Archaeomagnetic Dating

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SPECIAL FEATURE: ARCHAEOLOGYwww.iop.org/journals/physed

Archaeomagnetic datingPaul LinfordEnglish Heritage, Centre for Archaeology, Fort Cumberland, Fort Cumberland Road, Eastney, Portsmouth PO4 9LD, UK E-mail: paul.linford@english-heritage.org.uk

Abstract Some naturally occurring minerals possess a permanent magnetization. Certain processes such as sedimentation or kiln-ring can cause the particles in structures made of such materials to align themselves with the direction of Earths magnetic eld at the time. This direction has varied over the last few thousand years in ways that can be traced from known records, so it provides a means of dating such structures if they have remained in their original orientationas a kiln would, for example.

IntroductionArchaeomagnetic dating is less frequently used than other physical dating techniques, such as carbon-14 dating, owing to limitations on the materials to which it can be applied. Nevertheless, it can provide valuable, often precise, chronological evidence for archaeological features, particularly where no suitable remains are present to be dated by other means. It depends upon two important physical phenomena: (i) The Earth spontaneously generates a magnetic eld, which uctuates in intensity and direction with time. (ii) Under certain conditions naturally occurring magnetic minerals can become permanently magnetized according to the magnetic eld pertaining at that time.

The Earths magnetic eldThe details of the mechanism by which the Earths magnetic eld is generated are not completely understood. It appears to be associated with a region 3000 km beneath the planets surface in the outer core, which is composed of slowly churning molten iron and nickel. This layer is trapped between the solid inner core at the centre of the planet and the mantle, another0031-9120/04/020145+10$30.00

solid layer extending from 3000 km to about 40 km beneath the surface. It is now generally accepted that free electron circulation within the convecting outer core creates the magnetic eld, which behaves as a self-sustaining dynamo. The uid motions that drive this dynamo derive from the Earths rotation along with gravitational and thermodynamic effects in and around the core. This results in an approximately dipolar magnetic eld at the Earths surface, as if a large bar magnet was situated at its centre with its long axis aligned almost parallel with the Earths rotational axis (gure 1). Near the equator the direction of the eld lines is horizontal (parallel to the surface of the Earth); however, as one of the two magnetic poles is approached they rise out of or dip into the ground at an increasingly steep angle. This angle is known as the angle of dip or inclination. The positions of the Earths magnetic poles do not exactly coincide with its geographic poles, and at present the axis of the dipolar eld is inclined at 11.5 to the Earths rotational axis. Because of this, the direction indicated by a compass needle at an arbitrary point on the Earths surface will generally deviate from the direction of geographic, or true, north. The angle in the horizontal plane between magnetic north and true north is known as the magneticPHYSICS EDUCATION 39 (2)

2004 IOP Publishing Ltd

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magnetic north pole angle of dip, I

geographic north pole

S N

eddy current

currents generating main dipole field mantle fluid outer core

Figure 1. The Earths main dipolar magnetic field is depicted with dashed lines. This is generated by electric current circulation in the outer core (shown in red) and is similar to the field that would be produced by a bar magnet located at the Earths centre tilted off-vertical by about 11.5. Eddy currents near the core/mantle boundary perturb this main field. The angle of dip (or inclination) is the angle that the field lines make with the horizontal plane where they cut the Earths surface.

declination. Over geologic timescales the position of the magnetic poles appears to precess about the geographic poles. This movement is caused by the same forces that generate the Earths eld. Superimposed upon the generally circulatory movement of the magnetic poles is an apparently random element known as secular variation. This is believed to be due to eddy currents in the Earths uid core and to the movement of charged particles in the upper atmosphere. As a result, the magnetic pole position calculated from measurements of declination and inclination at one point on the Earths surface will not match exactly that calculated from measurements made at another position. For archaeomagnetic dating, this has the consequence of requiring the compilation of separate calibration curves of the variation of the Earths eld with time for different regions, each about 1000 km in diameter.146PHYSICS EDUCATION

As well as changes in the position of the magnetic poles, the intensity of the Earths eld varies with time. Evidence from magnetizations recorded in igneous rocks indicates that periodically, every few hundred thousand years, the eld intensity decreases to nothing then increases again with the polarity reversed (i.e. the north and south poles change places) (Hoffman 1988). Again, a random component of variation is superposed on this trend over shorter timescales. The eld intensity determines the strength of the attraction of a compass needle to the magnetic poles and the strength of magnetization acquired by magnetic minerals.

Remanent magnetismSome naturally occurring minerals are ferrimagnetic, possessing a permanent or remanent magnetization. By far the most prevalent of these areMarch 2004

Archaeomagnetic dating

Figure 2. Thermoremanent magnetization. Initially magnetic domains within a sample are magnetized in random directions that cancel out (top picture). As the sample is heated the domains demagnetize as the temperature exceeds their blocking temperatures (second and third pictures down). On cooling, the domains remagnetize in a direction close to the prevailing ambient magnetic field, resulting in a net magnetization within the sample (bottom two pictures).March 2004

the iron oxides, haematite, maghaemite and magnetite, which occur in most soils, clays and as trace components in many types of rock (Thompson and Oldeld 1986). In crystals of these minerals, quantum mechanical exchange interactions between neighbouring atoms force all the unpaired electron magnetic moments to align, resulting in a net spontaneous magnetization (Tauxe 2002). Such alignment will occur over a region of the crystal known as a magnetic domain and the shape and size of these domains will depend on the structure and size of the crystal as well as the impurities within it. Each may correspond to one physical grain of the mineral crystal (as occurs in haematite), or a single grain may be divided into several magnetic domains (as can occur in magnetite). Magnetization within each domain will be in one of the two directions parallel to its easy axis, a direction determined by domain shape and the underlying crystal structure. As each domain will typically be magnetized in a different, randomly orientated, direction, a macroscopic sample of the mineral containing a large number of domains will usually exhibit no net magnetization. However, if the mineral is heated, thermal agitation of the crystal structure leads to a diminution of the spontaneous magnetization in each domain until, at a certain critical temperature, known as the blocking temperature, it disappears entirely (gure 2). On cooling, each domain will remagnetize in one of the two directions parallel to its easy axis: that adopted will be the one closest to any ambient magnetic eld direction. Although most individual domains magnetizations will not be exactly aligned with the ambient eld direction, they will tend to favour it on average. Thus, after heating, the mineral will exhibit a net thermoremanent magnetization in the direction of the prevailing magnetic eld at the time it cooled. The blocking temperatures of different magnetic domains vary. Maximum blocking temperature is limited by the Curie temperature of the particular mineral involved (585 C for magnetite and 675 C for haematite) but considerations of grain size and crystal structure can reduce it below this limit. Indeed even without heating, and in the absence of an external magnetic eld, some domains will spontaneously reverse their directions over time. In the idealized case the probability, p , that this will happen varies asPHYSICS EDUCATION

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(Tauxe 2002, N eel 1955) p = et/ where = 1 Kv/kT . e C

Here C is a frequency factor around 1010 s1 , K is an anisotropy constant, v is the volume of the magnetic domain, k is Boltzmanns constant and T is the ambient temperature. The blocking temperature is dened as that temperature which results in a value of between 102 and 103 seconds. Naturally occurring samples of rocks and clays will contain a heterogeneous mineral composition and thus a spectrum of blocking temperatures. To be capable of retaining a stable thermoremanent magnetization, they must contain a high proportion of magnetic domains with blocking temperatures above 200300 C. In addidion to thermoremanent magnetization, a second mechanism, called depositional remanent magnetization, also occurs in archaeological contexts. This involves water-borne sediment particles that possess a weak overall magnetization, often because they are composed of thermoremanently magnetized minerals (although chemical effects during crystal growth can also result in remanent magnetization). If suspended in still water, they will attempt to rotate so that their directions of magnetization align with the prevailing magnetic eld (gure 3). As gravitational forces pull the particles to the bed of the body