Archaeomagnetic dating is a method of dating iron-bearing sediments that have been superheated—for example, the clay lining of an ancient hearth.
Archaeomagnetic dating works because the earth’s magnetic field "wanders," continually changing its position in response to changes in the flow of liquid iron in the planet's core.
By tracking and cross-dating past changes in the location of the magnetic field, geophysicists have reconstructed a series of magnetic polar positions extending back more than 2,000 years. This series of dated positions is known as the "archaeomagnetic reference curve." (Stacey Lengyel, 2010. The Pre–A.D. 585 Extension of the U.S. Southwest Archaeomagnetic Reference Curve. Journal of Archaeological Science 37:3081–3090.)
So how do scientists use the earth's wandering magnetic field to date archaeological sites?
It's all about clay. Certain clays have a naturally high iron (Fe) content. When these clays are heated to high temperatures, the iron in them aligns with the earth’s magnetic field at that moment. As the clay cools, the alignment of the iron “fixes,” preserving a record of the magnetic field at a specific time in the past.
At archaeological sites, hearths constructed of iron-bearing clays are ideal for archaeolomagnetic sampling because they were subjected to repeated hot firings. The iron in the clay realigned with every sufficiently hot fire, so it is the last hot fire in a hearth that archaeologists are able to date.
Learn more about earth's shifting magnetism in Earth's Inconstant Magnetic Field, a Science News article on NASA's website.
Dr. Eric Blinman, the director of the New Mexico Office of Archaeological Studies, explains archaeomagnetic dating in this Time Team America (PBS) video.
For more information about archaeomagnetic dating, see Paleomagnetic and Archaeomagnetic Dating on the University of California, Santa Barbara, website.