True polar wander (TPW), which can sometimes tilt entire planets or moons relative to their respective axes, is something we know. However, it's not clear how often this has occurred to Earth.
A new study now shows that one such tilting event occurred approximately 84 million years ago, when dinosaurs walked the Earth.
Researchers analyzed limestone samples taken from Italy during the Late Cretaceous period (100.5 - 65.5 Million years ago) in search of evidence of magnetic shifts that could point to the occurrence of TPW.
The evidence of true polar wander in Late Cretaceous is provided by Bacteria fossils that have been trapped in rock and formed chains of mineral magnetite. This may be a breakthrough in scientific debate that has been ongoing for decades.
It's amazing how true polar wandering looks. (Victor C. Tsai/Wikimedia commons/Public Domain).
The researchers explained in their paper that this observation is the largest-scale TPW ever documented. It challenges the idea that the spin axis was largely stable for the past 100 millions years.
Earth is composed of a solid inner core of metal and a liquid outer core of metal, with a crust (the top) and solid mantle. The liquid metal moves slowly on top. Rocks such as the ones here are evidence of Earth's magnetic field. It is generated by the outer core.
Polar wandering is when the geographical poles shift significantly, so that the Earth's outer wrapping tilts over. While the Earth's magnetic field does not change, however, shifting rocks will report different paleomagnetic data.
This data allows researchers to determine the distance between the North and South poles. A rock at the pole is a rock with a vertical field, while one at the equator is a rock with a horizontal field.
Joe Kirschvink from Japan's Tokyo Institute of Technology says, "Imagine looking up at Earth from space." True polar wander would appear like the Earth tilting on its side. But, what is actually happening is that the entire rocky shell of Earth's solid mantle is rotating around its liquid outer core.
Part of the extraction work. (Ross Mitchell)
The study found that Earth had a tilt of 12 degrees around 84 million year ago. This tilt was quickly corrected within 5 million years.
This means that the rocks and Italy themselves traveled towards the Equator before tipping back. A second study has shown that the climate change is causing a similar tilt.
According to the team, previous studies that suggested that true polar wander did not occur during the Late Cretaceous were incomplete. This is something that cannot be said about the new research.
Richard Gordon, a geophysicist from Rice University in Houston, said that "that is one reason why this study with its abundance and beautiful paleomagnetic information is so refreshing." He wasn't part of the study.
Nature Communications published the research.
