“In the hangi process the oven stones are heated to red or even white heat, well above the temperature at which they are magnetised. As they cool down in situ they gain a record of the magnetic field.”
Gillian Turner, geophysicist, Victoria University of Wellington.
An experimental hangi at Waiwhetu marae, held at Matariki in 2012, proved that the oven stones hold a record of the earth's magnetic field as they cool. Photo: Gillian Turner / VUW
Hangi stones from archaeological sites around New Zealand are providing an extraordinary window into how the magnetic field in the southwest Pacific has been changing.
They have revealed that over the past 600 years the declination, or direction of a compass needle, has varied significantly, moving from 5° west of North to today’s 20° east of North.
The inclination of vertical component of the magnetic field hasn’t changed so dramatically. “It’s steepened by 3° or 4° over that time,” says Gillian Turner, a geophysicist at Victoria University of Wellington.
The changing strength of the magnetic field is the hardest component to measure, and Gillian says that while it doesn’t seem to have changed much in the past 400 years there are exciting hints at some of the oldest archaeological sites (between 400 and 600 years ago), that the “field has gone through some bigger amplitude, more wild, changes in strength. And that’s something we’re seeing in present day places like the South Atlantic Ocean.”
“In the south Atlantic there’s a big anomaly in the magnetic field that’s allowing the solar wind particles to get lower in the atmosphere,” says Gillian. “They are affecting things like satellite systems.”
Gillian points out that there are variations in the magnetic field in different parts of the world depending on the behaviour of the earth’s core directly below. The hangi stone project is focusing on the magnetic field in the southwest Pacific.
Magnetism and hangi stones
How the earth’s magnetic field works
Bruce McFadgen, Rimpy Kinger and Gillian Turner holding ancient hangi stones that have been excavated from archaeological sites of various ages around New Zealand. Photo: RNZ / Alison Ballance
“The earth’s magnetic field looks, to a very good approximation, as if you’ve got a bar magnet – a very strong bar magnet – at the centre of the earth. It creates a field that encompasses the earth, and … it protects us from charged particles that come at really high speeds from out of the sun,” says Gillian. “Of course geophysicists know that [what we really have] in the core of the earth is a churning cauldron of liquid iron at high temperature, and there are electrical currents in that iron that actually cause the magnetic field. And for us that’s interesting as we know that it’s dynamic, changing with time, and it’s those changes, over years and even thousands of years, that are at the heart of this project.”
There are three components to the magnetic field: declination (which is the horizontal component that we see when we use a compass), inclination (the vertical component) and strength.
In New Zealand we have records of declination that go back 400 years to early European explorers such as Abel Tasman.
A little later sailors and explorers began using a dip needle to record inclination, and by 200 years ago people began measuring the strength of the magnetic field.
The New Zealand magnetic observatory has kept good records for the last century.
There are also natural records of the magnetic field, kept by any material that has become magnetised in the magnetic field. For example, lava flows record the direction and strength of the magnetic field as they cool in place. Grains of sediment falling onto a lake or sea floor also record the magnetic field.
The first part of this Marsden-funded project was to prove that hangi stones could, indeed, record the magnetic field.
With the help of Terese McLeod and archaeologist Bruce McFadgen, from the School of Maori Studies at Victoria University, a hangi was held at Waiwhetu Marae at Matariki. This showed that the technique worked, and then Bruce helped Gillian and PhD student Rimpy Kinger excavate hangi stones at archaeological sites of various ages.
Rimpy explains that a key part of the process was ensuring that the oven stones weren’t moved before their orientation was clearly marked on them. She says that volcanic stones from the North Island kept a much clearer record than sandstone from the South island.
Gillian Turner is the author of the book 'North Pole, South Pole: the Epic Quest to Solve the Great Mystery of Earth's Magnetism.'
Bruce McFadgen has featured on Our Changing World previously talking about peat, pumice and tsunamis on the Kapiti Coast.
