About 4.5 billion years ago, many scientists say, Earth had an encounter with Theia, another planetary object the size of Mars. When the two worlds collided in one big blow, the idea goes, debris shot into space, became trapped in the orbit of the young, damaged Earth, and led to the formation of our moon.
But the collision with Theia may have done more than that, according to a study published last month in the journal Geophysical Research Letters.. The impact may have given rise to something else: plate tectonics, the engine that drives the movement of Earth’s giant continental and oceanic plates and causes earthquakes, volcanic eruptions and the eventual change of our planet’s surface about every 200 million years.
Earth scientists have long studied and debated the origins of plate tectonics, and other theories have been put forward as well. Qian Yuan, a postdoctoral researcher at the California Institute of Technology and author of the new paper, and his colleagues argue for the Theia collision as a source of plate tectonics. They reason from computer simulations that the event produced the heat needed in Earth’s early days to start the process.
Tectonics begins with superheated plumes of magma from near the Earth’s core that rise and remain beneath the Earth’s plates. The plumes can weaken the crust and lava can erupt and push aside overriding plates.
Driven by the erupting lava, plates scrape and collide with each other. They can also dive under other plates and into the interior of the planet in a process called subduction.
In previous research, Yuan described continent-sized “blobs” about 2,000 miles beneath the Earth’s surface near the core. He and his team think these blobs are remnants of Theia that, forcibly delivered, created the heat needed to form the first tectonics-driving plumes. The giant blobs are believed to be connected to magma plumes, meaning the blobs could fuel plate tectonics.
“Simulations show that the catastrophic, moon-forming giant impact ignited the engine that powers plate tectonics,” Yuan said.
Another clue is in Western Australia. There, at a place called the Jack Hills, the rocks contain crystals that formed about 4.4 billion years ago—not long, geologically speaking, after Theia struck Earth.
Those crystals in Australia, called zircons, form only where plate subduction occurs, and subduction can only occur on a planet with active plate tectonics.
When Yuan learned that the zircons formed relatively soon after the Theia impact, he became convinced that the collision had something to do with the beginning of plate tectonics.
Bradford Foley, a geophysicist at Pennsylvania State University, thinks the idea of plate tectonics beginning with a planetary collision has merit. But it’s not the only way tectonics can arise, he says.
“The gigantic impact is a possible way to initially make the Earth’s core very hot,” he said. “It’s an interesting idea that I’d like to see published for the scientific community to debate, but could easily be oversold and overdramatized for the general public.”
An alternative explanation that the study doesn’t refute, he says, is that the initial formation of the planetary core made it hot enough for tectonic activity to begin.
The challenge, Yuan explained, lies in accurately representing the physical conditions of our planet more than 4 billion years ago.
“We are confident in our model, but does it really represent the entire Earth?” Yuan said. “That’s a question that needs to be explored by future tests.”