On a chilly Sunday in June, a group of scientists gathered around a mound of dirt in a grassy field. After some muttered consultation, Victoria University geophysicist Gillian Turner picked up a shovel and began digging. Big puffs of steam escaped into the air. Soon, brown stalks of charred ponga fronds became visible, a flame licked upwards, then sacks were lifted to reveal metal racks filled with foil-wrapped packages. A hangi. But Turner wasn’t that interested in the chicken, pumpkin and kumara packed inside the foil lunch containers – she was there for the rocks lining the hangi pit. In a project supported by the Marsden Fund, Turner is investigating the geomagnetic field of the southwest Pacific over the past 10,000 years. Archaeological hangi stones, she says, hold clues to the orientation and strength of the magnetic field since Maori arrived in New Zealand.
The orientation of the geomagnetic field – which is driven by electric currents in the Earth’s liquid iron core, and changes over time – is recorded in tiny grains of magnetic minerals, such as magnetite or haematite. “If we heat minerals above about 600°C, they are demagnetised. But as they cool back through this temperature, we get a collective magnetic alignment at the atomic level, and the grains become magnetised in the direction of the prevalent field,” says Turner. The experimental hangi was dug by tangata whenua of the Waiwhetu Marae and coordinated by Bruce McFadgen from Victoria University’s School of Maori Studies. As the rocks reached temperatures above 600°C, Turner theorised, the magnetic minerals would lose their initial magnetisation and, as they cooled, would be remagnetised in today’s magnetic field. To create the hangi, a large pit was dug, then covered with a pyramid of macrocarpa logs.
A selection of stones was placed on top of the pile. As the fire burnt, the rocks reached temperatures of up to 1100°C. “Then we removed the charred logs, piled ferns on top of the 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.” Once the rocks had cooled, Turner uncovered them, “just like you would in an archaeological dig, shovelling away a little bit of ash at a time so as not to disturb the stones”. She used plaster of paris and a compass to record the orientation of selected stones, so she can recreate their position in the lab. The next step is to drill samples from the rocks to enable measurement of their magnetisation direction and intensity. She plans to use the technique to determine the magnetisation of rocks from archaeological hangi sites. By using radiocarbon dating to determine the age of contemporaneous organic material found in the pits, she will create a record of New Zealand’s geomagnetic field over recent centuries.
When the geomagnetic field flips
The Earth’s magnetic poles are constantly on the move and sometimes flip – north becomes south and south becomes north – with the last geomagnetic reversal occurring 780,000 years ago. Seismologists recently suggested the Earth’s inner core is growing unevenly, and in a paper in Nature Geoscience (July 1, 2012), Peter Olson and Renaud Deguen from Johns Hopkins University calculate that this could lead to rapid shifts in the magnetic field, possibly culminating in a polarity reversal. Over the past 200 years, the Earth’s magnetic axis has shifted into the eastern hemisphere and the strength of the field has decreased significantly. These could be signs the planet is heading for a geomagnetic reversal, which would leave it exposed to the solar wind, with devastating effects on electrical and telecommunications networks.