True polar wander: A shift 84 million years ago (2024)

True polar wander: A major shift

The Tokyo Institute of Technology said on October 18, 2021, that one of its researchers helped uncover new evidence for what’s called true polar wander. It’s the still-controversial idea that the solid outer shell of Earth might have shifted relative to Earth’s spin axis some 84 million years ago, during the late Late Cretaceous period. A statement from Tokyo Tech said:

Hold on to your hats, because scientists have found more evidence that Earth tips over from time to time. We know that the continents are moving slowly due to plate tectonics. But continental drift only pushes the tectonic plates past each other. Scientists have debated for the past few decades whether the outer, solid shell of the Earth can wobble about, or even tip over relative to the spin axis. Such a shift of Earth is called true polar wander. But the evidence for this process has been contentious. New research published in Nature Communications … provides some of the most convincing evidence to date that such planetary tipping has indeed occurred in Earth’s past.

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

A controversial idea

Kirschvink, in particular, has long specialized in measuring remnant 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:

The technique has led [Kirschvink] to powerful, influential ideas.

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

‘He’s not afraid to go out on a limb,’ says Kenneth Lohmann, a neurobiologist who studies magnetoreception in lobsters and sea turtles at the University of North Carolina, Chapel Hill. ‘He’s been right about some things and not right about other things.’

True polar wander is not this

  • Not a geomagnetic reversal, or reversal of Earth’s magnetic field, known to have happened before in Earth history.
  • Not plate tectonics, which describes the large-scale motions of great land plates on Earth and is thought to be driven by the circulation of Earth’s mantle.
  • Not precession of the Earth, whereby our world’s axis of rotation slowly moves, tracing out a circle among the stars, causing the identity of our North Star to change over time.

True polar wander is this

True polar wander suggests that if an object of sufficient weight on Earth – for example, a supersized volcano or other weighty land mass – formed far from Earth’s equator, the force of Earth’s rotation would gradually pull the object away from the axis around which Earth spins. A supersized volcano far from Earth’s equator would create an imbalance, in other words. The statement from Tokyo Tech explained:

The Earth is a stratified ball. It has a solid metal inner core, a liquid metal outer core, and a solid mantle and overriding crust at the surface, which we live on. All of this is spinning like a top, once per day. Because the Earth’s outer core is liquid, the solid mantle and crust are able to slide around on top of it. Relatively dense structures, such as subducting oceanic plates and massive volcanoes like Hawaii, prefer to be near the equator. In the same way, your arms like to be out to your side when you’re spinning around in an office chair.

Joe Kirschvink was the first to propose true polar wander in 1997. And scientists do measure a small amount of true polar wander occurring today, very precisely, with satellites. But, for the past few decades, geologists have hotly debated whether large rotations of Earth’s mantle and crust occurred in Earth’s past. And a particular point of contention is whether such a large shift occurred 84 million years ago, during the Late Cretaceous. Public arguments about it have appeared in the journal Science, and at numerous science meetings, these scientists said. Kirschvink added:

Imagine looking at Earth from space. True polar wander would look like the Earth tipping on its side. And what’s happening is that the whole rocky shell of the planet—the solid mantle and crust—is rotating around the liquid outer core.

Fossil magnetism tells the tale

The statement from Tokyo Tech explained:

Despite this wandering of the crust, Earth’s magnetic field is generated by electrical currents in the convecting liquid nickel-iron metal of Earth’s outer core. On long time scales, the overlying wander of the mantle and crust does not affect the core. That’s because those overlying rock layers are transparent to Earth’s magnetic field.

In contrast, the convection patterns [a roiling motion, like you see in boiling water] in this outer core are forced to dance around Earth’s rotation axis. That means that the overall pattern of Earth’s magnetic field is predictable. The field spreads out in the same way as iron filings lining up over a small bar magnet. So these data provide excellent information about the direction of the North and South geographic poles. And the tilt gives the distance from the poles (a vertical field means you are at the pole, horizontal tells us it was on the equator).

Many rocks record the direction of the local magnetic field as they form, in much the same way that a magnetic tape records your music. For example, tiny crystals of the mineral magnetite – produced by some bacteria – line up like tiny compass needles. They get trapped in the sediments when the rock solidifies. This fossil magnetism can be used to track where the spin axis is wandering relative to the crust.

Gathering data in Italy

As a student, Ross Mitchell – the lead author of the new study – studied the geology of the Apennine Mountains of central Italy. So, for this study, he knew just the right rocks to sample for fossil magnetism. Tokyo Tech explained:

The international team of researchers then placed their bet. They bet that paleomagnetic data from limestones created in the Cretaceous (between ~145.5 and 65.5 million years ago) located in Italy would provide a definitive test. Scientists had already studied the magnetism of the younger rocks in the same area, nearly 50 years ago. Those data had indirectly led to the discovery of the asteroid impact that killed the dinosaurs.

The team said in their statement:

To test [our] hypothesis about true polar wander, paleomagnetic data with lots of redundancy are required. Prior studies, especially some claiming that true polar wander does not occur, have failed to explore enough data points.

The scientists’ statement also quoted Richard Gordon, a geophysicist at Rice University in Houston who wasn’t involved in the study. Gordon commented:

That 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 wander hypothesis predicted, the Italian data indicate an approximately 12-degree tilt of the planet 84 million years ago. The team also found that Earth appears to have corrected itself after tipping on its side. It appears that Earth reversed course and rotated right back, for a total excursion of nearly 25 degrees of arc in about five million years.

These scientists said:

Certainly, this was a cosmic yo-yo.

Bottom line: Researchers used the fossil magnetism of rocks in Italy to acquire a large amount of data. The data provided a record of true polar wander – a major shift in the outer shell of Earth relative to its spin axis – 84 million years ago.

Source: A Late Cretaceous true polar wander oscillation

Via Tokyo Tech

Deborah Byrd

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About the Author:

Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.

True polar wander: A shift 84 million years ago (2024)

FAQs

True polar wander: A shift 84 million years ago? ›

True polar wander: A major shift

What is the true polar wander event? ›

Abstract. The reorientation of Earth through rotation of its solid shell relative to its spin axis is known as True polar wander (TPW). It is well-documented at present, but the occurrence of TPW in the geologic past remains controversial.

What is the true polar wander effect? ›

Here, TPW is considered as a long-term (106 –107 year) process by which the solid Earth shifts in a secular manner beneath its spin axis. The resulting displacement of the rotational axis relative to an established geo- graphic reference grid gives rise to the term ''polar wander.

What is the polar wandering theory? ›

The apparent polar wander is the path that the magnetic pole appears to take according to the data on a continent. When multiple continents are moving relative to each other, the path their magnetic pole follows will be different from others.

What is the time period for Earth's polar wander? ›

Cases of true polar wander have occurred several times in the course of the Earth's history. It has been suggested that east Asia moved south due to true polar wander by 25° between about 174 and 157 million years ago.

What is the real reason for apparent polar wandering? ›

Thus, there can be three reasons for the existence of APW paths: A fixed pole and moving tectonic elements, called continental drift or plate motion (see Plate Tectonics); A moving pole and fixed tectonic elements, called true polar wander.

What is the true polar wander moon? ›

After each impact, the Moon gradually rebalanced itself around its spin axis. This kind of rebalancing, in which a planet or moon reorients itself while the spin axis continues to point in the same direction in space, is called true polar wander. In a study published in the Planetary Science Journal, David E.

When was the last magnetic reversal for the Earth? ›

The time intervals between reversals have fluctuated widely, but average about 300,000 years, with the last one taking place about 780,000 years ago.

What happens during a magnetic reversal? ›

By magnetic reversal, or 'flip', we mean the process by which the North pole is transformed into a South pole and the South pole becomes a North pole. Interestingly, the magnetic field may sometimes only undergo an 'excursion', rather than a reversal.

What is the pole fleeing theory? ›

Early last century Alfred Wegener proposed that the continents were once joined together into a single protocontinent called Pangaea. He believed the centrifugal force caused by the earths rotation caused this to move towards the equator and break up. He called this the “pole-fleeing" force.

What did the polar wandering study ultimately show? ›

Both groups found evidence that rocks had had moved relative to the earth's magnetic poles, so either the continents or the poles had moved.

How far does the Earth's magnetic North Pole wander in a year? ›

The most recent survey determined that the Pole is moving approximately north-northwest at 55km per year.

How long does it take for the Earth to change polarity? ›

It has always been a feature of our planet, but it has flipped in polarity repeatedly throughout Earth's history. Each time it flips – up to 100 times in the past 20 million years, while the reversal can take about 1,000 years to complete – it leaves fossilised magnetisation in rocks on Earth.

What is polar wandering best explained by? ›

Apparent polar wandering is best explained by the movements of continents. Instead of the Earth's magnetic poles significantly wandering or reversing, the continents themselves move, causing the perceived shift in the magnetic pole positions relative to the continents.

What did the phenomenon of polar wandering help explain? ›

Polar wandering refers to the motion of the entire solid Earth with respect to the spin axis, caused by centrifugal forces from Earth's orbit acting on mass anomalies in the upper mantle. It is closely linked to the supercontinent cycle.

What is the true polar north? ›

True north is the direction that points directly towards the geographic North Pole. This is a fixed point on the Earth's globe.

What did the discovery of polar wander curves show? ›

That polar-wandering curves for different continents (which show the paths of a magnetic pole with respect to a given continent) do not agree was one of the first important evidences for continental drift (the large-scale movements of continents and ocean basins relative to one another over geologic time).

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