Magnetic signatures hidden inside rocks tell us a lot about Earth’s magnetic field and the way continents and tectonic plates have shifted across millennia – but for some periods, the geological record doesn’t make much sense.
A fresh analysis of rocks from one of these periods, the Ediacaran (about 630-540 million years ago), aims to solve a long-standing mystery: why is the magnetic record from this time showing wild and chaotic variations in the magnetic field, as if the continents were speeding unusually quickly across the planet’s surface?
An international team of researchers led by Yale University has found that this glitch wasn’t from the continents acting strangely. It was Earth’s magnetic field.
Related: Scientists Reveal The Most Detailed Geological Model of Earth’s Past 100 Million Years
They ran a detailed, layer-by-layer analysis of volcanic rocks from the Anti-Atlas mountain region in Morocco, yielded a trove of data that was more precise in terms of the magnetic field positions and the dates of these changes.
Subsequent data modeling showed these magnetic shifts happened over mere thousands of years, not millions – even faster than was previously thought. The best explanation is an erratic magnetic field around the planet, rather than tectonic plates moving unreasonably fast.
“We are proposing a new model for the Earth’s magnetic field that finds structure in its variability rather than simply dismissing it as randomly chaotic,” says geologist David Evans of Yale University.
“We have developed a new method of statistical analysis of Ediacaran paleomagnetic data that we think will hold the key to producing robust maps of the continents and oceans from that period.”
The new data ruled out some previous theories about the Ediacaran, including the idea of true polar wander, where the entirety of Earth’s outer crust and mantle moves significantly, while the poles stay in the same place.
Through a comparison with sedimentary rocks formed over a longer time period than the volcanic samples, the researchers saw an average of magnetic pole positions, finding that they didn’t shift a huge amount across the Ediacaran.
Put all that together, and we have the best model yet for how Earth’s magnetic field behaved over this time: rather crazily. As for what may have triggered the abnormalities, the researchers suggest the continuing formation of Earth’s core may have been at least partly responsible.
As analysis methods improve, more and more of our planet’s history is being interpreted through the geological record we’ve been left with. Not only does this record show evidence of the movements of rocks over lengthy periods of time, it can also give us an idea of when our planet has been hit by visiting objects from space.
We know that during the Ediacaran, the first complex life forms started appearing, and life on the planet would’ve looked very different to how it is now. What’s more, it now looks likely the continents weren’t acting strangely – but Earth’s magnetic field was. The findings can now in turn be used to inform further research.
“If our proposed, new statistical methods prove to be robust, we can bridge the gap between older and younger time periods to produce a consistent visualization of plate tectonics spanning billions of years, from the earliest rock record to the present day,” says Evans.
The research has been published in Science Advances.
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