AGU: NY & New England’s old stone walls record history of Earth’s magnetic wanderings (+video)
Under the forests of New York and New England, a hidden tracery of tumbledown stone walls marks the boundaries of early American farms, long abandoned for city jobs and less stony pastures in the West.
Some old walls record past locations of Earth’s magnetic north pole. Geochemist and local history buff John Delano used old walls as references to reconstruct a history of our planet’s magnetic field in eastern North America in a new study published in AGU’s Journal of Geophysical Research: Solid Earth.
Delano discovered some walls mark boundary lines surveyed in the 18th and 19th centuries by magnetic compass. Using GPS and aerial lidar “light radar” images, he measured the walls’ present-day bearings with respect to True North and compared them to compass bearings for the boundary lines recorded by the surveyors (watch a video).
The discrepancy between Delano’s measurements and the historical compass readings is not the error of the early surveyors, but the magnetic declination at the time of the survey. The difference between True North, on the axis of Earth’s spin, and magnetic north shifts over time due to changes in Earth’s outer core. The angle of deviation is called magnetic declination, and its degree varies depending on where the compass holder stands on the globe.
This difference matters not just for bushwhackers making their way without a GPS device, but for military and commercial aircraft, ships, submarines and even smartphones, which still use Earth’s magnetic field for orienteering.
Modeling the wanderings of Earth’s magnetic field can provide clues to the enigmatic motions deep in the Earth that generate it.
Learn more and find multimedia illustrations of this study at:https://blogs.agu.org/geospace/2019/02/27/historic-stone-walls-record-history-of-earths-magnetic-wanderings/
This research paper is freely available through March 15th. A PDF copy of the paper can be downloaded at the following link:https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JB016655. Alternatively, journalists who would like a copy of the paper can contact Liza Lester, AGU Public Information Specialist at llester@agu.org or 202-777-7494. The study’s author, John Delano, emeritus professor at the State University of New York in Albany, can be reached atjdelano@albany.edu.