Cooking stones help reconstruct Earth's magnetic history

When it comes to creating a picture of the Earth's magnetic history, scientists have pretty good data on the strength and direction of magnetic fields in the Northern Hemisphere, thanks to the study of ancient pottery. But, for researchers looking to make a truly complete model, the problem has been the gaps in information in the southwest Pacific.

The reason for these gaps is that the ancient Maori from the area used cooking stones in pit ovens called hangi rather than pottery over fires. Scientists didn't believe the stones from the hangi would yield magnetic data like the pottery. Recent tests have since proved this wrong as they've shown the stones yield a wealth of magnetic history.

The news the stones can help complete the modelling is great news, but likely you're wondering what pottery and stones have to do with magnetic field in the first place. Here's how it works:

When pottery is fired, minerals in the clay are heated above the Curie temperature. This means the magnetic minerals lose their natural alignment or "magnetic spin" and become essentially demagnetized. The process is reversible however, and as the various minerals cool they become magnetized once again in the strength and direction of the prevalent field. This is part of the crucial data for the models

The Maori of the southwest Pacific didn't use pottery but rather hot cooking stones placed in pits that were then covered with layers of fern fronds soaked in water and baskets of food. These pit ovens, or hangi, were covered in dirt and then left to steam the food for hours.

Dr. Gillian Turner, from Victoria University, Wellington, New Zealand, has been working on a project to create a complete history that could reach back some 10,000 years. The question in her mind was whether the stones would achieve enough heat to reach the Curie temperature needed to demagnetize the stones, and in doing so provide the same kind of data as ancient pottery. So, she and her team created their own hangi to see if they could achieve the same behavior from the stones.

She explained to the BBC News:

"The Maori legend is that the stones achieve white hot heat. Well, red-hot is about 700° C (1292° F) and so white hot would be a good deal more than that. But by putting some thermocouples in the stones we were able to show they got as high as 1,100° C (2,000° F), which of itself is quite surprising. At that temperature, rock-forming minerals start to become plastic if not melt."
They then validated the re-magnetization by placing a compass on top of the cooled hangi stones. Sure enough they provided clear data on the magnetic field of the area.

With the theory of the hangi stones now proven, Dr. Turner and her team are now searching for ancient hangi sites across New Zealand hoping to find abandoned stones that could help fill in the gaps of data she was missing. To match the data with points in time she'll need to date the stone, but it's believed this can be done by radiocarbon analysis of charcoal left from the firewood used to heat the stones.

While ancient hangi sites are not expected to be found past the 1200s, that doesn't mean the value of these stones ends there. The Maori chose very specific stones — those made from andesite boulder found in the Central North Island. These were volcanic boulders, and were preferred because they didn't decompose in the hot fires.

Not only was this a cooking practicality for the ancient Maori, it is also a scientific boon for Dr. Turner. These volcanic boulders just happen to have a high concentration of magnetite, which is part of what helped in the successful trials of the hangi stone theory in the first place.

Where the knowledge about the volcanic boulders as the source will really come in handy however, is when Dr. Turner needs to look beyond the 1200s for data. She can now actually look to original volcanic rocks from the area, known to have erupted above the Curie temperature.

She'll also be looking at long-core lake sediments to help continue to fill in the gaps in the region's magnetic history.

Dr. Turner presented her findings at the American Geophysical Unions' fall meeting in San Francisco, the world's largest annual gathering of Earth scientists.

American Geophysical Union, via BBC News, LiveScience

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