In recent decades there has been a heated debate about the possibility that the solid outer layer of the Earth could wobble, or even tip over, relative to its axis of rotation. Such a shift is called ‘ true polar shift ‘, but the evidence for it is controversial. Now, a team of scientists publishes a new paper in the journal Nature Communications that provides some new evidence that such a planetary tilt would have occurred in Earth’s past.
This is how the polar shift would occur
Earth is a layered ball, with an inner core of solid metal, an outer core of liquid metal, and a solid mantle and crust predominant on the surface we live on. All of this spins like a top, going one full turn once a day. Because the Earth’s outer core is liquid, the solid mantle and crust can slide over it. Relatively dense structures, like subducting ocean plates and massive volcanoes like Hawaii, sit close to the equator, in the same way that your arms like to be to the sides when you swivel in an office chair.
Despite this wandering of the crust, the Earth’s magnetic field is generated by electrical currents in the liquid nickel-iron convection metal of the outer core. On long time scales, the displacement of the overlying mantle and crust does not affect the core, because those overlying rock layers are transparent to the Earth’s magnetic field. Rather, the convection patterns in this outer core are forced to ‘dance’ around the Earth’s axis of rotation, meaning that the overall pattern of Earth’s magnetic field is predictable, spreading in the same way as the iron filings lined up on a small magnetic bar. Therefore, these data provide excellent information about the direction of the north and south geographic poles, and the inclination gives the distance from the poles (a vertical field means you are at the pole, horizontal tells us you are at the equator) . Many rocks record the direction of the local magnetic field as they form, in the same way that a magnetic tape records its music. For example, the tiny crystals of the mineral magnetite produced by some bacteria actually line up like tiny compass needles and get trapped in sediment when the rock solidifies. This “fossil” magnetism can be used to track where the axis of rotation deviates relative to the crust.
“Imagine looking at Earth from space,” explains Tokyo Institute of Technology researcher Joe Kirschvink. “The true polar shift would look like the Earth was tilting on its side, and what is actually happening is that the entire rocky layer of the planet (the solid mantle and crust) revolves around the outer liquid core.” Although scientists can measure the true polar shift that occurs today very precisely with satellites, geologists still debate whether large rotations of the mantle and crust have occurred in the Earth’s past.
Did a true polar drift occur during the Cretaceous?
This is a highly controversial topic and numerous arguments for and against this theory have been published in the last thirty years. In this case, the researchers collected paleomagnetic data from limestone rock samples created in the Cretaceous (between approximately 145.5 and 65.5 million years ago) located in the Italian Apennine mountain range. The magnetism of younger rocks in the same area was studied almost fifty years ago and indirectly led to the discovery of the impact of the asteroid that killed the dinosaurs. Sarah Slotznick, co-author and geobiologist at Dartmouth College explains, “These Italian sedimentary rocks turn out to be special and very reliable because the magnetic minerals are actually fossils of bacteria that formed chains of magnetite.”
Kirschvink and his colleagues found, as the polar shift hypothesis had predicted, that the Italian data indicate a tilt of about 12˚ of the planet 84 million years ago. The team also found that the Earth may have corrected itself: after tilting on its side, the planet reversed its course and spun backward, moving a total of 25˚ of an arc in approximately five million years. A true cosmic yo-yo.
Text: Tokyo Institute of Technology
