A study published Monday (23) by two Chinese geologists helps clarify the mystery of how Earth’s rocky core floats within the liquid outer core, rotating independently of the planet’s crust. The highly metallic rocky core, driven by Earth’s magnetic field, used to rotate slightly faster than the Earth’s surface in a clockwise direction as seen from the South Pole, but the scientists’ study shows that this disruption has recently stopped and should be restarted soon, but in a clockwise direction.
Recently, the rotation of the Earth’s core has slowed
To study the rotation of the inner core, which is invisible to ordinary geological devices, scientists Xiaodong Song and Yi Yang of Peking University analyzed the frequency of “double” earthquakes: pairs of earthquakes that have a common origin. They occur when the shock wave generated by a tremor travels through the center of the Earth until it is detected elsewhere, with an almost identical shaking pattern.
By observing the differences between these pairs of earthquakes, it is possible to calculate the influence of the planet’s inner core on their occurrence, and thus to infer their rotation. The now-released research, detailed in an article by the scientists in the journal Nature Geoscience, provided the longest and most accurate series of measurements of the phenomenon, with results that surprised scientists.
“We analyzed data from the early 1990s and showed that seismic waves with trajectories that previously showed time differences have now stopped showing in the past decade,” Song and Yang wrote. “This consistent pattern across the globe indicates that the rotation of the inner core has recently stalled.”
The Earth’s core rotates in the opposite direction of the planet’s rotation
According to the scientists, the rotation of the inner core follows a different pattern than that of the surface because it is governed by two forces. While the magnetic field encourages rotation against the general direction of the planet’s rotation, the gravitational pull of Earth’s entire mass pulls the inner core in the other direction. Rotation misalignment occurs due to differences in the strength and direction of these two factors.
What would happen if the rotation of the Earth’s core reversed?
Although a huge event for geologists, the inner core’s rotation reversal is on an astronomically small scale, less than 1 degree per year, and doesn’t have much impact on life on the surface. Through scientists’ calculations of historical data, a relatively recent reversal may have occurred without scientists being able to measure it.
However, indications of this happening are found in historical data from seismographs in some parts of the world.
“We compared this newly observed pattern with seismic records of twin tremors in Alaska and the Sandwich Islands (in the South Atlantic) obtained since 1964, and it appears to be associated with a gradual inner-core reversal that will be part of the oscillation for about 70 years,” the scientists write.
When did the last reversal in the Earth’s core rotate?
This period is not fixed, the researchers say, and the last reversal appears to have occurred in the early 1970s. However, the degree of certainty about the estimate is not high, because tremor measurements at the time were less frequent and less accurate.
What made the Earth’s core reverse its rotation?
The theoretical model that the Chinese now say has no practical bearing on how geologists deal with earthquakes or geomagnetism, but it should help improve theories that explain how the Earth changes over the ages.
It is not clear whether the oscillation is related to, for example, the reversal of the Earth’s magnetic field, when the south and north poles reverse, about every 500,000 years. However, the roughly 70-year cycle may help explain other geophysical phenomena more accurately, such as microscopic fluctuations in day length and in the planet’s magnetic field.
“It is interesting to note that a similar multi-decadal periodicity is observed in the Earth’s climate system, particularly with respect to global temperature and sea level rise,” they report.
These changes are small compared to the severity of the warming caused by increased carbon dioxide today, but they allow us to better understand how the planet functions on a longer time scale.
The authors conclude that “the multi-decadal periodicity of the climate system may originate from oscillations in the mantle-core relationship.”
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