Credit: Vitalij Cerepok / EyeEm/ Getty images.
The Earth spin axis slowly shifts over time, taking tens of million years for just one degree of variation. In the process, the geographical location of the North and South magnetic poles changes accordingly. But geophysicists suspect that faster and more traumatic oscillations have occurred during the planet’s history, in particular at the end of the Cretaceous period.
To test the idea, an international team, led by Ross Mitchell from the Chinese Academy of Sciences, and including palaeontologist, Rodolfo Coccioni, from Università di Urbino, has spent seven years recording and analysing data from rock samples found in central Italy. “Stratigraphic sections from Furlo and Apiro in the Marche region are largely composed of ‘scaglia rossa’, a sedimentary rock full of microfossils that simplifies the dating process” Coccioni explains. Hematite and lodestone in those rocks have microscopic natural magnets embedded within, that retain signs of the position of magnetic poles in the past, even after millions of years.
The scientists collected data from two parallel stratigraphic samples dating to the same era, collected from the same site and analysed with the same methods, to ensure that the corresponding results were no accident. The data, published in Nature Communications1, show that in the Late Cretaceous period, about 84 million years ago, the Earth’s spin axis started to change, and Italy moved to lower latitudes. The scientists estimate that the variation reached about 12 degrees. Then the process reversed, and 78 million years ago the axis returned to its original position. “The axis probably took a round trip’” says Coccioni. “Our data fit with what we call a true polar wander, consisting in this notable shift in spin axis”.
The hypothesis had already been suggested in the past, but an older analysis on similar rocks from a site in Umbria in the 1970s seemed to disprove it. Now, Coccioni and his colleagues have analysed the rocks with more precise techniques that did not exist at the time, such as thermal demagnetization, that removes the disturbances caused by the current magnetic field, and superconducting magnetometers.
As for the causes, scientists suspect that tectonic plates drifted, and that an unbalanced weight distribution on the Earth surface forced the planet to tilt. “But more data from other parts of the world are needed to confirm that a true polar wander occurred” says Coccioni.
