Message #53738

[Daedalus]

Despite this limited history of direct observations, researchers can use various clues to estimate the size, strength, and polarity of Earth's magnetic field at many times in the past. For instance, some minerals that crystallize as lava cools can record the direction of the planet's magnetic field at the time the eruption occurred, says Steven T. Johnston, a geologist at the University of Victoria in British Columbia. In many cases, such information enables scientists to establish the latitude where pieces of Earth's crust originated and thereby infer their long-term tectonic motion, he notes.

As long as magnetized minerals aren't heated above a characteristic temperature known as a Curie temperature, the alignment of the magnetic materials contained therein remains intact. If the rocks are heated beyond the Curie temperature, which typically lies between 500°C and 600°C, the stored magnetic information gets scrambled, says Cathy Batt, a paleoarchaeologist at the University of Bradford in England. Then, when the rocks cool, their magnetic materials realign themselves with the planet's magnetic field (SN: 3/13/04, p. 174). Because fires usually are hotter than a mineral's Curie temperature, magnetic materials lining a hearth record the strength and direction of magnetic field lines at the last time a fire had been lit there—a finding of great interest to an archaeologist, for example.

By combining data gathered by geologists and archaeologists, researchers have tracked the motions of the magnetic poles for the past 7,000 years or so, says Mandea. During that time, the magnetic poles have wandered through all longitudes, roughly circling the geographic poles, she notes. While the north magnetic pole has remained well within the Arctic Circle, the south magnetic pole has recently roamed farther away from the south geographic pole and is now around 64°S.

Using information collected in Britain, mostly from England and Wales, Batt and her colleagues have compiled a record of how magnetic declination has varied in that region during the past 4,000 years. To provide a more useful comparison among sites, the researchers adjusted each measurement to replicate what the magnetic field would have been like at Meriden, England, a town about 150 km northwest of London. The model should be valid for any site within 500 km of that town, which is roughly the center of the England-Wales region, Batt and her colleagues note in the Feb. 16 Physics of the Earth and Planetary Interiors.

The team's data also include information about magnetic dip, the angle between the Earth's magnetic field lines and a horizontal plane. Only a few of the 858 sets of measurements, most notably the 238 data points taken at observatories since the 1600s, include data about paleointensity, or how strong the planet's magnetic field was at the time data were gathered. The combination of two or more of these parameters enables researchers to better estimate the age of an artifact when other clues don't provide a clear answer, says Batt.