A craton’s clues to Earth’s moving plates

New research suggests plate tectonics may have been well under way on Earth more than 3.2 billion years ago, adding a new dimension to the ongoing debate about exactly when they began influencing the early evolution of the planet. 

A team from Harvard University, US, says analysis of lingering magnetism in rocks from the Pilbara Craton, a stable block of crust in Western Australia, provides evidence for a large change in the latitude of the block relative to the Earth’s magnetic poles between 3.35 and 3.18 billion years ago. 

This would push back the date for the onset of modern plate tectonics from about 2.8 billion years go to 3.2 billion years ago.

It also supports the theory, the researchers say, that changes in the crust have resulted from continuous, uniform processes similar to those we observe today – or at least from intermittent switching between episodes of movement and immobility. 

“Basically, this is one piece of geological evidence to extend the record of plate tectonics on Earth farther back in Earth history,” says Alec Brenner, one of the lead authors of a paper in the journal Science Advances.

“Based on the evidence we found, it looks like plate tectonics is a much more likely process to have occurred on the early Earth and that argues for an Earth that looks a lot more similar to today’s than a lot of people think.”

Earth’s outer shell comprises about 15 rigid blocks of crust, and the movement of these plates shaped the location of the continents, helped form new ones and created landforms like mountain ranges. 

It also exposed new rocks to the atmosphere, which led to chemical reactions that stabilised Earth’s surface temperature over billions of years.

“We’re trying to understand the geophysical principles that drive the Earth,” says Roger Fu, another lead author. “Plate tectonics cycles elements that are necessary for life into the Earth and out of it.”

A craton is a thick and very stable piece of crust usually found in the middle of tectonic plates. The Pilbara Craton is close to 500 kilometres wide, and rocks there formed as early as 3.5 billion years ago.

Fu and Brenner took core samples from a portion of the craton called the Honeyeater Basalt, then examined them to find their magnetic history.

They comparing this with data collected by other researchers in nearby areas to date when the rocks shifted from one point to another, and found a drift of 2.5 centimetres a year.

Their work differed from most studies, they say, because they focused on measuring the position of the rocks over time rather than the chemical structures in the rocks that suggest tectonic movement.

In their paper, they point out they weren’t able to rule out a phenomenon called “true polar wander, which also can cause the Earth’s surface to shift. 

However, their results lean more towards plate tectonic motion, they say, because of the time interval of this geological movement.