Something Weird is Happening to Earth’s Inner Core
A controversial theory that the spinning of the inner core has slowed appears to be supported by a recent investigation. What might this imply for our comprehension of the internal dynamics of Earth?
Kindly, Reader:
Yes, there is a bizarre thing going on at the center of the Earth. A study published June 12 in Nature reveals that since about 2010, Earth’s inner core has been rotating more slowly than its mantle and surface based on decades of earthquake data. This discovery seems to support a contentious theory put forth last year that the inner core might have rotated in the opposite direction with respect to the mantle and surface—a change that might happen around every 35 years.
The most recent turnaround may have been hampered by something, according to the current study, according to geophysicist John Vidale of the University of Southern California in Los Angeles. “It’s retreating at a slower pace than its advance.”
The inner core continues to rotate in the same direction as the surface and mantle in an absolute sense. Envision a truck and bus traveling in the same direction alongside one another. When the truck slows down and the bus accelerates forward, the truck appears to be traveling backward from the bus’s point of view. However, it appears to a pedestrian like both cars are moving forward. In a similar vein, the inner core of Earth would appear to be rotating in the opposite direction than it did a few decades ago to someone standing on the planet’s surface.
While the 2023 study garnered media attention, other researchers had differing opinions. Some, such as seismologist Lianxing Wen of Stony Brook University in New York, contended that the data might be explained by the inner core’s surface moving in shape rather than the inner core rotating on its own. Some others thought that the rotation changed more quickly. An alternative examination of the 2023 data indicated an oscillation lasting 20 to 30 years, in contrast to a research Vidale coauthored the previous year that proposed an oscillation lasting 6 years.
Vidale and his colleagues looked at recurring earthquakes in the South Sandwich Islands, close to Antarctica, from 1991 to 2023. recurring earthquakes are those that strike the same location at different periods. Some of the seismic waves from these earthquakes made it all the way to the inner core of the planet. Waveforms are squiggly line graphs that represent the ground shaking that were captured by instruments in Alaska.
Vidale and his colleagues looked for matching waveforms that were months or years apart. If the Earth’s inner core rotates independently of its outer layers, waves from recurrent earthquakes should cross different regions of the core, resulting in unique waveforms. There should be some waveforms that are the same from before and after the turnaround if the 2023 study is accurate and the inner core’s rotation has reversed.
After comparing 200 waveforms, the scientists discovered 25 matches. According to these measurements, the inner core’s rotation relative to the mantle appears to have flipped around 2008, and it then rotated in the new direction at a rate that was less than half as fast.
Vidale speculates that the slower backtracking could mean that the mantle, which makes up around 70% of Earth’s mass, is exerting its gravitational attraction on the inner core, causing it to distort. The inner core may be kneaded by denser mantle pockets as it churns, warping the oscillation. It makes sense to believe that the outermost region of the inner core is soft because we know its surface is exactly near the melting point, according to Vidale.
Vidale concurs with the findings of the 2023 study after observing how the waveforms align over time: the inner core’s gyration most likely oscillates on a period of about 70 years.
Wen, on his part, maintains that the data may be entirely explained by the expansion and contraction of some regions of the inner core’s surface. Over the course of a few months, these areas may rise or fall by up to a kilometer, changing the waveforms of recurring earthquakes.
The Australian National University in Canberra’s geophysicist Hrvoje Tkalčić says, “It is very likely the truth is somewhere in between.” Although seismologists appear to agree that the rotation of the inner core is separate and varies, Tkalčić notes that “we need more data to find the ultimate truth.” Divergent viewpoints arise from the numerous assumptions that researchers must make on the inaccessible areas of Earth’s interior.
In the next years, some clarity might become apparent. Vidale’s team has reason to believe that the inner core may soon resume a robust phase of the cycle if its rotation oscillates at the frequency they suspect. The inner core appears to have rotated extremely swiftly for a brief period of time about 20 years ago, and Vidale predicts that it should soon do so again. “We can probably gain a better understanding of what transpired back then by keeping an eye on it for the next five or 10 years.”
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