The BBC, reporting from the annual meeting of the American Geophysical Union in San Francisco, summarises an interesting presentation on the significance of traditional Maori ovens, or hangi pits, in paleomagnetic research.

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IMAGE: Turner’s proof-of-concept hangi pit. Photo by Gillian Turner via the BBC.

The story begins earlier this year, in June, when Dr. Gillian Turner, a physicist at Victoria University in Wellington, worked with an archaeologist at the university’s school of Maori studies as well as some local indigenous people to build an experimental hangi using traditional techniques.

A hangi is essentially an earth- and rock-based version of a low-pressure, steam-assisted slow-cooker, with meat and vegetables placed in a pit for four hours, on top of superheated rocks and water-doused ferns, and under a layer of earth. The results are apparently delicious: meat is fall-off-the-bone tender, and infused with the subtle fragrance of the leaves and minerals that surround it.

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IMAGE: Turner used a pyramid of cypress logs to heat her stones. Photo via Genuine Maori Cuisine.

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IMAGE: A contemporary hangi under construction, using sacking to cover the food and protect it from the earth. The rocks have been heated, the leaves sprinkled with water, and the food placed on top; all that remains is to cover the whole thing with earth and leave it for four hours. Photo via Genuine Maori Cuisine.

Today, many Maori use wire baskets, cloth rather than leaves, or even hangi machines,” but the best stones, including volcanic andesites, greywackes, and quartzites, are still passed down through generations as family heirlooms.

And it is the stones rather than the food that Dr. Turner is interested in: her experiment was designed to see whether the rocks in a traditional Maori oven reach the Curie temperature, the point at which magnetic minerals become demagnetised.

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IMAGE: Measuring hangi rocks. Photo by Gillian Turner via the BBC.

Using thermocouples buried in her reconstructed oven, Turner found that the stones did indeed reach up to 1,100°C, and the rocks’ original magnetic polarisation was wiped clean. As the oven cooled down, however, the rocks became remagnetised in alignment with the current orientation and intensity of Earth’s magnetic field.

Every time a Maori oven is lit, in other words, its materials are rearranged to form a record of that particular moment’s geomagnetic conditions.

Taylor is now combing through records of archaeological digs across New Zealand to find abandoned hangi ovens, whose stones, combined with radiocarbon dating of the associated charcoal, will help her plot shifts in the Earth’s magnetic field over time.

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IMAGE: Earth’s magnetic field, as represented in an illustration by Tasa Graphic Arts.

In response to geodynamic forces that scientists still don’t fully understand, Earth’s North and South poles have reversed polarity several times, at varying intervals, even over the course of human existence. In fact, according to the British Geological Survey, we may be in the early stages of a reversal (or a less drastic “excursion”) right now, as the strength of the magnetic field appears to have declined by half since Roman times.

Unfortunately, no one knows exactly what happens during a reversal, or how fast, although we do know that the resulting increase in exposure to atmospheric radiation would pose a serious problem for GPS and other communications satellites, not to mention the birds, fish, and turtles who use geomagnetic sensing to migrate over long distances.

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IMAGE: Stills from an animation showing a 1995 model of geomagnetic reversal created by Gary Glatzmaier and Paul Roberts and published in Nature, via Wikipedia.

To that end, geophysicists are developing mathematical models of geodynamic variation that they then test against historical readings gathered from pottery, magnetic mineral-rich rock, and even ships’ records, from points all over the Earth’s surface. As the BBC reports, however, “crucial data from the southwest Pacific is missing,” and New Zealand’s Maori settlers were neither a compass-bearing nor a pottery-using people.

Fortunately, as Gillian Taylor’s experiment shows, their cooking technology can fill the gap. Just as the fossilised remains of the human diet allow paleontologists to construct climate and ecological histories, abandoned Maori kitchen appliances turn out to provide a record of geomagnetic history — an edible archaeology whose invisible markings add planetary significance to its cultural importance.