Information for archaeologists Information for Archaeologists Archaeomagnetic applications

This section is designed for an archaeologist with little or no previous experience of archaeomagnetism, but has, for example, found a burnt structure, such as a kiln or oven, and wishes to find out more about the archaeomagnetic technique.  Further details can be found by either clicking on the appropriate word in blue or by going back to the main menu. The glossary can be accessed for the definition of words in blue at any time and will then return to this section.

Archaeomagnetism is mostly known for magnetic dating of burnt materials in an archaeological context.  This will be discussed first, followed by archaeomagnetic dating of other materials, such as plaster, sediments, paint, etc., and then other applications, such as reconstruction and provenancing.

When any material containing magnetic grains is heated above some 700 °C, it loses any previous remanent magnetisation.  As it cools down, the magnetic grains acquire a direction of magnetisation that is the same as that of the Earth’s magnetic field at that time and an intensity of magnetisation that is proportional to the strength of that field.  This magnetisation, acquired during cooling, is called a thermal remanent magnetisation (TRM) and has the remarkable property that most of it is preserved for thousands of years (and vastly longer) unless it is reheated or chemically changed. Such materials therefore retain a record of the direction and intensity of the Earth’s magnetic field from the time that the magnetisation was originally acquired.  Direct observations of the present Earth’s magnetic field only go back for some 400 years (<200 years for its intensity), but show that both the direction and intensity of this field change with time (secular variation). Therefore measurements of the magnetic properties of ancient fired materials can be dated by comparison of their directional and intensity properties with the known record of the Earth’s magnetic field properties. It is immediately obvious that this means that materials less than 400 years can be dated by  comparison with any nearby geomagnetic observatory records.  For older times, the principles are, of course, the same. However, there are no direct human recordings of the Earth’s magnetic field and so our knowledge of the behaviour of the Earth’s magnetic field depends on obtaining archaeomagnetic records from archaeological sites that have been securely dated by other means.  Thus archaeomagnetism not only provides a dating method for archaeologists but also provides a unique record of one of the major geophysical properties of the Earth.  More importantly, as the database increases, i.e. the number of securely dated archaeomagnetic sites increases, the ability to provide more reliable archaeomagnetic dates increases.  This makes this dating method unique as the greater the amount of data, the more precise the method becomes – unlikely all other scientific dating methods.

Archaeomagnetic applications