True polar migration: a shift 84 million years ago

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Left, the earth rotates almost vertically in relation to its axis of rotation, as we know it today. On the right, a proposed shift due to real polar wandering. Image via USGS / Wikimedia Commons.

True Polar Migration: A Big Shift

The Tokyo Institute of Technology announced on October 18, 2021 that one of its researchers helped find new evidence for the so-called true polar migration. It is the still controversial idea that the solid outer shell of the earth was formed about 84 million years ago, during the late Late Cretaceous Period Period. A opinion from Tokyo Tech said:

Hold on to your hats as scientists have found more evidence that the earth tips over from time to time. We know that the continents move slowly because of plate tectonics. But the continental drift only pushes the tectonic plates past each other. Scientists have debated in recent decades whether the earth’s outer, solid shell can wobble or even tip over relative to the axis of rotation. Such an earth shift is called true polar migration. But the evidence for this process is controversial. New research released in Nature communication … provides some of the most compelling evidence yet that such a planet overturn actually occurred in Earth’s past.

geologist Ross Mitchell at the Institute of Geology and Geophysics in Beijing directed the new research. Joe Kirschvink of Tokyo Tech and also Caltech was part of the research team.

A controversial idea

Kirschvink in particular has long specialized in the measurement of residual magnetic fields in rock. These measurements can reveal the latitude at which the rock formed millions or billions of years ago. According to a 2016 article in science:

Technology led [Kirschvink] to strong, influential ideas.

[For example], 1992, he compiled evidence that glaciers nearly covered the earth more than 650 million years ago. He suggested that their subsequent withdrawal from Snowball Earth (a term he coined) sparked an evolutionary sweepstakes leading to the Cambrian explosion 540 million years ago. And he was prominent among a group of scientists who argued in the 1990s and 2000s that magnetic crystals in a famous Mars meteorite, Allan Hills 84001, were fossilized signs of life on the red planet …

“He’s not afraid to get on his feet,” says Kenneth Lohmann, a neurobiologist studying magnetoreception in lobsters and sea turtles at the University of North Carolina, Chapel Hill. “He was right about some things and wrong about others.”

Smiling friendly looking man with brown hair and graying mustache.
Geologist Joe Kirschvink from the Tokyo Institute of Technology and Caltech. “He’s not afraid to get on his feet,” remarked another scientist. Picture over Caltech.

True polar wandering is not this

  • Not a geomagnetic reversal or reversal of the Earth’s magnetic field known to have occurred previously in Earth’s history.
  • Not plate tectonics, which describes the large-scale movements of large land plates on earth and is driven by the circulation of the earth’s mantle.
  • No precession of the earth, as a result of which the axis of rotation of our world moves slowly and draws a circle between the stars, as a result of which the identity of our north star changes over time.

True polar wandering is this

True polar wandering suggests that if an object of sufficient weight on Earth – for example, an oversized volcano or other heavy land mass – formed far from the Earth’s equator, the force of the Earth’s rotation would gradually pull the object away from its axis that the earth turns. An oversized volcano far from Earth’s equator would become one Imbalance, in other words. The Tokyo Tech statement stated:

The earth is a layered sphere. It has an inner core made of solid metal, an outer core made of liquid metal, and a solid mantle and crust looming over the surface on which we live. All of this spins like a top once a day. Since the outer core of the earth is liquid, the solid mantle and crust can slide over it. Relatively dense structures, such as subduct oceanic plates and massive volcanoes like Hawaii prefer being near the equator. Likewise, your arms like to be stretched out to your sides when you turn in an office chair.

Joe Kirschvink was the first to suggest a true polar walk in 1997. And scientists are very accurately measuring a small amount of true polar migration that occurs today with satellites. But over the past few decades geologists have been hotly debating whether there have been major rotations of the Earth’s mantle and crust in the Earth’s past. And a particular point of contention is whether such a large shift occurred 84 million years ago, during the late Cretaceous period. There was public discussion about it in the magazine science, and at numerous science meetings, these scientists said.

Circle in the middle with loops, which are marked with N at the top and S at the bottom.
Scientists see the Earth’s magnetic field as dipole (ie bar magnet) positioned in the center of the earth and aligned with the earth’s axis of rotation. The record of the strength and direction of the earth’s magnetic field is preserved by many rocks from the time they were formed. This is called Paleomagnetismor fossil magnetism. Picture and caption above Geo.mtu.edu.

Fossil magnetism tells the story

The Tokyo Tech statement stated:

Despite this migration of the crust, the earth’s magnetic field is caused by electrical currents in the convective liquid nickel-iron metal of. generated Outer core of the earth. On long time scales, the overlying mantle and crust migration does not affect the core. This is because these overlying rock layers are transparent to the earth’s magnetic field.

In contrast are the convection patterns [a roiling motion, like you see in boiling water] in this outer core are forced to dance around the earth Axis of rotation. This means that the overall pattern of the Earth’s magnetic field is predictable. The field spreads like iron filings lined up over a small bar magnet. These data therefore provide excellent information about the direction of the geographic north and south poles. And the slope indicates the distance from the poles (a vertical field means you are at the pole, horizontal tells us it was on the equator).

Many stones record the direction of the local magnetic field as they are formed, much like a magnetic tape would record your music. For example, tiny crystals of the mineral Magnetite – produced by some bacteria – line up like tiny compass needles. They become trapped in the sediments when the rock solidifies. this fossil magnetism can be used to track where the spin axis moves relative to the crust.

Kirschvink added:

Imagine looking at Earth from space. True polar wandering would look like the earth is falling on its side. And what happens is that the entire rocky shell of the planet – the solid mantle and crust – rotates around the liquid outer core.

Men stand on a street and examine rock layers in the steep street section.
Scaglia Rossa limestone exposed near Furlo, Italy, in the northern Apennines. Limestone in this location accumulated at the bottom of a shallow sea in an arm of the ancient Mediterranean almost 85 million years ago, during the so-called Late Cretaceous Period. Image via Ross Mitchell / Tokyo Tech.

Collection of data in Italy

As a student, Ross Mitchell – the lead author of the new study – studied the geology of the Apennines in central Italy. So for this study he knew exactly the right rocks for fossil magnetism. Tokyo Tech explains:

The international research team then placed their bet. They bet that paleomagnetic data from limestones formed in Italy during the Cretaceous Period (~ 145.5 and 65.5 million years ago) would provide a definitive test. Scientists had already studied the magnetism of the younger rocks in the same area almost 50 years ago. These data had indirectly led to the discovery of the asteroid Impact that killed the dinosaurs.

The team said in their statement:

To test [our] True polar migration hypothesis, paleomagnetic data with a lot of redundancy is required. Previous studies, particularly some that claimed that true polar migration does not occur, have failed to examine enough data points.

The statement of the scientists is also quoted Richard Gordon, a geophysicist at Rice University in Houston who was not involved in the study. Gordon commented:

This is one reason why it is so refreshing to see this study with its abundant and beautiful paleomagnetic data.

A cosmic yo-yo

Ross, Kirschvink, and colleagues found that, as the true polar migration hypothesis predicted, the Italian data indicate that the planet was tilted about 12 degrees 84 million years ago. The team also found that the earth apparently corrected itself after tipping on its side. It appears that the earth has reversed course, precisely reversing itself in about five million years for a total displacement of nearly 25 degrees of arc.

These scientists said:

Surely this was a cosmic one yo-yo.

Close up of rock layers plus 8 small, round, dark holes in the rock.
High-resolution sampling on the road west of the Apiro Dam in the central Apennines in Italy. This particular locality crosses the boundary of a major geomagnetic reversal known as the Chron 33R / 33N transition, which was dated nearly 80 million years ago. An astonishingly high proportion of oriented samples from these sites provide excellent records of the ancient magnetic field at the time it was created.
Image via Ross Mitchell / Tokyo Tech.

Conclusion: Researchers used the fossil magnetism of rocks in Italy to obtain large amounts of data. The data provided a record of true polar migration – a large shift in the Earth’s outer shell relative to its axis of rotation – 84 million years ago.

Source: A real polar migration oscillation from the late Cretaceous period

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