Preamble
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.
Introduction
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.