Strange Patterns In Ancient Rocks Reveal Earth's Tumbling Magnetic Field, Not Speeding Continents

Magnetic deposits laid down during the Ediacaran Period, 630 million years ago to 541 million years ago, are not inexplicable fluctuations, some researchers argue. Instead, there is a pattern encoded in the rocks of Morocco’s Anti-Atlas Mountains consistent with rapid shifts in the planet’s magnetic field.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

The Earth’s magnetic field is stable enough that people used it to guide compasses until we invented GPS, but it’s not perfectly fixed. We can trace the wanderings of the north and south magnetic poles, moving up to 60 kilometers (36 miles) a year, and we know from magnetic records in rocks that in the past the poles have flipped many times, providing geologists with valuable tools. Yet strange as this behavior is, it’s nothing compared to what went on during the Ediacaran Period.

The mid-Ediacaran from 591-565 million years ago was marked by a dramatic weakening of the Earth’s magnetic field, sometimes to around one-tenth of its previous and subsequent strength. Some scientists have attempted to link this to the rise of the Ediacaran animals, the first large and mobile creatures to move through the waters. Whether or not that’s right, the anomalous behavior didn’t stop with weakening, becoming unstable as well.

The erratic magnetism has been recorded in rocks laid down at the time, but left a messy puzzle for geologists. Some have attributed the swift variations in field direction at certain locations to the tectonic plates racing around the planet at speeds far greater than today, carrying continents with them. It’s hard to identify what could provide the extraordinary forces that would be required, but the alternative, that the poles moved faster, has also presented puzzles.

Yale's Professor David Evans thinks the rocks of the Anti-Atlas not only tell us what happened, but could offer clues to why. “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,” Evans said in a statement. “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 Anti-Atlas is a range to the south of Morocco’s more famous High Atlas Mountains. Although the range is about 300 million years old, it contains rocks produced when volcanoes erupted during the Ediacaran Period, and the cooling lava took on the magnetic orientation of the field at the time. There is evidence that at the time, this was much further from the equator than today.

“Previous studies of rocks from this time period often employed traditional analytical tools that assumed the Earth’s magnetic field behaved similarly in the past as it does now,” said PhD student James Pierce. “We took a fresh approach. We were able to determine precisely how fast the Earth’s magnetic poles were changing by sampling for paleomagnetism at high stratigraphic (layer-by-layer) resolution and determining precise ages for these rocks.”

The team narrowed the period of wandering the rocks recorded to 568 to 562 million years ago, overlapping with the end of the field’s weakness. The rest of the Ediacaran didn’t show the same variation.

Although this approach helped reveal what happened, it made the question of how even more challenging, because the authors concluded changes thought to have taken place over millions of years happened in thousands instead. Unless the conclusions are badly wrong, that eliminates any chance the mountains could have moved a long way across the Earth in the interim – the poles must have shifted beneath instead.

Fortunately, however, the authors also found a clue to the reason, detecting a structure to the changes of magnetic orientation, albeit an odd one. They think that rather than shifting around the planet’s axis of spin, the poles flip-flopped as if they couldn’t make up their mind.

This still leaves the root of the mystery unsolved – why would the poles switch like this during one geologic period, but not before or afterwards? Another important question is how this movement relates to the weakness of the field previously and at the time.

However, while it might be hard to know where to start in answering those two questions, the work does at least make it easy for others to check the researchers’ conclusions using other lava deposits from the same era.

The authors also note that there is some evidence to suggest similar periods of magnetic weakness and rapid reversals in the Devonian and late Jurassic, implying repetition around 200 million years apart. It doesn’t seem this led to any mass extinction among the fish and dinosaurs of the respective eras, but we probably don’t have to worry about it for about 50 million years anyway.

“My entire career has been dedicated to charting the motions of continents, oceans, and tectonic plates over the Earth’s surface, throughout its history,” said Evans. “The Ediacaran Period in particular has posed a major barrier in that long-term goal, because global paleomagnetic data just didn’t make much sense. 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.”

The study is open access in Science Advances