SPECIAL FEATURE: ARCHAEOLOGY

www.iop.org/journals/physed

Archaeomagnetic datingPaul Linford

English Heritage, Centre for Archaeology, Fort Cumberland, Fort Cumberland Road,Eastney, Portsmouth PO4 9LD, UK

E-mail: paul.linford@english-heritage.org.uk

AbstractSome naturally occurring minerals possess a permanent magnetization.Certain processes such as sedimentation or kiln-firing can cause the particlesin structures made of such materials to align themselves with the direction ofEarth’s magnetic field at the time. This direction has varied over the last fewthousand years in ways that can be traced from known records, so it providesa means of dating such structures if they have remained in their originalorientation—as a kiln would, for example.

IntroductionArchaeomagnetic dating is less frequently usedthan other physical dating techniques, suchas carbon-14 dating, owing to limitations onthe materials to which it can be applied.Nevertheless, it can provide valuable, oftenprecise, chronological evidence for archaeologicalfeatures, particularly where no suitable remainsare present to be dated by other means. It dependsupon two important physical phenomena:

(i) The Earth spontaneously generates amagnetic field, which fluctuates in intensityand direction with time.

(ii) Under certain conditions naturally occurringmagnetic minerals can become permanentlymagnetized according to the magnetic fieldpertaining at that time.

The Earth’s magnetic fieldThe details of the mechanism by which the Earth’smagnetic field is generated are not completelyunderstood. It appears to be associated witha region 3000 km beneath the planet’s surfacein the outer core, which is composed of slowlychurning molten iron and nickel. This layeris trapped between the solid inner core at thecentre of the planet and the mantle, another

solid layer extending from 3000 km to about40 km beneath the surface. It is now generallyaccepted that free electron circulation within theconvecting outer core creates the magnetic field,which behaves as a self-sustaining dynamo. Thefluid motions that drive this dynamo derive fromthe Earth’s rotation along with gravitational andthermodynamic effects in and around the core.

This results in an approximately dipolarmagnetic field at the Earth’s surface, as if a largebar magnet was situated at its centre with itslong axis aligned almost parallel with the Earth’srotational axis (figure 1). Near the equator thedirection of the field lines is horizontal (parallelto the surface of the Earth); however, as one ofthe two magnetic poles is approached they riseout of or dip into the ground at an increasinglysteep angle. This angle is known as the angle ofdip or inclination. The positions of the Earth’smagnetic poles do not exactly coincide with itsgeographic poles, and at present the axis of thedipolar field is inclined at 11.5◦ to the Earth’srotational axis. Because of this, the directionindicated by a compass needle at an arbitrary pointon the Earth’s surface will generally deviate fromthe direction of geographic, or true, north. Theangle in the horizontal plane between magneticnorth and true north is known as the magnetic

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Figure 1. The Earth’s main dipolar magnetic field is depicted with dashed lines. This is generated by electriccurrent circulation in the outer core (shown in red) and is similar to the field that would be produced by a barmagnet located at the Earth’s centre tilted off-vertical by about 11.5°. Eddy currents near the core/mantle boundaryperturb this main field. The angle of dip (or inclination) is the angle that the field lines make with the horizontalplane where they cut the Earth’s surface.

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declination. Over geologic timescales the positionof the magnetic poles appears to precess about thegeographic poles. This movement is caused by thesame forces that generate the Earth’s field.

Superimposed upon the generally circulatorymovement of the magnetic poles is an apparentlyrandom element known as secular variation.This is believed to be due to eddy currents inthe Earth’s fluid core and to the movement ofcharged particles in the upper atmosphere. Asa result, the magnetic pole position calculatedfrom measurements of declination and inclinationat one point on the Earth’s surface will notmatch exactly that calculated from measurementsmade at another position. For archaeomagneticdating, this has the consequence of requiring thecompilation of separate calibration curves of thevariation of the Earth’s field with time for differentregions, each about 1000 km in diameter.

As well as changes in the position of themagnetic poles, the intensity of the Earth’s fieldvaries with time. Evidence from magnetizationsrecorded in igneous rocks indicates that periodi-cally, every few hundred thousand years, the fieldintensity decreases to nothing then increases againwith the polarity reversed (i.e. the north and southpoles change places) (Hoffman 1988). Again,a random component of variation is superposedon this trend over shorter timescales. The fieldintensity determines the strength of the attractionof a compass needle to the magnetic poles and thestrength of magnetization acquired by magneticminerals.

Remanent magnetismSome naturally occurring minerals are ferrimag-netic, possessing a permanent or remanent mag-netization. By far the most prevalent of these are

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Figure 2. Thermoremanent magnetization. Initiallymagnetic domains within a sample are magnetized inrandom directions that cancel out (top picture). As thesample is heated the domains demagnetize as thetemperature exceeds their blocking temperatures(second and third pictures down). On cooling, thedomains remagnetize in a direction close to theprevailing ambient magnetic field, resulting in a netmagnetization within the sample (bottom two pictures).

the iron oxides, haematite, maghaemite and mag-netite, which occur in most soils, clays and as tracecomponents in many types of rock (Thompson andOldfield 1986). In crystals of these minerals, quan-tum mechanical exchange interactions betweenneighbouring atoms force all the unpaired electronmagnetic moments to align, resulting in a net spon-taneous magnetization (Tauxe 2002). Such align-ment will occur over a region of the crystal knownas a magnetic domain and the shape and size ofthese domains will depend on the structure andsize of the crystal as well as the impurities withinit. Each may correspond to one physical grain ofthe mineral crystal (as occurs in haematite), or asingle grain may be divided into several magneticdomains (as can occur in magnetite). Magneti-zation within each domain will be in one of thetwo directions parallel to its easy axis, a directiondetermined by domain shape and the underlyingcrystal structure.

As each domain will typically be magnetizedin a different, randomly orientated, direction, amacroscopic sample of the mineral containinga large number of domains will usually exhibitno net magnetization. However, if the mineralis heated, thermal agitation of the crystalstructure leads to a diminution of the spontaneousmagnetization in each domain until, at a certaincritical temperature, known as the blockingtemperature, it disappears entirely (figure 2).On cooling, each domain will remagnetize inone of the two directions parallel to its easyaxis: that adopted will be the one closest to anyambient magnetic field direction. Although mostindividual domains’ magnetizations will not beexactly aligned with the ambient field direction,they will tend to favour it on average. Thus,after heating, the mineral will exhibit a netthermoremanent magnetization in the direction ofthe prevailing magnetic field at the time it cooled.

The blocking temperatures of differentmagnetic domains vary. Maximum blockingtemperature is limited by the Curie temperatureof the particular mineral involved (585 ◦Cfor magnetite and 675 ◦C for haematite) butconsiderations of grain size and crystal structurecan reduce it below this limit. Indeed even withoutheating, and in the absence of an external magneticfield, some domains will spontaneously reversetheir directions over time. In the idealized casethe probability, p, that this will happen varies as

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(Tauxe 2002, Neel 1955)

p = e−t/τ

where

τ = 1

CeKv/kT .

Here C is a frequency factor around 1010 s−1,K is an anisotropy constant, v is the volume ofthe magnetic domain, k is Boltzmann’s constantand T is the ambient temperature. The blockingtemperature is defined as that temperature whichresults in a value of τ between 102 and 103

seconds. Naturally occurring samples of rocksand clays will contain a heterogeneous mineralcomposition and thus a spectrum of blockingtemperatures. To be capable of retaining astable thermoremanent magnetization, they mustcontain a high proportion of magnetic domainswith blocking temperatures above 200–300 ◦C.

In addidion to thermoremanent magnetiza-tion, a second mechanism, called depositionalremanent magnetization, also occurs in archaeo-logical contexts. This involves water-borne sedi-ment particles that possess a weak overall mag-netization, often because they are composed ofthermoremanently magnetized minerals (althoughchemical effects during crystal growth can alsoresult in remanent magnetization). If suspendedin still water, they will attempt to rotate so thattheir directions of magnetization align with theprevailing magnetic field (figure 3). As gravita-tional forces pull the particles to the bed of thebody of water they will form a layer magnetizedin the direction of this ambient field. As moresediment accumulates above this layer, frictionalforces caused by its weight lock the magnetizedparticles in place so that they are no longer free torotate and realign themselves. This situation canoccur when alluvial sediments accumulate on rel-atively still lakebeds and a stratigraphic sequenceof magnetic layers results, recording changes inthe Earth’s magnetic field over time.

Archaeomagnetic dating techniquesIf the past history of the changes in theEarth’s magnetic field is known, there are twoprincipal ways to date an archaeological artefact,structure or deposit that has acquired a remanentmagnetization at some time in the past.

Figure 3. Depositional remanent magnetization.Sediment particles, each with a weak magnetization,settle out of still water. As they fall through the watercolumn they rotate to align their internal magnetizationdirections with the Earth’s field (upper three pictures).Once settled on the bed of the body of water, the weightof sediment accumulating on top of the particles locksthem in place, leaving a layer magnetized in the directionof the Earth’s field.

The first method is to infer from the strengthof the magnetization in the artefact, the intensity

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Figure 4. Movement of the apparent position of the Earth’s magnetic north pole with time, based upon archaeomagneticmeasurements made in the UK. The picture on the left shows movement from the Bronze Age until 500 AD, theright-hand picture shows movement since then.

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of the Earth’s field at the time that it acquiredits remanence. As the direction of the field isnot involved, this has the attraction that it allowsportable archaeological objects such as potsherdsto be dated. A technique for doing this wasdeveloped by Thellier and Thellier (1959). Thisinvolves repeatedly heating the sample in a zerofield to a number of increasing temperature stagesand measuring the intensity of the magnetizationremaining after each stage. Once all themagnetization is removed, the process is repeatedbut with the sample exposed to a known referencemagnetic field during each heating. From acomparison of the results it is possible to infer thestrength of the Earth’s field at the time the samplewas originally heated in antiquity. However, manymeasurements are required for this technique,as compared with directional dating, and moresources of error are involved. These can arisefrom: sample inhomogeneity, sample anisotropy,differences between the original and laboratoryfiring atmosphere and differences between theoriginal and laboratory heating and cooling rates(Tarling 1983).

Given these problems, most archaeomagneticresearch in the UK has been directed towards thedevelopment of a technique involving the directionof the Earth’s magnetic field. This involvesestablishing the magnetic north pole positionindicated by the declination and inclination of

the field within the sample and determining whenin the past magnetic north was in that position.Clearly, the artefact must have remained in exactlythe same position as it was when it acquiredits remanence, limiting the types of object thatcan be dated to non-portable structures. To thisend, analysis of lake sediment data (depositionalremanence), a large number of well datedarchaeological structures (thermoremanence), aswell as direct compass measurements from thelast 400 years, has led to the construction ofthe United Kingdom archaeomagnetic calibrationcurve (Clark et al 1988).

Based upon these data, figure 4 shows thevariation in the apparent position of the magneticnorth pole as viewed from the United Kingdomover the past 3000 years. Thus, in principle,the date at which an unmoved archaeologicalobject acquired its remanence can be inferredby measuring the declination and inclination ofits magnetization, determining the correspondingmagnetic pole position and comparing this withthe dated timeline.

What can be dated?

Directional archaeomagnetic dating imposes threeconstraints on the types of archaeological featuresthat can be dated. They must

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Figure 5. Base of a Roman pottery kiln (∼1.5 m indiameter) constructed of fired clay, discovered atHeybridge in Essex and dated to the second centuryAD.

(i) contain magnetic minerals capable ofcarrying a stable remanent magnetization,

(ii) have experienced a remanence-inducingevent at some time in their history (e.g.heating above a blocking temperature ornon-turbulent sediment deposition),

(iii) have remained undisturbed since acquiringthe remanence so that its direction is stillmeaningful.

Hence, it is mostly fired structural features thatare suitable for analysis. Remains of furnaces andkilns are best suited. These are typically composedof clay, tile or brick, all of which usually containsuitable magnetic minerals. Furthermore, duringtheir operation, these features reach temperaturesin excess of 700 ◦C, above the Curie temperaturesof all the remanence-carrying minerals. Forexample, figure 5 shows the base of a Roman kilnconstructed of fired clay discovered at Heybridgein Essex. However, it is not always necessaryfor such high temperatures to be reached and theremains of domestic hearths and ovens can oftenbe dated, even though they tend to possess weakermagnetizations. The example shown in figure 6is a medieval hearth composed of ironstone fromBurton Dasset in Warwickshire. Similarly, burntor heated natural soil that has lain beneath a fireor fired structure can also be suitable in someinstances. Dates have been obtained from the firedclay soil beneath Tudor glass-making furnaces atBagot’s Park in Staffordshire (figure 7). At thissite, the remains of the furnaces themselves hadbeen removed in the 1960s to allow the area to beploughed. With all thermoremanent features it is

Figure 6. Medieval hearth constructed of verticallystacked tiles at Burton Dasset in Warwickshire.Archaeomagnetism demonstrated that it was last usedat the time of the documented abandonment of thesettlement in the late fifteenth century.

important to bear in mind that each time they arefired their magnetization will be reset. Hence, theevent dated by archaeomagnetic analysis will bethe final firing of the feature.

Features possessing depositional remanenceare less commonly encountered. However, wherea waterlogged ditch has filled due to slowaccumulation of sediment, such as the exampleshown in figure 8 from Yarnton in Oxfordshire, itis sometimes possible to date the time at which thesediment accumulated. Here, it was establishedthat a prehistoric drainage ditch fell out of use andsilted up during the Iron Age. Pictured in figure 9is a Palaeolithic sequence of marine sands fromBoxgrove in West Sussex. Here a stratigraphicallyrelated sequence of archaeomagnetic directionswas obtained, showing the changes in directionof the Earth’s field during the time over which thesediment layers accumulated (∼500 000–200 000BP). This sequence is too old to date bycomparison with present UK calibration databut it has established that the site is less than780 000 years old (when the last magnetic polarityreversal occurred). With all depositional remanent

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Figure 7. Fired clay soil originally beneath a Tudorglassmaking furnace at Bagot’s Park in Staffordshire.Archaeomagnetic dates on soils beneath 15 suchfurnaces have contributed to knowledge of theeconomics of glassmaking in 16th century England.

Figure 8. Ditch section at Yarnton in Oxfordshire.Archaeomagnetic analysis showed that the sedimentfilling the ditch accumulated between 200 and 100 BC,indicating that it had fallen out of use by this time.

magnetizations the event being dated is the timewhen the sediment particles became locked intoposition within the sediment column. This is

Figure 9. Marine sediment sequence laid down duringthe Palaeolithic period at Boxgrove in West Sussex.Sampling from different heights within the sequencehas revealed a history of magnetic field changes overthousands of years.

often tens of years after they first settled out ofsuspension and can be much longer (Tarling 1983).Thus archaeomagnetism is most useful for datingolder, prehistoric, sediments where such time lagsare less significant.

Sampling and measurement procedureSince the direction of magnetization within anarchaeological feature must be measured relativeto true north and the horizontal plane, it isnecessary to orient each sample before it isextracted or moved. For well consolidated featuresthis is typically done by attaching a horizontallylevelled marker to the sampling position withepoxy resin (figure 10). Levelling is achievedvia a bull’s-eye spirit level whilst the resin sets,then an arrow is marked onto the disk denotingthe direction of true north. A gyro-theodolite can

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Figure 10. Sampling archaeomagnetic features.Horizontally levelled markers are attached to thematerials to be extracted in the top picture. In thebottom picture, the true north direction is beingtranscribed onto each marker with the aid of agyro-theodolite.

be used for this (a device capable of finding thedirection of true north using the precession of abuilt-in gyroscope), although a magnetic or suncompass bearing will often suffice. Once theseprocedures have been completed the sample canbe removed. Less well consolidated sediments

are usually sampled by enclosing a short pillar ofsediment within a specially manufactured plasticcylinder which can be oriented as above, thenremoved and sealed. Ten to twenty samples mustbe extracted from different parts of each featureto be dated, to average out random perturbationsin the recorded magnetization direction caused bymaterial inhomogeneity and other factors.

Laboratory measurement is usually carriedout using a spinner magnetometer in whichsamples are spun within a pickup coil or ringfluxgate (figure 11). In such magnetometersthe sample is placed in a magnetically shieldedmeasurement chamber to exclude the influence ofexternal magnetic fields. The sample sits on aplatform atop a shaft, which is then turned at afixed speed to rotate the sample about its verticalaxis. The rotating magnetic field caused by thesample’s magnetization generates an electricalcurrent in collecting coils wound around themeasurement chamber, using the same principleas a dynamo. The magnitude of the currentgenerated is proportional to the strength of thesample’s magnetization in the horizontal plane.By remeasuring the sample in three orthogonalorientations it is possible to determine the totaldirection and strength of its magnetization.

For rapid measurement of very weaklymagnetized samples, palaeomagnetic laboratoriesemploy cryogenic SQUID magnetometers. Intro-duction of a magnetized sample into a supercon-ducting ring causes a persistent current to flow thatis proportional to the magnetic moment along theaxis of the ring. Again, three measurements inorthogonal orientations are needed to completelydetermine the magnetization direction. Moreinformation about these and other types ofmagnetometer can be found in Collinson (1983).

Dating precision and limitationsWith the present calibration data for the UK it ispossible to date archaeological features to withinabout 50 years with 95% confidence. However,the potential precision of the technique varies withthe rate at which the magnetic pole position wasmoving at different periods. Where movementwas rapid, features can be dated to within a smallertime window than in periods where movementwas slow. This places a limit on the relativeprecision with which the dates of features fromdifferent periods can be determined, regardless

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Figure 11. Spinner magnetometer used to measure the magnetization within a sample by placing it on a rotatingplatform inside a measuring coil or ring fluxgate. The current generated by the rotating magnetic dipole withinthe sample will be proportional to the strength of its magnetization.

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of the quality of the calibration evidence. Inaddition, it also appears that the magnetic pole hasreoccupied the same positions at different timesover the last 3000 years. This means that thecalibration curve crosses itself and can lead touncertainty as to which of two dates is correctfor a particular remanence direction. At present,consideration of related archaeological evidenceis often used to resolve such dilemmas, but itis hoped that current research will allow fieldintensity measurements to be used to distinguishbetween the possibilities in future.

ConclusionThis article has briefly overviewed magnetismas an archaeological dating tool; however, ofnecessity, much information has been omitted.More detailed discussions can be found in Aitken(1990) and, at a more technical level, in Tarling(1983). Magnetism is also used in other ways inarchaeology. The use of portable magnetometersas a prospecting tool is now well establishedin British archaeology, and magnetic analysis ofsediments is increasingly being applied to studyenvironmental changes in antiquity (Thompsonand Oldfield 1986). Magnetic properties have

also been used to determine the provenanceof obsidian (McDougall and Tarling 1983) andceramics (Rasmussen 2001) and remanence andother magnetic properties have been used todetermine the firing temperatures of pottery (Coeyet al 1979). This is far from an exhaustive list ofthe possibilities that have already been investigatedand new applications of archaeomagnetism willdoubtless emerge in the future.

Related websites

• C M Batt: AARCH—ArchaeomagneticApplications for the Rescue of CulturalHeritagewww.bradford.ac.uk/acad/archsci/aarch/

• G A Glatzmaier: The Geodynamowww.es.ucsc.edu/∼glatz/geodynamo.html

• S Lengyel: North AmericanArchaeomagnetismwww.u.arizona.edu/∼slengyel/amag2 new-sw 001.htm

• R Nave: Magnetic Field of the Earthhyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html

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• Scientific American: Ask the Experts:Geology: Why does the Earth’s magneticfield flip...?www.sciam.com/askexpert question.cfm?articleID=00010D87-BB04-1C71-9EB7809EC588F2D7&pageNumber=1&catID=3

Received 14 October 2003, in final form 19 December 2003PII: S0031-9120(04)70307-XDOI: 10.1088/0031-9120/39/2/002

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McDougall I and Tarling D H 1983 The magneticsourcing of obsidian samples from Mediterraneanand Near Eastern sources J. Arch. Sci. 10 441–52

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Tarling D H 1983 Palaeomagnetism: Principles andApplications in Geology, Geophysics andArchaeology (London: Chapman and Hall)

Tauxe L 2002 Paleomagnetic Principles and Practice(Dordrecht: Kluwer Academic)

Thellier E and Thellier O 1959 Sur l’intensité duchamp magnétique terretre dans le passéhistorique et géologique Ann. Géophys. 15285–376

Thompson R and Oldfield F 1986 EnvironmentalMagnetism (London: Allen & Unwin)

Paul Linford has worked for the EnglishHeritage Ancient MonumentsLaboratory, now part of the Centre forArchaeology, for 17 years and is jointhead of the Geophysics Team. Hisspecialisms are archaeomagnetic dating,electrical and magnetic prospecting andgeophysical data processing.

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