archaeomagnetic dating

Dating method based on the palaeomagnetism of archaeological materials such as baked clay structures (ancient hearths, kilns, ovens). Firing temperatures in excess of 565°C activated iron-bearing minerals such as hematite, causing the particles to realign with the Earth's magnetic field existing at that time and creating a permanent record of its direction and intensity. As the field changes over time, local and regional chronologies of field direction can be built up and independently dated.

Comparison of an unknown field direction with a regional chronology can provide an archaeological age. However, regional variations mean that a separate master sequence is needed, and structures must remain on the site.

A master sequence has been developed for Chinese pottery, and the method has been used to confirm the origins of Japanese porcelain-making at Arita, northwestern Kyushu Island, AD 1603–27.

Complete reversals in the Earth's magnetic field have been put in sequence from 700,000 years ago, and can aid the dating of early hominid sites if used with other techniques.

Over the past four centuries, data from London Observatory records and navigation diaries show that the direction of movement of the Earth's magnetic field has been consistently westwards at a rate of 0.23° longitude per year, but there is no simple pattern evident in earlier times.