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Joukowsky Institute for Archaeology & the Ancient World
Brown University
Box 1837 / 60 George Street
Providence, RI 02912
Telephone: (401) 863-3188
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Archaeomagnetism
Archaeomagnetic dating uses variations in the direction and intensity of the earth’s magnetic field to date ceramics containing weakly magnetic minerals. The magnetic minerals within clay and clay soil are aligned randomly prior to being heated. When pottery is fired, the heat allows the minerals to align themselves with the earth’s magnetic field, a property which is preserved as they cool in the earth’s magnetic field. Because of the relative weakness of the magnetic force operating on the minerals, only partial alignment is achieved – some particles remain in random alignment. If enough contemporaneous samples are collected, an examination of their magnetic state can provide a mean vector indicating the location of the geomagnetic pole at the time the clay was fired. This information is translated into a date of firing by comparing the indicated location of the geomagnetic pole to a region-specific secular variation curve, which provides a timeline of movement of the pole relative to a specific location.
Sources:
“Dating In Exposed and Surface Contexts”, ed.: Beck, Charlotte. University of New Mexico Press: Albuquerque, NM, 1994.
Michels, Joseph. “Dating Methods in Archaeology”. Seminar Press, New York: NY, 1973.