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1. A Dictionary ofEarthSciencesthird editionEdited bymichael
allaby1
2. 3Great Clarendon Street, Oxford ox2 6dpOxford University
Press is a department of the University of Oxford.It furthers the
Universitys objective of excellence in research, scholarship,and
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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
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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
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Publisher Services, Pondicherry, IndiaPrinted in Great Britainon
acid-free paper byClays Ltd, St Ives plcISBN 97801992119441 3 5 7 9
10 8 6 4 2
3. From the Preface to the First EditionSetting intellectual
boundaries that would serve to dene 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
scienticdisciplines.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
dening 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
gures, 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 rst 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 rst editionhas
been scrutinized and many have been amended to bring them up to
date.
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 Oxfords
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-tionarys
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. ContentsContributors and Advisers xiA Dictionary of Earth
Sciences 1appendix a: Stratigraphic Units as Dened 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. 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, Goldsmiths 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. aa See lava.AABW See antarctic bottom water.AAC See
antarctic convergence.Aalenian A *stage in the European
Middle*Jurassic (175.6171.6 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 longerows (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 tted to its
environment as a conse-quence of the characters it inherits,
whichhave been ltered by *natural selection inprevious
environments. Because present en-vironments seldom differ greatly
from recentpast environments, adaptive tness can re-semble
*adaptation. In this sense, however,adaptation appears to imply
advance plan-ning, or design, which is misleading.Abb 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 chiey by air temper-ature, wind
velocity, *humidity, rainfall, and*solar radiation. Ablation on
snowelds isalso inuenced 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. respect to a xed frame of reference. Variousframes of
reference have been used, includ-ing those dened 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 rst appear inthe
regional pollen rain their prolic 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 speciedwavelength. 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 coefcient
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 articial 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 at, deep-oceanoor, commonly
50250m in height and afew kilometres in width. They are most
typi-cal of the *Pacic Ocean oor at depths of30006000m.abyssal
plain Smooth, almost level area ofthe deep-ocean oor 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 oor to about 40cm/s, raisinglarge amounts of
ne 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 oor.
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. a3 accretionary basinAcanthodii (acanthodians) Class of
primi-tive, fossil sh, characterized by the presenceof a true bony
skeleton (see bone), a *heterocer-cal tail *n, a persistent
*notochord, *ganoidscales, and stout spines in front of the ns.
Theacanthodians lived from the *Silurian to the*Permian Period and
may be related to ances-tors of the more modern bony
sh.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 dened 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 foridentication 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 accumulatedrst 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 (1mv2, 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 lls
10. aaccretionary lapilli 4with *debris-ow 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 ne 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 ow processes.accretionary
leve See lava leve.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 ordecit of temperature with respect to adened
mean value and expressed as an accu-mulation over a given period,
e.g. a month,season, or year. For example, a datum valueof 6C is
used as a critical temperature forsustained vegetation growth,
against whichaccumulated surpluses or decits may
bemeasured.accumulation zone That part of a
*glacierwherethemeanannualgainof*ice,*rn,andsnowisgreaterthanthemeanannualloss.Thezone
consists of stratied rn 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 ve 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 MgO), makingthe diagrams
particularly useful for showingassemblage variations in
metamorphosed,*basic, *igneous rocks, and impure *lime-stones.
However, each of these componentshas to be modied slightly to
account for thepresence of other, minor components in therock. Such
modication leads to: A (Al2O3 Na2O K2O); C (CaO [(10/3)P2O5] CO2);
andF (FeO 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 denetriangular 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 pels hair.achondrite Rare
stony *meteorite lacking*chondrules and with low
nickel-ironpassivemarginocean accretionarywedgefore-arc
volcanicarcoceaniclithosphereAccretionary wedge
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 BrnstedLowry 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 Brnsted 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 ve standardranges of soil acidity (less than pH 4.5,
ex-tremely acid; 4.55.0, very strongly acid;5.15.5, strongly acid;
5.66.0, medium acid;and 6.16.5, slightly acid). Surface *soil
hori-zons of acid *brown earths have a reaction ofpH 5.0 or
less.acme zone(peak zone, ood 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 () and the acoustic velocity (v) for agiven
rock mass; Z v. The *reectioncoefcient 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 modied form.Acrisols A reference soil group in the
soilclassication 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,5240m in diameter, inside which
dinoag-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 Mg 0.9
to 0.5, belonging to the*tremolite*ferroactinolite series of
Ca-richamphiboles; sp. gr. 3.03.4; *hardness 56;*monoclinic; light
greenish-grey to darkgreen; white *streak; *vitreous *lustre;
habit*acicular, often brous and felted; *cleavage*prismatic, good
{110}; occurs widely in low-to medium-grade *schists and some
*igneousrocks. The asbestiform variety is called
12. *nephrite and such felted forms were used inthe past for
insulation and re-resistant ma-terials, but the development of
asbestosis inworkers has severely restricted their
use.Actinopterygii (ray-nned sh) A sub-class of the *Osteichthyes
(bony sh, seebone), comprising the ray-nned sh, whichinclude the
majority of living bony sh of seaand fresh water. The *ns are
composed of amembranous web of skin supported by a vary-ing number
of spines and soft rays. Theyappeared rst 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
anarticial 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, Pacic-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 anAndino-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 theexchange or
cycling pool.active remote sensing *Remote sensingwhich is based on
the illumination of a sceneby use of articial 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 coefcient () 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: ( 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:ap/p*.Inanidealsolutiontheac-tivity coefcient 1, and
the activity of thecomponent is equal to its mole fraction.
Ingeneral, the greater the amount of dissolvedmaterial, the lower
the activity coefcients ofeach of the species present.Actonian A
*stage of the *Ordovician(453454Ma 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 sufcient explanationfor
past geomorphological phenomena, al-though the rate of activity of
these processesmay have varied. The theory was rst
clearlyexpressedin1749byG.L.L.*Buffon(170788),and was the essential
principle of *uniformi-tarianism as presented in 1830 by C.
*Lyell(17971875).Actinopterygii 6a
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. Adamis 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 dened. In Britain the
best-known exam-ple occurs at Shap Fell in Cumbria.AdamsWilliamson
equation Equationdescribing a fundamental relationship be-tween
seismic velocities (vp and v), the *grav-itational acceleration
(g), and the adiabaticchange in density (d) within the *Earth
(as-suming only hydrostatic pressure) as a func-tion of radium
(dr):d =gdrn2 (4/3)n2This 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 favourablegenetictraitsbreedmoreprolicallythanthoselacking
these traits (genotypic adaptation), orthey may involve non-genetic
changes in
indi-viduals,suchasphysiologicalmodication(e.g.acclimatization) or
behavioural changes (phe-notypic adaptation). Compare abaptation.
2. Inanevolutionarysense,thatwhichtsanorgan-ism both generally and
specically 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 Darwinsnches in the Galpagos
Islands resultedin a proliferation of *species. 2. Term
usedsynonymously with *cladogenesis by someauthors.adaptive zone
The adaptive specializa-tion(s) that t 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 lters,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 (5421300Ma 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, rst 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. 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 uid, and
with-out any exchange of energy with the sur-rounding uid. 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 Lifor Mg2in the *pyroxenes,*amphiboles, and
*micas.adobe A silty *clay, often calcareous, foundin dry,
desert-lake basins. This ne-grained*sediment is usually deposited
by desertoods 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 (20) (23 20
15km);mass1.91106kg;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.aedichnia 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 y cases,spiders larders consisting of
concentrationsof insects, insect remains, and spiders.aegirine
*Pyroxene mineral, NaFe3Si2O6;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 aegirineaugite. 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 Earths 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. 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 outow 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 ow ofair caused by irregularities in the
surface(which may be of a solid, or of air of differentdensity)
over which the ow takes place.aerological diagram Diagram to
demon-strate variations with height of the
physicalcharacteristicsoftheatmosphere,particularlyits temperature,
pressure, and *humidity.aeromagnetic survey Survey of theEarths
magnetic eld, based on data from*magnetometers towed behind
aircraft or sus-pended below helicopters. These instrumentsmeasure
the total intensity of the *geomag-netic eld or, occasionally,
components of thiseld.Theresultingmeasurementscanthenbecompared
with theoretical models for thevalue of the eld 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 own 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 astinger in the tail. In cases where the
magne-tometerisonboard,in-boardcoilsystemscom-pensate for the
aircrafts own magnetic eld.Aeronian A *stage (436439 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 (0.0110m) 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, reect,or
scatter radiation. Stratospheric aerosols,mainly sulphate particles
resulting from vol-canic *eruptions, may reduce
*insolationsignicantly. About 30% of tropospheric dustparticles are
the result of human activities.See atmospheric structure; mie
scattering;rayleigh scattering; and volcanic dust.Atosauria 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 eld
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 ve 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 Al2O3FeOMgO face fromeither the *muscovite or K-feldspar
point on
16. the Al2O3FeO 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
modied 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) elds
(*sferics)in the audio-frequency range (11000Hz)generated by
thunderstorms to investigatelateral changes in the *resistivity of
theEarths 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 dening 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
(1.30.9Ma) of four*interglacial *stages in N. America, followingthe
*Nebraskan glacial episode, and approxi-mately equivalent to the
*Donau/Gnz inter-glacial of Alpine terminology. Climatically itwas
marked by mild summers and winterswarmer than those in present-day
N. America.Agassiz, Jean Louis Rodolphe (180773)A Swiss geologist
who worked initially on fos-sil sh, 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 sufx
-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 specic 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 negrained. An agglomerate may be the productof a volcanic
explosion and therefore a *pyro-clastic rock, but often the term
agglomerateis applied to brecciated volcanic rocks of un-certain
origin. Those deposits may rangefrom vent *breccias to debris from
mudowor 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 60m.aggradation The general accumulation ofunconsolidated
sediments on a surface,which thereby raise its level. A large range
ofAFMAG EM system 10a
17. a11 AGUmechanisms may be involved, including *u-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, lter beds,
and in some manufactur-ing processes as ux. 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 Specic 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 freezethaw 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 ne
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, sh-likevertebrates,
with sucker-like mouths, includ-ing the extant lampreys, slime-eels
andhagsh,andsomeoftheearliestprimitivever-tebrates, with heavily
armoured forms, e.g.*Cephalaspis (see also osteostraci), Pteraspis
(seeheterostraci), and Jamoytius (see anaspida).They appeared rst
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 identied 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)(14941555) The author of works on geologyand mineral
classication, and of the rstcomprehensive 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. Agulhas current Part of the large-scalecirculation of the
southern Indian Ocean. It isa surface-water current that ows off
the eastcoast of southern Africa between latitudes25S and 40S 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 ight 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 airwater 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)identied 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 modied 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 (180192) 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 onscientic issues created,
for the rst time, therole of a professional scientic civil
servant.Airy model A model to account for *iso-stasy which in the
*lithosphere assumes aconstant density (c 2 670kg/m3), but inwhich
topographic elevations (h) are com-pensated by the presence of
roots replacinghigh-density *mantlerocks(m 3300kg/m3)by
lower-density lithospheric rocks. Thedepth of the root (d) is equal
to hc/( c).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,calledtheAiryphase.Aitken
nuclei counter Device for the esti-mation of the concentration of
particles withradii of more than 0.001m 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,solidparticlewitharadiusoflessthan0.2m.Most Aitken
nuclei are about 0.5m. Onaverage, their concentration varies from
lessthan 1000/cm3over oceans to 150000/cm3inAgulhas current
12amountainoceanconstant densitydense regionAiry model
19. a13 albiteepidoteamphibolite faciesurban areas. See aitken
nuclei counter; andnucleus.AIV See aggregate tests.AIW See
antarctic intermediate water.kermanite See melilite.akl 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). Akl 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
rst 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 at oor, which maybe occupied by
a lake. Alases are well devel-oped in Siberia (the word alas is of
Yakutianorigin) where they can occupy 4050% of theland
surface.Alaska current Oceanic water boundarycurrent produced by
the deection of the*N. Pacic current by the N. American conti-nent.
It ows 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
7090km. The baselies on the *mantle and this boundary is
the*Mohorovicic discontinuity.albedo The proportion of *insolation
that isreectedbackfromtheEarth,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 is827% (over 30%
for yellowing deciduous for-est in autumn); for cities and rock
surfaces,1218% (over 40% for chalk and light-colouredrock and
buildings); for sand up to 40%; forfresh, at 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
classication 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 (501513 Ma ago) of
theMiddle *Cambrian of N. America, equivalentto the *St
Davids.Albian *Stage (99.6112Ma 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.albiteepidoteamphibolite facies Aset of
metamorphic *mineral assemblagesthat is produced by the
*metamorphism ofa wide range of initial rock types underthe same
metamorphic conditions, and iswinddirectionAkl dune
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 specic mineral assemblage, eventhough they are all being
metamorphosedunder the same conditions. The variation ofmineral
assemblage with starting rock com-position reects a particular
range of pres-sure, temperature, and P(H2O). Experimentalstudies of
mineral PT stability elds 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 ne 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 nal
stagesof crystallization of a *granite body. Thisvapour, which can
carry high concentrationsof Nain 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
4SiO2 2Na2NaAlSi3O8 Ca2; anorthite quartz sodium (in aqueous
solution) albite 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-oor *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 (3336 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
ows westwards southof the Aleutian Islands and parallel to,
butnorth of, the *N. Pacic current. The watermass is a mixture of
water from the*Kuroshio and *Oyashio currents. See alsoalaska
current.Aleutian low Region of the N. Pacic, 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 Pacic equivalent of
*Ice-land low, used in the Atlantic.Aleutian Trench The oceanic
*trenchwhich marks the boundary between the*N. American Plate and
the *Pacic Plate. The*subduction of the Pacic 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 theLowerMiddle
*Llandovery.alexandrite See chrysoberyl.Alsols (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.Alfvn waves Magnetohydrodynamicwaves that are produced
by coupling forcesbetween the *geomagnetic eld and highlyconductive
uids. Alfvn waves travel alongmagnetic eld lines when jets of
highlyconductive uid or charged particles owacross the eld lines.
The waves were discov-ered by the Swedish astrophysicist HannesOlof
Gsta Alfvn (190895).albite twin 14a
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-pendexterclassication.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 insufcient 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 lter can beapplied, which
removes frequency compo-nents above the *Nyquist frequency.Alisols
A reference soil group in the soilclassication 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.alkaliaggregate
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 ne-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. 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 nal temperature, a range ofperthites may result,
from coarse (*perthite),representing perthites formed during a
largedrop in temperature, to ne (*microperthite),and nally to very
ne (*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 900C. 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 7.47.8 asmildly alkaline;
7.98.4 as moderately alka-line; 8.59.0 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 (014) 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.3.44.2; *hardness 5.06.5;
*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. a17 allopatric speciationsame relative position (locus) on
homologous*chromosomes.allelomorph Term that is commonly short-ened
to *allele.Allens rule A corollary to *Bergmannsrule and *Glogers
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.Allerd 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 ora 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 classication of *Folk,
allochemsare often found together with a *carbonatemud *matrix
(*micrite) and may subse-quently have *pore spaces lled by
sparry*calcite *cement (*sparite). Common al-lochems include
skeletal fragments (*bio-clasts), *ooids, *peloids, and
*intraclasts.allochemical A *limestone dened by the*Folk
classication as comprising *allochemswith either a sparry *calcite
*cement(*sparite), or a *microcrystalline *calcite (*mi-crite)
*matrix. Limestones lacking allochemsare dened by Folks
classication 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 *massow, particularly used in relation to *lime-stones deposited
by mass ow.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 sufcient 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. 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 dened and identiedfrom 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 oodplain. Theterm is also applied to surface
ow, 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
6.57.5; *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.alnite An *intrusive, *basic, *igneous,*carbonatite rock,
distinctive in possessingprimary *calcite, and consisting of
*melilite(1); *biotite (1); and *pyroxene, calcite, and*olivine
(1). *Feldspar is not present in therock, its place being taken by
the mineralmelilite which has the general formula:X2YZ2O7; where X
Ca, Na; Y Mg, Al; Z Si,Al. The type location for this rock is Aln
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 2. 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.alphaprotonX-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 Pathnder
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
alphaproton 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. 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 nally becomes purplish. The
same se-ries of colours in reverse order is seen onmountains in the
west at sunrise.AlpineHimalayan 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, reectits
outer effects.Alportian The nal stage (318.1324.5 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-eld 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 cloudclassication.Altonian A *stage (16.517.5
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, brous, or
uniform. It may becomposed of ice crystals as well as
waterdroplets. See also cloud classication.alumstone See
alunite.alunite (alumstone) *Mineral, KAl3(SO4)2(OH)6; sp. gr.
2.62.8; *hardness 3.54.0; *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 difcult 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(262 146 134km), the irregular shape
sug-gesting a rigid body. Its mass is 7.17 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
theHartmannTanaka Model and 3.55Gy to thepresent in the NeukumWise
Model, and di-vided into three epochs: Lower Amazonian(1.800.70 or
3.552.50Gy); Middle Amazon-ian (0.700.25 or 2.500.70Gy); and
UpperAmazonian (0.250.00 or 0.700.00Gy).