Earth's Inner Core Stopped Spinning Faster Than the Surface Around 2009

In 1936, the Danish seismologist Inge Lehmann noticed that earthquake waves arriving on the far side of the planet behaved as if they had passed through something solid in the middle of Earth's molten core. She published a one-page paper titled simply "P′" and announced the existence of the inner core. It was a ball of iron and nickel about 1,221 km in radius, sitting at roughly 5,400–5,700 K and 330 gigapascals — pressures so absurd that iron stays solid despite being hotter than the surface of the sun.

What Lehmann couldn't tell, but later seismologists could, is that this ball is not locked to the rest of the planet. In 1996, a study of seismic waves from repeating earthquakes near the South Sandwich Islands suggested the inner core was super-rotating, drifting eastward roughly half a degree per year faster than the mantle and crust above it. Subsequent work refined the figure and added a complication: the rotation isn't constant.

In January 2023, Yi Yang and Xiaodong Song at Peking University reported that the differential rotation appeared to have slowed to nearly zero around 2009 and was possibly reversing. By comparing decades of doublet earthquakes — pairs of quakes from the same fault, years apart — they argued the inner core oscillates on a roughly 70-year cycle, swinging slightly ahead of and slightly behind the rest of the planet. None of this is dangerous; it just means the deepest piece of Earth has its own slow weather.