According to many scientists, about 4.5 billion years ago, Earth met another planet, Theia, which was about the size of Mars. When the two worlds collided violently, pieces of it were sent flying into space and trapped in the orbit of the young, damaged Earth, which is thought to have led to the formation of the Moon.
But a study published last month in Geophysical Research Letters suggests the collision with Theia may have had a much bigger impact than that.. The collision may have given rise to something else: plate tectonics, the force that drives the movement of Earth's huge continental and oceanic plates, causing earthquakes, volcanic eruptions and ultimately reshaping the Earth's surface about every 200 million years.
Earth scientists have long studied and debated the origin of plate tectonics, with many theories put forward. Qiang Yuan, a postdoctoral researcher at the California Institute of Technology and author of the new paper, and his colleagues argue that the Theia impact is the origin of plate tectonics. From computer simulations, they infer that the impact generated the heat needed to start the process on Earth's early planet.
Tectonics begins when a plume of hot magma rises from near the Earth's core and lodges beneath the Earth's plates, weakening the crust and allowing lava to erupt and push the overlying plates away.
Erupting lava causes plates to rub against each other and collide, and can also cause plates to slide beneath other plates, even into the Earth's interior, in a process called subduction.
In previous research, Dr. Yuan described continent-sized “blobs” floating about 2,000 miles below the surface near the Earth's core. He and his team believe that these chunks are remnants of Theia, which collided violently to generate the heat needed to form the plumes that caused the first tectonic events. The giant chunks are thought to be connected to plumes of magma, meaning they may be the driving force behind plate tectonics.
“Simulations suggest that a catastrophic moon-forming giant impact ignited the engine that drives plate tectonics,” Dr Yuan said.
Another clue comes from Western Australia, where a site called Jack Hills contains crystal-containing rocks that formed about 4.4 billion years ago — not long after Theia hit Earth, geologically speaking.
The crystals found in Australia, called zircons, can only form in places where there is plate subduction, but plate subduction only occurs on planets with active plate tectonics.
When Dr Yuan learned that the zircons formed relatively soon after the Theia impact, he became convinced that the impact had something to do with the start of plate tectonics.
Penn State geophysicist Bradford Foley thinks there's merit to the idea that plate tectonics begins with a planetary collision, but he says that's not the only way tectonics can start.
“A giant impact is one possible way that Earth's core could have been initially very hot,” he said. “It's an intriguing idea, and I'm glad it's been brought out for discussion in the scientific community, but it may be overblown and overdramatized for the general public.”
Another explanation, which the study doesn't rule out, is that when the planet's core first formed, it may have gotten hot enough that tectonic activity began, he said.
The challenge, Dr Yuan explained, is to accurately recreate the physical conditions on Earth more than four billion years ago.
“We have confidence in our model, but does it really represent the whole Earth?” Dr Yuan said. “That's a question that remains to be explored in future tests.”