Sun, 09 Dec 2012 14:02 UTC
The stones inside these ovens reach temperatures as high as 1,100C. This heat is high enough that the magnetic minerals in these stones will align themselves with the current magnetic field direction.
The Earth's magnetic field extends from the core of our planet out to the solar winds. It is sometimes easy to think of this field as a giant bar magnet running from the North to South Pole. Unlike a solid bar magnet, the Earth's field is created by the interaction of the molten metal core at the center of our planet, so the field doesn't align with "true" North exactly. It is currently off by about 11 degrees. Moreover, it can - and has - flipped polarity in the past. The Earth's field protects the planet from collision with solar particles and erosion by the solar winds.
"Earth's magnetic field is important to us," Professor John Tarduno, from the University of Rochester told the European Geosciences Union meeting in 2010. Tarduno's team developed techniques for studying tiny magnetite minerals trapped inside the crystals of volcanic rock, which align themselves with the magnetic field, and then lock into place once the rocks' host environment temperatures drop below 580C.
Tarduno's team has been investigating the magnetic record in volcanic rocks located in Africa, India and Australia that might retain records as ancient as 3.6 billion years ago.
Dr. Gillian Turner and her team from Victoria University, Wellington, in New Zealand are interested in fleshing out the map of these historical magnetic lines in more recent times.
"We have very good palaeomagnetic data from across the world recording field strength and direction - especially in the Northern Hemisphere," Gillian Turner told BBC News recently. "The southwest Pacific is the gap, and in order to complete global models, we're rather desperate for good, high-resolved data from our part of the world."
To this end, the team has been leading an archaeological search to find sites with abandoned hangi ovens. The stones in these ovens could shed light on Earth's magnetic behavior going back hundreds of years. The team's project, reported at the American Geophysical Union (AGU) Fall meeting, hopes to retrieve magnetic field data stretching back over 10,000 years.
For relatively current information, say the last few centuries, Turner would normally have turned to pottery. When pottery is fired, the minerals in the clay are heated above the Curie temperature - the temperature above which a ferromagnetic substance loses magnetism - and are demagnetized. The cooling minerals become magnetized again in the direction of the current field. The strength of the magnetism in these minerals is directly related to the strength of that field.
There is just one small problem with Dr. Turner's usual methods. The original Maori settlers in New Zealand - some 700 to 800 years ago - did not use pottery. Turner has found a rather fascinating alternative source of data, however.
Turner's team experimented with some modern day hangi, testing their ability to reach the necessary Curie temperatures to reset the magnetism of the rocks.
"The Maori legend is that the stones achieve white hot heat," she explained. "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 1,100C, which of itself is quite surprising. At that temperature, rock-forming minerals start to become plastic if not melt."
When the stones were removed from the ovens, the researchers placed a compass on top of the cooling rocks to establish that re-magnetization had taken place. The Maori were, and still are, particular about the stones used in their hangi ovens, one of the most popular types being an andesite boulder found in Central North Island.
"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," the Wellington scientist said. "But there are some sedimentary rocks which we can use also."
With the heat range of the ovens established and the re-magnetization proven, the team is scouring New Zealand for archaeological digs that have uncovered hangi ovens. To establish the critical date of the ovens, the team is using radiocarbon analysis of the charcoal left from the firewood used to light the oven.
Because of the relative recentness of Maori settlements, the hangi stones can only elucidate the magnetic record to the 1200s. To go further back, Turner and her team will need to find other sources.
"We're also studying volcanic rocks because they're erupted above the Curie temperature. And the other source of information is lake sediments. Long-core sediments can give us a continuous record at specific places."
One of the reasons this research is so important is the possibility of a polarity reversal, as mentioned earlier. In a Johns Hopkins University study published this past July in Nature Geoscience, researchers revealed a reversal might be happening right now.
Earth's magnetic field periodically changes direction, making the magnetic North and South poles reverse position. Considering our current reliance on electronic equipment, this could be disastrous for human society. Cell phones, compasses, computers - anything with electronic circuits or magnets - would be affected by a polarity shift.
Geologic evidence shows the field flips about once every 450,000 years. The last reversal happened about 780,000 years ago, so looking at the average interval, we are overdue. Sometimes, though, there is as much as a million years between reversals.
To know if a reversal is imminent, scientists first have to understand the underlying mechanism that causes it. Peter Olson and Renaud Deguen from Johns Hopkins used seismic imaging of the Earth's core to reveal lopsided growth. The core, normally thought to be spherical, isn't even close. The eastern hemisphere of the core is currently bigger than the western hemisphere.
This deformation lines up with the axis of the magnetic field, as the line linking the North and South magnetic poles doesn't pass directly through the center of the Earth. It is currently about 300 miles east of center. Until about 200 years ago, the magnetic axis was firmly in the western hemisphere for at least the last 10,000 years, making this movement a relatively recent event - in geologic terms, at least.
The team's research indicates a link between quick shifts of the magnetic axis and past reversals, suggesting that a magnetic reversal is already underway. The scientists are fast to point out, however, that the core is simply too chaotic to say anything with certainty yet. The new model suggests the reversal is possible, if remotely.
The good news is that magnetic reversals take place on geologic time scales, taking anywhere from 1,000 to 10,000 years to complete. Even if the team's hypothesis is correct and we are 200 years into the next reversal, any serious effects on human technology should be centuries away, let alone in our lifetime.