Hangi Stones Hold Clues to Earth’s Past Geomagnetic Field
Victoria University geophysicist Gillian Turner is measuring the magnetisation of hangi stones as part of a project to determine the geomagnetic field of the southwest Pacific over the past 10,000 years.
Geophysicists don’t usually study hangi pits, but Gillian Turner believes these traditional Māori ovens could hold valuable data about the Earth’s past geomagnetic field.
“All rocks, including those favoured by Māori for hangi stones, contain small amounts of magnetic minerals such as magnetite,” says Turner.
“In the preparation of a hangi, the stones are heated to very high temperatures, and the grains of magnetic minerals lose their magnetisation. As they cool, they become remagnetised by the prevalent magnetic field. We hope that, by careful measurement of the magnetisation of a hangi stone, we will be able to retrieve both the direction and the strength of the magnetic field in which it cooled.”
In June 2012, Turner tested this theory using an experimental hangi dug by tangata whenua of the Waiwhetu Marae and coordinated by Bruce McFadgen, an archaeologist from Te Kawa a Māui - the School of Māori Studies.
To create the hangi, a large pit was covered with a pyramid of macrocarpa logs. A selection of stones – including andestites, greywackes and quartizes – was placed on top of the pile.
As the fire burned, the rocks recorded temperatures of up to 1100 degrees, says Turner.
“Then we removed the charred logs, piled ferns on top of the hot rocks, hosed water onto the ferns to produce steam, put the food on top of that and covered it over. For the next four hours the food cooked.”
The stones at the bottom of the hangi pit lose their magnetisation as they are heated, and are remagnetised in the prevalent magnetic field as they cool.
As the food cooked, so did the rocks. Once they were cool enough to touch, Turner recorded the orientation of each rock, then took them back to her laboratory to use a magnetometer to measure the direction and intensity of the magnetisation. Turner now plans to use this technique to determine the magnetisation of rocks from historical hangi pits around New Zealand. By using radiocarbon dating to determine the age of organic material found in the pits, Turner will create a record of the strength and direction of the geomagnetic field over time.
This archaeomagnetism project is part of a larger project, supported by the Royal Society of New Zealand’s Marsden Fund, in which Turner is investigating the magnetic field of the southwest Pacific over the last 10,000 years. As well as hangi stones, Turner and her students will be looking at the magnetisation of volcanic rocks, lake and marine sediments, and ancient Lapita pottery from the Pacific Islands.
The magnetic field is driven by electric currents in the Earth’s liquid iron core, and changes gradually over time, says Turner. Models of the earth’s magnetic field require data from all over the earth’s surface. Turner’s project will provide crucial data from the southwest Pacific.