Oxford dictionary earth science

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Oxford dictionary earth science

  1. 1. A Dictionary ofEarthSciencesthird editionEdited bymichael allaby1
  2. 2. 3Great Clarendon Street, Oxford ox2 6dpOxford University Press is a department of the University of Oxford.It furthers the University’s objective of excellence in research, scholarship,and education by publishing worldwide inOxford New YorkAuckland Cape Town Dar es Salaam Hong Kong KarachiKuala Lumpur Madrid Melbourne Mexico City NairobiNew Delhi Shanghai Taipei TorontoWith offices inArgentina Austria Brazil Chile Czech Republic France GreeceGuatemala Hungary Italy Japan Poland Portugal SingaporeSouth Korea Switzerland Thailand Turkey Ukraine VietnamOxford is a registered trademark of Oxford University Pressin the UK and in certain other countriesPublished in the United Statesby Oxford University Press Inc., New York© Oxford University Press 1990, 1999, 2003, 2008The moral rights of the authors have been assertedDatabase right Oxford University Press (maker)First published 1990First issued as an Oxford University Press paperback 1991Second edition 1999Reissued with new covers and corrections 2003Third edition 2008All rights reserved. No part of this publication may be reproduced,stored in a retrieval system, or transmitted, in any form or by any means,without the prior permission in writing of Oxford University Press,or as expressly permitted by law, or under terms agreed with the appropriatereprographics rights organization. Enquiries concerning reproductionoutside the scope of the above should be sent to the Rights Department,Oxford University Press, at the address aboveYou must not circulate this book in any other binding or coverand you must impose this same condition on any acquirerBritish Library Cataloguing in Publication DataData availableLibrary of Congress Cataloging in Publication DataData availableTypeset by SPI Publisher Services, Pondicherry, IndiaPrinted in Great Britainon acid-free paper byClays Ltd, St Ives plcISBN 978–0–19–921194–41 3 5 7 9 10 8 6 4 2
  3. 3. From the Preface to the First EditionSetting intellectual boundaries that would serve to define ‘geology’ has neverbeen simple. As long ago as 1830, in his Principles of Geology, Charles Lyell ex-pressed the view that geologists should be well versed in chemistry, natural phi-losophy, mineralogy, zoology, comparative anatomy, and botany. For at least acentury and a half those who would study the structure and composition of theEarth have had to familiarize themselves with a wide range of scientificdisciplines.Strictly speaking, the word ‘geology’ describes all studies of the Earth. Tradi-tionally, however, ‘geology’ has come to mean the study of rocks.T. C. Chamberlin used the name ‘Earth sciences’ to embrace astronomy, cos-mogony, and cosmology as well as the traditional disciplines, and AlfredWegener (originally a meteorologist) also used it, but it was not until the 1960sthat it began to gain a wider currency. Within ten years it was widely accepted,used sometimes in the singular, nowadays commonly in the plural. When, inthe late summer of 1985, our friends at the Oxford University Press invited us tocompile a dictionary of terms used in the topics directly related to studies of theEarth, it was clear that it should be a dictionary of ‘Earth sciences’.We had to begin by defining the term for our own purpose. We examined theway it was used by other authors, assembled a kind of consensus, and deter-mined that our dictionary should include terms from climatology, meteorology,economic geology, engineering geology, geochemistry, geochronology, geomor-phology, geophysics, hydrology, mineralogy, oceanography, palaeoclimatology,palaeoecology, palaeogeography, palaeontology, pedology, petrology, the phi-losophy and history of the Earth sciences including brief biographical notes ofimportant figures, planetary geology, sedimentology, stratigraphy, structuralgeology, tectonics, and volcanology.The task of a dictionary is descriptive, not prescriptive. It records words andexpressions that are in current use and explains the meanings attached to them,but it does not impose those meanings or seek to dictate what a correct usageshould be. As recorders, we express no opinions.We would emphasize that the book is meant to be used as a dictionary. In nosense is it intended to be a textbook in its own right.Preface to the Second EditionWork on the first edition of The Concise Oxford Dictionary of Earth Sciences was com-pleted in the summer of 1988, and the book was published in 1990. Manychanges and advances have occurred during the years that have passed since theinitial compilation and we are glad to have been given the opportunity to takeaccount of them in a second edition.The revision has been extremely thorough. Every entry from the first editionhas been scrutinized and many have been amended to bring them up to date.
  4. 4. vii Preface to the Third EditionIn preparing the new edition we have been assisted by Dr Robin Allaby. Hiscontribution has greatly strengthened the revision and we are very grateful forhis help.We also wish to thank Professor D. H. Tarling and Dr C. D. Gribble. They eachscrutinized a long list of entries, revising them where necessary.Finally, we thank Nigel May, the science librarian at the library of the Univer-sity of Plymouth, for allowing us to make use of the library facilities.Ailsa AllabyMichael AllabyPreface to the Third EditionThe second edition of Oxford’s Dictionary of Earth Sciences was published eightyears ago, in 1999, and although the dictionary was reprinted with correctionsin 2003, it was possible then to make only minor changes. This edition is entirelynew. Every entry has been scrutinized and brought up to date where necessary.New entries have been added and the number of diagrams has increased.The new entries added in the second edition related predominantly to the ex-pansion of planetary exploration and satellite technology. Those entries havebeen updated. Many of the new entries in this edition serve to expand the dic-tionary’s coverage of geomorphological terms.It is in the nature of dictionaries to grow longer with each revision, and I makeno apology for the fact that this one conforms to that rule. Such growth is un-avoidable, because the language itself is always growing, but when new termsare introduced it does not mean that older terms are discarded. Even terms thatdo become obsolete do so gradually, surviving for many years in established andmuch loved texts.The first edition was compiled with the help of many contributors and advis-ers, and a smaller number of contributors assisted in preparing the second edi-tion. The value of their hard work endures, and I gladly and fully acknowledgeit, for without it there would have been no dictionary to revise. I have workedalone in preparing this third edition, however, so if any new errors have crept inthey are mine alone.Michael AllabyTighnabruaich, Argyllwww.michaelallaby.com
  5. 5. ContentsContributors and Advisers xiA Dictionary of Earth Sciences 1appendix a: Stratigraphic Units as Defined in theNorth American Stratigraphic Code, 1983 635appendix b: Time-Scales 637appendix c: Wind Strength 642appendix d: SI Units, Conversions, and Multiples 644appendix e: List of useful websites 646Bibliography 647
  6. 6. Contributors and AdvisersAilsa AllabyMichael AllabyRobin Allaby, University of Manchester Institute of Science and TechnologyDr Keith Atkinson, Camborne School of MinesDr R. L. Atkinson, Camborne School of MinesDr T. C. Atkinson, University of East AngliaDr A. V. Bromley, Camborne School of MinesDenise CrookJ. G. Cruickshank, Department of Agriculture for Northern Ireland, BelfastDr P. Francis, Open University; Lunar and Planetary Institute, HoustonProfessor K. J. Gregory, Goldsmith’s College, University of LondonDr C. D. Gribble, University of GlasgowDr Colin Groves,* Australian National UniversityDr W. J. R. Harries, University of PlymouthProfessor M. Hart, University of PlymouthProfessor Emeritus H. H. Lamb,* University of East AngliaJohn MacadamDr R. J. T. Moody, Kingston UniversityDr J. Penn, Kingston UniversityDr John M. Reynolds, Reynolds Geo-Science Ltd.Dr D. Rolls, Kingston UniversityDr I. RoxburghDr N. A. Rupke, Wolfson College, OxfordDr Stuart Scott, University of PlymouthDr B. W. Sellwood, University of ReadingDr P. J. C. Sutcliffe, Kingston UniversityProfessor D. H. Tarling, University of PlymouthJoan TaylorProfessor S. R. Taylor, Australian National UniversityDr R. J. Towse,* Kingston UniversityDr I. Tunbridge, University of PlymouthDr C. E. Vincent, University of East AngliaProfessor Brian F. Windley, University of LeicesterAndrew Yelland, Birkbeck College, London*Contributor to The Oxford Dictionary of Natural History whose earlier entries have been trans-ferred to this book.
  7. 7. aa See lava.AABW See antarctic bottom water.AAC See antarctic convergence.Aalenian A *stage in the European Middle*Jurassic (175.6–171.6 Ma, Int. Commissionon Stratigraphy, 2004). See also dogger.AAV See aggregate tests.Ab See alkali feldspar.abandoned channel A former streamchannel through which water no longerflows (e.g. a *cut-off).abandonment facies association A *fa-cies association formed under conditions ofrising sea level, when *clastic deposition hasceased and sediment is deposited very slowly.abapical A directional term meaning awayfrom the shell *apex.abaptation The process by which an organ-ism is fitted to its environment as a conse-quence of the characters it inherits, whichhave been filtered by *natural selection inprevious environments. Because present en-vironments seldom differ greatly from recentpast environments, adaptive fitness can re-semble *adaptation. In this sense, however,adaptation appears to imply advance plan-ning, or design, which is misleading.Abbé refractometer See refractometer.abiogenesis Development of living organ-isms from non-living matter; as in the sup-posed origin of life on Earth, or in the conceptof spontaneous generation, which was onceheld to account for the origin of life butwhich modern understanding of evolution-ary processes (see evolution) has renderedoutdated.abiotic Non-living; devoid of life. Comparebiotic.ablation 1. Removal of snow and ice bymelting and by direct alteration from thesolid to the gaseous phase (sublimation). Therate of loss is controlled chiefly by air temper-ature, wind velocity, *humidity, rainfall, and*solar radiation. Ablation on snowfields isalso influenced by aspect, depth of snow, andthe nature of the underlying surface. Abla-tion *till is the glacial debris that may be re-leased. The ablation zone of a glacier is thatarea in which losses, including *calving, ex-ceed additions. 2. Removal of *rock mater-ial, especially by wind action.ablation till See ablation 1; and till.ablation zone See ablation.aboral Away from the mouth; on the oppo-site side of the body from the mouth.abrasion (corrasion) The erosive (see ero-sion) action that occurs when *rock particlesof varying size are dragged over or hurledagainst a surface. Some common agents ofabrasion are the *bed load of streams, rockdebris embedded in the bases of *glaciers,and *sand and *shingle transported by windor waves.abrasion ramp A gentle, seaward slope,with a gradient of approximately 1°, in anintertidal *shore platform that is caused bywave *abrasion. The removal of material bywave action leaves the base of the cliff ex-posed, leading to further cliff retreat. Abra-sion ramps usually terminate on the seawardside where the sea depth reaches about 10m.absolute age (true age) The age of a geo-logic phenomenon measured in presentEarth years, rather than its age relative toother geologic phenomena (compare relativeage). The term ‘absolute age’ has been consid-ered rather misleading, as the means formeasuring ages (*radiometric dating, *den-drochronology, *varve analysis) are subject toexperimental error and the dates obtainedare not precise. The alternative term ‘appar-ent age’ has been suggested. See also datingmethods; and geochronology.absolute humidity See humidity.absolute plate motion The motion ofa lithospheric *plate (see lithosphere) withA
  8. 8. respect to a fixed frame of reference. Variousframes of reference have been used, includ-ing those defined by *hot spots, no net torqueof all the plates, and palaeomagnetic (seepalaeomagnetism) Euler poles (see pole ofrotation).absolute pollen frequency (APF) *Pollendata from sediments, expressed in terms ofthe absolute numbers for each *species,*genus, or *family, per unit volume of sedi-ment and, where deposition rates are known,per unit time. In certain circumstances thisapproach gives clearer information than doesthe traditional way of expressing pollen dataas *relative pollen frequencies (RPFs). APFs areparticularly useful in site comparisons inwhich one or more high pollen producersvary. For example, when trees first appear inthe regional pollen rain their prolific pollenmay, in an RPF method, give the impression ofdeclining herbaceous species, whereas exami-nation by an APF method will show constantvalues for herb species.absolute porosity See porosity.absolute temperature Temperaturemeasured using the *Kelvin scale.absolute vorticity See vorticity.absolute zero See kelvin scale.absorptance The ability of a material to ab-sorb *electromagnetic radiation of a specifiedwavelength. See also absorptance band.absorptance band The range of wave-lengths of *electromagnetic radiation whichare absorbed by a material. See also absorp-tance.absorption The amount of seismic energylost during transmission, by conversion toheat. The absorption coefficient is the frac-tional loss of energy over a distance of one*wavelength; hence higher-*frequency sig-nals are attenuated more readily than thoseof lower frequencies over the same path. Typ-ical values for *rocks range from 0.25 to0.75 dB per wavelength.abstraction (extraction) The artificial re-moval of water from a well, *reservoir, orriver.Abukama-type metamorphism The*recrystallization of *rocks under a high *ge-othermal gradient so that at any given tem-perature the pressure is relatively low. Theterm originally referred to a belt of *meta-morphic rocks stretching south-westwardsfrom the Abukama Plateau in Japan, andcharacterized by the development of *an-dalusite and *sillimanite in rocks that wereoriginally *shales (*pelites). This belt liesparallel to, and on the continental side of,a high-pressure metamorphic belt.abundance zone See acme zone.ABW See arctic bottom water.abyssal hills Relatively small topographicfeatures of a dominantly flat, deep-oceanfloor, commonly 50–250m in height and afew kilometres in width. They are most typi-cal of the *Pacific Ocean floor at depths of3000–6000m.abyssal plain Smooth, almost level area ofthe deep-ocean floor in which the gradient islikely to be as low as 1:10000. The coveringsediments are usually thin deposits of a*pelagic ooze or *distal *turbidite.abyssal storm (benthic storm) A largepulse of energy, possibly transferred from thesurface, that accelerates *contour currentson the ocean floor to about 40cm/s, raisinglarge amounts of fine sediment.abyssal zone Zone of greatest ocean depth,i.e. below a depth of 2000m. This zone liesseaward of, and deeper than, the *bathyalzone, and covers approximately 75% of thetotal ocean floor. It is the most extensiveEarth environment, cold, dark, with slow-moving currents (less than a few centimetresper second), supporting *fauna that typicallyare black or grey, delicately structured, andnot streamlined.Acadian orogeny A phase of mountainbuilding affecting an area from the northernAppalachians in what is now New York Stateto the Bay of Fundy in maritime Canada (thename refers to the colony of Acadie in that re-gion of French Canada). It occurred in the*Devonian about 390Ma ago, although theprecise date and duration are uncertain, andwas most intense east of the Taconic area (seetaconic orogeny). It was caused by the west-ward movement of the Avalon *terrane. Seeappalachian orogenic belt.Acado-Baltic Province See atlanticprovince.acanthodians See acanthodii.absolute pollen frequency 2a
  9. 9. a3 accretionary basinAcanthodii (acanthodians) Class of primi-tive, fossil fish, characterized by the presenceof a true bony skeleton (see bone), a *heterocer-cal tail *fin, a persistent *notochord, *ganoidscales, and stout spines in front of the fins. Theacanthodians lived from the *Silurian to the*Permian Period and may be related to ances-tors of the more modern bony fish.Acanthograptidae See dendroidea.Acanthostega See ichthyostega.acceleration *Evolution that occurs byincreasing the rate of ontogenetic (see on-togeny) development, so that further stagescan be added before growth is completed.This form of *heterochrony was proposed byE. H. Haeckel as one of the principal modes ofevolution.acceleration, gravitational See gravita-tional acceleration.accelerograph (earthquake seismometer)An instrument used to measure *earthquakemovements that are too strong for more sen-sitive *seismometers to register accurately.An accelerograph contains three *accelerom-eter heads aligned to measure movement inthree directions. The accelerograph is oftenconnected directly to the Internet.accelerometer A device whose output isdirectly proportional to acceleration. Ac-celerometers are used in the measurement ofthe motion of a ship, helicopter, or aircraftduring *gravity surveys. A *seismometer ormoving-coil *geophone can also function asan accelerometer.accessory, lithic See lithic fragment.accessory cloud A small cloud that isseen to be associated with a much largercloud belonging to one of the ten cloud gen-era (see cloud classification). *Pileus, *tuba,and *velum are accessory clouds.accessory mineral A *mineral *phasewithin a rock whose presence does not affectthe root name of the rock. For instance, theroot name ‘granite’ is defined by the presenceof *quartz, *alkali feldspar, and *mica. Theseare the ‘*essential minerals’. The presence ofthe mineral *sphene does not affect the rootname and hence would be an example of anaccessory mineral. *Apatite and *zircon arealso common accessory minerals.accessory plate (sensitive tint) In opticalmicroscopy, a plate used to determine theoptical properties of *minerals. *Quartz,*mica, and *gypsum are the common miner-als used to determine the slow and fast *vibra-tion directions that relate to the two*refractive indices of an *anisotropic mineral.The terms ‘length-fast’ and ‘length-slow’ maythen be assigned to a given mineral foridentification purposes. A wedge of quartz(quartz wedge) is used to determine the orderof*interferencecolourexhibitedbyamineral.accidental lithic See lithic fragment.accommodation space The space inwhich sediment may accumulate.accommodation zone A region of inter-meshed *normal faults, with very complexgeometry, that lies between the boundaryfaults of a series of interlinked half-grabens(see graben) in an area of crustal extension.accordion fold See chevron fold.accretion 1. Process by which an inorganicbody grows in size by the addition of newparticles to its exterior. It is the mechanismby which primitive planetary bodies are be-lieved to form as a result of the accumulationof minute, cold, homogeneous particles(homogeneous accretion). An alternative hy-pothesis is that iron-rich cores accumulatedfirst and were later surrounded by silicatematerial (heterogeneous accretion). Homoge-neous accretion yields a planet that initiallyhas the same composition from centre to sur-face; heterogeneous accretion yields a planetthat has a layered structure from the start.2. The accumulation of sediments from anycause, representing an excess of depositionover *erosion.3. The addition of continental material to apre-existing continent, usually at its edge. Theuse of ‘accretion’ in this sense has evolvedfrom theories of *nucleation to newer theo-ries of the horizontal addition of *allochtho-nous *terranes of initially coherent bodies ofcontinental *rock, usually more than 100km2in area, which can collide, rotate, and frag-ment as they become sutured to a continent.accretional heating The heating of bodiesorbiting a star due to bombardment bysmaller objects, the kinetic energy of theimpacting body (1⁄₂mv2, where m is mass and vvelocity) being released mainly as heat.accretionary basin A small basin, muchsmaller than a *fore-arc basin, that developson the top of an *accretionary wedge and fills
  10. 10. aaccretionary lapilli 4with *debris-flow material, *turbidites, andvolcanic rocks.accretionary lapilli Pellets of *ash, rang-ing in size from 2mm to 64mm, which com-monly exhibit a concentric (‘onion skin’)internal structure. The *lapilli are formedby the accretion of very fine ash aroundcondensing water droplets or solid particles,particularly in steam-rich eruptive columns(see eruption). Once formed they can be trans-ported and deposited by *pyroclastic fall,*surge, or flow processes.accretionary levée See lava levée.accretionary prism See accretionarywedge.accretionary wedge (accretionary prism)A tectonically thickened wedge of *sedimentfoundonthelandwardsideofsome*trenches.The accretionary wedge consists of oceanicsediment scraped off the subducting *plate(see subduction), plus sediment derived fromlandwardanddepositedinthetrench.Slicesofsediment are added to the wedge by *under-thrusting and the trench migrates seaward,the continuation of this process producing an*inversion.accumulated temperature Surplus ordeficit of temperature with respect to adefined mean value and expressed as an accu-mulation over a given period, e.g. a month,season, or year. For example, a datum valueof 6°C is used as a critical temperature forsustained vegetation growth, against whichaccumulated surpluses or deficits may bemeasured.accumulation zone That part of a *glacierwherethemeanannualgainof*ice,*firn,andsnowisgreaterthanthemeanannualloss.Thezone consists of stratified firn and snow to-gether with ice from frozen meltwater. Itslower boundary is the *equilibrium line.ACD See aragonite compensation depth.ACF See acf diagram; and autocorrelation.ACF diagram A three-component, triangu-lar graph used to show how metamorphic*mineral assemblages vary as a function of*rock composition within one *metamor-phic facies. Besides SiO2, the five most abun-dant oxides found in *metamorphic rocksare Al2O3, CaO, FeO, MgO, and K2O. The threecomponents plotted on ACF diagrams areA (Al2O3), C (CaO), and F (FeO ϩ MgO), makingthe diagrams particularly useful for showingassemblage variations in metamorphosed,*basic, *igneous rocks, and impure *lime-stones. However, each of these componentshas to be modified slightly to account for thepresence of other, minor components in therock. Such modification leads to: A (Al2O3 −Na2O − K2O); C (CaO − [(10/3)P2O5] − CO2); andF (FeO ϩ MgO − Fe2O3 − TiO2). The minerals*quartz and *albite are assumed to be pre-sent in the rocks and are not shown on the di-agram. *Tielines connect minerals whichcoexist in equilibrium and can thus definetriangular areas in which three minerals arein equilibrium in the rock, lines on whichtwo minerals are in equilibrium in the rock,and points at which one mineral is in equilib-rium in the rock (in addition to the ubiqui-tous quartz and albite). See afm diagram.achnelith See pelé’s hair.achondrite Rare stony *meteorite lacking*chondrules and with low nickel-ironpassivemarginocean accretionarywedgefore-arc volcanicarcoceaniclithosphereAccretionary wedge
  11. 11. a5 actinolitecontent. It is more coarsely crystalline than a*chondrite. Basaltic achondrites resembleterrestrial *lavas.achromatic line In the three-dimensionalgraph which plots quantities of the three*additive primary colours contributing to*pixels against each other, the line whichruns at 45° to the axes. Pixels which plot closeto this line will not be strongly coloured andmay be subject to *decorrelation stretching.acicular Pointed or needle-shaped.acid According to the Brønsted–Lowry the-ory, a substance that in solution liberates hy-drogen *ions or protons. The Lewis theorystates that it is a substance that acts as anelectron-pair acceptor. An acid reacts with a*base to give a salt and water (neutralization),and has a *pH of less than 7.0. The theory wasproposed in 1923 by the Danish physicalchemist Johannes Nicolaus Brønsted and theBritish chemist Thomas Lowry, and indepen-dently by the American theoretical chemistGilbert Newton Lewis.acidophile An *extremophile (domain*Archaea) that thrives in environmentswhere the *pH is below 5.0.acid rain Precipitation with a *pH of lessthan about 5.0, which is the value producedwhen naturally occurring carbon dioxide,sulphate, and nitrogen oxides dissolve intocloud droplets. The effects of increased acid-ity on surface waters, soils, and vegetation arecomplex.acid rock *Igneous rock containing morethanabout60%*silica(SiO2)byweight,mostofthesilicabeingintheformofsilicateminerals,but with the excess of about 10% as free*quartz. Typical acid rocks are *granites, *gra-nodiorites, and *rhyolites. Compare basic rock;andintermediaterock.Seealsoalkalinerock.acid soil*Soil having a *pH less than 7.0. De-grees of soil acidity are recognized. Soil is re-garded as ‘very acid’ when the reaction is lessthan pH 5.0. The *USDA lists five standardranges of soil acidity (less than pH 4.5, ex-tremely acid; 4.5–5.0, very strongly acid;5.1–5.5, strongly acid; 5.6–6.0, medium acid;and 6.1–6.5, slightly acid). Surface *soil hori-zons of acid *brown earths have a reaction ofpH 5.0 or less.acme zone(peak zone, flood zone, epibole,abundance zone) An *informal term for abody of *strata containing the maximumabundance of a particular *taxon occurringwithin the stratigraphic range of that taxon,and after which the *zone is named.acoustic impedance (Z ) The product ofdensity (ρ) and the acoustic velocity (v) for agiven rock mass; Z ϭ ρv. The *reflectioncoefficient for an interface is governed by thecontrast in the acoustic impedances of thetwo adjacent *rock masses.acquired characteristics Characteristicsthat are acquired in the lifetime of an organ-ism, according to early evolutionary theoristssuch as *Lamarck. Lamarck further sug-gested that traits acquired in one generationin response to environmental stimuli wouldbe inherited by the next generation. Thusover several generations a particular type oforganism would become better adapted (seeadaptation) to its environment. The kinds ofacquisition envisaged by Lamarck and theirheritability are now discredited, althoughthere has been a recent revival of some as-pects of Lamarckism in modified form.Acrisols A reference soil group in the soilclassification scheme used by the *FAO.Acrisols are *acid soils with an argic B hori-zon (see argic horizon) having a *cation-exchange capacity of less than 24 cmolc/kg.acritarchs Hollow fossil structures,5–240µm in diameter, inside which dinoflag-ellates and single-celled algae survived dryperiods. They range from *Precambrian to*Recent times. They are found in marinestrata, although some non-marine examplesare reported from Recent beds. Acritarchs areused in *correlation and to distinguish on-shore from offshore *sediments.Acrothoracica See cirripedia.acrozone See range zone.actinium series See decay series.actinolite A member of the *amphiboles,Ca2(Mg,Fe)5(Si4O11)2(OH,F)2, with the ratioFe/Fe ϩ Mg ϭ 0.9 to 0.5, belonging to the*tremolite–*ferroactinolite series of Ca-richamphiboles; sp. gr. 3.0–3.4; *hardness 5–6;*monoclinic; light greenish-grey to darkgreen; white *streak; *vitreous *lustre; habit*acicular, often fibrous and felted; *cleavage*prismatic, good {110}; occurs widely in low-to medium-grade *schists and some *igneousrocks. The asbestiform variety is called
  12. 12. *nephrite and such felted forms were used inthe past for insulation and fire-resistant ma-terials, but the development of asbestosis inworkers has severely restricted their use.Actinopterygii (ray-finned fish) A sub-class of the *Osteichthyes (bony fish, seebone), comprising the ray-finned fish, whichinclude the majority of living bony fish of seaand fresh water. The *fins are composed of amembranous web of skin supported by a vary-ing number of spines and soft rays. Theyappeared first during the *Devonian.activation analysis See neutron activa-tion analysis.activation energy (energy of activation)The energy that must be delivered to a systemin order to increase the incidence within it ofreactive molecules, thus initiating a reaction.active geophysical methods Geophysi-cal exploration methods which require anartificial signal to be generated. For example,exploration seismology, some *electromag-netic techniques, *electrical resistivity, *re-mote sensing, and *induced polarization aresaid to be active geophysical methods. Theterm is contrasted with *passive geophysicalmethods.active layer Seasonally thawed surfacelayer between a few centimetres and about3mthick,lyingabovethepermanentlyfrozenground in a periglacial environment. Itmay be subject to considerable expansion onfreezing,especiallyifsilt-sizedparticlesdomi-nate, with important engineering implica-tions.See alsomollisols;andpermafrost.active margin(seismic margin)Themarginof a continent that is also a *plate margin. Thealternative term, ‘Pacific-type margin’, indi-cates the range of features (e.g. *earthquakes,andesitic (see andesite) volcanic chains, off-shore oceanic *trenches, and young foldmountains) which may be associated with ac-tive margins. Some authors distinguish an‘Andino-type margin’, involving an oceanicand a continental plate, from a ‘Japan-typemargin’, involving an oceanic plate and an*island arc. The term ‘Mediterranean-typemargin’ is also in use, although to a lesser ex-tent, to signify the coincidence of continentaledges and plate margins in a *collision zone.active methods See active geophysicalmethods.active pool The part of a *biogeochemicalcycle in which the nutrient element underconsideration exchanges rapidly between thebiotic and abiotic components. Usually theactive pool is smaller than the *reservoirpool, and it is sometimes referred to as the‘exchange’ or ‘cycling’ pool.active remote sensing *Remote sensingwhich is based on the illumination of a sceneby use of artificial radiation. An example is*radar. Compare passive remote sensing.activity A broadly used term which refers tothe rate or extent of a change associated withsome substance or system. For example, itmay be the tendency of a metal high in theelectromotive series to replace another metallower in the series, e.g. magnesium displac-ing copper from most of its compounds. Itmay also be used to describe the rate of decayof atoms by radioactivity.activity coefficient (γ) The ratio of chemi-cal activity (i.e. the effective concentration, a)of a component in a solution, to the actualmole fraction (X) present in solution: (γ ϭ a/X).Values for activities are determined experi-mentally in a number of ways, including mea-suring the ratio of the *vapour pressure (p) ofa known concentration of the substance in so-lution to the vapour pressure (p*) of the puresubstance:aϭp/p*.Inanidealsolutiontheac-tivity coefficient ϭ 1, and the activity of thecomponent is equal to its mole fraction. Ingeneral, the greater the amount of dissolvedmaterial, the lower the activity coefficients ofeach of the species present.Actonian A *stage of the *Ordovician(453–454Ma ago) in the Upper *Caradoc, un-derlain by the *Marshbrookian and overlainby the *Onnian.actual evapotranspiration (AE) Theamount of water that evaporates from thesurface and is transpired by plants if the totalamount of water is limited. Compare poten-tial evapotranspiration.actualism The theory that present-dayprocesses provide a sufficient explanationfor past geomorphological phenomena, al-though the rate of activity of these processesmay have varied. The theory was first clearlyexpressedin1749byG.L.L.*Buffon(1707–88),and was the essential principle of *uniformi-tarianism as presented in 1830 by C. *Lyell(1797–1875).Actinopterygii 6a
  13. 13. a7 adhesion wartsacuity The ability of a human to discern spa-tial variation in a scene.ACV See aggregate tests.Adam The postulated male ancestor for allmodern humans, who lived in Africa betweenabout 100000 and 200000 years ago. ‘Adam’is based on a change in the human Y chromo-some that occurred at that time in one de-scendant of Adam and is now present in allhuman males, except for some Africans. Seealso mitochondrial eve.adamantine Of mineral *lustre, brilliant,like a polished diamond.adamellite A rock of granitic composition(see granite) characterized by the presence of*quartz, *plagioclase feldspar, and potassicfeldspar (see alkali feldspar) accompanied by*biotite and/or *hornblende. The two feldspartypes occur in approximately equal propor-tions, the plagioclase composition lyingwithin the oligoclase range. The name is de-rived from the type locality of Adamello in theTyrol where granites of this type were origi-nally defined. In Britain the best-known exam-ple occurs at Shap Fell in Cumbria.Adams–Williamson equation Equationdescribing a fundamental relationship be-tween seismic velocities (vp and vω), the *grav-itational acceleration (g), and the adiabaticchange in density (dρ) within the *Earth (as-suming only hydrostatic pressure) as a func-tion of radium (dr):dρ =gρdrn2ρ − (4/3)n2ωThis equation is directly applicable to thelower *mantle and outer *core, but is invalidwhere the composition is variable, the pres-sure is not hydrostatic, or the increase in pres-sure is not adiabatic.adapical A directional term meaning to-wards the shell *apex.adaptation 1. Generally, the adjustmentsthat occur in animals in respect of their envi-ronments. The adjustments may occur by *nat-ural selection, as individuals with favourablegenetictraitsbreedmoreprolificallythanthoselacking these traits (genotypic adaptation), orthey may involve non-genetic changes in indi-viduals,suchasphysiologicalmodification(e.g.acclimatization) or behavioural changes (phe-notypic adaptation). Compare abaptation. 2. Inanevolutionarysense,thatwhichfitsanorgan-ism both generally and specifically to exploit agiven environmental zone.adaptive radiation 1. A burst of evolution,with rapid divergence from a single ancestralform, resulting in the exploitation of an arrayof habitats. The term is applied at many *tax-onomic levels, e.g. the radiation of the mam-mals at the base of the *Cenozoic refers to*orders, whereas the radiation of ‘Darwin’sfinches’ in the Galápagos Islands resultedin a proliferation of *species. 2. Term usedsynonymously with ‘*cladogenesis’ by someauthors.adaptive zone The adaptive specializa-tion(s) that fit the *taxon to its environment,e.g. feeding habits.addition rule (Weiss zone law) With refer-ence to crystallographic notation, the rulestating that the indices (see miller indexes) oftwo *crystal faces in the same *zone alwaysadd up to the indices of a face bevelling theedge lying between them. The rule may beused to index faces on a *stereogram, or facesat the intersection of two zones.additive primary colours The spectralcolours red, green, and blue, which, whenmixed together by projection through filters,can be used to produce all other colours.None of the primary colours can be producedby combinations of the other two. See alsosubtractive primary colours.adductor muscles See muscle scar.Adelaidean A *stage (542–1300Ma ago) ofthe Upper *Proterozoic of south-eastern Aus-tralia, underlain by the *Carpentarian andoverlain by the Hawker (*Cambrian).Adelaidean orogeny A late *Proterozoicand *Ordovician phase of mountain building,affecting what is now southern Australia, inwhich *sedimentary rocks of the AdelaideanSystem were raised by severe thrusting andoverfolding, first in the south and later alongthe northern margin of the system.adhesion ripples See adhesion warts.adhesion warts (adhesion ripples) A *sed-imentary structure consisting of an irregu-lar, wart-like or blistered, *sand surface,formed by the wind blowing dry sand over amoist surface. The warts tend to be slightlyasymmetrical, with steeper sides in the up-wind direction.
  14. 14. adiabat The rate at which a *parcel of aircools as it rises and warms as it descends, asindicated by two lines (dry adiabat and wetadiabat) on a *tephigram.adiabatic Applied to the changes in temper-ature, pressure, and volume in a *parcel ofair or liquid that occur as a consequence ofthe vertical movement of the fluid, and with-out any exchange of energy with the sur-rounding fluid. See also dry adiabatic lapserate; and saturated adiabatic lapse rate.adit Horizontal or nearly horizontal tunnelfrom the surface into a mine, for entry,drainage, or exploration.admission The substitution of a *trace ele-ment for a major element with a similar*ionic radius but a higher *valency duringthe crystallization of a *magma, e.g. the sub-stitution of Liϩfor Mg2ϩin the *pyroxenes,*amphiboles, and *micas.adobe A silty *clay, often calcareous, foundin dry, desert-lake basins. This fine-grained*sediment is usually deposited by desertfloods which have eroded wind-blown *loessdeposits. The term is of Spanish origin.adoral On the same side of the body as themouth.Adrastea (Jupiter XV) A jovian satellite (a*moom) that orbits within the main ring ofJupiter; it and *Metis may be the source of thematerial comprising the ring. Both are consid-ered too small to suffer tidal disruption, buteventually their orbits will decay. Adrastea isone of the smallest satellites in the solar sys-tem. It was discovered in 1979 by David Jewitt.Its diameter is 20km (Ϯ20) (23 ϫ 20 ϫ 15km);mass1.91ϫ106kg;meandistancefromJupiter129000km.adsorption The attachment of an ion, mol-ecule, or compound to the charged surface ofa particle, usually of *clay or *humus, fromwhere it may be subsequently replaced orexchanged. Ions carrying positive charges(e.g. those of calcium, magnesium, sodium,and potassium) become attached to, or ad-sorbed by, negatively charged surfaces (e.g.those of clay or humus).adsorption complex Various materials ofthe soil, mainly *clay and *humus and to alesser degree other particles, capable of ad-sorbing ions and molecules.adularia See alkali feldspar.advection The horizontal transfer of heatby means of a moving gas (usually air).adventive cone See parasitic cone.AE See actual evapotranspiration.aedifichnia A category of *trace fossils thatcomprises structures in full relief that wereconstructed by organisms from raw materi-als, e.g. mud nests of wasps, caddis fly cases,spiders’ larders consisting of concentrationsof insects, insect remains, and spiders.aegirine *Pyroxene mineral, NaFe3ϩSi2O6;sp. gr. 3.5; *hardness 6; *monoclinic; green-ish-black or brown; occurs as fairly short,*prismatic crystals in *igneous and *meta-morphic rocks. A variety intermediate incomposition between aegirine and augite iscalled ‘aegirine–augite’. See also augite; andclinopyroxene.Aegyptopithecus zeuxis A genus andspecies of early *catarrhine primates, knownfrom abundant remains, including severalnearly complete skulls, from the early*Oligocene of the Jebel al-Qatrani Formation,Fayum, Egypt. The size of a small, living mon-key, it had a long tail and could jump frombranch to branch. It possessed the dental andsome of the cranial characteristics of livingcatarrhines, but lacked many of the other cra-nial and most of the postcranial diagnosticfeatures, and so represents a time when ca-tarrhines had separated from other primates,but remained more primitive than livinghominoids (*Hominoidea) or Old World mon-keys and it could have been ancestral to livingcatarrhines.aeolian abrasion The erosion of a surfacethat is caused by bombardment with looseparticles carried by the wind.aeolianite General term for the sedimen-tary products of wind (aeolian) deposition.aeolian processes (eolian processes) Theerosion, transport, and deposition of ma-terial due to the action of the wind at or nearthe Earth’s surface. Aeolian processes are attheir most effective when the vegetationcover is discontinuous or absent.aeolian ripple (eolian ripple) A ripple onthe surface of a sedimentary rock that iscaused by saltating grains. Aeolian rippleshave a wavelength approximately equal tothe *saltation path of the grains, and theyusually have no internal *cross-lamination.adiabat 8a
  15. 15. a9 AFM diagramAeolis Quadrangle A region of Marsformed in the Late *Noachian or Early *Hes-perian Epoch, containing both extensionaland compressional land-forms and *valles,some of which may be outflow channels, butsome of which may be tectonic rift features.aerial photograph A photograph takenfrom an aircraft. In hydrology, false-colour in-frared photographs are used to determine thewetness and temperature of soils and to de-tect *springs.aerial photography The taking of aerialphotographs of rock exposures and of theground surface for purposes of geologic inter-pretation. The photographs may be taken ver-tically, or at a high-oblique or low-obliqueangle, and may be assembled like a mosaic toprovide a picture of a large area. Stereoscopiccameras (two cameras within a single body)may be used to produce pairs of pictures thatprovide three-dimensional pictures whenobserved through a stereoscopic viewer. Seephotogeology.aerobic 1. Of an environment: one in whichair (oxygen) is present. In the case of a deposi-tional environment, one with more than 1mlof dissolved oxygen per litre of water. Compareanaerobic; and dysaerobic. 2. Of an organ-ism: one requiring the presence of oxygen forgrowth, i.e. an aerobe. 3. Of a process: onethat occurs only in the presence of oxygen.aerodynamic roughness Uneven flow ofair caused by irregularities in the surface(which may be of a solid, or of air of differentdensity) over which the flow takes place.aerological diagram Diagram to demon-strate variations with height of the physicalcharacteristicsoftheatmosphere,particularlyits temperature, pressure, and *humidity.aeromagnetic survey Survey of theEarth’s magnetic field, based on data from*magnetometers towed behind aircraft or sus-pended below helicopters. These instrumentsmeasure the total intensity of the *geomag-netic field or, occasionally, components of thisfield.Theresultingmeasurementscanthenbecompared with theoretical models for thevalue of the field and the differences (*mag-netic anomalies) can be interpreted in termsof changes in the magnetic properties of therocks below the survey line or grid. Themagnetometers are usually flown with otherinstrumentation, e.g. *radiometric and elec-tromagnetic, at the lowest practicable con-stant height above the ground. Usually themagnetometer is housed in a ‘bird’ towed be-hind the aircraft, or in a wing-tip pod, or in a‘stinger’ in the tail. In cases where the magne-tometerisonboard,in-boardcoilsystemscom-pensate for the aircraft’s own magnetic field.Aeronian A *stage (436–439 Ma ago) of theLower *Silurian (*Llandovery Period) under-lain by the *Rhuddanian and overlain by the*Telychian.aerosol Colloidal substance, either naturalor man-made, that is suspended in the air be-cause the small size (0.01–10µm) of its parti-cles makes them fall slowly. Aerosols in the*troposphere are usually removed by *pre-cipitation and their *residence time is mea-sured in days or weeks. Aerosols that arecarried into the *stratosphere usually re-main there much longer. Troposphericaerosols may act as *Aitken nuclei but thegeneral effect of aerosols is to absorb, reflect,or scatter radiation. Stratospheric aerosols,mainly sulphate particles resulting from vol-canic *eruptions, may reduce *insolationsignificantly. About 30% of tropospheric dustparticles are the result of human activities.See atmospheric structure; mie scattering;rayleigh scattering; and volcanic dust.Aëtosauria Mainly *Triassic group of prim-itive thecodontian (‘tooth-in-socket’) reptiles(see thecodontia). They resembled heavily ar-moured crocodiles, and appear to have beenspecialized herbivores or possibly omnivores.They grew up to 3m long, and their armourplating comprised rows of bony *plates.AFC See assimilation-fractional crystal-lization.AF demagnetization See alternatingmagnetic field demagnetization.AFM diagram A three-component, triangu-lar graph used to show how metamorphic*mineral assemblages vary as a function of*rock composition within one *metamorphicfacies. Besides SiO2, the five most abundant ox-ides found in *metamorphic rocks are Al2O3,CaO, FeO, MgO, and K2O. The three compo-nents plotted on AFM diagrams are derivedfrom a tetragonal diagram, with species Al2O3,K2O, FeO, and MgO, and are ideal for showingmineral assemblage variations as a function ofthe composition of *pelites. Mineral and rockcompositionsplottingwithinthisdiagramareprojected on to the Al2O3–FeO–MgO face fromeither the *muscovite or K-feldspar point on
  16. 16. the Al2O3–FeO edge. The components of the di-agram are thus A (Al2O3), F (FeO), and M (MgO),with the projection geometry being accom-modatedonspeciallyscaledaxes.Eachofthesecomponents has to be modified slightly to ac-count for the presence of other, minor compo-nents in the rock, leading to: A (Al2O3 − 3K2O);F (FeO − TiO2 − Fe2O3); and M (MgO). The miner-als *quartz and *albite are assumed to be pre-sent in the rocks and are not shown on thediagram. As in *ACF diagrams, *tielines con-nect minerals which coexist in equilibrium.AFMAG EM system Audio-Frequency Mag-netic ElectroMagnetic method, which usesnatural electromagnetic (EM) fields (*sferics)in the audio-frequency range (1–1000Hz)generated by thunderstorms to investigatelateral changes in the *resistivity of theEarth’s surface.African Plate One of the present-day majorlithospheric *plates, consisting of the conti-nental mass of Africa surrounded, except tothe north, by *oceanic crust and oceanic*ridges. To the north, a complex picture ofcollision and *subduction zones and *trans-form faults has been postulated for theboundary with the *Eurasian Plate and vari-ous minor plates, e.g. the *Aegean Plate. Thenorthern part of the African Plate also con-tains remnants of the oceanic crust of*Tethys. To the north-east the Red Sea is in-terpreted as an actively forming ocean, at theyoung stage of the *Wilson cycle, while theE. African *rifts, partially defining what iscalled by some the ‘Somali Plate’ to the east,may be at the embryonic stage of ocean devel-opment, or possibly a stillborn ocean.aftershock A seismic event that occurs afteran*earthquake,usuallywithindaysorweeks.Although often of small *magnitude, after-shocks can be more destructive as buildingsand structures have already been weakened.Aftonian The earliest (1.3–0.9Ma) of four*interglacial *stages in N. America, followingthe *Nebraskan glacial episode, and approxi-mately equivalent to the *Donau/Günz inter-glacial of Alpine terminology. Climatically itwas marked by mild summers and winterswarmer than those in present-day N. America.Agassiz, Jean Louis Rodolphe (1807–73)A Swiss geologist who worked initially on fos-sil fish, Agassiz is better known for his*glacial theory (1837). He met *Buckland in1840, and persuaded him that *drift depositsin Britain were evidence of a glacial epoch. In1846 he moved to the USA to become profes-sor of zoology and geology at Harvard, wherehe founded the Museum of ComparativeZoology (1859).agate (mocha stone) Variety of chalcedonicsilica (SiO2) that is *cryptocrystalline. It issimilar to *chalcedony except that impuri-ties of iron and manganese may give it a dis-tinct colour banding which is frequentlyprecipitated in concentric zones. Moss agatecontains delicate, fern-like, dendritic pat-terns. Agates may be cut and polished as dec-orative stones.age 1. The interval of geologic time equiva-lent to the *chronostratigraphic unit ‘*stage’.Ages are subdivisions of *epochs and maythemselvesbesubdividedinto*chrons.Anagetakes its name from the corresponding stage,so like the stage name it carries the suffix ‘-ian’(or sometimes ‘-an’); the term ‘age’ is capital-ized when used in this formal sense, e.g. ‘*Ox-fordian Age’. 2. An *informal term to denotea time span marked by some specific feature,e.g. ‘*Villefranchian mammalian age’.ageostrophic wind The vector differencebetween the *geostrophic and the actualwinds.agglomerate Coarse-grained volcanicrock with rounded to subangular fragments.These fragments are mainly larger than 2cmin size, but the mixture of fragments is typi-cally ill sorted and the *matrix may be finegrained. An agglomerate may be the productof a volcanic explosion and therefore a *pyro-clastic rock, but often the term ‘agglomerate’is applied to brecciated volcanic rocks of un-certain origin. Those deposits may rangefrom vent *breccias to debris from mudflowor lahar deposits.agglutinateAconstituentoflunarsoilscom-prising glass-bonded *aggregates, which con-sistof*glassesandrockandmineralfragmentswelded together by glass. These aggregatesform during the impact of micrometeoritesintolunarsoils.Theirabundanceinalunarsoilisanindexofexposuretomicrometeoritebom-bardment, and hence to soil maturity. The av-eragesizeofagglutinatesinmaturesoilsvaries,but tends toward a mean of 60µm.aggradation The general accumulation ofunconsolidated sediments on a surface,which thereby raise its level. A large range ofAFMAG EM system 10a
  17. 17. a11 AGUmechanisms may be involved, including *flu-vial, *aeolian, marine, and *slope processes.aggregate 1. In the building and construc-tion industry, a mixture of mineral sub-stances (bulk *minerals), e.g. sand, gravel,crushed *rock, stone, slag, and other materi-als (e.g. colliery spoil, pulverized fuel ash)which, when cemented, forms *concrete,mastic, mortar, plaster, etc. Uncemented, itcan be used as road-making material, railwayballasts, filter beds, and in some manufactur-ing processes as flux. In road-making, aggre-gate mixed with *bitumen is called ‘coatedstone’, and different physical characteristicsare required for the different layers compris-ing the road *pavement. Fine aggregate is lessthan 6.35mm in diameter, coarse aggregategreater than 6.35mm. See aggregate tests;and pavement.2. Group of soil particles adhering togetherin a cluster; the smallest structural unit, orped, of soil. Aggregates join together to makeup the major structural soil units.aggregate abrasion value See aggre-gate tests.aggregate crushing value See aggre-gate tests.aggregate impact value See aggregatetests.aggregate tests Specific tests used to de-termine the suitability of *aggregates forspecial purposes. There are tests for: (a) shapeand texture (the angularity number), to deter-mine whether particles have a large angle offriction with good bonding properties; (b) sizeand grading, to determine whether particleswill pack well; (c) moisture content, to dis-cover whether materials absorb so muchwater that freeze–thaw action might causethe break-up of structures; (d) rock density,which may affect the economics of an opera-tion; (e) strength, determined by subjectingthe rock to hammering in a standard test andmeasuring the percentage of fine materialproduced (the aggregate impact value, orAIV); (f) resistance to crushing (the aggregatecrushingvalue,orACV),measuredinasimilarmanner; (g) resistance to abrasion, measuredby standard equipment to give the aggregateabrasion value (AAV)—the lower the AAV, themore resistant the rock; and (h) resistance topolishing, measured in the laboratory to givethe polished stone value (PSV)—the higher thePSV, the greater the resistance to polishingand therefore skidding, and the more valu-able the material.aggregation 1. Process in which soil parti-cles coalesce and adhere to form soil aggre-gates. The process is encouraged by thepresence of bonding agents such as organicsubstances,*clay,ironoxides,andions(e.g.cal-cium and magnesium). 2. Progressive attach-ment of particles (e.g. ice or snow) or dropletsaround a nucleus, thereby causing its growth.Aglaophyton major See rhynia.Agnatha (phylum *Chordata, subphylum*Vertebrata) Superclass of jawless, fish-likevertebrates, with sucker-like mouths, includ-ing the extant lampreys, slime-eels andhagfish,andsomeoftheearliestprimitivever-tebrates, with heavily armoured forms, e.g.*Cephalaspis (see also osteostraci), Pteraspis (seeheterostraci), and Jamoytius (see anaspida).They appeared first during the *Ordovician.Agnostida An order of *Trilobita that livedfrom the Lower *Cambrian to Upper *Ordovi-cian. Most were blind, lacking sutures, andtypically are found rolled up. They had a sub-equal *cephalon and *pygidium. There weretwo suborders. They are important strati-graphic markers.agric horizon Mineral-soil diagnostic hori-zon formed from an accumulation of *clay,*silt, and *humus, which has moved downfrom an overlying, cultivated soil layer. It is a*soil horizon created by agricultural man-agement, and is identified by its near-surfaceposition,andby*colloids accumulated in thepores of the soil.agrichnion (pl. agrichnia) A *trace fossilcomprising a burrow that formed the perma-nent dwelling of an organism and was used totrap or culture smaller organisms for food oruse them in chemosymbiosis.Agricola, Georgius (Georg Bauer)(1494–1555) The author of works on ‘geology’and mineral classification, and of the firstcomprehensive record of mining, De Re Metal-lica (1556). Using Roman sources and contem-porary German knowledge, his books becamebasic reference material for two centuries.agrometeorology The study of the rela-tionship between conditions in the surfacelayers of the atmosphere and those in the sur-face of the Earth, as this affects agriculture.AGU See american geophysical union.
  18. 18. Agulhas current Part of the large-scalecirculation of the southern Indian Ocean. It isa surface-water current that flows off the eastcoast of southern Africa between latitudes25°S and 40°S in a south-westerly direction.Flow velocity varies seasonally between 0.2and 0.6m/s.ahermatypic Applied to corals that lackzooxanthellae (symbiotic unicellular *algae)and that are not *reef-forming.AIPG See american institute of profes-sional geologists.airborne dust analysis Sampling anddetermination of airborne particles. This tech-nique requires size segregation of the particlesand a device for collection during updrafts inorder to obtain only local particles. Modernequipment sucks dust directly off vegetationfor analysis.airborne gravity survey A regional*gravity survey undertaken from the air.Such surveys are now rapid and precise be-cause of the development of *gravimeters ca-pable of being compensated for changes inthe motion and flight path of an aircraft, par-ticularly a helicopter.airgun A seismic source which discharges abubble of highly compressed air into water.Airguns are most commonly used in marineseismic exploration, but can also be used as adown-hole seismic source.air-lift pump A device composed of twopipes, one inside the other. Air is blowndown the inner pipe, which is slightlyshorter than the outer pipe. The result ofthis is to push an air–water mixture up thegap between the two pipes. This is a usefulpump for obtaining samples from very smalldiameter boreholes.air mass (airmass) Large body of air (some-times of oceanic or continental proportions)identified primarily by an approximatelyconstant wet-bulb-potential temperature(i.e. the lowest temperature to which the aircan be cooled by the evaporation of waterinto it). The temperature and *humidity char-acteristics of an air mass, which are roughlythe same within the one air mass at a partic-ular latitude and height, are modified byand modify the atmospheric environmentthrough which the air mass passes.air wave A sound wave which travelsthroughtheairfromaseismicshot.Thespeedof such a wave is approximately 330m/s.Airy, George Biddell (1801–92) A Cam-bridge astronomer and mathematician, Airybecame Astronomer Royal in 1835. He investi-gated planetary motion and tides, and stud-ied the Earth and its density, using gravitymeasurements. His name is used to describeone version of the theory of *isostasy. Hiswide-ranging advice to the government onscientific issues created, for the first time, therole of a professional scientific civil servant.Airy model A model to account for *iso-stasy which in the *lithosphere assumes aconstant density (ρc ϭ 2 670kg/m3), but inwhich topographic elevations (h) are com-pensated by the presence of ‘roots’ replacinghigh-density *mantlerocks(ρm ϭ3300kg/m3)by lower-density lithospheric rocks. Thedepth of the root (d) is equal to hρc/(ρ − ρc).See alsoprattmodel.Airy phase When a high-frequency seismicwave is superimposed on a low-frequencyground wave, the two frequencies graduallyapproach one another until they merge, atwhichpointtheyformasinglewavewitharel-ativelylargeamplitude,calledthe‘Airyphase’.Aitken nuclei counter Device for the esti-mation of the concentration of particles withradii of more than 0.001µm in a sample of air.Air is made to expand in a chamber: thiscauses it to cool. Water vapour in it condenseson to particles, forming a mist whose opacityallows estimation of the number of particlespresent. See also aitken nucleus.Aitken nucleus Suspended, atmospheric,solidparticlewitharadiusoflessthan0.2µm.Most Aitken nuclei are about 0.5µm. Onaverage, their concentration varies from lessthan 1000/cm3over oceans to 150000/cm3inAgulhas current 12amountainoceanconstant densitydense regionAiry model
  19. 19. a13 albite–epidote–amphibolite faciesurban areas. See aitken nuclei counter; andnucleus.AIV See aggregate tests.AIW See antarctic intermediate water.åkermanite See melilite.aklé French term for a network of sand*dunes found especially in the western Sa-hara. The basic unit of the network is a sinu-ous ridge, at right angles to the wind, madeup of crescent-shaped sections which alter-nately face the wind (linguoid) and back tothe wind (barchanoid). Aklé patterns requirewinds from one direction, and a large quan-tity of sand.aktuopalaeontology A branch of *taph-onomy in which experimenters observe thedecay of recently living organisms under nat-ural conditions.alabaster See gypsum.alar The first lateral protosepta (see septum)on either side of the *cardinal septum. Theterm is used in descriptions of the septal de-velopment of the rugose corals (*Rugosa),and may also be applied to *fossulae whichoccur in a similar position.alas A *thermokarst depression with rela-tively steep sides and a flat floor, which maybe occupied by a lake. Alases are well devel-oped in Siberia (the word ‘alas’ is of Yakutianorigin) where they can occupy 40–50% of theland surface.Alaska current Oceanic water boundarycurrent produced by the deflection of the*N. Pacific current by the N. American conti-nent. It flows in a north-westward directionalong the south-eastern margin of Alaska.The Alaska current is also called the *Aleut-ian current in some texts.A-layer The seismic layer corresponding tothe *crust of the Earth. It varies in thicknessfrom a few kilometres to 70–90km. The baselies on the *mantle and this boundary is the*Mohorovicˇic´ discontinuity.albedo The proportion of *insolation that isreflectedbackfromtheEarth,fromthetopsofthe clouds, and from the atmosphere, with-out heating the receiving surface. It averagesabout 30%, but varies widely according to thesubstance and texture of the surface, and theangle and wavelength of the incident radia-tion. The value for green grass and forest is8–27% (over 30% for yellowing deciduous for-est in autumn); for cities and rock surfaces,12–18% (over 40% for chalk and light-colouredrock and buildings); for sand up to 40%; forfresh, flat snow up to 90%; and for calm wateronly 2% in the case of vertically incident radi-ation but up to 78% where there is a low angleof incidence. The albedo for cloud surfaces av-erages 55%, but can be up to 80% for thick*stratocumulus.Albeluvisols A reference soil group in thesoil classification scheme used by the *FAO. Al-beluvisols have an argic B horizon (see argichorizon) with an irregular upper boundary(a condition sometimes known as ‘tonguing’).Alberta low Storms common in Alberta,Canada, and associated with heavy rain andsnow. The storms form as a result of *cycloneregeneration after passage over the CanadianRockies: as they move eastwards they bringvery cold conditions, with blizzards.Albertan A *series (501–513 Ma ago) of theMiddle *Cambrian of N. America, equivalentto the *St David’s.Albian *Stage (99.6–112Ma ago) in the *Cre-taceous, underlain by the *Aptian, and over-lain by the *Cenomanian. It is known tocontain a great variety of *molluscs, with the*gastropods in particular being useful *zonalindicators between continents. The Gault andSpeeton Clays of England are Albian.albic Applied to an almost white soil inwhich there is little *clay or oxides coatingthe sandy or silty particles. The albic *hori-zon lies at or below the surface.albite See alkali feldspar; and plagioclasefeldspar.albite–epidote–amphibolite facies Aset of metamorphic *mineral assemblagesthat is produced by the *metamorphism ofa wide range of initial rock types underthe same metamorphic conditions, and iswinddirectionAklé dune
  20. 20. typically characterized by the develop-ment of the mineral assemblage *albite–*epidote–*hornblende in rocks of *basic *ig-neous composition such as *basalts. Otherrocks of contrasting composition, e.g. *shalesor *limestones, would each develop theirown specific mineral assemblage, eventhough they are all being metamorphosedunder the same conditions. The variation ofmineral assemblage with starting rock com-position reflects a particular range of pres-sure, temperature, and P(H2O). Experimentalstudies of mineral P–T stability fields indicatethat the *facies represents a range of low-pressure, moderate-temperature conditions.See amphibolite.albite twin The *plagioclase feldspars, par-ticularly *albite (NaAlSi3O8), are frequentlytwinned on the albite law (see twin law) wherethe *twin plane and composition plane is(010). This twinning is often repeated to give aseries of fine lamellae, seen in the hand speci-mens as striations (particularly on the basalplane); such twinning is usually called ‘*poly-synthetic’ or ‘*lamellar’ twinning.albitization The partial or complete re-placement of pre-existing *plagioclase or *al-kali feldspar by albite. There are a number ofways in which this can be achieved. A com-mon process involves the residual water-richvapour released during the final stagesof crystallization of a *granite body. Thisvapour, which can carry high concentrationsof Naϩin solution, rises through the granitebody and reacts with the feldspars present inthe granite, converting them to albite whichis stable under the lower temperature vapour-rich conditions. A typical reaction that par-tially or completely replaces plagioclasewould be: CaAl2Si2O8 ϩ 4SiO2 ϩ 2Naϩ→2NaAlSi3O8 ϩ Ca2ϩ; anorthite ϩ quartz ϩsodium (in aqueous solution) → albite ϩ cal-cium (in aqueous solution). This type of reac-tion, where a rock simmers in its own juices,is termed a ‘*deuteric reaction’. Another wayin which albitization can be achieved is bythe reaction of ocean-floor *basalts with seawater in thermal circulation cells within thebasalt layer of the *oceanic crust.alcove A steep-sided hollow eroded by astream from an exposed rock face.alcrete See duricrust.Aldingan A *stage (33–36 Ma ago) in theLower *Tertiary of south-eastern Australia,underlain by the *Johannian, overlain by the*Janjukian, and roughly contemporaneouswith the *Bartonian and *Priabonian Stages.alete See spore.Aleutian current (Sub-arctic current) Theoceanic current that flows westwards southof the Aleutian Islands and parallel to, butnorth of, the *N. Pacific current. The watermass is a mixture of water from the*Kuroshio and *Oyashio currents. See alsoalaska current.Aleutian low Region of the N. Pacific, nearthe Aleutian Islands, where the average valueof atmospheric pressure is low, owing to thefrequency of low-pressure systems (cyclones)moving into and occupying the region. Anyone of these systems, when present on an in-dividual day, may be called ‘an Aleutian low’.Some of them are intense, others much lessso. The term is the Pacific equivalent of ‘*Ice-land low’, used in the Atlantic.Aleutian Trench The oceanic *trenchwhich marks the boundary between the*N. American Plate and the *Pacific Plate. The*subduction of the Pacific Plate changes fromnormal to oblique from west to east alongthe trench, with the boundary becoming a*transform fault before subduction contin-ues in the *Kuril Trench. Towards the easternend of the Aleutian Trench there is an in-creasingly wide *accretionary wedge, and anabsence of andesitic *volcanoes.Alexandrian A *series of the Lower *Sil-urian of N. America equivalent to theLower–Middle *Llandovery.alexandrite See chrysoberyl.Alfisols (grey-brown podzolics) An orderof mineral soils that have *clay-enriched or*argillic B *horizons; are alkaline to interme-diate in reaction, with the *base saturation inthe B horizon more than 35%; are usually de-rived from base-rich parent materials; and aredrier than −15 bars moisture potential for atleast three months when plants could grow.Alfvén waves Magnetohydrodynamicwaves that are produced by coupling forcesbetween the *geomagnetic field and highlyconductive fluids. Alfvén waves travel alongmagnetic field lines when jets of highlyconductive fluid or charged particles flowacross the field lines. The waves were discov-ered by the Swedish astrophysicist HannesOlof Gösta Alfvén (1908–95).albite twin 14a
  21. 21. a15 alkali feldsparalga (pl. algae) Common (non-*taxonomic)nameforarelativelysimpletypeofplantwhichis never differentiated into root, stem, andleaves; which contains chlorophyll a as the pri-mary photosynthetic pigment; which has notrue vascular (water-conducting) system; andin which there is no sterile layer of cells sur-rounding the reproductive organs. The algaerange in form from single cells (*Protista) toplants many metres in length; algae can befound in most habitats on Earth, althoughthe majority occur in freshwater or marineenvironments. See bacillariophyceae; charo-phyceae; chlorophyceae; chrysophyceae;dinophyceae; phaeophyceae; and rhodo-phyceae.algal bloom Sudden growth of algae in anaquatic ecosystem. It can occur naturally inspring or early summer when primary pro-duction exceeds consumption by aquaticherbivores (see primary productivity). Algalblooms may also be induced by nutrientenrichment of waters due to pollution.algal limestone See leighton-pendexterclassification.algal mat A sheet-like accumulation of blue-green algae (*Cyanobacteria) developed inshallow marine *subtidal to *supratidal envi-ronments, as well as in lakes and swamps. Thealgae cover the *sediment surface, and will inturn trap sediment to produce a laminatedalternation of dark, organic-rich algal layersand organic-poor sediment layers. See alsostromatolite.alginite See coal maceral.Algonkian A *Precambrian *system (VanEysinga, 1975) of equivalent time period tothe *Proterozoic.aliasing A distortion in the frequency ofsampled data produced by insufficient sam-pling per wavelength, which can result inspurious frequencies. When the samplingrate is too low to represent the wave-form ac-curately, then aliasing will occur. To avoidaliasing, the sampling frequency should be atleast twice that of the highest-frequency com-ponent contained within the sampled wave-form. Alternatively, an anti-alias filter can beapplied, which removes frequency compo-nents above the *Nyquist frequency.Alisols A reference soil group in the soilclassification scheme used by the *FAO. Al-isols have an argic B horizon (see argic hori-zon) with a *cation-exchange capacity ofmore than 24cmolc/kg clay and a *base satu-ration of less than 50% within 100 cm of thesoil surface. Alisols have a high concentrationof aluminium.alkali–aggregate reaction A chemicalreaction that can lead to damage in *concretestructures. Free lime (CaO) in *cement reactswith CO2 in the atmosphere to precipitateCaCO3 around the cement grains. This pro-tects them from *weathering and also givesan alkalinity level (*pH higher than 7.0)which helps to protect steel from corrosion. Ifthe aggregate contains soluble *silica, how-ever, new minerals may precipitate by reac-tion between the aggregate and the cement.These may absorb water, causing the concreteto swell and eventually crack. Water enteringthese cracks may cause rusting of reinforce-ment bars and repeated wetting and dryingmay eventually destroy a structure.alkali basalt A fine-grained, dark-coloured,volcanic rock characterized by *phenocrystsof *olivine, titanium-rich *augite, *plagio-clase, and iron oxides. For similar SiO2 con-centrations, alkali basalts have a highercontent of Na2O and K2O than other *basalttypes such as *tholeiites. They are also char-acterized by the development of *modal*nepheline in their *groundmass (only seenwith the highest powered lens on a petrologi-cal microscope) and normative nepheline(Ne) in their *CIPW norms. Alkali basalts aretypically found on updomed and rifted *con-tinental crust, and on oceanic islands such asHawaii and Ascension Island.alkalic See alkaline.alkali-calcic series See calc-alkaline.alkalic series See calc-alkaline.alkali feldspar A group of *silicate miner-als that contain the alkali metal elementspotassium and sodium. The normal feldsparminerals (including the calcium-bearing vari-eties) can be plotted on a chemical basis intoa triangle which has KAlSi3O8 (potassiumfeldspar, sanidine, orthoclase (Or), or micro-cline), NaAlSi3O8 (sodium feldspar, albite, orAb), and CaAl2Si2O8 (calcium feldspar, anor-thite, or An) at the three apices. The alkalifeldspars are represented by the edge of thetriangle joining KAlSi3O8 and NaAlSi3O8 andthese minerals may also contain up to 10% byweight of the third phase (CaAl2Si2O8). At high
  22. 22. temperatures the alkali feldspars show com-plete *solid solution between the potassiumand sodium *end-members, but as the tem-perature drops unmixing occurs and potas-sium feldspar and sodium feldspar separateout to produce a perthitic texture. Dependingupon the final temperature, a range ofperthites may result, from coarse (*perthite),representing perthites formed during a largedrop in temperature, to fine (*microperthite),and finally to very fine (*cryptoperthite), rep-resenting perthites invisible to the naked eyeand often invisible under the microscope, butobserved by *X-ray diffraction (XRD) tech-niques. If the amount of potassium exceedsthat of sodium, then potassium feldspar is thehost and sodium feldspar occurs within thehost mineral as *blebs, irregular patches, etc.In the alkali feldspars, perthitic texturesoccur in the compositional range Or85Ab15 toOr15Ab85 (or Or85 to Or15). K-feldspar (KAlSi3O8)is the general name for the *monoclinic,potassium-rich end-member: sp. gr. 2.6;*hardness 6; white, sometimes with a reddishtint; *vitreous *lustre; crystalline, *pris-matic, with simple twins (see crystal twin-ning). It is an *essential constituent of *acid*igneous rocks and *arkoses and is used inthe manufacture of glazes, porcelain, andpottery. Microcline has the same physicalpropertiesandcompositionasorthoclase,butis *triclinic and is characterized by ‘cross-hatched’ twinning. It is greyish-white, butbright green in the variety known as ‘ama-zonstone’ (‘amazonite’). Anorthoclase is verysimilar to microcline, but the amount ofsodium exceeds that of potassium. Crystaltwinning is common particularly along the*pericline and albite laws. Sanidine is thehigh-temperature variety of orthoclase andthe inversion temperature is at 900°C. It oc-curs in quickly cooled lavas. Adularia is a vari-ety of microcline, but with up to 10% sodiumsubstituting for potassium. It may show anopalescent play of colours to give a varietyknown as ‘moonstone’. Albite (NaAlSi3O8) isthe sodium-rich end-member of both the al-kali feldspars and the *plagioclase feldspars.The semi-precious moonstone, with its char-acteristic bluish sheen or *schiller, is an ex-ample of a perthitic alkali feldspar.alkaline (alkalic) 1. Having a *pH greaterthan 7.0. 2. See alkaline rock.alkaline rock *Igneous rock containing arelatively high concentration of the alkali(lithium, sodium, potassium, rubidium,caesium, and francium) and alkaline earthmetals (magnesium, calcium, strontium,barium, and radium). Both silica-saturatedand silica-undersaturated varieties exist, ex-pressed in the presence of *alkali feldsparsand *feldspathoids respectively. Alkali *fer-romagnesian minerals are usually present,and their identity depends on the composi-tion of the rock. Igneous rocks of the alkalinesuite span the composition range from *basicto *acid, and may be *intrusive or *extrusive.alkaline soil Soil with a *pH greater than7.0. Degrees of soil alkalinity are recognized.The *USDA lists soils with pH 7.4–7.8 asmildly alkaline; 7.9–8.4 as moderately alka-line; 8.5–9.0 as strongly alkaline; and morethan 9.0 as very strongly alkaline. Soil is notregarded as highly alkaline unless the reac-tion is between 8.0 and 10.0. The full range ofthe pH scale (0–14) is not used in soils, as thereaction of most soils is between pH 3.5 andpH 10.0. A *base saturation of 100% indicatesa pH of about 7.0 or higher.alkaliphile An *extremophile (domain*Archaea) that thrives in environmentswhere the *pH is above 9.0.allanite (orthite) *Mineral, with the for-mula (Ca,Ce,Y,La,Th)2(AlFe)3Si3O12(OH); sp. gr.3.4–4.2; *hardness 5.0–6.5; *monoclinic;light brown to black; pitchy to *sub-metallic*lustre; faintly radioactive; *crystals nor-mally *prismatic, often *tabular, sometimes*massive; *cleavage imperfect {001}; oftenoccurs as an *accessory mineral in graniticrocks, *syenites, *gneisses, and *skarns.Alleghanian orogeny A phase of moun-tain building, that began in the Early *Car-boniferous and was completed by the endof the *Permian, caused by the collision be-tween N. America and Africa. It formed partof the general WSW to ENE *Hercynian belt.The orogeny affected the Lower *Palaeozoic*basement and Lower Permian strata alongthe western margin of the southern andcentral parts of the Appalachian Mountainsextending from what is now Pennsylvaniato Alabama, with effects as far north asNew Brunswick and Newfoundland. Seeappalachian orogenic belt.allele Common shortening of the term ‘al-lelomorph’. One of two or more forms of a*gene arising by mutation and occupying thealkaline 16a
  23. 23. a17 allopatric speciationsame relative position (locus) on homologous*chromosomes.allelomorph Term that is commonly short-ened to ‘*allele’.Allen’s rule A corollary to *Bergmann’srule and *Gloger’s rule, holding that a race ofwarm-blooded species in a cold climate typi-cally has shorter protruding body parts (nose,ears, tail, and legs) relative to body size thananother race of the same species in a warmclimate. This is because long protrudingparts emit more body heat, and so are disad-vantageous in a cool environment, but ad-vantageous in a warm environment. The ideais disputed, critics pointing to many otheradaptations for heat conservation whichprobably are more important, notably fat lay-ers, feathers, fur, and behavioural adapta-tions to avoid extreme temperatures.Allerød Late glacial (i.e. late *Devensian) pe-riod marking a prolonged warmer oscillationor *interstadial during the general phase ofice retreat in NW Europe. *Radiocarbon dat-ing suggests it lasted from about 12000BP to10800BP. Pollen records for the NW Euro-pean area indicate a cool temperate flora withbirch (Betula species) widespread, in markedcontrast to the preceding and following,colder, *Dryas, phases.allochem The collective term for particles(grains) which form the framework in me-chanically deposited *limestones. In thelimestone classification of *Folk, allochemsare often found together with a *carbonatemud *matrix (*micrite) and may subse-quently have *pore spaces filled by sparry*calcite *cement (*sparite). Common al-lochems include skeletal fragments (*bio-clasts), *ooids, *peloids, and *intraclasts.allochemical A *limestone defined by the*Folk classification as comprising *allochemswith either a sparry *calcite *cement(*sparite), or a *microcrystalline *calcite (*mi-crite) *matrix. Limestones lacking allochemsare defined by Folk’s classification of lime-stones as *orthochemical limestones or *au-tochthonous *reef rocks.allochthon A body of rock that has beentransportedtoitspresentposition,usuallyovera considerable distance. See allochthonous.allochthonous Not indigenous; acquired.In the Earth sciences the term is applied togeologic units that originated at a distancefrom their present position. Such displace-ment may be due to lateral thrusting andoverfolding, or to gravity gliding. Compareautochthonous.allochthonous terrane See terrane.alloclast A *clast produced by subter-ranean, igneous processes that break up pre-existing volcanic rocks. Compare autoclast;epiclast; and hydroclast.allocyclic mechanisms Events respons-ible for the accumulation of sediments thatare external to the sedimentary system itself(e.g. sea-level changes, tectonic activity, or cli-mate). Compare autocyclic mechanisms.allodapic Applied to materials deposited byturbidity (see turbidity current) or *massflow, particularly used in relation to *lime-stones deposited by mass flow.alloformation See allostratigraphicunits.allogenic Applied to minerals, or othercomponents of a rock, that have been derivedfrom pre-existing rocks and transportedsome distance to form part of the presentunit; e.g. *quartz grains in a *sandstone. Com-pare authigenic.allogenic stream Stream originating out-sideaparticularareaandwhosecontinuationis inconsistent with its new surroundings.Type examples are the Nile and Indus, whosedischarges are sufficient to carry themthrough arid regions, and the EuropeanNeretva, which is large enough to pass overpermeable limestone.allogroup See allostratigraphic units.allomember See allostratigraphic units.allometry Differential rate of growth suchthat the size of one part (or more) of the bodychanges in proportion to another part, or tothe whole body, but at a constant exponentialrate. For example, the antlers of the extinct*Irish elk (Megaloceros giganteus), the largest ofall deer, grew 2.5 times faster than the rest ofits body to reach an adult span of up to 3.5min the largest individuals. Allometry may inother cases be negative, leading to compara-tively smaller parts.allopatric speciation Formation of new*species from the ancestral species as a resultof the geographic separation or fragmenta-tion of the breeding population. Genetic
  24. 24. divergence in the newly isolated daughterpopulations ultimately leads to new species;divergence may be gradual or, according topunctuationist models, very rapid. See alsopunctuated equilibrium.allopatry The occurrence of *species indifferent geographic regions. When closely re-lated species are separated, differences be-tweenthemthatminimizedtheircompetitionfor food, shelter, or other resources usuallydecrease (i.e. the characteristics converge). Theprocess is called character displacement andmay be morphological or ecological.allophane (kandite) *Clay mineral of the*kaolinite group, Al2Si2O5(OH)2; whitish;amorphous, non-crystalline; occurs along*faults or *joint planes in a variety of rocks.allostratigraphic units Allogroups, allo-formations, and allomembers; these are sub-divisions of sedimentary structures that arethe subject of *allostratigraphy.allostratigraphy The study of sedimen-tary strata that can be defined and identifiedfrom the discontinuities bounding them,and that can be mapped.allotriomorphic See anhedral.allowable bearing pressure The *bear-ing capacity that takes account of boththe weight of the built structure and thestrength of the geological structure beneaththe foundation to ensure that geological de-formation and the settlement of the build-ing remain within limits the building cantolerate. The allowable bearing capacity (qa)is always equal to or less than the *safe bear-ing capacity (qs).alluvial Applied to the environments, ac-tion, and products of rivers or streams. Allu-vial deposits (alluvium) are *clastic, *detritalmaterials transported by a stream or riverand deposited as the river floodplain. Theterm is also applied to surface flow, as in *al-luvial fans, *bajadas, etc.alluvial cone See alluvial fan.alluvial fan (alluvial cone) Mass of sedi-ment deposited at some point along a streamcourse at which there is a sharp decrease ingradient, e.g. between a mountain range anda plain. Essentially, a fan is the terrestrialequivalent of a river-delta formation.alluvium An *alluvial deposit.almandine Member of the *garnet group of*minerals, Fe3Al2(SiO4)3; sp. gr. 4.25; *hard-ness 6.5–7.5; *cubic; red, brown-red, or black;greasy to vitreous *lustre; most common*crystals are dodecahedra, and many are ir-regular grains; widely distributed in *meta-morphic and *igneous rocks, and in beach*sands and *placers. Transparent crystals areused as *gemstones, and the mineral is usefulin general as an abrasive.alnöite An *intrusive, *basic, *igneous,*carbonatite rock, distinctive in possessingprimary *calcite, and consisting of *melilite(1⁄₃); *biotite (1⁄₃); and *pyroxene, calcite, and*olivine (1⁄₃). *Feldspar is not present in therock, its place being taken by the mineralmelilite which has the general formula:X2YZ2O7; where X ϭ Ca, Na; Y ϭ Mg, Al; Z ϭ Si,Al. The type location for this rock is Alnö is-land off the coast of Sweden.alpha decayCertainradionuclides(radioac-tive *nuclides) decay by the spontaneousemission of alpha particles from their nuclei.The alpha particle is composed of two protonsand two neutrons and has a charge of ϩ2. Italso has an appreciable mass and its ejectionfrom the nuclide creates a certain amount ofrecoil energy in the nucleus. The total energy(Ex) created by alpha decay is, therefore, thesum of the kinetic energy of the particle, therecoil energy given to the new *nucleus andthe total energy of any emitted *gamma rays.See also radioactive decay.alpha diversity Diversity among membersof a species within a single population.alpha-mesohaline water See halinity.alpha–proton–X-ray spectrometer(APXS) A set of instruments carried on Rus-sian *Vega and *Phobos missions and by So-journer, the rover vehicle carried on the 1997*Mars Pathfinder mission, that measures theelemental chemistry of surface materials.The sensor head of the instrument containscurium, as a source of alpha particles, analpha particle detector, a proton detector,and an X-ray detector. The head is placed incontact with a sample and remains there for10 hours. Alpha particles of known energybombard the sample. Scattered alpha parti-cles, protons from alpha–proton reactions,and X-rays produced by excitation by thealpha particles of the atomic structure of thesample are measured by the detectors. Theenergy spectrum of detections by all threeallopatry 18a
  25. 25. a19 Amazonianinstruments is then recorded and transmit-ted to Earth.alpine glow At sunset, beginning as theSun nears the horizon, mountains exposed todirect sunlight in the east, particularly ifsnow-covered, assume a series of colourschanging from yellow-orange to a rosy pink,which finally becomes purplish. The same se-ries of colours in reverse order is seen onmountains in the west at sunrise.Alpine–Himalayan orogeny Period ofmountain building that affected both north-ern and southern margins of the ancient*Tethyan ocean. It began in the *Triassic, butreached its high point during the Late*Oligocene and *Miocene. The Alps are an ob-vious testament to this orogeny, while thegentle folds of northern France, and theWeald and London Basin in England, reflectits outer effects.Alportian The final stage (318.1–324.5 Maago) of the *Serpukhovian epoch, underlainby the *Chokierian.alteration A change produced in a rock bychemical or physical action.alteration halo A border of minerals pro-duced by *hydrothermal *alteration in therock surrounding a *vein.alternating current The current output,with a sinusoidal wave-form, from an alterna-tor or dynamo.alternating-magnetic-field demagne-tization (AF demagnetization, thermalcleaning) A common method for demagnetiz-ing (see demagnetization) rock samples thatis widely used in *palaeomagnetism and *ar-chaeomagnetismbecauseofitssimplicityandbecause it produces no chemical change inthe samples. It can cause problems associatedwith *anhysteretic and *rotational rema-nences, and is only fully suitable for *mag-netite-bearing rock samples.altiplanation Process of relief reduction orplanation (i.e. the smoothing of the surface)under periglacial conditions. Two mecha-nisms are involved: destruction of upstandingrelief features by *gelifraction or *nivation,and accumulation of debris in depressions oras terraces. In many areas only partial altipla-nation has been achieved, with the emer-gence of altiplanation terraces, such as thoseof Cox Tor on Dartmoor, England.altocumulus From the Latin altum (height)and cumulus (heap). A genus of cloud com-posed largely of water droplets, and consist-ing of grey-white sheets, or banded layers androlls, which may also be broken up into cells.Sometimes it has a banded appearance, occa-sionally giving a mackerel-sky effect; this isprobably associated with strong vertical windshear in middle altitudes. See also cloudclassification.Altonian A *stage (16.5–17.5 Ma ago) in theUpper *Tertiary of New Zealand, underlainby the *Otaian, overlain by the *Cliffdenian,and roughly contemporaneous with theupper *Burdigalian Stage.altostratus From the Latin altum (height)and stratus (spread out). A genus of cloud con-sisting of greyish sheets or layers; the cloudmay be striated, fibrous, or uniform. It may becomposed of ice crystals as well as waterdroplets. See also cloud classification.alumstone See alunite.alunite (alumstone) *Mineral, KAl3(SO4)2(OH)6; sp. gr. 2.6–2.8; *hardness 3.5–4.0; *trig-onal; white, sometimes grey to reddish; white*streak; *vitreous *lustre; *crystals rare,*habit *massive; *cleavage basal {0001},distinct; *fracture uneven, *conchoidal;slightly astringent taste. Occurs as a *sec-ondary mineral in volcanic rocks containingpotassic *feldspars altered by sulphuric-acidsolutions. It is difficult to distinguish from*dolomite, *anhydrite, and *magnesite.alveolus See belemnitida.A/m See amperes per metre.Amalthea (Jupiter V) The jovian satellitewith the closest orbit to Jupiter. Its surfacecolour is reddish, apparently because of sul-phur emitted from *Io. Its diameter is 189km(262 ϫ 146 ϫ 134km), the irregular shape sug-gesting a rigid body. Its mass is 7.17 ϫ 1018kg;mean distance from Jupiter 181000km. It ra-diates more heat than it receives from the Sun.Amarassian See kazanian; and tatarian.Amazonian A division of *areological time,lasting from 1.80Gy to the present in theHartmann–Tanaka Model and 3.55Gy to thepresent in the Neukum–Wise Model, and di-vided into three epochs: Lower Amazonian(1.80–0.70 or 3.55–2.50Gy); Middle Amazon-ian (0.70–0.25 or 2.50–0.70Gy); and UpperAmazonian (0.25–0.00 or 0.70–0.00Gy).
  26. 26. amazonite See alkali feldspar.amazonstone See alkali feldspar.amb See ambulacrum.amber Fossil conifer resin which is brittleand hard, translucent to transparent, and yel-low to brown in colour. It is found in *sedi-ments or on the shore and takes a fine polish.ambient pressure Atmospheric pressurein the surrounding air.ambient temperature The dry-bulb tem-perature prevailing in the surrounding air.ambitus The outline or edge of an echinoid(*Echinoidea) when seen from above orbelow. Usually it is the place where the*plates of the *test are at their widest.ambulacral Applied to those areas of thebody of an *echinoderm that bear *tube feet.ambulacral groove See ambulacrum.ambulacrum (amb) In *Echinodermata, anarea of the body surface (covered in mostclasses by calcitic (see calcite) *plates), thatoverlies one of the radial canals of the internalwater vascular system, and bears the *tubefeet. In some echinoderms, e.g. Asteroidea,*Blastoidea,and*Crinoidea,theambulacrumis marked by a deep linear depression, the am-bulacral groove. Typically, echinoderms havefive ambulacral areas, or a multiple of five. Seeechinoidea.Ambulocetus natans The most completelyknown early cetacean, described in 1994 byJ. G. M. Thewissen, S. T. Hussain, and M. Arif,from Lower to Middle *Eocene beds inPakistan. It is known by parts from most ofthe skeleton, showing that it had a longneck, relatively long hind limbs, and fiveseparate (hoofed) digits on each limb. It wasthe size of a sea lion.amensalism An interaction of speciespopulations, in which one population is in-hibited while the other (the amensal) is unaf-fected.American Geophysical Union (AGU) Asociety with approximately 35 000 membersthat was founded in 1919 and is based inWashington, DC. It is the leading academicbody for geophysicists.American Institute of ProfessionalGeologists (AIPG) The body that certifiesprofessional geologists in the United States,based on their competence, integrity, andethical standards. The AIPG presents testi-mony and position papers to legislators andagencies at federal and state level on matterspertaining to the interests and employmentopportunities of geologists. Through its con-tacts with equivalent bodies in other coun-tries, the AIPG provides American geologistswith access to professional registration inthose countries.American Province See pacific province.Amersfoort An *interstadial in the last*glaciation of the Netherlands during theearly *Devensian (somewhere between 60000and 70000Ma). The July temperature (basedon floral evidence) was perhaps 15–20°C.amesite See chamosite.amethyst See quartz.amino acid Organic compound containingan acidic carboxyl (COOH) group and a basicamino (NH2) group. They constitute the fun-damental building blocks of peptides andproteins and are classified either (a) as neu-tral, basic, or acidic (according to their *pH),or (b) as non-polar, polar, or charged (accord-ing to their electrical configuration).amino group The chemical group −NH2.ammonites Family of *Ammonoidea.Ammonoidea (ammonoids) (phylum*Mollusca, class *Cephalopoda) Subclass ofcephalopods which generally have *planispi-rally coiled, septate shells (see septum). Char-acteristically the shells are tightly coiled andplanispiral, although some are coiled looselyamazonite 20aambulacralspineambulacrumambulacralgroovemouthAmbulacrum
  27. 27. a21 amphibolite faciesor spirally; the *protoconch is globular; theshells may be either *involute or *evolute.Some forms have marked *ventral *keels;ribs and nodes may also be present. The*siphuncle is variable but mainly ventral inposition. *Sutures are often very complex.*Camaral deposits are absent. The Am-monoidea were probably tetrabranchiate(four-gilled) cephalopods. They constitute thelargest cephalopod subclass, with 163 fami-lies including the ammonites, in which thesuture lines form very complex patterns; theceratites, in which part of the suture line isfrilled; and the goniatites, with relativelysimple suture lines. They range in age from*Devonian to Upper *Cretaceous. All mem-bers are now extinct. See also aperture; apty-chus; foramen; phragmocone; and venter.ammonoids See ammonoidea.amnion See amniotic.amniotic Applied to a type of developmenttypicalofhigher*vertebrates(*reptiles,*birds,and *mammals), in which the amnion (a pro-tective membrane) surrounds the embryo in abag of (‘amniotic’) fluid. Evolutionarily, the am-nion is *primitively associated with a shell andiscapableofgaseousexchange;itsdevelopmentthus enabled eggs to be laid on dry land forthe first time in vertebrate evolution. Compareanamniotic.amorphous cloud Continuous cover oflow, featureless cloud (e.g. *nimbostratus),often producing rain.amosite See anthophyllite; and asbestos.amperes per metre (A/m) The SI unit ofmagnetic moment per unit volume. 1A/m ϭ103gauss.Ampferer subduction See a-subduction.Amphibia (amphibians) Class that appearedfirst in the *Devonian, having evolved fromrhipidistian (lobe-finned) fish (see rhipidistia).They flourished in the *Carboniferous and*Permian. During the *Triassic some forms,e.g. Mastodontosaurus, grew to 6m long, andthe first modern types were established. Todaythe amphibians are represented by just threegroups, of which the Urodela (salamanders)andAnura(frogsandtoads)arethebestknown(the third group, the caecilians (Apoda), areworm-like and burrowing). Most amphibiansare found in damp environments and theyoccur on all continents except Antarctica.amphibians See amphibia.amphiboles A group of *minerals possess-ing double chains of silicon–oxygen [SiO4]tetrahedra with a composition of [Si4O11]n run-ning parallel to the *crystallographic axis;i.e. parallel to the *prism zone of a crystal.The double chains are held together bymonovalent, divalent, or trivalent cations, ofwhich Naϩ, Ca2ϩ, Mg2ϩ, Fe2ϩ, Al3ϩ, and Fe3ϩarethe most important; hydroxyl ions alsooccur. There are three main groups of am-phibole minerals: (a) calcium-poor amphi-boles with the general formula X2Y5[Z4O11]2(OH,F)2, where X ϭ Mg or Fe2ϩ, Y ϭ Mg, Fe2ϩ,Fe3ϩ, Al3ϩ, etc., and Z ϭ Si or Al; (b) the cal-cium-rich amphiboles with the general for-mula AX2Y5[Z4O11]2(OH,F)2, where A ϭ Na, X ϭCa, Y ϭ Mg, Fe2ϩ, Fe3ϩ, Al, etc., and Z ϭ Si or Al;and (c) the alkali amphiboles in which Na ϾCa and with the general formula AX2Y5[Z4O11]2(OH,F)2, where A ϭ Na or K, X ϭ Na (or Na andCa), Y ϭ Mg, Fe2ϩ, Fe3ϩ, Al, etc., and Z ϭ Si orAl. Calcium-poor amphiboles include the *or-thorhombic amphiboles (called the orthoam-phiboles) and include *anthophyllite andgedrite, but the other two groups are *mono-clinic and include the common *horn-blendes *tremolite and *actinolite, as well asthe sodium-rich varieties such as *glauco-phane and *riebeckite. Amphiboles are com-mon rock-forming silicate minerals thatoccur in *intermediate and *alkaline *ig-neous rocks and also in many regional *meta-morphic rock types.amphibolite A medium-grained, dark-coloured, regional *metamorphic rock com-posed of *hornblende and *plagioclase withminor *epidote, *sphene, *biotite, and*quartz. The rock may show a well-developedplanar or linear alignment of elongate horn-blende crystals as a result of suffering de-formation at the same time as *regionalmetamorphism. These alignments define*fabrics within the rock known as *schistosityand *lineation respectively. Amphibolites areformed by medium-grade *metamorphism of*basic *igneous rocks such as *basalts, both*extrusive and *intrusive types.amphibolite facies A set of metamorphic*mineral assemblages produced by the*metamorphism of a wide range of startingrock types under the same metamorphicconditions and typically characterized bythe development of the mineral assemblageandesine (*plagioclase)–*hornblende in

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