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archaeomagnetism

Index

The field of study concerned with ancient geo-magnetic phenomena and the
use of archaeological material in determining past variation in the
earth's magnetic field is called archaeomagnetism. These variations have
been recorded in London, Paris and Rome over the past four centuries. The
fact of these variations in the earth's magnetic field from time to time
in direction and intensity are the basis of archaeomagnetic dating. This
magnetism occurs naturally and is called fossil magnetism. (Tarling 145)

A main property in these magnetic particles in rocks is that they may
acquire a remnant at some specific points in time in space. Since the
earth's geometric field changes in direction and intensity in time and
space the time of the acquisition can be determined by the comparison of
the magnetic particles with already existing records of the past geometric
field at the locality of which the specimen was found.

The values of declination, inclination and intensity define the magnetic
field at any point on the earth's surface. These readings are by no means
permanently fixed. The earth's magnetic field is indeed dynamic and does
shift. At present the declination for London changes by approximately
1degree every decade. The angle of dip is also subject to shifting.

When past variations of the earth's geomagnetic field have already been
established, archaeomagnetic dating can be as accurate as the original
dating of the record itself because of the comparison of the magnetic
properties in the specimen and that of the geomagnetic field at that
historic time to the geomagnetic field at the same place at the present
time. But even when these records are absent archaeomagnetic dating is
usually possible. This can be achieved by identifying differences in the
magnetic parameters of the geomagnetic field of the two times. If these
differ and are related to the past geomagnetic field, then the samples are
of different age, while similar parameters of the geomagnetic field would
indicate a similar age between the two specimens.

This method is fine for relative dating, but in order to have more exact
dates, the long term changes in the earth's geomagnetic field, also called
secular variations, or "Master Curves" (Tarling p. 149)of dated secular
changes need to be constructed by finding the rate of change of the
geomagnetic field from archaeomagnetic studies of materials that have
already been dated by other means. Now because observatory studies of the
geomagnetic field only extend back for 400 years at the most, only
relatively recent material can be dated by direct comparison. So for older
specimens, archaeomagnetic dates are determined by finding a rate of the
geomagnetic field by comparing the pole location of an archaeomagnetic
sample with a master curve of a polar movement that has been constructed
from independently dated material with mutually geomagnetic poles.

Therefore, an archaeomagnetic date depends not only on the collected
sample and curve summary, but also on the set of independently dated pole
positions that go into making the curve.

Magnetic information is also recorded in ferromagnetic elements in baked
clay which have kept their position on cooling from the last firing of the
clay. This means that baked clay, used for thousands of years in the
construction of hearths, ovens and kilns, contains a weak but permanent
magnetization which can be measured to determine the magnetic intensity
and declination at the time of its last cooling. The thermoremnent
magnetism (TRM) of baked clay is gained from the magnetic properties of
magnetite and hematite, iron-oxides that make up on the average of 6-8% of
the earth's crust. In raw clay magnetic particles of these minerals are
aligned to form magnetic domains, or crystals.

Before clay is baked these domains (which may be thought of as tiny bar
magnets) are orientated in random directions. They do, however, tend to
balance each other out magnetically by turning domains next to them in
opposing directions, thus, balancing out the magnetic effect of the lump
of clay. If the temperature is raised to over several hundred degrees
Celsius, the thermal agitation of the crystals allows some of the domains
to be aligned by the earth's magnetic field. When the clay cools their
directions remain fixed, and there is a weak permanent magnetization in
the same direction as the earth's field.

This magnetization can then be measured and compared to a regional time
scale developed from TRM readings from artifacts of known age, similar to
the concept used for dendrochronology.

When considering which burnt features are suitable for dating by this
method, the archaeologist's first consideration should be whether the
feature has been moved since its last firing. A reliable sample may come
from the floor or wall base of a kiln or oven. Before the sample is
extracted it must be marked with its exact orientation to geographic
north. Archaeomagnetism is capable of providing results that are very
precise on samples of archaeological features up to about 10,000 years
old.

Sources

Tarling, D.H. Paleomagnetism: Principles and Applications in Geology,
Geophysics and Archaeology. Chapman and Hall Pr. New York. 1984.

Irving, E. Paleomagnetism and its Application to Geological and
Geophysical Problems. John wiley and Sons Inc., New York/London/Sydney.
1964.

WWW. Colostate.edu, Jeffery L. Eighmy. Archaeomagnetism;A Handbook for the
Archaeologist.