The Earth’s geographic poles – the points where the imaginary axis of the planet’s rotation is located – may be far less immutable than previously known, with a new study finding that ancient sea-level changes provide a clue to their movement.
Earth has two main pairs of poles: its magnetic poles, resulting from its magnetic field, and its geographic poles based on its rotation. The magnetic poles are constantly in movement and are often hundreds of kilometres away from the geographic north and south poles. They can even flip altogether.
The geographic poles are much more stable. But they can move a little, through a phenomenon called true polar wander. This is driven by the way plate tectonics and mantle convection slowly redistribute mass in Earth’s mantle and affect the planet’s spin.
When the spin of the planet is thrown out of whack, the solid outer layers of the globe begin to move in order to help Earth find a new stable spin equilibrium. This has the effect of moving the geographic poles.
“The crust and the mantle effectively slip over the core during a true polar-wander event,” says Mathew Domeier at the University of Oslo in Norway.
However, until now much of the evidence for true polar-wandering events has come from ancient magnetic data locked inside rocks that formed millions of years ago, and this evidence has yielded confusing and conflicting results about the frequency and scale of the events.
So, Domeier and his team took a different approach. They analysed the geological record of sea-level change looking for distinctive signals that might be expected during a true polar-wander event.
Because of the way ocean water can more quickly adjust to the centrifugal potential changes than the rocky sea floor, true polar-wander events are expected to leave a telltale pattern. Sea level will rise in one half of the northern hemisphere and fall in the other half, and it will also rise in one half of the southern hemisphere and fall in the other half.
The northern and southern hemisphere signals are reversed, however, so if it is the western side of the northern hemisphere that experiences sea-level rise, it will be the eastern side of the southern hemisphere that does so, too, creating a beach ball-like global pattern.
“This is a very strange pattern that is not caused by other Earth processes,” he says.
On the time scales we experience, the events would be unnoticeable. But over longer periods of geological time, a wandering pole changes the distribution of land and sea.
For instance, from their analysis of ancient sea-level patterns, Domeier and his colleagues discovered that the geographic poles moved dramatically, over periods of around 10 million years, four times in the geological past: at 20, 90, 140 and 190 million years ago.
“By the timescales of human experience, the movement may not seem fast,” says Domeier. “We cannot estimate the exact speed, but we can recognise events where the rate of polar motion exceeded about 0.6 degrees per million years.”
He says this is faster than the rate at which most tectonic plates move – fast enough to potentially have an effect on climate systems.
What’s more, we know that true polar wandering continues today, although maybe not at the rate seen during the four fast bursts in the geological past.
“[It] is measurable by satellites at a rate of about 10 cm per year, but most people are unaware of this. So these events are totally unrecognisable according to our everyday experience,” says Domeier.
Sabin Zahirovic at the University of Sydney says very few people are aware that Earth’s 2800-km-thick outer shell of the crust and mantle are in motion with respect to Earth’s spin axis.
The study demonstrates that clues in the coastline change provide an independent fingerprint of true polar wander.
“This is a very exciting development that helps us link very deep Earth processes to changes in long-term climate and sea level on the planet,” says Zahirovic.