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Common geoscientific terms and their definitions.



accelerator mass spectrometry: commonly known as AMS; a method for measuring 14C, 10Be and other isotopes including those produced by cosmic rays [accelerator mass spectrometer]

accretion: the process of [1] adding sediment to a river, beach or offshore area; [2] adding terranes to a landmass; [3] forming planets

acidic: relating to fluids with pH less than 7 [acid]

acoustics: the study and measurement of sound waves

active volcano: a volcano that is currently erupting, or has erupted within the past few thousand years and may do so again in the future

aggregate: crushed rock, gravel or sand used for concrete or road making

alga: a primitive plant without complex structures [algae]

alkali feldspar: a group of light-coloured rock-forming potassium–sodium–aluminium silicate minerals.

alkaline: relating to [1] fluids with pH greater than 7 [alkali]; [2] igneous rocks with high concentrations of the alkali metals lithium, potassium, sodium, rubidium, cesium and/or radium.

alluvial: deposited by rivers [alluvium]

Alpine Fault: the c. 500 kilometre long transform fault that forms the Pacific–Australian plate boundary along western South Island

ammonite: an extinct mollusc with a multi-chambered, coiled, single calcareous shell—a relative of living nautiloids

amphibole: a group of dark-coloured, rock-forming, calcium–iron–magnesium silicate minerals

andesite/andesitic: volcanic rock (or lava) containing 54 to 62% silica and moderate amounts of iron and magnesium.

Antarctic Circumpolar Current: the world's largest and strongest ocean current, which moves eastwards around Antarctica and links with other major ocean currents'

Antarctic Intermediate Water: cold, low salinity water that forms in subantarctic regions before sinking to between 700 and 1200 m ocean depth and spreading northward towards the equator

Antarctic Polar Front: the oceanic boundary between less-saline Antarctic Surface Water (equatorwards) and more-saline Circumpolar Surface Water (polewards)'. The Polar Front lies near the centre of the Antarctic Circumpolar Current, between about 50° and 65° South

Antarctic–Australia Ridge: the part of the mid-ocean ridge system that separates the Australian and Antarctic plates. It begins south of New Zealand at the Macquarie Triple Junction and extends west into the Indian Ocean

anticline: a fold in rock layers, generally convex upwards, where the oldest rocks are in the core

aquifer: a porous rock unit containing water that can be extracted

aragonite: a calcium carbonate mineral with the same chemical composition as calcite, but with a different crystal form [CaCO3]

arc front: an array of volcanoes directly above the upper zone of partial melting in a subduction zone

archaeology: the study of human history and prehistory through the excavation of sites and the analysis of physical remains

argon-40/argon-39 dating: commonly known as Ar–Ar dating; a variation of potassium–argon dating, in which the concentration of stable 40Ar, created from radioactive 40K in a reactor, is compared with the concentration of stable 39Ar

ash: fine particles of pulverized rock (tephra) erupted from the vent of a volcano. Particles smaller than 2 mm in diameter are termed as ash.

ashfall: volcanic ash that has fallen through the air from an eruption plume. 

asthenosphere: a weak layer of the mantle below the lithosphere, which deforms plastically when subjected to stress

atmosphere: the envelope of gases surrounding the Earth

Australian Plate: the tectonic plate that includes Australia, the North Island and western South Island. It is also known as the Indo-Australian Plate'

axial ranges: the continuous chain of mountains in eastern North Island including (south to north) the Rimutaka, Tararua, Ruahine, Kaimanawa–Kaweka, Huiarau, Ikawhenua and Raukumara ranges. In South Island, the axial mountain chain includes (from south to north) the mountains of Fiordland, the Southern Alps, Spenser Mountains and St Arnaud Range

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back arc: the region, generally of rifting and subsidence, behind a volcanic arc

ballistic projectiles: Ballistic projectiles are pieces of rock thrown from a volcanic vent in an eruption. These rocks fall rapidly to the ground so rarely reach more than about 3 kilometres radius from the vent. There are two main types, volcanic blocks (large angular solid rock fragments) and volcanic bombs (thrown out as molten rock and smoothed or streamlined during flight)

ballistics: tephra particles larger than 64 mm (including ‘blocks’ and ‘bombs’) that are ejected from a volcanic vent in any direction without being affected by wind. They rarely reach more than about 3 kilometres radius from the vent.

basalt: volcanic rock (or lava) containing less than 54% silica, commonly producing more effusive, runny and less explosive lava [basaltic]

basaltic andesite: a volcanic rock intermediate in composition between basalt and andesite, common in volcanic arcs

basement rocks: crystalline, indurated or metamorphosed rocks that underlie the softer sedimentary or volcanic cover rocks, and which are generally older than c. 100 Ma in Zealandia

base surge: a pyroclastic surge, or base surge, is a type of pyroclastic density current that has high gas content, is turbulent, and the material (including rocks, tephra, gas and steam) is well mixed. They often occur during volcanic eruptions that start in a lake or in a coastal environment, that involves interaction between water and hot magma. Base surges move out horizontally from the base of an eruption column, and are less constrained by topography than pyroclastic flows. In very large eruptions, base surges can travel several kilometers from the vent, and are lethal

bathymetry: the variation in depth of a water body, such as an ocean or a lake. Bathymetry is usually shown using contour lines of equal depth, or isobaths [bathymetric]

bedding: layers of rock of varying thickness and character [bed; bedded]

belemnite: an extinct, squid-like mollusc with an internal bullet-shaped, calcareous shell

benthic: bottom dwelling—in marine or fresh water

bentonite: a clay formed largely by alteration of volcanic ash, which swells in volume when water is added to it—a sodium–calcium–aluminium–magnesium silicate

beryllium-10 dating: a method for determining the age of [1] sedimentary deposits and [2] exposed surfaces using the in situ build-up over time of cosmogenic 10Be

biodiversity: the variety of plant and animal life— usually the number of species—in a region

biogenic ooze: a fine-grained soft sediment, usually on the deep-ocean floor, consisting mainly of microorganisms

biological productivity: the production of nutrients in the ocean

biology: the study of living organisms [biological]

biostratigraphy: the use of fossils to establish the order of rock succession [biostratigraphic]

biota: the plant and animal life of a region

bivalve: a mollusc with a hinged calcareous double shell—for example, a clam

black smoker: a submarine hydrothermal vent that emits metal-rich fluids with grey-black particles of iron–manganese compounds

block and ash flow: a fast-moving avalanche of volcanic ash, hot gas and potentially large rocks, often caused from the collapse of an over-steepening lava flow front or lava dome. These can travel at tens of kilometres per hour, can be hundreds of degrees in temperature and can travel distances of several kilometres. A type of pyroclastic density current

block: angular fragment of solid rock ejected during a volcanic eruption, with diameters of over 64 mm. A type of ballistic

bomb: fragment of molten or semi-molten rock erupted from a volcano, with a diameter of over 64 mm. Because of their semi-molten state, bombs are often modified in shape during their flight or upon impact. A type of ballistic

brachiopod: commonly known as a lamp-shell; a filter-feeding animal with a hinged double, asymmetrical, calcareous shell

break-out flood: a sudden flood of water (and usually rocks, sediment/tephra and debris), caused by the collapse of a natural or man-made dam that is containing a large body of water. The ‘dam’ may include a volcano crater lake rim, or a landslide or debris that has blocked a waterway, which then collapses. Very large volumes of water may be released at once, causing a hazard to areas downstream.

breccia: a coarse-grained sedimentary rock with angular clasts

bryozoan: a colonial filter-feeder that forms a calcareous branched or encrusted skeleton [bryozoa]

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caesium-137 dating: a method for determining the age of sediment or soil using the occurrence of 137Cs generated in atmospheric nuclear tests

calcareous: comprising or containing significant amounts of calcium carbonate

calcite: a commonly occurring, colourless, white or light-coloured calcium carbonate mineral [CaCO3]

caldera: a volcanic depression formed by the collapse of the ground above a magma chamber, which empties during very large volcanic eruptions. The diameter of a caldera many be times larger than the size of the individual vents

calibration: the assignment of numerical ages to (1) the boundaries of geological time intervals; (2) dates derived using relative chronological methods, such as radiocarbon [calibrate]

carbonate: a commonly-occurring ion consisting of one atom of carbon and three atoms of oxygen (CO3). Combined with other elements, it forms common minerals such as calcite (calcium carbonate), dolomite (calcium magnesium carbonate) or siderite (iron carbonate)

carbon dioxide: a gas composed of carbon and oxygen [CO2] and a key greenhouse gas; the dissolved form is bicarbonate [HCO3-]. A common volcanic gas

carbon isotopes: there are three naturally-occurring isotopes of carbon (C), the stable isotopes 12C and 13C and the radioactive isotope 14C (radiocarbon)

carbon monoxide: a colourless and odourless gas composed of carbon and oxygen [CO]. A common volcanic gas

cement: a substance that holds together particles such as sand grains, pebbles or shell fragments. Common cementing substances in rocks are calcite, silica (silicon dioxide) or limonite (iron oxide) [cementation]

Cenozoic Era: the time span that started 65 million years ago and continues to the present day

CFC dating: a method for determining the age of groundwater using the occurrence of industrially produced chlorofluorocarbons

chalk: a soft, light-coloured, fine-grained limestone

chemistry: the study of the elements, the compounds they form and the reactions they undergo [chemical]

chemosynthesis: the process by which microbes convert inorganic compounds to carbohydrates or sugars without sunlight

chert: a hard flinty rock composed mainly of siliceous microfossils

chloride: the dissolved form of chlorine, and a common constituent of geothermal fluids [Cl-]

chronology: an arrangement of events in the order of their occurrence [chronological; chronometer]

clast: a rock fragment or grain broken or eroded from a larger rock. Many sedimentary rocks are called clastic rocks, because they are accumulations of debris eroded from older rocks

clay: [1] groups of (i) layered silicate minerals including kaolinite, illite, halloysite and smectite, (ii) oxide minerals including gibbsite, goethite and haematite, and (iii) extremely small minerals allophane and ferrihydrite; [2] a very fine-grained product of the chemical or physical break-down of rock and deposits [claystone]

closure temperature: the temperature below which a mineral retains the daughter products from in situ radioactive decay

coal: compressed plant remains—peat, lignite, subbituminous, bituminous and anthracite (in order of increasing carbon, and decreasing water, contents)

coccolithophorid: a microscopic single-celled marine plant that secretes calcareous plates [coccolith]

concretion: a hard, often spherical deposit formed by precipitation of minerals from solution within fine-grained rocks

condensate: hydrocarbon gases that condense into a liquid as pressure and temperature are reduced

Conduit: a passage followed by magma within a volcano (part of the under-ground plumbing)

conglomerate: a coarse-grained sedimentary rock with rounded clasts

conodont: a tooth-like phosphatic microfossil

continental crust: the 20–50 kilometre thick crust that underlies the continents

continental drift: plate tectonic movement of continents about the globe

continental shelf: the gently sloping submerged margin of a continent between the shore and the uppermost continental slope

continental slope: the region between the continental shelf and the deep-ocean floor

Country rocks: existing rock beneath a volcano that is intruded by and surrounds an injection of magma (which is also known as an igneous intrusion). During explosions fragments of country rock can be included in the ejecta

convection: continuous transfer of heat by slow circular movement [convective]

convergence (tectonic): the coming together of plates, usually resulting in mountain building or subduction [converge; convergent]

core: [1] the innermost part of a planetary body; [2] a sample of rock or deposits obtained by drilling or coring

correlation: the process of relating rocks or sedimentary deposits of the same essential character or age [correlate]

Coromandel Volcanic Zone: the volcanic arc active from c. 18 to 2 Ma along the present-day Coromandel Peninsula [Coromandel Arc]

cosmogenic isotopes: isotopes produced by primary cosmic rays in the atmosphere or by secondary cosmic rays in minerals contained in exposed rock surfaces

COSPEC: COrrelation SPECtrometer, used to measure a particular gas species, in this case sulphur dioxide in the gases from erupting Ruapehu. High SO2 levels indicate actively degassing magma at shallow depths.

cover rocks: rocks formed on top of basement rocks since about 100 Ma in Zealandia

Crater: a commonly circular depression formed by either explosion or collapse at a volcanic vent, from which volcanic material is erupted.

crust: the outermost shell of the Earth [crustal]

crustacean: an aquatic, crab-like animal with a hard external skeleton [crustacea]

crustal assimilation: the incorporation of rock material into a magma as it moves through the crust

crystal fractionation: the crystallisation and settling out of minerals from a magma, modifying the magma’s composition

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Dacite: a type of volcanic rock with a silica content above 62%, intermediate in composition between andesite and rhyolite. Mt. Tauhara at Taupo and Mt. Edgecumbe (Putuaki) near Kawerau are examples of dacite volcanoes [dacitic]

Debris Avalanche: an avalanche or slurry consisting of unsorted rock, water and other material (such as fragmented cold and hot volcanic rocks, snow/ice and trees). Debris avalanches can move rapidly, and commonly occur on volcanoes.

deep ocean: the abyssal depths beyond the base of the continental slope [deep sea]

deformation: the process of folding, faulting, shearing, compressing or stretching of rocks or soft sediments due to tectonic stress, or as a result of gravitational movements (e.g. landslides). At a volcano, deformation consists of ground movement in a vertical (up and down) or horizontal (sideways) direction. If ground deformation does occur, the rate of ground movement is usually slow (e.g., on the order of millimetres per year), but on rare occasions the rate can be higher (cm per month).

diatom: a microscopic single-cell form of alga with a siliceous shell

dike: a narrow intrusion, usually of magma, that cuts across the natural bedding or layering of the host rock

dinoflagellate: a single-celled, mobile plant-like organism (alga) with a resistant-walled cyst

dinosaur: large extinct reptile from the Mesozoic Era

dip: the inclination of a planar feature such as a layer within a rock, or a fracture plane in a rock. Usually measured in degrees, where 0° is horizontal, 20° is gentle, 65° is steep and 90° is vertical [dipping; also see strike]

discharge: the volume of flow of a moving liquid or gas. Examples include a river, a spring, a gas blow from a hydrothermal vent or flow from an artificial channel or pipe. Commonly measured in litres per second or cubic metres per second

DNA: deoxyribonucleic acid—the ‘genetic code’ molecule found in living tissues

dolomite: a light-coloured mineral composed on calcium magnesium carbonate. Commonly occurs as a magnesium-rich variant of limestone

dormant volcano: a volcano that has not erupted for thousands of years, but may do so again at some stage in the future.

DSIR: New Zealand’s Department of Scientific and Industrial Research (1926–1992)

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earthquake: a sudden motion or trembling in the crust caused by the abrupt release of accumulated stress along a fault

earthquake magnitude: commonly denoted as M; the size of an earthquake based on the original Richter scale

earthquake swarm: a group of many earthquakes of similar size occurring closely clustered in space and time with no dominant main shock

earthquake wave: the energy radiated from an earthquake source in the form of P-, S- and other waves [seismic wave]

East Pacific Rise: the part of the mid-ocean ridge system east of New Zealand that separates the Pacific and Antarctic plates

echinoderm: a spiny-skinned marine animal that has five-part radial symmetry—for example, starfish, seaurchin

ecology: the study of the relationships of living things to each other and to their environment [ecological; ecologist]

economic minerals: geological materials that can be utilised profitably, including oil, gas, coal, metallic and non-metallic minerals, and water

ecosystem: a community of interacting organisms and their physical and chemical environment

effusive flow: a quiet, non-explosive eruption [effusion; extrusion]

ejecta: material that erupted by a volcano, including tephra

electrical conductivity measurement: a geophysical method based on the ease with which current can pass through the ground [electrical resistivity]

electron spin resonance (ESR) dating: a method for determining the age of sediment using the change in magnetism in crystal lattices resulting from electrons dislodged by natural radiation

epicentre: the point on the Earth’s surface directly above an earthquake source

eruption: the arrival of fragmented material, the effusion of lava, or both, to the surface of the Earth (or other planetary bodies) by a volcano.

eruption hazards: eruption hazards depend on the volcano and eruption style, and may include explosions, ballistics, pyroclastic density currents, lava flows, lava domes, landslides, ash, volcanic gases, lightning, lahars, tsunami, and/or earthquakes

eruption plume: a cloud of gases, steam and tephra that rises from the vent, driven by thermal convection and gas pressure during predominantly explosive volcanic eruptions. An energetic type of an eruption ‘plume’ is an eruption ‘column’. If it is of sufficient volume and velocity, an eruption column may reach many kilometres into the stratosphere, where wind may carry it long distances. Eruption columns can collapse and form pyroclastic density currents. Tephra falls out of an eruption plume with small particles travelling further away from the vent than larger particles. Eruption plumes are influenced by the wind at different heights above the vent

estuary: a coastal embayment or river mouth where fresh and marine water mix [estuarine]

evolution: [1] a biological process relating to change in form over time, leading to the rise of new species; [2] a non-biological process relating to gradual change, often from a simple to a more complex form [evolutionary; evolve; evolving]

exclusive Economic Zone: the 200 nautical mile-wide offshore area adjacent to a country’s coastline, in which it has exclusive economic rights

explosions: during a volcanic eruption, the sudden decompression of hot, pressurised volcanic gas can cause a volcanic explosion, or blast. A shock wave is often caused, which on rare occasions can blow down trees and break windows at nearby buildings, and is usually accompanied by a loud boom. Explosions are typically accompanied by the ejection of ballistics, gas and steam from the vent

extension (tectonic): the pulling apart of sections of the crust at faults or plate boundaries [extend; extensional]

extinct volcano: a volcano that has not erupted for over 10,000 years, and is unlikely to do so again in the future, because the volcano no longer has a magma supply.

extinction: the dying out of an entire class, family, genus or species

extrusion: the emission of magma at the earth's surface. Also, the structure or form produced by the process (e.g. lava flow, volcanic dome). The term ‘extrusion’ is more commonly used for effusive emissions (i.e., lava), than for explosive eruptions

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fall deposit: Mantling blanket of pyroclastic particles (ash, scoria, pumice, etc) erupted explosively and transported through the atmosphere before falling back to the ground

fault: a major fracture or dislocation along which the crust has moved

fauna: the animal life of a region or geological interval

feldspar: a group of light-coloured potassium–sodium– calcium–aluminium silicate minerals—the most common mineral type in crustal rocks

ferromanganese deposit: an iron–manganese-rich nodule or crust formed in the deep ocean

fiord: a glacier-cut valley that has been flooded by the sea

fissure: elongated fracture or crack on a volcano. Fissures can host eruptions, which typically consist of lava flows or lava fountains, and can be kilometres long

flank Eruption: an eruption from the side of a volcano (in contrast to a summit eruption).

flood plain: low- and flat-lying areas adjacent to a river, containing river-transported sediment deposited during floods

flora: the plant life of a region or geological interval

fold: a curve or bend in a planar geological structure such as bedding—varieties include anticline, syncline

food web: a complex set of feeding interactions and energy flows between species in an ecosystem

foraminiferan: a single-celled micro-organism with a calcareous shell [foraminifera]

forensics: scientific tests or techniques used in the investigation of crimes

formation: a body of rock unified by origin, age or composition

fossil: the remains, trace, or imprint of an ancient organism

fracture: a brittle crack in the crust

frequency of occurrence: the long-term average return period for earthquakes, volcanic eruptions, tsunamis, floods etc.

fumarole: a vent or hole in the ground through which steam and other gases emit

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gamma radiation: naturally-occurring radiation resulting from decay of radioactive elements in minerals

gas hydrate: an ice-like crystalline solid formed from a mixture of water and natural gas, usually methane

gastropod: a mollusc with a single, asymmetrical shell that is usually spiral-coiled—for example, a snail

gene pool is the complete genetic potential (all of the available alleles) within a species or population

genus: in Linnaean classification, the second-lowest level used to group organisms, and consisting of one or more related species—for example, Agathis, the genus to which kauri trees belong

geochemistry: the study of the chemistry of the crust and mantle [geochemical; geochemist]

geochronology: [1] in general usage, the study and measurement of geological time; [2] with respect to radiometric dating, the determination of the time that a mineral first crystallised [geochronological; geochronologist]

geodetics: the study and measurement of ground movement

geothermal system: the natural transfer of heat within a confined volume of the Earth’s crust where heat is transported from a ‘heat source’ to a ‘heat sink’, usually the ground surface. Types of geothermal systems include hydrothermal systems and magmatic-hydrothermal systems.

Geographic Information System: commonly known as GIS; a computer-based information system that uses a set of procedures and tools to store, integrate, manipulate, query, model and display geographically referenced and associated data

geography (physical): the study of the Earth’s physical features, resources and climate

geological hazards: natural events that threaten human existence, including earthquakes, floods, meteorite strike, tsunamis and volcanic eruptions [natural hazards]

geological map: a map that contains primarily geological information [geological mapping]

geological structure: the general disposition, attitude, arrangement or relative positions of rock masses

geological time: the time from the formation of the Earth 4.6 billion years ago to the present

geological timescale: a chronological arrangement or sequence of geological events—key timescales include the Global Geochronological Scale; the Geomagnetic Polarity Time Scale; the New Zealand Geological Timescale

geology: the study of the composition, structure and origin of the solid Earth; sub-disciplines include geochrononology, engineering geology, mineralogy, petrology, paleontology and stratigraphy [geological; geologist]

geomagnetic polarity: the configuration of the north and south magnetic poles of the geomagnetic field

geomagnetic stripes: a set of parallel magnetic anomalies that increase in age away from a mid-ocean ridge

geomagnetism: the study and measurement of the Earth’s magnetic field—the geomagnetic field

geomorphology: the study of the physical features of the Earth’s surface [geomorphological; geomorphologist]

geophysics: the study of the physical properties of the Earth [geophysical; geophysicist]

geoscience: the study of geology, geochemistry, geophysics, geomorphology, soil science and some aspects of hydrology, meteorology and oceanography [geoscientific; geoscientist]

geothermal: relating to [1] natural heat within the crust; [2] heat from hot groundwater or steam used for power generation

geothermometer: specific chemical compositions in minerals or fluids that indicate the temperature, at equilibrium, of a system

geyser: an eruption of hot water and steam from a hydrothermal system. It is usually of cyclic occurrence, and ejects only small amounts of solid material. The ejection mechanism is volume change due to boiling, as opposed to ejection of water because of artesian pressure alone.

glaciation: a phase of globally cooler climate, with growth or expansion of glaciers or ice sheets in high-altitude or high-latitude land areas, and lowering of sea level by up to 120 m due to the build-up of glacier ice on land. During the last 800,000 years, Earth's climate has experienced periods of glaciation (or glacials) alternating with interglaciations (inrterglacials), on an approximate 100,000 year cycle. About 18,000 years ago, the last glaciation ended, and the climate reached full interglacial conditions about 11,700 years ago

glacier: a slowly moving mass of ice, formed where more snow accumulates over winter than can melt in summer. Excess snow compacts into glacier ice and flows downhill to lower warmer altitudes or, in polar regions, to iceberg-calving margins (e.g. Ross Ice Shelf in Antarctica) or arid zones where ice evaporation (sublimation) exceeds the flow rate (e.g. Dry Valleys of Antarctica). Glacier types include mountain valley glaciers (for example; Franz Josef Glacier or Tasman Glacier in the South Island) or ice sheets such as exist today on parts of Iceland or in Antarctica

Global Positioning System: commonly known as GPS; a satellite-based system for accurately determining locations on the Earth’s surface

global warming: an increase in global average temperature resulting from natural processes or human activity

Gondwana: the ancient super-continent in the Southern Hemisphere from which Zealandia split 83 million years ago [also known as Gondwanaland]

gradual evolution: an evolutionary pattern where new species develop as a result of gradual continuous changes throughout their history.

granite: a coarse-grained, crystalline plutonic rock rich in silica, alumina and alkali metals [granitic; granitoid]

graptolite: a dendritic, colonial fossil with a protein-like skeleton

gravity measurement: a geophysical method based on measurement of the attractive force of the Earth, which varies with altitude, latitude and the density of local rock masses

gravity anomaly: the difference between measured gravity and the expected (modelled) gravity at a site. Small gravity anomalies can indicate the presence of a subsurface magma body

greenhouse gas: an atmospheric gas, such as carbon dioxide or methane, which traps heat and causes global warming

greywacke: an indurated medium-grained sedimentary rock composed mainly of quartz, feldspar and rock fragments; often used collectively to mean greywacke plus argillite, a major constituent of basement terranes

ground shaking: the ground motion caused by an earthquake

ground truthing: the use of field observations to verify data obtained by remote sensing and office-based interpretations

groundwater: subsurface water contained in pores and fissures in rock beneath soil, most of which is beneath the water table

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habitat: the natural home of an organism

half-life: the time for half of an amount of a radioactive isotope to decay

Hawaiian eruption: basaltic eruptions with very low viscosity (i.e. runny) magma. These eruptions often include fire fountains and lava flows

hazard map: a map showing areas of relative risk for a particular geological hazard

hazard zones: the GNS Hazard Zones are a series of zones set up around Mt Ruapehu to indicate areas prone to various hazards (lava bombs, etc)

high frequency earthquakes: a type of earthquake caused by the fracture of a competent, usually cold, rock body.

Hikurangi subduction system: the subduction zone east of the North Island, where the Pacific Plate descends beneath the Australian Plate

hot springs: a surface feature of a geothermal system, where warm or hot water flows out of the ground

hybrid earthquakes: a type of earthquake caused by fracture of a competent body of rock, usually involving a mixture of hot gases and fluids. The mixed lower frequencies result from low rupture velocities and path affects

hydrocarbons: the chemical components of oil, natural gas and coal

hydrogen isotopes: the isotopes of hydrogen, namely 1H (hydrogen), 2H (deuterium) and 3H (tritium)

hydrogen sulphide: a poisonous, strongly odorous gas composed of hydrogen and sulphur [H2S]. A common volcanic gas

hydrology: the study of land-based water systems, including rivers, lakes and groundwater

hydrophone: an instrument for detecting sound and seismic waves underwater

hydrothermal: relating to naturally occurring hot water—often referred to as ‘thermal’ at tourist locations—whose minimum temperature is higher than the ambient mean annual temperature

hydrothermal activity: manifestations seen at the surface of geothermal systems. Hydrothermal activity may include hydrothermal eruptions, fumaroles, gas/steam emissions, steaming ground, geysers, hot springs and streams, and hot pools (including mud pools)

hydrothermal eruption: an eruption ejecting steam and some solid material. Energy is derived only from a convecting hydrothermal system, not magma.

hydrothermal system: a type of geothermal system where heat transfers from a heat source (e.g., magma) to the surface by ‘free convection’, involving meteoric fluids, with or without traces of magmatic fluids. Liquids discharged at or near the surface are replenished by meteoric water derived from the outside, that is drawn in by the rising fluids. A hydrothermal system consists of 1) a heat source, 2) a reservoir with thermal fluids, 3) a surrounding ‘recharge region’, and 4) a heat discharge area at the surface with manifestations (e.g., fumaroles, hot springs)

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ice age: an extended interval of cold glaciations and warmer interglacials, the most recent of which began c. 2.6 million years ago

ice core: a core of ice drilled from a glacier or ice sheet

ice sheet: a vast extent of ice covering a landmass [ice cap]

igneous: the type of rocks formed during volcanic activity, both above and below the ground surface

ignimbrite: a volcanic deposit formed by a pyroclastic flow. Ignimbrite layers can be soft and full of pumice such as the land surface around Taupo (from the Taupo eruption 1800 years ago); or hard rock where the deposit has been thick and hot enough for the particles to fuse together (eg at Bulli and Te Toki Points on Lake Taupo).

impermeability: the inability of rocks or sediments to transmit fluid [impermeable]

interglaciation: a warm period between glaciations— the most recent (present) interglaciation is called the Holocene

intrusion: the process of emplacement of magma in pre-existing rock. The term also refers to the igneous rock mass formed in this way

invertebrate: an animal without a backbone

ionise: to give atoms an electrostatic charge [1] in a chemical reaction; [2] in a mass spectrometer [ion; ion source]

ironsand: black sands containing heavy minerals rich in iron—for example, magnetite, titanomagnetite and ilmenite

isotopes: forms of an element with differing numbers of neutrons [isotopic]

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ka: thousands of years ago (a point in time)

Kermadec Arc: the chain of largely submarine volcanoes northeast of New Zealand, including the Kermadec Islands

K/T boundary: the Cretaceous–Paleogene boundary (derived from ‘Cretaceous–Tertiary’), marked by a mass extinction of dinosaurs and other species

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lahar: a volcanic mudflow – a flow of water-saturated, typically dense volcanic material that resembles a flow of wet concrete. Lahars usually flow down topographical lows (i.e. valleys), however, they may overtop banks. A lahar may be caused by the rapid melting of ice/snow by an eruption or from an eruption ejecting crater lake water. It may also be unaccompanied by an eruption, such as by the collapse of a crater lake wall, or through remobilisation of volcanic material due to heavy rain. Lahars can travel well over 100 km from the source, and can be dangerous to downstream populations who are unaware of the approaching hazard. Due to the large amount of sediment carried by a lahar, water channels (and other nearby flat land) can rapidly fill with deposited sediment, causing long-term flooding issues. They are also highly erosive, and can cause a lot of damage to bridges and other infrastructure, entraining all material in their paths.

landslide: the down-slope movement of rock and soil under the influence of gravity

lapilli: literally, "little stones" that have been erupted from a volcano. They are round to angular erupted rock fragments (tephra) measuring 2 to 64 mm in diameter, and may be ejected in either a solid or molten state.

Last Glacial Maximum: commonly known as the LGM the time of maximum global ice volume and minimum temperature between c. 24,000 and 18,000 years ago (extended by some scientists to 28,000–18,000 years ago in New Zealand)

Last (Otiran) Glaciation: : the most recent glaciation encompasses Stages 2 to 4 inclusive of the Marine Oxygen Isotope Scale (MIS), beginning at c. 71,000 years ago and ending at the start of the Holocene epoch at 11,700 years ago. In New Zealand, this glacial phase is known as the Otira Glaciation [last glacial; Otiran]

lava: molten rock that has reached the Earth's surface and been thrown out of or has flowed from a volcano or volcanic vent. Molten rock that is still underground is called magma

lava dome: a steep-sided pile of viscous (i.e. sticky) lava at a volcanic vent. The surface is often rough and blocky as a result of fragmentation of the cooler outer crust during growth of the dome. Lava domes can collapse and cause block and ash flows

lava flow: magma which has reached the surface during a volcanic eruption and flows effusively away from the vent. The term is most commonly applied to the flowing rock that emits from a crater or fissure, however it also refers to cooled and solidified rock formed in this way. Lava varies in viscosity (runniness and therefore speed of movement), chemistry and temperature

lava fountain: a pillar-like jet of lava that can reach heights of tens to hundreds of meters above a volcanic vent. Some lava fountains erupt from long fissures, producing a curtain-like lava fountain. Lava fountains are a common feature of basaltic eruptions; forming scoria deposits and feeding lava flows.

lead isotopes: isotopes of lead, expressed as ratios of 206Pb, 207Pb or 208Pb and 204Pb, and used to determine the source and evolutionary history of rocks

lead-210 dating: a method for determining the age of sediment using the radioactive decay of the short-lived isotope 210Pb to stable 206Pb

lightning: an electrostatic discharge that is often seen in volcanic ash plumes. The lightning can be cloud-to-cloud (intracloud), or cloud-to-ground, which can be hazardous

limestone: a sedimentary rock composed of more than 50 percent calcium carbonate

liquefaction: the process where a saturated soil loses strength and behaves as a liquid due to an applied stress, usually severe earthquake shaking

lithic: particle of previously formed rock (i.e., from country rock). Lithics are often found in volcanic eruption deposits

lithification: to turn become a rock by chemical or physical means [lithify]

lithological: related to the physical character of a rock [lithology]

lithosphere: the outer shell of the Earth, consisting of the crust and upper mantle [lithospheric]

low frequency earthquakes: a type of earthquake caused by fluid pressurisation processes (such as the formation or collapse of bubbles in fluid), and the fracture of less competent rock with the involvement of a mixture of hot gases and fluids

luminescence dating: a method for determining the age of sediment by measuring the amount of radiation it has received from its surroundings

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maar: a volcanic crater formed by a phreatic or phreatomagmatic eruption. Typically the eruption occurs in a wet or low lying area. These craters are often wide, but shallow.

Ma: millions of years ago (a point in time)

macrofossil : a fossil typically more than one millimetre across that is easily visible to the naked eye

mafic: magma with a silica content of less than about 55%

magma: molten or partly molten rock beneath the surface of the earth. Magma that reaches the surface erupts as lava or pyroclasts from a volcano

magma chamber: a reservoir beneath a volcano that contains magma

magmatic-hydrothermal system: a type of geothermal system where ascending magmatic (primary) fluids commonly mix with meteoric (secondary) fluids, including sea water

magnetism: the magnetic property of rocks or minerals

mantle: the part of the Earth’s interior between the core and the crust, composed of peridotite

mantle hot spot: a site of high heat flow from the mantle, usually associated with intra-plate volcanic activity

mantle wedge: the wedge-shaped region of mantle that lies between the subducting plate and over-riding plate

marine regression: retreat of the sea from the land marking a fall in relative sea level

marine transgression: an advance of the sea across the land marking a rise in relative sea level

Marlborough fault system: the system of faults in northern South Island between the Alpine Fault and the North Island fault system

mass extinction: a globally-significant event where large numbers of unrelated species suddenly die out

mass spectrometry: a method for separating ions of different mass, used for radiometric dating and isotope ratio measurement [mass spectrometer]

Mesozoic Era: the time span between 250 and 65 million years

metallurgy: the study of the production, purification and properties of metals

metamorphism: the alteration of rocks by heat and/or pressure [metamorphic; metamorphosed]

methane: a hydrocarbon—a major constituent of natural gas, and a key greenhouse gas [CH4]

mica: a variably-coloured group of layered, sheet-like silicate minerals that include muscovite and biotite

microfossil: a fossil typically less than one millimetre across that can usually be studied only under a microscope

micro-organism: a microscopic life form [microbe]

mid-ocean ridge: a submarine ridge system associated with seafloor spreading, in which basalt rises to the surface from the mantle

Milankovitch cycle: a repeating cycle of climatic fluctuation due to changes in the Earth’s orbit and in the tilt and wobble of the Earth’s axis

mineral: [1] a constituent of a rock with a specific chemical composition, or range of compositions [mineralogy; mineralogist]; [2] in an economic sense, a profitable geological commodity [mineralised; mineralisation]

mitigation: the reduction of the social and economic effects of geological hazards

MMI: the Modified Mercalli Intensity (MMI) scale is a measure of the intensity of shaking at the Earth’s surface caused by an earthquake

moa: a large extinct flightless bird known only from New Zealand

mollusc: an invertebrate, typically with a calcareous shell—for example, a paua, scallop or squid

moraine: rock debris transported by a glacier and deposited along the glacier sides and front

mudstone: a fine-grained sedimentary rock of mixed silt and clay-sized grains [mud]

multi-beam sonar: a method for the detailed mapping of the seafloor based on multiple reflection of sound waves

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nannofossil: a tiny calcareous fossil, best studied using an electron microscope

nautilus: a mollusc with a multi-chambered coiled, single calcareous shell with a simpler internal chamber than an ammonite [nautiloid]

neoteny: when youthful characteristics are retained by the adult organism due to arrested development.

nitrate: the oxidised form of nitrogen, a major polluter of groundwater [NO3-]

normal fault: a fault in which the upper, hanging wall moves down with respect to the lower, footwall

North Island fault system: the system of faults between Cook Strait and the Bay of Plenty, including the Wellington and Wairarapa faults

Northland Arc: a volcanic arc active between 23 and 15 million years ago on the Northland peninsula

nutrient: a substance that provides essential nourishment for the maintenance of life

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oceanic crust: the 5–10 kilometre-thick crust generated at mid-ocean ridges that underlies the oceans

oceanography: the multidisciplinary study of the oceans [oceanographic; oceanographer]

ore body: a metallic or non-metallic mineral deposit that can be mined at a profit

orogenic: relating to [1] mountain building; [2] ore formation in metamorphic or sedimentary terranes

orographic: relating to precipitation caused by airflow over mountain ranges or hilly terrain

ostracod: a tiny free-swimming, aquatic crustacean with a hinged, double calcareous shell

oxide: a compound of oxygen with another element

oxygen isotopes : the stable isotopes of oxygen, expressed as 16O/18O (δ 18O), used either as an indicator of the source of rock material, the temperature of seawater or cave water or, when compared to a calibrated paleo-seawater curve, the age of fossil remains

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Pacific Plate: the tectonic plate that includes much of the Pacific Ocean and eastern South Island

pakohe: the Māori word for an indurated (‘baked’) mudstone

paleoclimate: ancient climate as determined from the geological record [paleoclimatic]

paleomagnetism: past geomagnetic field information that is preserved in rocks [paleomagnetic]

paleontology: study of life in the geological past [paleontological; palaeontologist]

Paleozoic Era: the time span between 540 and 250 million years ago

palynology: the study of spores, pollen, dinoflagellates and other acid-resistant organic structures [palynological; palynologist]

peat: plant material decomposed in water and partly carbonised

permeability: the ability of rocks or sediments to transmit fluid through interconnected pores or fractures [permeable]

petroleum: crude oil and natural gas

petrology: the study of rocks [petrologist]

Phanerozoic eon: The major unit of geological time in which abundant life has existed. It began about 542 million years ago.

phosphate: a phosphorous-bearing compound present in bones and cement [PO43-]

photosynthesis: the process by which plants and some micro-organisms use sunlight to convert carbon dioxide and water to carbohydrates and oxygen.

phreatic eruption: a volcanic eruption that is caused by heating and ‘flashing’ of water to steam, produced when magma comes into contact with water, but only country rock or overburden is ejected (i.e. no juvenile magmatic material). The energy of the eruption comes from magma or magmatic gas

phreatomagmatic eruption: an explosive volcanic eruption that results from the sudden interaction of surface or subsurface water and magma. Magma or lava may contact water, or water may be introduced during an eruption. Fragments of fresh magma (i.e. juvenile igneous material) are erupted, and ashfall is usually wet and sticky

physics: the study of the properties and interaction of matter and energy [physical]

phytoplankton: planktic plants such as algae, diatoms and coccolithophorids that help regulate the concentration of carbon dioxide in the ocean and atmosphere

pillow lava: lava in the shape of pillows with glassy margins, which is formed when lava erupts effusively into water

plagioclase: a group of light-coloured, rock-forming, sodium–calcium silicate minerals of the feldspar family

planktic: relating to organisms that float or drift in marine or fresh water [plankton]

plate (tectonic): a rigid segment of the crust and upper mantle [lithosphere]

plinian eruption: a volcanic eruption with a powerful, convecting eruption column reaching up to 45 km high, usually requiring high viscosity magma (i.e., thick, sticky magma such as dacite and rhyolite). Plinian eruptions often lead to the formation of pyroclastic density currents. A subplinian eruption is a lower magnitude and intensity version of the Plinian eruption. It can also result in pyroclastic density currents

plume: [1] a body of convecting fluid travelling upward through the mantle or crust; [2] a dense cloud of black, mineral-laden ‘smoke’ expelled from hydrothermal vents on the seafloor; [3] a suspension of muddy sediment discharged onto the continental shelf by rivers; [4] a concentration of pyroclastic (tephra) particles, aerosols and gases from an erupting volcano

plutonic: related to magma that has solidified deep in the crust [pluton; plutonism]

polar: regions between the Arctic or Antarctic circles and the north and south poles, respectively

polarised light: light that vibrates in a single plane, producing diagnostic interference colours in microscope thin sections

pollen: a fine powdery substance dispersed from a flower’s anther—the microscopic male reproductive cell of plants

porosity: the state of having open pore spaces in rocks

potassium-40–argon-40 dating: commonly known as K–Ar dating; a method for determining the age of rocks and minerals using the radioactive decay of 40K to stable 40Ar

pounamu: commonly known as greenstone; the Māori word for the mineral nephrite jade

ppm: parts per million. Used to describe dilute concentrations

progenesis: an early onset of reproductive maturity. It is when the organism reaches maturity when it is in its juvenile state.

prospect: a mineral occurrence that requires further investigation to determine if it is economic to mine

pumice: a light-weight volcanic rock (usually pale-coloured), formed by the expansion of gas in frothy lava during an eruption. Pumice commonly floats on water due to the high number of bubbles (vesicles) in each rock, and can travel further than other rocks of a similar size during an eruption due to its low density. It is often composed of rhyolite

punctuated evolution: an evolutionary pattern in which new taxa (types of plants or animals) appear relatively suddenly via major shifts in characteristics within very short time spans and then remain constant in those characters for a period of time. (Shown by the graph on the right)

Puysegur subduction system: the subduction system southwest of New Zealand where the Australian Plate descends beneath the Pacific Plate

P-wave: a primary (pressure) earthquake wave—the first to arrive at a seismograph station

pyroclast: erupted material which starts out hot, and consists of fragmented rock material formed by a volcanic explosion. Literally ‘fiery fragments’. Pyroclastic fall is when pyroclasts fall back to earth [pyroclastic]

pyroclastic surge: see base surge

pyroclastic density current: fast-moving, lethal, hot clouds of ash, rocks and gas, caused by a volcanic eruption. They are controlled by gravity, moving laterally and usually down topographical lows at high speeds (usually between 40 to 100 km per hour). They can travel a few hundreds of metres to kilometers from the source. In large but rare caldera-forming eruptions they can travel 10’s of kilometres. They are sometimes referred to as ‘PDCs’, and types include pyroclastic flows, and pyroclastic [base] surges.

pyroclastic flow: a lethal, turbulent mixture of hot gases and rock fragments that can move at high speed (usually between 40 to 100 km per hour, but they can travel at over 500 km per hour) down the sides of the volcano or outwards from a vent. Pyroclastic flows are a type of pyroclastic density current. They usually follow topographical lows (i.e., flow down valleys), however they can overtop banks if mobile enough. They are often generated by the collapse or partial collapse of an eruption column

pyroclastic surge: a hot, gas-charged, fast moving, ground-hugging flow produced by an explosive eruption, where particles are in a dilute suspension throughout the flowing mass

pyroxene: a group of black, magnesium–iron–calcium– aluminium silicate minerals

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quaternary: the period of Earth's history from about 2 million years ago to the present. Rocks and deposits of that age may also be referred to as Quaternary deposits.

quartz: a commonly occurring, variably-coloured mineral composed of silicon dioxide (SiO2), also known as silica. A rock composed largely of quartz is called quartzite [quartzose, silicious]

quiescence: periods of inactivity between volcanic eruptions, seismicity, or tectonic activity

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radioactive decay: the breakdown of some isotopes by splitting of the atomic nucleus and the emission of subatomic particles

radiocarbon (carbon-14) dating: commonly known as carbon dating; a method for determining the age of carbon-bearing materials less than c. 60,000 years old, using the concentration of 14C calibrated against atmospheric 14C

radiolarian: a siliceous single-celled marine microorganism [radiolaria]

radiometric dating: a method for determining the age of geological materials using radioactive isotopes

remote sensing: the acquisition of information by remote means using, for example, satellites or ships

reserve: a mineral resource that has been defined to a high level of confidence and is extractable at a profit

reservoir: a porous volume within a rock formation containing fluids

resource(s): [1] economic mineral reserves plus all other known mineral deposits that may eventually become available; [2] mineral and energy sources that may be of economical use to society

return period: the average time between hazardous events

reverse fault : a fault in which the upper, hanging wall moves up with respect to the lower, footwall

rhyolite: volcanic rock or highly viscous magma, light coloured, with a high silica content (typically more than 69%). It is found as pumice, ignimbrite, lava or obsidian.

rift: a large region within a plate that is being pulled apart (extended) horizontally [rifting; rifted]

risk: the likelihood of suffering adverse consequences from being exposed to a hazard

rubidium–strontium dating: commonly known as Rb–Sr dating, this method for determining the age of rocks uses the decay of long-lived radioactive isotope 87Rb to stable isotope 87Sr

rupture: displacement along a fault-line

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saline: relating to a fluid with moderate to high concentrations of salts [haline; salinity]

samarium-147–neodymium-143 dating: commonly known as Nd–Sm dating; a method for determining the age of rocks using the radioactive decay of the long-lived isotope 147Sm to stable 143Nd

sandstone: a medium-grained sedimentary rock with cemented grains more than 60 micrometres in size [sand; sandy]

satellite altimetry: the use of satellites to determine the height of the Earth’s land or ocean surface

satellite telemetry: the use of satellites to transmit data

scarp: a cliff or bank formed by upward movement along a fault trace

schist: a foliated, metamorphosed rock derived typically from sandstone or greywacke

science: studies conducted on objective principles involving systematic observation and experimentation

scoria: a frothy basaltic rock, full of small gas bubbles (70-85% void space), produced during weak explosive volcanic eruptions. It is irregular in form, and typically a black or red colour. It is usually heavier, darker, and more crystalline than pumice, and is common in the Auckland Volcanic Field

scoria cone: a small volcanic hill formed mainly of scoria erupted from a central vent

seamount: a submarine mountain rising 1000 metres or more above the seafloor

seafloor: the submarine surface below the seawater column [ocean floor]

seafloor spreading: the concept that new oceanic crust is created at mid-ocean ridges, causing lateral movement of the crust

sea level: the average height of the ocean’s surface, usually expressed as ‘mean sea level’, which is the average mid-point between high tide and low tide at any location. Generally taken as an elevation of 0 m and the reference datum for the elevation of the land surface, global sea level is approximated across the whole planet (including areas far from the sea) by the ‘geoid’, a geodetic reference surface’

sea level (eustatic): this is the ‘global sea level’; a true change in sea level, such as a rise due to melting of land-based ice, or a fall due to an increased volume of land-based ice, is called a eustatic change

sea level (relative): a change in sea level at any particular location relative to the position of the shoreline or depth of the water. A fall in relative sea level may be due to uplift of the crust, build-up of sediment on the sea floor or a true eustatic change; a rise in relative sea level may result from subsidence of the crust, erosion of sea floor sediment or a eustatic change

seal: a natural layer of impermeable rock that prevents seepage of oil, natural gas, water or other fluids from an underground hydrocarbon reservoir

sector collapse: the large-scale collapse of the top or flank of a volcano, which produces a debris avalanche. Sector collapse can result from the intrusion of magma causing ground deformation, and may be triggered by an earthquake. Sometimes a sector collapse can result in an explosive eruption, due to exposing and depressurising the subsurface magma. An example of a sector collapse causing an eruption is the 1980 eruption at Mount St Helens (USA).

sediment drift: a sedimentary deposit near the base of the continental slope formed by boundary currents

sediment: a deposit composed of mineral or rock particles transported by water, ice, wind, or gravity [sedimentary; sedimentologist]

sedimentology: the study of sediment and sedimentary processes

sedimentary basin: an area where notable thicknesses of sediment have accumulated, commonly due to subsidence of the crust

seiche: ("saysh") rhythmic waves produced in an enclosed or partially enclosed body of water. Wind-generated seiches are common on large lakes and enclosed seas, taking the form of a slow rise and fall of water level over periods of many minutes. Earthquake-shaking can generate seiches in lakes, bays, harbours or even swimming pools. Volcanic explosions may also generate seiches

seismic: relating to earthquakes [seismicity]

seismicity: Seismic activity; earthquakes and other shaking (tremors).

seismic reflection: the use of seismic vibration waves to explore the subsurface distribution of rock types or faults. The reflection of waves off, or refraction along, underground layers or structures, reveals the depths and positions of changes in rock density or character. This geophysical investigation method can utilise vibrations of natural earthquakes, or more commonly used an artificial source of vibration, such as explosions or sound waves

seismic tomography: the use of earthquake (seismic) waves to produce images of the Earth’s interior

seismograph: an instrument for measuring earthquake (seismic) waves

seismology: the study of earthquakes [seismological; seismologist]

self-sufficiency (energy resources): the proportion of domestic production relative to usage

shear: to dislocate and grind within a fault zone [shearing; shear belt]

shield volcano: a large, broad volcano with a flattened dome shape, typically made up of basaltic lava flows. Although they are modified by erosion, the extinct volcanoes of Akaroa and Lyttelton on Banks Peninsula are examples.

shock wave: a special type of pressure wave that can be caused by volcanic explosions. On rare occasions shock waves can blow down trees and break windows at nearby buildings, and is usually accompanied by a loud boom.

silica: the common name of silicon dioxide (SiO2). Typically, it is found as the mineral quartz. Where silica crystals are very fine grained (cryptocrytalline), they form minerals such as opal and the chalcedony group, including agate. Sinters around hydrothermal vents are rich in silica. Organisms such as diatoms and radiolarian have silica skeletons. Accumulations of their remains (biogenic silica) form deep sea-floor oozes and where compacted into rock are called chert, or diatomite if composed mainly of diatoms. In magma, silica content has a great influence on the viscosity of the molten rock; basalt is poor in silica and is very hot and flows freely, while rhyolite is rich in silica, is cooler and more viscous. Erupting rhyolite is more prone to violent explosions as gas struggles to escape from the sticky lava [siliceous, silicic]

silicate: minerals whose key building-block is the silicon tetrahedron (SiO4). The bulk of the rocks forming the Earth’s crust are composed of silicate minerals, such as feldspar, olivine, pyroxene, amphiboles and mica

silicon-32 dating: a method for determining the age of ice or sediment using the build-up over time of the cosmogenic isotope 32Si

slate: moderately metamorphosed mudstone, which breaks into thin, parallel-sided slabs. Where subjected to more intense metamorphism, mudstones are converted into fine-grained schist rock

slip-rate: the average net rate of movement of crust along a fault

soil: the zone of intense biological activity immediately beneath the land surface. Soils vary in thickness and properties depending on their age and the parent materials from which they are formed. A typical soil profile in New Zealand consists of an A horizon, composed of dark, organic rich material that is the locus of invertebrate animal activity (e.g. worms) and bacterial action, passing downwards to a transition zone of somewhat disturbed and weathered material (B-horizon) resting on the undisturbed parent rock or deposit (C-horizon). The soil zone supports rooted plants, and is essential to life through recycling of nutrients, carbon and oxygen

source: [1] also known as the focus; the initiation point of an earthquake rupture; [2] the origin of crustal fluids—meteoric, magmatic or connate, including groundwater; [3] the origin of hydrocarbons leading to the formation of oil and gas reservoirs; [4] the location of the original material making up a rock

species: a basic level of life-form in the Linnaean classification—a group of organisms capable of interbreeding and producing fertile offspring—for example, Agathis australis is the species name for kauri trees

spore: the microscopic reproductive cell of many plants, fungi and algae, designed for dispersal and survival

stable isotope: a non-radioactive isotope of an element

stasis: morphologic constancy of a species through a stratigraphically significant time interval

steam eruption: usually small eruptions consisting mostly of steam. Rocks and ash might also be erupted, but no fresh magma is deposited. Steam eruptions include hydrothermal eruptions

strain: the proportional change in shape or size resulting from the application of stress

strata: layers of sedimentary rock visually separable from those above and below [stratum; stratification]

stratigraphy: the order and relative position of rock units or other geological deposits in sequence [stratigraphic]

stratovolcano: a volcano composed of interlayered lava flows and pyroclastic deposits (i.e., erupted rock fragments). They are typically andesite volcanoes, and look like inverted cones (such as Ngauruhoe and Taranaki/Egmont). Also known as a stratocone

stress: the force per unit area acting on any surface within a 3D object

strike: the direction of a horizontal line on a planar feature such as a layer within a rock, or a fracture plane in a rock. Usually measured as a compass bearing, in degrees. Strike in combination with dip defines the orientation, or ‘attitude’, of any planar geological feature [striking; also see dip]

strike-slip: displacement parallel to the strike of a fault

strombolian eruption: basaltic-andesitic eruptions that are characterised by a series of small explosive eruptions

strontium isotopes: isotopes of strontium, expressed as the ratio 87Sr/86Sr, used either as an indicator of the source and evolutionary history of rock material or, when compared with a calibrated paleo-seawater curve, the age of calcareous fossils

subduction: the process by which an oceanic plate moves down into the mantle at a convergent plate margin. A subduction zone is where those two tectonic plates meet, and is the most common place for volcanoes to form [subduct; subduction zone]

submergence: the process of sinking below sea level [submerge]

submersible: a mini-submarine, remotely-operated vehicle or automatically-operated underwater vehicle used in seafloor studies

subsidence: the downward movement of the ground surface with respect to another point (such as sea level, or a different location). At a volcano, this may be caused by a change in volume of a magma body or the pressure produced by it. Subsidence can be on the order of millimetres to metres at a volcano, and is measured by surveying techniques (levelling, GPS or InSAR, etc)

subterranean: beneath the Earth’s surface

subtropical: the regions bordering the tropics to the south or north

succession: the natural time-order of rocks or fossils

sulphur: a yellow non-metal often found in volcanic or geothermal areas [S]; compounds are sulphides [S6-] or sulphates [SO42-]

sulphur dioxide: a colourless, choking poisonous gas often associated with volcanic or geothermal areas [SO2]

surge: see base surge

S-wave: a shearing earthquake wave—the second to arrive at a seismograph station

syncline: a fold in rock layers, generally concave upwards, where the youngest rocks are in the core

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Taupo Volcanic Zone: the c. 100 kilometre wide by c. 350 kilometre long volcanic region of central North Island extending north from Ruapehu volcano to beyond White Island volcano; the ‘older TVZ’ was active from c. 2 Ma to 340 ka, and the ‘younger TVZ’ has been active since c. 340 ka

taxonomy: the theory and practice of describing and classifying biota, rocks, soil etc.

tectonic: relating to the formation of large-scale structural features [tectonism; plate tectonics]

tectonic earthquakes: Earthquakes occurring in the crust of the earth and caused by movement on fault lines.

tephra: solid material of all types and sizes that are erupted from a volcanic vent and that travel through the air

terrestrial: [1] land-based; [2] Earth-based

terrigenous sediment: sediment derived from the land

Thermohaline Circulation: the global system of oceanic currents that is driven by density contrasts, due to changes in heat and salt

thin section: a thin slice of a rock or mineral that can be analysed under a petrological microscope

thrust: a gently inclined reverse fault, in which the upper, hanging wall moves up with respect to the lower, footwall

tilt: the angle created between a reference point and another point when the second point moves relative to the first. Often this is a measure of the change in shape of a volcano

transform fault: a strike-slip fault that terminates abruptly at a major transverse feature such as a midocean ridge

trap: an underground natural feature that allows oil, natural gas, water or other fluids to accumulate. Examples include structural traps such as anticlinal domes containing an impermeable layer of rock that traps fluids or gas in an underlying permeable rock layer, or where an impermeable fault zone cuts across a porous rock layer, trapping its fluids or gases under the fault. Stratigraphic traps may occur where a lensoid layer of permeable rock is overlain and cut off by a more continuous younger layer of impermeable rock

tremor: a continuous vibration of the ground around active volcanoes. It has a low amplitude, and is often associated with magma movement

trench: a long, deep depression of the ocean floor marking a subduction plate boundary

trigger: a phenomenon or process that gives rise to a geological event such as an earthquake (triggered by build-up of strain in the crust), landslide (e.g. triggered by a rainstorm or by an earthquake) or a volcanic eruption (triggered by rise of magma)

trilobite: an extinct marine arthropod common in the Paleozoic Era

tritium dating: a method for determining the age of groundwater using the concentration of tritium (3H) produced by cosmic rays, and in particular by atmospheric nuclear tests in the 1950s and 1960s

tsunami: a surge of water with a long wavelength produced by the displacement of a body of water. Causes of tsunami include an earthquake causing offset (uplift or subsidence) of the sea bed, a volcanic eruption, or a large landslide (including sector collapse). The height of a tsunami is influenced by the morphology of the coastline that it travels towards. The speed of a tsunami ranges between 10-100 km/hr in shallow areas, and up to 800 km/hr when crossing deeper waters. Landslides or icefalls into lakes or fiords may also generate tsunami.

tuff ring: A wide low ring of pyroclastic material, surrounding a volcanic vent.

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UNCLOS: the United Nations Convention on the Law of the Sea

unconformity: a break in time within a sequence of rock layers. An unconformity may represent a period when no deposition took place, or may represent an interval of erosion that stripped away some rock layers, before deposition recommenced. Where the rock layers were tilted by tectonic movements and truncated by erosion before younger rock layers were deposited, bedding in the older, truncated layers dips more steeply than those of the younger layers on top; Such a configuration is called an angular unconformity [unconformable]

unrest: volcanoes are said to be in a state of volcanic unrest when magma or its associated fluids are interacting with country rock, groundwater or hydrothermal fluids to produce notable signs. Those signs may include various types of seismicity (earthquakes), ground deformation, degassing (the release of magmatic gases through the ground) and/or changes in geothermal systems. Other processes that are not related to magma movement, such as tectonic fault movement or hydrothermal system changes, often appear very similar to signs of volcanic unrest

unrest hazards: volcanic unrest hazards occur on and near the volcano, and may include steam eruptions, volcanic gases, earthquakes, landslides, uplift, subsidence, changes to hot springs, and/or lahars

uplift: the raising of rocks from their place of formation. The upward movement of the ground surface at a volcano is termed uplift. This may be caused by a change in volume of a magma body or the pressure produced by it. Uplift at a volcano can be on the order of millimetres to metres per year, and is measured by [surveying techniques] (levelling, GPS or InSAR, etc)

uranium–lead dating: a method for determining the age of rocks using the radioactive decay of long-lived isotopes of uranium to stable lead [thorium–lead dating]

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vent: the opening at the earth's surface through which volcanic materials are erupted, or were erupted in the past

vertebrate: an organism with a backbone

vesicularity: containing pores created by gas bubbles in volcanic rocks [vesicle; vesicular]

viscosity: a measure of the resistance for a liquid to flow (water has low viscosity while toothpaste has a higher viscosity.) [viscous]

vog: Smog of volcanic origin, composed of volcanic ash and gases.

volcanic alert level: a system that defines the current level of activity at New Zealand's active volcanoes

volcanic arc: a chain of volcanoes, tens to hundreds of kilometres long, overlying a subduction zone

volcanic centre: a group of volcanoes that are related to each other, and are clustered in space (e.g., Okataina Volcanic Centre, Taupo Volcanic Centre)

volcanic earthquakes: earthquakes (up to about Magnitude 5.5) that are caused by the movement of magma within the crust.

volcanic environment hazards: hazards that may exist on or near a volcano, even though there may not be an eruption or volcanic unrest taking place. These hazards may include hydrothermal activity, earthquakes, landslides, volcanic gases, and/or lahars

Volcanic Explosivity Index (VEI): a measure of the magnitude and energy of volcanic eruptions based on several parameters, including eruption duration and style, volume of eruptive products, and the eruption column height

volcanic field: an area of small volcanoes that form instead of a single large volcano. They are generally basaltic, and each is usually produced by a single episode of volcanic activity. An example is Auckland Volcanic Field

Volcanic gases: magma deep in the earth contains dissolved gases. As the magma rises closer to the ground surface, these gases are released and, because they are so mobile when compared to the sluggish liquid magma, they rise to the surface and are discharged through vents, fumaroles, and the soil. The gas temperatures, absolute amounts, and relative proportions of different gases give information on the state of the magmatic system. There are many types of volcanic gases, with the most common being water vapour (H2O); sulphur as sulphur dioxide (SO2) or hydrogen sulfide (H2S); nitrogen, argon, helium, methane, carbon monoxide and hydrogen

volcanic massive sulphide: deposits of metal sulphide minerals, commonly rich in copper-zinc-lead, formed in association with undersea volcanic hydrothermal vents. Ancient examples are found in some rock sequences and may provide minable ore deposits. Modern examples are seen in ‘black smoker’ hydrothermal vents on the sea floor in volcanically active areas

vulcanian eruption: a small to moderate-sized explosive eruption style that forms an eruption plume reaching 10-20 km high. The short but violent eruptions last from seconds to minutes, eject blocks, bombs and other tephra, and may cause shock waves.

volcano: a vent in the surface of the Earth through which magma and associated gases erupt, and the form or structure that is produced by the deposits or the eruption process.

volcanology: the study of volcanoes [volcanic; volcanism; volcanological; volcanogenic; volcanologist]

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water table: the upper surface of a zone of watersaturated rock or sediment

weathering: the chemical or physical break-down of rocks on the Earth’s surface

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X, Y

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Zealandia: also known as the New Zealand continent; the mainly submergent continental mass that split from Gondwana at c. 83 Ma

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