About 4.5 billion years ago, according to many scientists, Earth encountered Theia, another Mars-sized planetary object. When the two worlds collided in a massive impact, the thinking goes, the debris shot out into space, orbiting the young, damaged Earth and giving rise to our moon.
But according to a study published last month in the journal Geophysical Research Letters, the collision with Theia may have caused more than that.. 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, causing earthquakes, volcanic eruptions and eventual reshaping of our planet’s surface roughly every 200 million years.
Earth scientists have long studied and debated the origins of plate tectonics, and other theories have been offered. 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 the source of plate tectonics. From computer simulations, they deduce that this event produced the heat needed in Earth’s early days to get the process going.
Tectonics begins with superheated plumes of magma from near the Earth’s core rising and settling beneath the planet’s plates. Plumes can weaken the crust and lava can erupt and push aside the overlying plates.
Driven by erupting lava, the plates scrape and crash into each other, and they can also sink under other plates and into the planet’s interior in a process called subduction.
In earlier research, Yuan described continent-sized “balls” floating about 3,000 miles below the Earth’s surface near the core. He and his team think these blobs are the remnants of Theia, which, when violently ejected, generated the heat needed to form the first tectonic clouds. The giant blobs are thought to be connected to plumes of magma, meaning they could fuel plate tectonics.
“Simulations show that the catastrophic giant impact that formed the Moon ignited the engine that drives plate tectonics,” Yuan said.
Another clue is in Western Australia. There, in a place called Jack Hills, the rocks contain crystals that formed about 4.4 billion years ago—geologically speaking, not long after Theia hit Earth.
These crystals in Australia, called zircons, only form where plates are subducting, and subduction can only occur on a planet with active plate tectonics.
Once Yuan learned that the zircons formed relatively soon after the Theia impact, he became convinced that the collision had something to do with the onset of plate tectonics.
Bradford Foley, a geophysicist at Pennsylvania State University, thinks the idea of ​​plate tectonics starting with a planetary collision has merit. But that’s not the only way tectonics can start, he says.
“A giant impact is one possible way to make Earth’s core very hot to begin with,” he said. “It’s an interesting idea that I’m glad was published for the scientific community to discuss, but it can easily be oversold and overdramatized to the general public.”
An alternative explanation, not disproved by the study, is that the initial formation of the planet’s core may have caused it to be hot enough for tectonic activity to begin.
The challenge, Yuan explained, is to accurately represent the physical conditions of our planet more than 4 billion years ago.
“We have confidence in our model, but does it really represent all of the real Earth?” Yuan said. “That’s a question future tests will explore.”