Earth is the only place in the Solar System where plate tectonics – the creation, movement, and destruction of continental and oceanic masses of crust – are known to still operate. They ensure that Earth’s volcanic activity, earthquake patterns, and even its entire surface remain unique.
It’s important to remember that plate tectonics weren’t always around, though – and neither were our present-day continents. In fact, a new Nature study suggests that just after the planet’s formation, there was just one hypercontinent covering the entire planet like a crusty shell.
This also means that plate tectonics, which are essentially driven by heat escaping the hellish depths of the planet, did not start immediately after Earth first formed 4.5 billion years ago. Most researchers assumed that it did, but this team claims that, for a while, the surface of the planet cooled enough to form an outer crust.
“Models for how the first continental crust formed generally fall into two groups: those that invoke modern-style plate tectonics and those that do not,” co-author Michael Brown, a professor of geology at the University of Maryland, said in a statement. “Our research supports the latter – a 'stagnant lid' forming the planet's outer shell early in Earth's history.”
In order to get to this conclusion, the team found some of the oldest continental rocks on the planet. Hailing from Western Australia, they are around 3.5 billion years old. They’re granites with a mineral composition that resemble the sort of stuff you’d find near Mount St. Helens or Mount Fuji – volcanoes that today require plate tectonics to grow.
At a glance, then, these granites imply that plate tectonics were probably operating for at least 3.5 billion years – and with plate tectonics come multiple continents and basins.
The team, however, wondered if this wasn’t true at all, and that you could make these granites without invoking plate tectonics.
Plate tectonics didn't really get started on Earth for around 1-1.5 billion years. Budkov Denis/Shutterstock
Nearby ancient basalt rocks – the very same type that form around Hawaii – were also looked at, and the team wondered if there was a way to turn them into the plate tectonic-linked granites. Using a careful series of modelling experiments, the team concluded that this was possible so long as the planet was incredibly hot very close to the surface.
There’s only one way this shallow, high-temperature zone could have existed – if the planet was entirely covered in a crustal shell that trapped the heat inside. This would have allowed the basalts to sufficiently melt and transform into the granites, all without the need for plate tectonics.
This means that plate tectonics, which would have broken up the shell, did not begin for at least a billion years after the planet first formed – and that Earth’s first continent was global in size.