The earth beneath our feet consists of a rocky crust, a semi-solid mantle and an iron core that’s believed to be as hot as the surface of the sun. The outer core is liquid, but the immense pressure at the center of Earth causes the inner core to be compressed into a solid ball of metal. Earth’s inner core spins, rotating in the same direction as the surface. However, the inner core can rotate independently of the surface because it is suspended in the liquid outer core.
Until recently, the inner core was believed to be spinning slightly faster than the surface of the earth. However, a study published in Nature Geoscience found evidence that the inner core has recently slowed its rotation — matching the spin of Earth’s surface — and may soon be rotating even more slowly. Is this a sign of the end of days? Not quite.
The study found evidence that Earth’s inner core has slowed down before, back in the 1960s, and then sped up again. Researchers believe the inner core’s rotational speed oscillates a bit like a pendulum, slowing down and speeding up over a 70-year cycle. Thus, the speed of the inner core matches the spin of Earth’s surface every 35 years or so, with the next occurrence expected to take place in the mid-2040s.
However, this interpretation of the data is controversial, largely because so little is known about the center of Earth. Let’s dig into what we do know.
Studying Earth’s Inner Core
The inner core sits about 3,200 miles (~5,100 kilometers) below Earth’s surface and is roughly the size of the dwarf planet Pluto. The inner core composition is mostly iron with some nickel, and the temperature is about 9,392 °F (5,200 °C), according to Space.com.
Given these facts about the inner core, studying it takes some ingenuity and patience since it cannot be directly observed. The most useful information has come from earthquakes and even nuclear explosions, which send seismic waves in all directions through the earth, including through the center of it.
This seismic activity is constantly monitored by seismometers located around the world. Some types of seismic waves can travel through solids but not liquids, and seismic waves travel at different speeds depending on the density and temperature of the rock. Therefore, they are used like an X-ray to learn about the inner core composition.
Danish seismologist Inge Lehmann used this technique to discover the very existence of Earth’s inner core, publishing her work in 1936.
Rotation of the Inner Core
In 1996, two researchers at Columbia University in New York found that the time it took for seismic waves to travel through Earth’s center had changed over the course of three decades, as reported in Nature. To explain this result, they proposed that the inner core was spinning slightly faster than Earth’s surface. Therefore, seismic waves that had originated from the same location — but years apart — were traveling through slightly different sections of the inner core.
One of these researchers used the same technique to examine seismic data from the 1990s and 2000s, again finding that the time it took for the seismic waves to travel through the inner core changed over time. When they looked at similar seismic data from 2009 to 2021, they were surprised to find very little change in the timing of seismic waves, suggesting that the inner core was rotating at the same speed as the surface of Earth.
By looking at earlier records of seismic activity, they found the same pattern of very little difference in the 1960s and 1970s. This led them to conclude that the rotation of the inner core regularly speeds up and slows down over the course of about 70 years, matching the speed of Earth roughly every 35 years.
These findings were published on January 23, 2023, and immediately misinterpreted by many in the popular press. Headlines declared that Earth’s inner core had stopped spinning and would soon be reversing direction. The authors issued a correction on January 31 to clarify that Earth’s core was still spinning in the same direction but more slowly.
Effects on Surface Dwellers
Slight changes in the rotation of the inner core have almost no effect on daily life. The researchers did report that their proposed 70-year oscillation coincides with minute changes to the length of days and Earth’s magnetic field.
The length of a day is known to increase by milliseconds over centuries because of friction caused by the moon’s tidal forces. When these predictable fluctuations are removed, there are still minuscule changes to the length of days that appear to track with the predicted 70-year oscillations in the inner core’s rotation.
These oscillations might also affect and be affected by the electromagnetic forces produced by the molten outer core. The heat energy released by the inner core is believed to cause the outer iron core to move. This movement generates electrical currents that produce a magnetic field that surrounds the planet. This magnetic field protects living things from damaging cosmic radiation and is essential for life as we know it. One hypothesis is that the electromagnetic forces produced by the outer core also influence the spin of the inner core.
There’s Still Much to Learn
Not enough is known about the center of Earth to draw any solid conclusions. Just a year earlier, a publication in Science Advances proposed a 6-year oscillation for the core’s rate of rotation. Others argue that changes to the surface of the inner core over time are a more plausible explanation for the seismic data.
As one of the scientists who proposed the 6-year oscillation told the New York Times, “No matter which model you like, there’s some data that disagrees with it.” But even so, the pieces will eventually come together into one clear picture as research continues, and one day, we may fully understand everything about Earth and its inner workings.
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