Our planet's magnetic field is constantly changing and evolving, making it impossible to know how the field behaved at any given point in the past. The only real way to study ancient magnetism is through our ancestors' love of fire.
Paleomagnetism has had an incredibly important role in the history of geology, as it was research into Earth's past magnetic fields that helped establish the existence and movement of the tectonic plates in the sixties and seventies. Paleomagnetic data continues to be crucial as a dating tool for human settlement sites and all sorts of fossil remains.
To establish an accurate paleomagnetic chronology, researchers rely on human pottery-making. Such activities involve firing minerals to temperatures exceeding the Curie Temperature, which is the point above which objects become demagnetized. When the objects cool, the finished pot remagnetizes, locking in a record of the direction and strength of the magnetic field in the region at that point in time and space.
This line of research has yielded fantastically accurate data for much of the Northern Hemisphere, but the Southern Hemisphere is trickier — partially because so much of it is covered by oceans, but also some Pacific cultures didn't make pottery. The Maori, who first settled New Zealand around 1200 CE, are one such example of a culture that left behind no pottery and, seemingly, no paleomagnetic data.
But the Maori are famous for hāngi, large steam pit ovens using heated rocks. As Dr. Gillian Turner of Wellington's Victoria University explains to the BBC, ancient Maori folklore suggests the hāngi stones might well have been hot enough to go past the Curie temperature:
"The Maori legend is that the stones achieve white hot heat. Well, red hot is about 700 degrees 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, which of itself is quite surprising. At that temperature, rock-forming minerals start to become plastic if not melt. The Maori prefer these volcanic boulders because they don't crack and shatter in the fire, and from our point of view they're the best because magnetically they behave better - they're formed with a high concentration of magnetite."
Dr. Turner and her fellow researchers experimented with modern hāngi to confirm that the rocks did indeed exceed the Curie temperature, which means that all rocks from ancient hāngi sites potentially hold a wealth of information on Earth's magnetic fields. By radiocarbon dating charcoal recovered at the hāngi sites, Dr. Turner can identify exactly when the rocks were first fired. For more, check out the full story over at BBC News.
Image by matteo.laghetto on Flickr.