Earth’s core

Why Does the Earth’s Core Mysteriously Wobble Every 8.5 Years?

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earth's core, artwork

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  • Every 8.5 years or so, the Earth emits a signal—the result of a “wobble” occurring in the inner core.
  • To explain this phenomenon, scientists from Wuhan University analyzed polar movement and length-of-day changes, and concluded that the inner core is 0.17 degrees off of the rotational axis of the mantle.
  • Although this idea goes against the conventional theory of the core-mantle relationship, it could help explain other geomagnetic phenomena.

Few things are more vital to our existence—yet less understood—than the Earth’s inner core. Surrounded by a cocoon of liquid metal (aka the outer core), this solid mass of nickel and iron at the planet’s heart is what creates the Earth’s magnetic field, which protects all life from harmful solar radiation. Despite this essential, life-sustaining function, the inner core remains a huge mystery for geologists eager to understand the deep workings of the world.

For example, scientists had noticed that the inner core tends to wobble, but had no clue why. Now, scientists at Wuhan University hope to at least check this one seismic oddity off the mystery list. Their study, published this month in the journal Nature Communications, argues that this “Inner Core Wobble” (ICW) occurs every 8.5 years and implies that the static tilt of the core is 0.17 degrees different from the rotation axis of the mantle. The team came to this conclusion by studying polar motion (PM) and length-of-day changes (ΔLOD).

“This implies a potential eastward differential rotation angle of the inner core of less than 1 degree and misalignment in the symmetry axes of the lower mantle/core-mantle boundary layer with the upper mantle,” Wuhan University research and study co-author Hao Ding said in an interview with Phys.org. “These deviations offer valuable constraints for the 3D density model of the mantle and question assumptions in the liquidity-core oblate, highlighting potential deviations from a perfectly spherical form calculated using traditional theories.”

In another study from 2018, Ding first discovered evidence of the 8.5-year harmonious signal while studying the subtle movement of Earth’s poles over time. The core’s tilt, as compared to the mantle, goes against the typical hypothesis that the tilt of those two layers are in sync. Ding also thinks that the origin of this “signal” could explain why the Earth’s magnetic field changes over time.

“The static tilt may also lead to a certain change in the shape of the liquid core, resulting in a change in the fluid motion and a corresponding change in the geomagnetic field,” Ding said in the interview.

While this theory explains some of the mysterious oddities surrounding the Earth’s inner core, it’s only one theory among several. As Vice notes, one idea points the finger instead at the push-and-pull forces between the Earth’s magnetic field and its gravitational field. Another focuses on the irregular shape of the core itself.

And while humans can’t simply travel to the center of the Earth (despite what sci-fi may tell you) geologists will continue analyzing the subtle hints—whether earthquake rumbles or harmonic wobbles—to figure out the amazing, life-sustaining machinations at the heart of the planet we call home.

Why Earth’s Inner Core May Be Slowing Down

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The planet’s solid inner core might rotate at a different rate than the rest of the planet, and that rate might be changing

Why Earth's Inner Core May Be Slowing Down

Earth’s core structure. Elements of this image were furnished by NASA.

The spin of Earth’s inner core may have slowed, with the heart of the planet now rotating at a slightly more sluggish clip than the layers above, new research finds. The slowdown could change how rapidly the entire planet spins, as well as influence how the core evolves with time.

For the new study, published in the journal Nature Geoscience, scientists used a database of earthquakes to probe the behavior of Earth’s solid inner core over time. The inner core sits suspended like a ball bearing in the molten-metal ocean of the outer core. Because of this liquid cocoon, the “ball bearing” may not spin at the same rate as the rest of the planet. Over the years, some researchers have found that the core rotates slightly faster than the mantle and crust, a condition called “super rotation.” But studies have not returned consistent numbers, with the first study to observe differential core rotation estimating that the inner core rotates up to one degree faster per year than the rest of the planet; others found an annual speedup of just tiny fractions of a degree.

These differences aren’t dramatic. The variation in rotation time between the inner core and the rest of Earth is very minor. Nor are the differences a threat to life on the surface: In contrast to the 2003 science-fiction movie The Core, there’s no need to call in a crack team of geophysicists and astronauts to drill to the center of our planet and start blowing things up. At most, the inner core rotation might influence Earth’s overall spin and contribute to fluctuations in the planet’s magnetic field. Each year the core expands by about a millimeter, as some of the molten iron in the outer core solidifies, seismic studies have shown. The solidification also drives the circulation of the outer core, which, in turn, creates the planet’s magnetic field. The rotation of the inner core could influence this solidification process in ways that are not yet fully understood, thus impacting the magnetic field, says study author Xiaodong Song, a geophysicist at Peking University in China.

The rotation might also matter for how the inner core grows over billions of years, says John Vidale, a geophysicist at the University of Southern California who was not involved in the study, but who has researched core rotation.

The catch, however, is that no one really knows how fast the inner core spins. In the new study, Song and geophysicist Yi Yang, also at Peking University, found that the core appeared to hold a steady spin, faster than the overall spin of Earth, between the 1970s and the early 2000s. Around 2009, though, that spin rather abruptly slowed to match Earth’s speed and then perhaps slowed so much that the rest of the planet now spins faster, Song says.

Song and Yang measured this spin by using pairs of almost-identical earthquakes that originated at the same spots, separated only by time. Because the quakes are nearly identical, their shock waves should also look identical when they travel through the core and back out, where they are detected by seismometers around the planet—that is, unless the core itself changes and alters the path of one earthquake’s waves relative to the other. If the core is spinning differently than the rest of the planet, identical earthquake waves that happen months or years apart will hit the core at slightly different spots and therefore bounce back with some subtle differences. The researchers compared quake waves going back to 1964 to track the changes in how the core might be moving over time. If they’re right, the spin of the core now lags that of the overall planet by a tiny amount.

“We are hypothesizing that this [slowed rotation] will continue in the coming years and decades, and we should be able to see that in [our] relatively short human time frame,” Song says.

The new findings likely won’t end the debate over the inner core. The work is well done and does an admirable job of combining different data, Vidale says. But there are several competing explanations for what’s going on. For example, Vidale’s research hints that the core may alter its rotation every six years or so, while researchers Guanning Pang and Keith Koper reported a single “lurch” in the early 2000s and little change since in a 2022 study. “I don’t view [the new work] as entirely conclusive,” Vidale says.

Lianxing Wen, a geodynamicist at Stony Brook University, who was not involved in the new study, also researches the core’s spin. He doesn’t believe the inner core spins any differently than the rest of the planet. A better explanation for the changes in seismic waves that travel through the core, Wen says, is that the surface of the inner core isn’t smooth like a ball bearing but rather uneven and constantly changing. “We believe the inner core has a shifting topography that best explains observed temporal changes of seismic waves that reflect off the inner core,” he says. The new research, Wen says, misinterprets these changes as caused by the core’s spin rather than to its fluctuating surface.

Fortunately, Song says, the seismic monitoring of Earth is better than ever, yielding far richer data about the planet’s interior than in earlier decades. By continuing to watch earthquake waves, the researchers should be able to show whether they’re right about the inner core’s spin.

“The exciting news,” Song says, “is that we don’t have to wait too long.”

Earth’s Core Appears to Have Stopped Spinning, Scientists Say

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Spin Cycle

According to a new study, the Earth’s inner core appears to have stopped spinning.

While that may sound bad, as Vice reports, scientists say it’s not actually a big deal.

The new findings, as detailed in a paper published in the journal Nature Geoscience, support the theory that the core comes to a halt and reverses direction every 60 to 70 years.

Measuring seismic waves from quakes deep beneath the surface, the researchers found that the Earth’s inner core “may be experiencing a turning-back in a multidecadal oscillation,” suggesting that there was “another turning point in the early 1970s.”

Flip It and Reverse It

The reasons for this switch back, the scientists believe, involve the Earth’s magnetic field — causing the planet’s mostly liquid outer core to move, thereby forcing the inner core to rotate — and gravity.

“The mantle and inner core are both highly heterogeneous, so the gravity between their structures tends to drag the inner core to the position of gravitational equilibrium, so-called gravitational coupling,” lead authors Yi Yang  and Xiaodong Song, a pair of researchers at Peking University, told Vice.

“If the two forces are not balanced out, the inner core will accelerate or decelerate,” they explained.

Molten Metal Onion

The theory could explain a number of observed phenomena, from cycles in the Earth’s climate system to the shifts in the length of a single day.

In fact, “the gravitational coupling between the inner core and the mantle may cause deformation at the Earth’s surface, which would affect the sea level,” the researchers told Vice, tying it to a number of other systems including “the global atmospheric circulation and temperature.”

Yang and Song are now working on building out their model, understanding how these mechanisms work, and how the inner core’s rotation will change going forward.

But for now, it’s a tantalizing glimpse into a possible relationship between the deepest inner workings of our planet and the effects it may have on life back on the surface.

Volcanoes Erupting All Over The World: Is Something Happening To The Earth’s Core?

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Within the last few months, there has been a lot of talk regarding volcanic eruptions, whether it was regarding volcanoes that have recently erupted or those that are expected to erupt in the near future. Just last week, one of the most powerful volcanoes found within Europe erupted for the second time in the last year.

Volcanic eruptions occur when magmas rise through the cracks and weak points in the Earth’s crust. As the pressure builds below the surface, once an opening releases the pressure due to a plate movement, the magma explodes, causing a volcanic eruption. However, with so much volcanic activity taking place throughout the world, we must question why the Earth is releasing so much magma. What exactly is taking place beneath Earth’s crust that would suddenly cause this much activity?

Despite the fact that Mount Etna may have been one of the only eruptions to make headlines, the fact is, that volcanic eruptions are presently taking place all over the world. Volcano Discovery, a website which reports on volcanic activity throughout the world shows 35 volcanos that are currently erupting or have recently erupted.

Included in those that have recently erupted, or that are currently active, is the Barren Islandvolcano located in India. In 1991, the volcano erupted for the first time in over 150 years, and since then, it has erupted intermittently.

Iceland is also experiencing an increase in volcanic activity. Four of Iceland’s volcanos are now showing increased amounts of activity, that would indicate the likelihood of yet another eruption. Included in those are Katla , which is now experiencing more activity than it has seen in over four decades. Pall Einarsson, a geophysicist stated that “Katla has been unrestful since this autumn.”

The others that are showing more activity are Hekla, Grímsvötn and Bárðarbunga.

Mexico’s Colima volcano has also recently erupted in February. The massive 12,500-feet volcano in Tuxpan, western Mexico has experienced an increase in eruptive activity since last October. The volcano itself is located just 30 kilometers away from a residential area that has a population of around 300,000 people.

To top the list, Ethiopia’s “gateway to hell” has also seen a surge in activity within the past few months. While this particular volcano has been active for over 100 years, it recently began baffling scientists after a variety of cracks were detected on the surface of the volcano. On top of that, its lava lakes have begun to overflow, which in turn has caused it to begin oozing red-hot magma.

And to put it into perspective, take a look at the following graphs depicting activity which occurred in the previous century compared to our current activity.

major

major2

Obviously, this massive increase in volcanic activity is not just taking place within one region. Even Campi Flegrei, a super volcano in Italy, appears to be preparing for a massive eruption. If it does, millions of people could die, and a countless number of others would be left in devastation. For whatever reason, something strange and mysterious is taking place beneath the ground we walk on.

 Has Mother Earth decided to enact some sort of self-defense mode in response to our constant exploitation of her resources? While scientists have been searching for an explanation behind the upsurge in volcanic activity, some have theorized that the increase is a normal response to Earth’s natural shift. Others, however, believe that it could have something to do with other forms of climate change as well. As it stands, we can only speculate as to what could be causing this massive shift beneath Earth’s crust. But as scientists continue to research volcanos and their activity charted over time, perhaps they will be able to better understand what is causing our planet to violently react.

Ironing out the details of the Earth’s core

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Identifying the composition of the earth’s core is key to understanding how our planet formed and the current behavior of its interior. While it has been known for many years that iron is the main element in the core, many questions have remained about just how iron behaves under the conditions found deep in the earth. Now, a team led by mineral-physics researchers at the California Institute of Technology (Caltech) has honed in on those behaviors by conducting extremely high-pressure experiments on the element.

“Pinpointing the properties of iron is the gold standard—or I guess ‘iron standard’—for how the core behaves,” says Jennifer Jackson, assistant professor of mineral physics at Caltech and coauthor of the study, which appears in the December 20 issue of Geophysical Research Letters. “That is where most discussions about the deep interior of the earth begin. The temperature distribution, the formation of the planet—it all goes back to the core.”

To learn more about how iron behaves under the extreme conditions that exist in the earth’s core, the team used diamond anvil cells (DAC) to compress tiny samples of the element. The DACs use two small diamonds to squeeze the iron, reproducing the types of pressures felt in the earth’s core. These particular samples were pressurized to 171 Gigapascals, which is 1.7 million times the pressure we feel on the surface of the earth.

To complete the experiments, the team took the DACs to the Advanced Photon Source at Argonne National Laboratory in Illinois, where they were able to use powerful X-rays to measure the vibrational density of states of compressed iron. This information allows the researchers to determine how quickly sound waves move through iron and compare the results to seismic observations of the core.

“The vibrational properties that we were able to measure at extraordinarily high pressures are unprecedented,” says Jackson. “These pressures exist in the earth’s outer core, and are very difficult to reproduce experimentally.”

Caitlin Murphy, a graduate student in Jackson’s group and first author of the paper, says the group was happy to find that their data set on the vibrational properties of iron evolved smoothly over a very wide pressure range, suggesting that their pressure-dependent analysis was robust, and that iron did not encounter any phase changes over this pressure range. To help achieve these successful measurements at high pressures, the group used some innovative techniques to keep the iron from thinning out in the DACs, such as preparing an insert to stabilize the sample chamber during compression. Additionally, they measured the volume of the compressed iron sample in situ and hydrostatically loaded the iron sample with neon into the sample chamber.

“These techniques allowed us to get the very high statistical quality we wanted in a reasonable amount of time, thus allowing us to obtain accurate vibrational properties of compressed iron, such as its Grüneisen parameter,” says Jackson. “The Grüneisen parameter of a material describes how its total energy changes with compression and informs us on how iron may behave in the earth’s core. It is an extremely difficult quantity to measure accurately.”

The team was also able to get a closer estimate of the melting point of iron from their experiments—which they report to be around 5800 Kelvin at the boundary between the earth’s solid inner core and liquid outer core. This information, combined with the other vibrational properties they found, gives the group important clues for estimating the amount of light elements, or impurities, in the core. By comparing the density of iron at the relevant pressure and temperature conditions with seismic observations of the core’s density, they found that iron is 5.5 percent more dense than the solid inner core at this boundary.

“With our new data on iron, we can discuss several aspects of the earth’s core with more certainty and narrow down the amount of light elements that may be needed to help power the geodynamo—the process responsible for maintaining the earth’s magnetic field, which originates in the core,” says Jackson.

According to Murphy, the next step is to perform similar experiments alloying iron with nickel and various light elements to determine how the density and, in particular, the vibrational properties of pure iron are affected. In turn, they will be able to evaluate the amount of light elements that produce a closer match to seismic observations of the core.

“There are a few candidate light elements for the core that everyone is always talking about—sulfur, silicon, oxygen, carbon, and hydrogen, for instance,” says Murphy. “Silicon and oxygen are a few of the more popular, but they have not been studied in this great of detail yet. So that’s where we will begin to expand our study.”

The study, “Grüneisen parameter of hcp-Fe to 171 GPa,” was funded by the California Institute of Technology, the National Science Foundation, and the U.S. Department of Energy. Bin Chen, a former postdoctoral scholar in Jackson’s lab, and Wolfgang Sturhahn, senior technologist at NASA’s Jet Propulsion Laboratory and visiting associate at Caltech, were also coauthors on the paper.

Source: California Institute of Technology