Archaeomagnetic dating is the study and interpretation of the signatures of the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...

at past times recorded in archaeological materials. These

paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...

signatures are fixed when

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

materials such as

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

cool below the

Curie point In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...

, freezing the magnetic moment of the material in the direction of the local magnetic field at that time. The direction and magnitude of the magnetic field of the Earth at a particular location varies with time, and can be used to constrain the age of materials. In conjunction with techniques such as

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

, the technique can be used to construct and calibrate the geomagnetic polarity time scale. This is one of the dating methodologies used for sites within the last 10,000 years. The method was conceived by E. Thellier in the 1930s and the increased sensitivity of

SQUID True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...

magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...

s has greatly promoted its use.

Instances of use

The

has two main components. The stronger component known as the Earth's poles, reverses direction at irregular intervals. The weaker variations are the Earth's magnetic map. Within these weaker areas the local directions and intensities change gradually (secular variation). A compass does not point to the true

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Ma ...

but to a direction that is a function of the

North Magnetic Pole The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...

and the local secular variation to yield a

magnetic declination Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) an ...

. The magnetic declination at any given time can be frozen into a clay formation that contains magnetite and is heated above the

. In general, many cultures used long-term fire hearths made of clay bricks, or a space lined with clay, that were baked into place by use. These artifacts of occupation can yield the magnetic declination from the last time they were fired or used. Archaeomagnetic dating was described in the 1992 publication “Paleomagnetism: Magnetic Domains to Geologic Terranes.” By Robert F. Butler.

Methodology

Archaeomagnetic dating requires an undisturbed feature that has a high likelihood of containing a remnant magnetic moment from the last time it had passed through the

. This involves sufficient mass to take samples from, and a suitable material with adequate magnetite to hold the remnant magnetism. In addition, the feature needs to be in an area for which a secular variation curve (SVC) exists. Once the paleodirections of enough independently dated archaeological features are determined, they can be used to compile a secular variation record for a particular region, known as an SVC. The Archaeomagnetic Laboratory at the Illinois State Museum has secular variation curves for the southwest, mid-continent and southeast United States. Additional data points from archaeomagnetic samples with corresponding dating techniques such as tree ring dating or

carbon-14 Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and co ...

dates, help refine the regional curves.

Technique

A number of samples are removed from the feature by encasement in non-magnetic plaster within non-magnetic moulds. These samples are marked for true north at the time of collection. The samples are sent to an Archaeomagnetic Laboratory for processing. Each of the samples is measured in a spinner

to determine the thermal remanent magnetism of each sample. The results are statistically processed and an

eigenvector In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denoted ...

is generated that shows the three-dimensional

that will yield a location for the North Pole at the time of the last thermal event of the feature. Data from this feature is compared to the regional secular variation curve in order to determine the best-fit date range for the feature's last firing event.

References

Archaeomagnetic dating - guidelines

English Heritage English Heritage (officially the English Heritage Trust) is a charity that manages over 400 historic monuments, buildings and places. These include prehistoric sites, medieval castles, Roman forts and country houses. The charity states that i ...

booklet (p. 33), 2006 (a popular introduction with illustrations and references) * Herries, A.I.R., Kovacheva, M., Kostadinova, M., Shaw, J., 2007. ''Archaeo-directional and -intensity data from burnt structures at the Thracian site of Halka Bunar (Bulgaria): The effect of magnetic mineralogy, temperature and atmosphere of heating in antiquity'', Physics of the Earth and Planetary Interiors. 162, 199–216.

External links

Archaeomagnetic Laboratory at the Illinois State Museum

The U. S. National Geophysical Data Center (NGDC) data of paleopole positions and paleomagnetic directions.

Instances of use

Methodology

Technique

See also

References

External links

Further reading