About 4.5 billion years ago, many scientists say, Earth had a close encounter with Theia, another Mars-sized planetary body. When the two worlds collided in a big crash, the thinking goes, debris shot out into space, caught 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 research 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, causing earthquakes, volcanic eruptions and the eventual reshaping 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 sit beneath the planet’s plates. The plumes can weaken the Earth’s crust and lava can erupt and push the overlapping plates aside.
The erupting lava causes plates to grind against each other and collide with each other. They can also dive under other plates and into the planet’s interior, a process called subduction.
In previous research, Dr. Yuan ‘blobs’ the size of a continent, floating about 2,000 kilometers below 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 drives plate tectonics,” said Dr. Yuan.
Another clue is in Western Australia. There, in a place called the Jack Hillsrocks contain crystals that formed about 4.4 billion years ago – not long, geologically speaking, after Theia hit Earth.
The crystals in Australia, called zircons, only form where plates are subducting. Subduction can only occur on a planet with active plate tectonics.
When Dr. Yuan heard that the zircons formed relatively soon after the Theia impact, and 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 there is merit to the idea that plate tectonics begins with a planetary collision. But it’s not the only way tectonics can start, 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 discuss, but it 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, explained Dr. Yuan lies in accurately portraying the physical conditions of our planet more than four billion years ago.
“We are confident in our model, but does it really represent the entire real Earth?” said dr. Yuan. “That’s a question that needs to be explored by future tests.”
