Plate tectonics began somewhere between 3 and 3.5 billion years ago, and as result, Planet Earth became a world full of volcanoes, earthquakes, oceans, and continents. We understand the processes that drive it, but how did it all begin?
A new Nature Geoscience study has a rather remarkable answer: massive asteroid and meteorite impacts kickstarted the formation of mountains, valleys and everything in between. They may have even helped trigger the formation of our planet’s magnetic field, without which life on Earth would all but cease to exist.
The international study, led by researchers at Macquarie University (MU), looked further back in geological time than most, right to the beginning of our planet, 4.5 billion years ago. Shortly after the planet formed from the embers of the Solar System’s violent birth, it developed a shell – a singular crust – that covered the entire world.
Then, between 4.1 to 3.5 billion years ago, the remnants of failed planets and comets began to rain down on the larger worlds out there, including our own. This was known as the Late Heavy Bombardment, and the team wanted to know if it had any effect on plate tectonics.
Setting up a cutting-edge computer model of the Hadean eon Earth, the team simulated several massive impacts on the crusted-over, superheated planet – including one proto-planetary body that was 1,700 kilometers (1,056 miles) across.
“Our results indicate that giant meteorite impacts in the past could have triggered events where the solid outer section of the Earth sinks into the deeper mantle at ocean trenches,” lead author Associate Professor Craig O’Neil, from MU, said in a statement.
The process being described there is subduction, the very same process that leads to the destruction of tectonic plates, the formation of mountains, volcanoes, and the world’s most powerful earthquakes. Tectonic plates can’t drift around if older ones aren’t being destroyed, which means that the very first moment that subduction occurred, plate tectonics officially began.
The researchers also suggest that such impacts – if they were powerful enough to induce subduction – could have caused pandemonium in Earth’s liquid outer core too. Like a cannonball landing on a frozen lake, the molten material trapped beneath the solidified surface would have been pushed around and mixed up.
This would have exacerbated the already massive temperature difference between the inside of the planet and the outer layers. Even if enormous convection currents in the outer core were already going, this would have only amplified them.
These liquid metal convection currents are responsible for generating the planet’s magnetic field – so there’s an argument to be made that without these giant impact events, that wouldn’t exist either.
It’s worth pointing out that this is all based on a computer simulation. Physical evidence for literally anything on Earth older than 4 billion years old is impossible to find, as it was all destroyed during those hellish early days of planetary evolution.
This means that all scientists can do is use simulations, and observations of other planets, to speculate, albeit conservatively and carefully.
“We know that meteorite impacts had a huge effect on the inner solar system at this time,” O’Neill said, adding that “you only need to look at the Moon to see that. What isn’t clear was how our own impact history might have affected the planet’s evolution.”
Nevertheless, if true, this hypothesis paints a beautifully paradoxical picture of our world, one in which frequent acts of unadulterated destruction were inadvertently responsible for kickstarting plate tectonics, the generation of a magnetosphere – and, by consequence, life itself.
