Archaeometry

Because shifts in the molten core of the planet cause Earth's magnetic field to vary, and because this causes our planet's magnetic North Pole to change position over time, magnetic alignments in archeological specimens can be used to date specimens.

In paleomagnetism, rocks are dated based on the occurrence of reversal's in Earth's magnetic poles. These types of pole reversals have occurred with irregular frequency every hundred thousand years or so in Earth's history. Geologists collect samples to be analyzed by drilling into bedrock, removing a core, and noting the relative alignment to Earth's present magnetic field. The sample is then analyzed in the laboratory to determine its remnant magnetism—the pole's alignment when the sample crystallized. Using a compiled master chronology of pole reversals, scientists can then date the specimen. Because the time between pole reversals is so large, this technique can only be used to date objects to an accuracy of a few thousand to tens of thousands of years. The technique has been used to date human remains in the Siwalki Hills of India, in the Olduvai Gorge in Kenya, and in the Hadar region of Ethiopia.

Archaeomagnetism makes use of the fact that the magnetic North Pole has shifted position over time. When clay in an object is heated to a sufficiently high temperature, the iron particles in the clay will align to the magnetic pole. If the clay has remained undisturbed since it was fired, it will indicate the position of the pole when it was made. Archaeomagnetism can therefore be used to date fixed objects such as lined fire pits, plaster walls, and house floors. Other techniques, such as radiocarbon dating and dendrochronology, can be used to date wood from the fire. By comparing data, a master curve showing the position of the magnetic North Pole over time can be generated. This master curve then provides a basis for assigning dates to undated clay samples based on where their remnant magnetism indicates the pole was when they were fired. Because the pole position can be determined rather exactly for the last 100,000 years or so, dates for materials of this age and younger can be quite accurate. However, disturbances occur at times in the earth's magnetic field at various geographical locations, so it has been necessary to develop separate master curves for different regions. In the southwestern United States, where dendrochronology was used to help calibrate the master curve, archaeomagnetism can yield dates with accuracy as great as +/- 50 years, a precision unmatched by radiocarbon dating.