyry that contains disseminated sulfide minerals (chalcopyrite, pyrite, sphalerite and so on), and can therefore form an ore for copper, gold, iron or molybdenum. In most deposits the grade is very low, and without various later enrichment processes mining would be uneconomic.

Pressure Dissolution: The dissolution of minerals due to increased pressure, the most common occurrence is in calcareous rocks such as limestones, where pressure forcing shells or grains against one another creates dissolution and causes jagged planes (stylolites) to develop where dissolution has occurred.

Primary Mineral: A mineral formed at the same time as the rock it occurs in.

Pseudomorph: A mineral that has the same crystal form appearance as another mineral, and which has usually developed by replacing the original mineral.

Pull Apart Basin: A basin that forms in strike slip fault zones, where the fault line steps laterally, leaving an area that is being extended by the movement of the two sides. These typically develop into small sags, however large faults, and a large distance between the two strands can result in larger structures, like the Hanmer Basin, developing. Uplift at the edges of the basin occur to accommodate the extension within the basin. The opposite case, which occurs when the faults are moving in the opposite direction, is an uplifted area, that can develop into a flower structure.

Pumice: Vesicular igneous rock of rhyolitic composition, formed by ‘bubbling’ of volatile gasses in the magma just prior to eruption.

Pyroclastic Rocks: Rocks that are formed from pyroclastic material, which is material formed by volcanic explosion, and comprising ash (commonly welded by the heat during eruption and deposition), glass fragments, fragments of rock from the volcano and surrounding area, fragments of magma, and any other fragments that the flow of pyroclastic material from the vent has picked up on the way. Pyroclastic deposits are commonly formed by collapse of eruption columns.

Pyroxenite: Coarse grained ultramafic igneous rock formed almost entirely of pyroxene, with minor amphibole or olivine.


Quartzites: A rock composed almost entirely of quartz, either a metamorphic rock consisting of equigranular recrystallized quartz, or a sedimentary rock composed of quartz grains cemented with silica.


Radial Dykes: Dykes that radiate from a volcanic centre or cone.

Radiometric Dating: The various methods of calculating the age of a rock or mineral by using the decay of radioactive elements, by measuring the amount of daughter isotope present and knowing the rate of decay for the original element. Dating using carbon-14 is used in relatively recent time, while isotopes of potassium and argon, uranium and lead are all used to determine ages for older minerals and rocks.

Red Beds: Sedimentary rocks that are red coloured due to the presence of iron oxide (mostly hematite) coating the grains. The presence of the hematite in the sediment is thought to be related to the aridity of the climate, as the iron would probably have been carried away in solution in a more humid climate.

Reef: A ridge or mound-like structure. In the sea reefs are formed by layers of sedentary and encrusting animals that precipitate hard calacareous skeletons, and the debris formed by the erosion of those skeletons. In modern reefs the dominant organisms are corals, but algae, bryozoa and invertebrate shellfish form an important part, and were more common in fossil reefs. Reef is also used to describe a mineralized vein by old miners, Reefton is so named because the region has many reefs with gold mineralisation

Regional Metamorphism: Also dynamothermal metamorphism. Metamorphism that occurs across a very broad region, associated with mountain building, compression, folding and continental collisions. Regional metamorphism involves the growth of new minerals under increasing heat and pressure as rocks are buried ever deeper in the crust. The first stage is the development of cleavage, parallel fractures in the rock roughly perpendicular to the direction of maximum compression. Cleavage develops most completely in mudrocks because platy clay minerals rotate to lie parallel to the cleavage planes, so that the rock splits along these planes completely and finely. Slate is formed in this way. Under increasing heat and pressure new minerals start to form and fine varieties of mica and chlorite grow. These platy minerals grow parallel to the cleavage and form ‘schistosity’, planes of platy minerals, in rocks called phyllites. With increasing grade (heat and temperature) of metamorphism, minerals become coarser and schists are formed. Schists are characterized by well developed schistosity and visible crystals. Crystal size continues to increase and schistosity develops into layers of dark micaceous minerals separated by layers of pale quartz and feldspar. This is called foliation and these rocks are gneisses. Increasing grade from here results in very coarse grained rocks and eventually melting and the formation of igneous rocks. Some minerals grow faster and to larger sizes within metamorphic rocks, these are called porphyroblasts, and are analogous to phenocrysts in igneous rocks. Changes in the orientation of the rocks in relation to the compression results in different generations of cleavage and schistosity developing, resulting in crenulated cleavage and minor folding of cleavage planes, these features are common in the Otago Schist.

Remnant Magnetism: The magnetism of a rock that is fixed and is independent and unchanged by other magnetic fields, like the earth’s magnetic field. Remnant magnetism is usually controlled by magnetic minerals orientated in the direction of the local magnetic field during their ccrystallisationor deposition. Because the remnant magnetic field reflects the earth’s during the rock’s formation, measuring that field can help determine the age of the rock (by tracking back the changes in the earth’s magnetic field through time), and the latitude of the rock when it was deposited, so that paleomagnetism studies can help with both dating and plate tectonic reconstructions.

Resistivity Methods: Geophysical method of exploration where an electrical current is inserted into the ground and the effect of this is measured at some distance away. This method is particularly useful for searching for insulators and conductors, such as beds of clay, water tables or metal deposits, or for abrupt changes in permeability such as disturbed ground (graves for example).

Rheology: The study of the deformation and particularly the flow of matter. When rocks become heated sufficiently to fuse or are infused with new material they can flow and deform in response to the pressure and stresses in their environment, a process called rheomorphism.

Rhyolite: Extrusive igneous rock of the same composition as granite, which is typically fine grained with phenocrysts of quartz, and commonly shows banding caused by flow while partially molten.

Richter Scale: A numerical scale of the magnitude of an earthquake or the amount of energy released by an earthquake event. The scale is logarithmic, with each level being 30 times more powerful than the previous level on the scale. The scale is commonly used in conjunction with the Modified Mercalli scale of earthquake intensity, which measures the effects of an earthquake or how it is felt at the surface, rather than the actual size. The effect of an earthquake at the surface depends on the distance to the focus (place of movement in the earth's crust) both along the surface of the earth and into the earth’s crust. Thus deep earthquakes are not as damaging as shallow earth quakes as much of the energy release is absorbed by the intervening rock.

Rift: A long narrow trough-like feature in the crust, of regional extent (as opposed to a graben), caused by regional extension, strike-slip faulting or volcanism. Continental rifting causes rift valleys to develop, which can develop into basins, and eventually into sea-ways floored with oceanic crust. Extension related to the presence of a strong upwelling in the crust commonly result in triple radiating rift valleys developing (triple junctions), of which only two can continue rifting as the stresses prevent the third opening fully. The ‘failed rift’ is called an aulacogen. An example of current rifting on this model is the Red Sea – Gulf of Aden – East African Rift system, where the EAR is an aulacogen. An older example is the rift valleys that now form the floor of the Atlantic Ocean, and the failed arm that is occupied by the Benue River in Nigeria.

Ripple: A corrugation on the surface of a sediment caused by current flow depositing grains on the lee face and eroding from the up-current side, resulting in small scale cross-beds. Ripples can be used as indicators of current direction and even current type and intensity.

Roche Moutonee: An elongated, rounded mounded of bedrock that has been shaped by a glacier, with the long axis parallel to the flow of the glacier, a rounded upstream side and a rough and steep downstream side.

Rock: An aggregate of one or more minerals in a body, or a body of undifferentiated mineral material or solid organic matter (coal). Also used to refer to prominent bare rock masses along coastlines or isolated in bodies of water.

Rockfall: See landslides.

Rudite: Sedimentary rock that is composed of grains or clasts larger than 2 mm in diameter (coarser than sand – granules, pebbles and boulders), for example conglomerate, breccia and calcirudite.


Sandstone: A clastic sedimentary rock composed of sand sized grains or clasts (between 1/16 mm and 2 mm), the consolidated or lithified equivalent of sand.

Sand Volcano: Eruption of sand from the subsurface, commonly associated with liquefaction and shaking due to earthquakes, wave action, impacts of one sort or another.

Scarp: See escarpment.

Schist: Regional metamorphic rock distinguished by the parallel growth of platy micaceous minerals in planes orientated perpendicular to the maximum compression direction (schistosity), and growth of other minerals inbetween layers of micaceous minerals.

Scoria: A vesicular rock, with holes caused by escape of gas bubbles just prior to eruption of the lava, typically occurring on the crust of lava flows, heavier and darker than pumice and likely to be of basaltic or andesitic composition.

Scour and Fill Structure: Alternate excavation and refilling of a channel, for example in a tidal situation or by streams in flooding events.

Seafloor Spreading: The increasing of the oceanic crust by upwelling of magma along rift-zones in the middle of the oceans called mid-ocean ridges, and by intrusion into the lower levels of the crust.

Sediment: Fragments, clasts or grains of solid rock or mineral material that has been deposited from suspension in water, ice or wind, or by direct precipitation of minerals from solution, and forms layers of loose, unconsolidated material. Examples are sand, gravel, mud, till.

Sedimentary Rocks: Rocks that have formed from the lithification of sediments.

Sedimentology: The study of sediments and sedimentary rocks, and the processes by which they form.

Seismicity: Earth movements or earthquakes and the likelihood that earthquakes will occur in an area.

Seismograph: An instrument that records seismic waves (waves in the earth’s crust generated by earthquakes).

Seismology: The study of earthquakes and the structure of the earth by using naturally generated waves and actual earthquakes and artificially generated seismic waves.

Shale: Fine grained, well indurated, clastic sedimentary rock, composed of silt, clay or mud, with a finely laminated structure giving it a natural splitting characteristic.

Shear: Deformation caused by stress which causes parts of a rock body to slide past each other, creating ground up rock particles and other deformation along the shear plane, and allowing fluids to penetrate the rock mass accelerating weathering.

Sheetflood: A thin but continuous film of moving water extending over a large area in an arid region, not confined to channels, usually occurring after a period of sudden rainfall and for a short period of time.

Shelf: A flat area or ledge of rock or an area of relatively flat area flooded by shallow marine waters and covered by a thin sedimentary cover.

Shell: The hard, rigid outer covering of an animal, commonly calcareous (for example brachiopods or bivalves) but could be chintinous (crustaceans) or siliceous (diatoms).

Shield Volcano: A volcano that has broad dome shape, with gently sloping sides, built up from overlapping or interfingering lava flows of basaltic composition.

Shock Metamorphism: Changes occurring in rocks and minerals as a result of high-pressure shock waves caused by meteorite impacts, laboratory experiments and nuclear device detonation.

Shoreface: Seaward from the low-tide shoreline and permanently covered by water, with active wave action moving sediments from the shoreline around.

Silcrete: Surficial sands and gravels cemented by silica, either by circulating groundwaters or surface waters rich in silica.

Silicates: Minerals whose structure is based on sio4 tetrahedra. Silicate minerals are classified based on their crystal structure, into neosilicates (isolated silica tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), cyclosilicates (rings of tetrahedra) and tectosilicates (matrices of tetrahedra).

Silicification: Silica being introduced into or replacing grains within a rock, especially in fine grained forms of silica such as chalcedony or opal.

Siliciclastic: Clastic sedimentary rocks that are dominated by silicate minerals or detrital material and not carbonate or organic material.

Sill: In igneous rocks, a sill is a tabular body of intruded rock that parallels the planar structure of the surrounding rock, most usually a sediment. In sedimentary basins, a sill is a ridge that divides one basin from another, or separating a gulf or fjord from the open sea. Sedimentary sills may be controlled structurally or be composed of drifts of sediment from previous episodes of deposition.

Silt: A particle finer than sand and coarser than clay, that is between 1/16 mm and 1/256 mm, or an aggregate of rock or mineral particles with the majority in this size range, but maybe including some coarser and finer particles. Clay minerals can be silt sized.

Siltstone: Lithified silt.

Sinistral: To do with the left; inclining or spiralling to the left.

Sinkhole: A collapse of an underground cave or cavity caused by dissolution or removal of material. Characteristic of limestone areas and karst landscapes where dissolution creates caves with then collapse and create areas of closed drainage.

Sinter: Siliceous deposition around a spring, usually a hot spring, either a smooth or porous rock, where the porosity is created when silica coats plant fragments around the spring, and when the plants rot spaces are left. The calcareous equivalents are tufa (porous) and travertine (smooth).

Skarn: A rock produced when igneous magma intrudes into a limestone or marble, and the zone of mixing results in lime-bearing silicates and precious gemstones.

Slab Pull: A theory as to why spreading zones continue to create new ocean crust even at great distances from the original zone of upwelling, it holds that the dragging force of the subducting plate descending into the crust pulls the spreading zone apart.

Slate: A low grade regional metamorphic rock, produced when mudstones are compressed so that the clay minerals align themselves parallel to cleavage planes, so that the rock splits easily into thin slabs.

Slaty Cleavage: Cleavage developed during low-grade regional metamorphism where very fine mica minerals crystallise and clay minerals realign to parallel the cleavage, giving the rock a slaty appearance.

Slickensides: Polished and striated rock surface within a fault plane where striations are caused by fault movement and indicate the direction of movement.

Slump: Gravitational movement of material downslope either on land or under the sea, or the mass produced by such movement. Slumps on land are types of landslides that generally involve some sort of backwards rotational movement.

Soil: Weathered rock material mixed with transported sediments and organic matter from plants growing on the surface, bioturbated by the action of plants and animals, and the natural zone in which most plants grow. Different types of soil develop above different rock types and in different weathering conditions or climatic zones. In terms of engineering geology, ‘soil’ is used to describe all unconsolidated materials above the bedrock.

Sorting: The processes by which particles with a specific characteristic (usually size, but also shape or weight) are separated from other particles by some type of transportation, or the result of that separation.

Spatter, Spatter Cone: Spatter forms from very fluid droplets of magma solidifying and accumulating around the surface of a vent. A spatter cone develops around a vent or along fissures by accumulation of this material, and is usually steep-sided and composed of solidified basaltic lava droplets.

Spheroidal Weathering: The development of spherical shaped blocks of weathered rock which are detached from a rock mass by the action of chemical weathering along multiple joint sets attacking a block from all sides.

Spherulitic Texture: When a rock is composed of numerous spherules, which are rounded masses of acicular crystals, commonly feldspar, radiating from a central point. Spherulites are thought to represent explosive crystallisation probably relating to an abrupt change in temperature or pressure of a magma.

Stalactite, Stalagmite: Cylindrical or conical deposits of calcareous material deposited from dripping or flowing water (another variety is flowstone), either hanging from the ceiling of a cave (stalactite), or standing up from the floor of a cave (stalagmite). Commonly they develop in tandem, with the drips falling from the stalactite depositing more calcite on the stalagmite, until they meet to form a column. Early stalactites are often tubes, with water running down the inside, while later stalactites are cones with water running down the outside. Repeated patterns of water flow can result in curtains of water developing. Deposition of calcite in this way results in chemical sedimentary limestones.

Stockwork: A network of veins containing economic minerals, where the veins are close enough together that the whole body can be mined.

Strata, Stratification: A stratum (many strata) is a layer of sedimentary rock that is easily identifiable from the layers above and below, stratification means the layering or bedding in the sedimentary rock, but is also used to refer to any layering, such as within snow and ice deposits, or within a body of water such as a lake or the sea.

Stratigraphy: The study of rock strata, their origin and organisation, or the arrangement of strata in terms of location and position within a sequence of strata.

Stratovolcano: A volcano constructed on alternating layers of lava and pyroclastic deposits. It is generally intruded at shallow levels by numerous dikes and sills, lava and pyroclastic deposits are ejected from a central vent or a parasite vent in the side of the volcano.

Streak Test: Where a mineral is scraped across a ceramic plate to determine the colour of its streak, the colour of the powdered mineral which is often diagnostic of the mineral composition.

Striation: One of a set of scratches, generally parallel that are inscribed on a rock surface by glaciers (glacial striations), streams (drag marks) or faulting (slickensides).

Strike and Dip: Strike and dip are used to define planes, such as bedding planes, foliation or schistosity planes, fault planes or any other surfaces. Strike is the direction of a horizontal line on the surface of the plane (think of the intersection of the surface of a body of water with an inclined plane within it), and dip is angle between horizontal and the surface of the plane, measured perpendicular to the line of strike as this is where the maximum dip occurs.

Stromatolite: Cyanobateria that build mats and trap sediment, some which secrete lime and all build mounds, also the laminated structures built by the bacteria, both in modern day examples (Shark Bay) and as fossils in old (Precambrian) rocks.

Strombolian Eruption: A type of volcanic eruption in which jets or fountains of fluid basaltic lava are erupted from a central crater.

Structural Geology: The study, description, classification and analysis of structures, generally of small to medium (outcrop to map) scale, including folding, faulting and other deformation.

Subduction: When one plate descends into the mantle beneath the other plate, causing compression, uplift and faulting in the overlying plate. Numerous earthquakes occur where the subducting plate interacts with the overriding plate and the mantle, these earthquakes define a ‘Benioff Zone’ of increasing depth beneath the overriding plate. Melting of the water-rich descending plate creates rising magma plumes and volcanoes in the overriding plate.

Subsidence: Sinking of a part of the surface of the earth in relation to the surrounding areas, at any rate, or at any scale. Natural processes such as faulting, compaction, solution, volcanic magma withdrawal and so on can all cause subsidence, as well as man’s mining, water and oil pumping or other activities.

Supercontinent: A large continent composed of several continent fragments joined together, and used to refer to continents that existed in the geologic past from which the present continents have been derived.

Supergene Enrichment: Near surface processes of oxidation and dissolution which remove metals from rock near the surface, carry the ions downwards and reprecipitate the metals in rocks beneath the surface, commonly in similar rocks to those exposed at the surface, thus enriching the concentrations of metals already present and often upgrading a deposit to the status of an ore. Supergene enrichment is especially important in porphyry copper deposits, as the disseminated mature of the copper sulphides means that without enrichment the grade would be too low for economic mining.

Superposition, Principle of: Superposition is the order in which sedimentary rocks or strata occur one above the other, with the oldest strata at the base of the pile and the youngest strata at the top. The principle or law of superposition merely states that in any sedimentary succession that has not been overturned, the oldest rock will be at the bottom and the youngest at the top.

Syenite: Plutonic igneous rock which contains abundant alkali feldspar, a little plagioclase feldspar, minor mafic minerals especially amphibole and little or no quartz. Trachyte is the extrusive equivalent.

Syncline: A concave up fold where the youngest rocks are in the middle layers.

Synform: A concave up fold where the age of the layers is not known.


Tectonics: The study of the structure of the crust, especially the deformation that has occurred and is occurring in the crustal rocks. This is a broader view than structural geology, which tends to deal with smaller scale features.

Tephra: Any material ejected in a volcanic eruption and carried through the air, including ash, scoria, pumice, bombs, lapilli and blocks.

Terrane: A fault-bounded block of rocks with a common history, that is separate and distinct from neighbouring blocks of rock, and assumed to be in fault contact with nearby terranes.

Terrace: Terraces are flat surfaces of sand, gravel and silt, that form in river systems or marginal marine areas. Terraces may be degradational (caused by erosion and downcutting) or aggradational (caused by excess sediment supply), or caused by uplift due to tectonic movement.

Thermoluminescence: A property of certain minerals that they release light when heated, caused by energy released from electron displacements within the crystal lattice.

Tholeiite: A type of basalt that has orthopyroxene as well as clinopyroxene and plagioclase.

Thrust Fault: A fault that has a very low angle (less than 45°) surface and a reverse motion (hanging wall moves up the fault plane).

Till: Unsorted and unstratified fragmental material deposited directly by a glacier, that has a range of clast size from clay to very large boulders. Material that has been reworked by outwash or marginal streams becomes stratified and sorted and can be called outwash deposits. Till is usually found in glacial moraines, the ridges that accumulate at the front end and at the sides of glaciers. Till may also occur beneath a glacier or dumped from the surface of a glacier by rapid retreat or melting. Compacted or lithified till is called tillite.

Timescale: see geological timescale

Torlesse Group: A group of conglomerates, sandstones and mudstones deposited between the Permian and Cretaceous periods in deepwater offshore of New Zealand, and buried, compressed, deformed and uplifted during the various episodes of convergence that have occurred in the New Zealand region during the last 250 million years. The sandstones are generally rich in rock fragments and feldspar and have a characteristic grey, sugary appearance, so that they are commonly called ‘greywacke’. The group forms the basement (hard, lithified to metamorphose rock underlying softer, younger sedimentary rocks and sediments) under much of Otago, Canterbury, Marlborough and much of the North Island. It is particularly well exposed in mountainous regions where the cover rocks and sediments have been removed by erosion, but also occurs beneath other areas. Well developed jointing and cleavage in the older parts of the group result in easily fractured outcrops causing landslides and slips and other stability problems especially in mountainous regions.

Trachyte: The extrusive equivalent of syenite, generally porphyritic with phenocrysts of alkali feldspar.

Transcurrent: A large strike-slip fault with a steeply inclined fault plane.

Transform Fault: A strike-slip fault along which the displacement stops of changes type, usually associated with mid-ocean ridges, where the faults connect segments of sea-floor spreading.

Transgression: The incursion of the sea over the land or any change in water depth that brings more offshore environments onto areas that were formerly nearshore or shallow areas, or shifts the shoreline further away from the centre of a marine basin. The opposite of regression.

Trench: A long, narrow depression of any kind, including those dug by people for drainage or for investigation of the subsurface, those excavated by natural stream flows, tectonic depressions between mountain ranges, or those that develop parallel to a continental margin at the boundary between a subducting plate and the overriding plate.

Trend and Plunge: The orientation of a linear feature or a line. Trend is the direction or compass bearing of the linear feature, and plunge is the angle between the line and horizontal. Trend can also be used in its mathematical sense, as the smooth direction of a numerical series or population.

Tufa: Chemical precipitate of calcium carbonate, precipitated around springs or streams, and containing numerous vesicles or holes where vegetation coated with calcite has rotted away. The hard, smooth variety without large pores is called travertine. The siliceous version is called sinter.

Tuff: Consolidated pyroclastic volcanic material, including ash, lapilli, bombs etc.

Turbidity Current: A dense current, usually dense because it contains a lot of sediment, that moves swiftly downslope and spreads across relatively horizontal regions. Generally turbidity currents occur in deep ocean water, where they are triggered by destabilisation of a portion of the sediment deposited on the continental shelf, and flow down the continental slope (often in canyons carved by successive similar flows), and spread out on the abyssal plain. The deposits of turbidity currents, ranging from conglomerates, through bedded graded sandstones to mudstones are termed turbidites.


Ultramafic Rocks: Rocks that are composed almost exclusively of mafic minerals, such as pyroxenite (mostly pyroxene) or dunite (mostly olivine).

Unconformity: A significant break in the geological record or of sedimentary deposition, represented by a contact surface between two rock units of different ages. The break in deposition may be the result of a temporary cessation in sediment supply in which case a paraconformity develops, which may also be cemented or colonised by animal communities. Short breaks in deposition that are within the range of normal processes (such as minor storm erosion, slack tides etc) are called diastems. If the break in deposition is accompanied by erosion, then the surface is a disconformity. Where tectonic deformation has occurred and the underlying beds have been tilted and eroded prior to the overlying beds being deposited, then the surface is an angular unconformity. When sediments were deposited directly on some sort of crystalline rock, the surface is called a nonconformity. Unconformity can be used to refer to the time period missing, the relationship between unconformable rock units, and the surface of contact between unconformable rock units.

Underplating: The process by which molten mantle material becomes attached to the underside of a plate, thus increasing the thickness of the plate.

Underthrust: A thrust fault where the footwall has moved downwards relative to the hanging wall, rather than the hanging wall moving up the footwall as occurs in most thrust faults.

Uniformitarianism: The principle that the processes that we see actively modifying the surface of the earth today have acted at the same rate and approximately the same intensity throughout geological time, and that all of the past geological events can be explained by the action of these processes without calling on catastrophic global events (this does not include meteorite impacts, which are included as a present day process). The key phrase is that “the present is the key to the past”. The opposite view, that the earth has developed as a series of quiescent periods interspersed by cataclysms is called catastrophism.

Uplift: The process of raising an area of the crust relative to surrounding areas (rate of uplift for example) or an area that has been raised relative to surrounding areas, by faulting or folding movements.


Winnowing: Separation of fine particles from coarser particles by the action of a current (usually wind or water) passing over them.


Xenolith: A block of rock of any size included in an intrusion that is of foreign origin and probably derived from surrounding country rock or rocks through which the intruding magma has passed.