Few of us give much thought to Earth’s swirling, spinning contents until some sudden movement, an earthquake or a volcanic eruption, jolts us to our senses.
Geoscientists, though, are a little more clued into the dynamics of Earth’s guts, and have just discovered that Earth’s solid inner iron core – which usually spins within a near-frictionless molten outer envelope – appears to have slowed to a grinding halt.
Before anybody panics and searches for a copy of a terrible 20-year-old science fiction movie predicting such an event in hopes of inspiring a solution, it’s not the first time record of such an event. It’s not even the first in recent history.
“We show surprising observations that indicate the inner core has nearly ceased its rotation in the recent decade and may be experiencing a turning-back in a multidecadal oscillation, with another turning point in the early 1970s,” geophysicists Yi Yang and Xiaodong Song of Peking University in Beijing write in their published paper.
We’ve only known for a few short decades that Earth’s inner core rotates in relation to the mantle above it, since it was confirmed in 1996 by Song and fellow seismologist Paul Richards at Colombia University. Before their work, the idea that Earth’s inner core rotates separately from the rest of the planet was an unproven theory, predicted by an unproven model of Earth’s magnetic field.
Since then, earth scientists have been trying to figure out – from a distance of 5,100 kilometers (or 3,170 miles) – how fast or slow the inner core spins.
At first, the inner core was thought to make a full revolution every 400 years, driven by electromagnetic torque and balanced by gravitational pull. But other scientists soon theorized that it spins much slower, taking 1,000 years or more to fully revolve.
The speed of this rotation, and whether it varies, is still debated today. Yet the inner core carries on its merry way, unaware of the raucous debate above.
Weighing back in, Song returned to the same method he and Richards used to infer that the inner core rolls around. In 1996, the duo tracked seismic wave readings from repeated earthquakes called doublets that traversed through the inner core, from the south Atlantic to Alaska, between 1967 and 1995.
Had the inner core not moved, the shock waves should have traversed the same path. But Song and Richards showed that the seismic waves got a fraction of a second faster from the 1960s to 1990s.
Now, in the new study with Yang, Song has revisited that old data, comparing it to more recent patterns of near-identical seismic waves which suggest the inner core has slowed to a stop – and could even be reversing.
They found that since around 2009, paths that previously showed significant temporal variation have exhibited little change as seismic waves coursed through the core and out the other side. Any time difference had disappeared.
“This globally consistent pattern suggests that inner-core rotation has recently paused,” Yang and Song write.
It also seems that this recent stalling of the inner core is associated with a rotation reversal, Yang and Song say, the solid iron sphere slipping back the other way as part of a seven-decade oscillation.
Based on their calculations, a small imbalance in the electromagnetic and gravitational forces would be sufficient to slow, and then reverse the inner core’s rotation as observed.