Nature
PUBLISHED
Direct evidence of the movements of tectonic plates has been found in some of the world’s oldest rocks, in the Pilbara Craton, Western Australia. This evidence dates back 3.5 billion years; the geologists presenting it say continental movements may have worked quite differently back then.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Plate tectonics shapes life on Earth, from pushing up mountain ranges to separating species onto distant continents. It also brings plenty of death in the form of earthquakes. Many planetary scientists argue that the conditions for life itself would be more hostile, if not outright impossible, without the recycling of materials through the mantle that plate tectonics provides. All of which makes it a vital question in planetary science as to when these movements started.
Currently, it is very much an unanswered question. Some indirect evidence has been used to suggest tectonic plates are almost as old as the Earth’s 4.6 billion years. At the other extreme are suggestions that the whole thing started 0.7 billion years ago, at the end of the era known as the “boring billion”. Dr Alec Brenner of Yale University thinks that range can now be greatly narrowed.
“With this study, we're able to say three and a half billion years ago, we can see plates moving around on the Earth surface,” Brenner, lead author of the study, said in a statement.
The Pilbara Craton formed around 3.8 billion years ago, and provides us with some of the earliest evidence of life. The oldest crystals also lie nearby, although Canada has even older rocks. As part of his PhD at Harvard, Brenner participated in an expedition led by Professor Roger Fu to study the magnetism of the Pilbara rocks, which also found evidence of a previously unknown meteor impact.
Using 931 rock samples from more than 100 sites, the team identified changes in the direction of the Pilbara Craton’s magnetic fields with the time at which rocks were formed. The magnetism in the rocks reflects the local field at the time they cooled, which in turn is a result of the planet-wide geomagnetic field. The variations the team found were not just the reversals in direction we see when the Earth’s magnetic field flips, but more subtle changes in orientation.
By progressively heating the rocks to almost 600°C (1,100°F), by which point all magnetism was lost, the authors could differentiate between signals created at different times.
“We took a really big gamble,” said Brenner, now a postdoc at Yale. “Demagnetizing thousands of cores takes years. And boy, did it pay off! These results were beyond our wildest dreams."
These changes required either the magnetic poles to have wandered dramatically, or for the Craton to speed southward from 3.48 billion years ago. If the poles were wandering so dramatically at the time, we should see evidence elsewhere, and this has not been found.
Consequently, what is now the East Pilbara moved 24 degrees of latitude in 30 million years, landing up at 77° S, further south than much of Antarctica today. At its peak, the region appears to have been moving around 47 centimeters (19 inches) a year. That’s about seven times more rapid than any plate today. Evidence for even faster motion 2.77 billion years ago has been presented, but the cause remains a mystery.
The Pilbara also appears to have rotated clockwise by more than 100 degrees in that time, which is even more out of step with recent continental shifts. However, it doesn’t seem things were always this frenetic. Soon afterwards, the Pilbara moved too little to be measured for 25 million years, as if exhausted from its previous activity.
Not all the continents – such as they were at the time – were speeding around like this. Previous studies have found that South Africa’s Barberton Greenstone Belt, one of the few places in the world with rocks of similar age, was nearly stationary at the time.
The current form of plate tectonics is known as an “active lid” in contrast to the “stagnant lid” or “sluggish lids” some geologists think existed in the Earth’s earliest days – although not all agree. A planet where at least one continent was moving faster than anything today may need a new name. The authors consider the possibility of an “episodic lid”, and the delightfully named “Plutonic squishy-lid”, but more continents will need to be tested to provide an answer.
Nevertheless, this work appears to rule out the stagnant lid still being in place 3.5 billion years ago. “We're seeing motion of tectonic plates, which requires that there were boundaries between those plates and that the lithosphere wasn't some big, unbroken shell across the globe, as a lot of people have argued before,” Brenner said. “Instead, it was segmented into different pieces that could move with respect to each other.”
Although they were looking for continental movement, not geomagnetic reversals, the team nevertheless found evidence of the oldest flip in the planetary field ever identified, 3.46 billion years ago. The team think such reversals were rarer than in recent times. “It's not by itself conclusive, but it suggests that maybe the dynamo was in a slightly different regime than today,” Fu said.
The fact that no other planet or moon in the Solar System appears to have plate tectonics has left planetary scientists wrestling with the question of why Earth is different, and if this has always been the case. The first part remains unanswered, but it seems that for most of the Earth’s history, plate tectonics has been a feature.
The study is published in Science.
