Earth’s core rocks beneath our feet: it moves slightly back and forth every six years 

Earth's core rocks beneath our feet: it moves slightly back and forth every six years

Earth’s core rocks beneath our feet: it moves slightly back and forth every six years 

Scientists have discovered that the inner core of our earth fluctuates somewhat compared to the rest of the globe. Every six years it moves a little faster against the Earth’s mantle and crust, and then a little slower. That would also explain why a 24-hour period every six years gets a little longer, then shorter again.

The earth rotates on its axis every day. A stable movement that we don’t notice, you would think. But now it’s not as stable as we thought: The core of our globe speeds up and slows down every few years, according to a new study published in the scientific journal Science Advances. It was one of the theories that scientists had considered for decades, but for which evidence had never been found. 

Hot sphere iron

The Earth’s core, as the word implies, is the innermost part of the Earth, almost 3,000 kilometers deep. This core consists of an inner part that is solid, with a liquid outer core around it. Both consist of metals, mainly iron. The outer core is liquid because the temperature is thousands of degrees high, but in the inner core, the pressure is so high that the metal solidifies into a sphere. That sphere is about the size of Pluto and grows slowly 

That sphere rotates, just like our entire globe. But in the 1990s, scientists discovered that the core doesn’t rotate quite a in sync with the rest of the Earth. This is possible because the mass around the inner core is liquid. It rotates slightly faster than the Earth’s mantle and crust, about one degree per million years. A fixed pattern, they thought.

Now, surprisingly, scientists have found that that core doesn’t consistently do that. According to new research, the inner core may fluctuate slightly in relation to the outer core, moving a little faster for six years, then a little slower. “The inner core moves under our feet, and seems to go back and forth for several miles every six years,” said American professor of Earth Sciences John E. Vidale, co-author of the new study .

The Soviet Union and Antarctica

How do you measure the movement of such a hot mass, thousands of kilometers deep? To do this, scientists work with seismographs, devices that measure the movement of earthquakes or explosions. Nuclear tests are also traced in this way.

When a number of these devices are set up near each other as a network, researchers can follow the movement precisely, also in history. What’s more, they use the measurements that took place during underground experiments with nuclear bombs decades ago. 

“The vibrations caused by an experiment with an atomic bomb go all the way through the Earth’s crust and mantle and bounce like a ping-pong ball against the hard inner core, back to the surface,” explains seismologist Koen Van Noten of the Royal Observatory. They are thus also influenced by the movement of the Earth’s core. If the vibration landed a little differently after the bounce than where you’d expect it to, the researchers know that Earth’s core was moving differently, and in what direction. 

The Earth’s inner core moves beneath our feet Professor John. E. Vidale, co-author of the study

For example, the researchers of this study had previously tried out a new method on data from experiments that the Soviet Union had carried out in the Arctic island group Nova Zemlya from 1971 to 1974. Now they did the same with data from US experiments on Amchitka Island from 1969, off Alaska, and saw that the core was spinning just a little slower than the rest. So a fluctuating speed.

Length of a day

Yet this theory has already been circulating in science before, as a possible explanation for this phenomenon: the length of a 24-hour period varies minimally. Every six years, the Earth takes about 0.2 seconds less to rotate on its axis, and then longer again. So a fluctuating duration, which can be explained by the fluctuating core. But so far there has been no tangible evidence pointing in that direction.

“It was always thought that the core moves faster than the surface and therefore pulls along,” Van Noten summarizes. “But if it moves in the other direction, the earth slows down for a while. And then it takes a day longer.”

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