Archaeomagnetic dating
Encyclopedia
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field
at past times recorded in archaeological materials. These paleomagnetic
signatures are fixed when ferromagnetic materials such as magnetite
cool below the Curie point
, freezing the magnetic moment of the material in the direction of the local magnetic field at that point in 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
, 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.
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
but to direction that is a function of the North Magnetic Pole
and the local secular variation to yield a Magnetic declination
.
The Magnetic declination at any given time can be frozen into a clay formation that contains magnetite and is heated above the Curie point
. 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.
The Archaeomagnetic dating was described in the seminal 1992 publication “Paleomagnetism: Magnetic Domains to Geologic Terranes.” By Robert F. Butler.
. 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
dates, help refine the regional Curves.
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.
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
at past times recorded in archaeological materials. These paleomagnetic
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
signatures are fixed when ferromagnetic materials such as magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
cool below the Curie point
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
, freezing the magnetic moment of the material in the direction of the local magnetic field at that point in time. The direction and magnitude of the magnetic field of the Earth at a particular location varies with time
Geomagnetic reversal
A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse...
, and can be used to constrain the age of materials. In conjunction with techniques such as radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
, the technique can be used to construct and calibrate the geomagnetic polarity time scale. This is one of the Dating methodologies
Dating methodology (archaeology)
Dating material drawn from the archaeological record can be made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence relative to datable contexts...
used for sites within the last 10,000 years.
Instances of use
The Earth's magnetic fieldEarth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
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
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
but to direction that is a function of the North Magnetic Pole
North Magnetic Pole
The Earth's North Magnetic Pole is the point on the surface of the Northern Hemisphere at which the Earth's magnetic field points vertically downwards . Though geographically in the north, it is, by the direction of the magnetic field lines, physically the south pole of the Earth's magnetic field...
and the local secular variation to yield a Magnetic declination
Magnetic declination
Magnetic declination is the angle between magnetic north and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is a synonym, and is more often used in navigation...
.
The Magnetic declination at any given time can be frozen into a clay formation that contains magnetite and is heated above the Curie point
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
. 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
Magnetic declination
Magnetic declination is the angle between magnetic north and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is a synonym, and is more often used in navigation...
from the last time they were fired or used.
The Archaeomagnetic dating was described in the seminal 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 Curie pointCurie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
. 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
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a 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 colleagues , to date archaeological, geological, and hydrogeological...
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 molds. 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 Magnetometer to determine the Thermal Remnant Magnetism of each sample. The results are statistically processed and an Eigenvector is generated that shows the three-dimensional Magnetic declinationMagnetic declination
Magnetic declination is the angle between magnetic north and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is a synonym, and is more often used in navigation...
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.