About 4.5 billion years ago, many scientists say, Earth had an encounter with Theia, another Mars-sized planetary object. When the two worlds collided in a big impact, the mind goes, the debris was shot into space, locked in the orbit of the young Earth, damaged it, 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 triggered 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 ultimate remaking 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 offered. Qian Yuan, a postdoctoral researcher at the California Institute of Technology and an author of the new paper, and his colleagues make the case for the Theia collision as the 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 rising and sinking beneath the planet’s plates. Pillars can weaken the crust and lava can erupt and push aside the overriding plates.
Driven by the erupting lava, the plates slide past and collide with each other, and they can also sink under other plates and into the interior of the planet in a process called subduction.
In an earlier study, Yuan described continent-sized “blobs” floating about 2,000 miles below Earth’s surface near the core. He and his team think those plumes are remnants of Theia that, violently dispersed, created the heat needed to form the first tectonic plumes. The giant plumes are believed to be associated with magma plumes, meaning the plumes could fuel plate tectonics.
“The simulations show that the giant cataclysmic 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—not long, geologically speaking, after Theia hit Earth.
Those crystals in Australia, called zircon, only form where there is plate subduction, and subduction can only happen 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 impact had something to do with the initiation of plate tectonics.
Bradford Foley, a geophysicist at Pennsylvania State University, thinks the idea of plate tectonics starting from a planetary collision has merit. But that’s not the only way tectonics can start, he says.
“The giant impact is a possible way to make the Earth’s core very hot to begin with,” he said. “It’s an interesting idea that I’m happy to see published for the scientific community to debate, but it could easily be oversold and overdramatized for the general public.”
An alternative explanation that the study does not rule out, he says, is that the initial formation of the planetary core may have made it hot enough to initiate tectonic activity.
The challenge, Yuan explained, is in accurately representing the physical states of our planet from more than 4 billion years ago.
“We have confidence in our model, but does it really represent all of the real Earth?” Yuan said. “This is a question that will be explored by future tests.”