Some 4.5 billion years ago, a Mars-sized object smashed into the nascent Earth, spinning off our moon. Now a team of scientists proposes this giant impact did even more: The collision left behind mysterious blobs within Earth’s interior that may have helped kick off plate tectonics — the geologic process that fuels earthquakes, volcanoes and generally allows life to exist on our planet.
The idea, fleshed out with computer modeling in a new study in the journal Geophysical Research Letters, is an attempt to answer one of the most fundamental questions about our home world.
We live on the only planet known to have a surface made up of rocky plates that slide around and crash into each other at boundary zones as the superhot interior churns. This subterranean drama usually goes unnoticed on human time scales, except when an earthquake strikes or a volcano erupts. But most experts agree this process is absolutely essential to life as we know it because it helps the planet cycle carbon, which is important to keeping the climate habitable.
What experts don’t agree on is how plate tectonics started.
The new paper builds on a previous idea that attempts to explain a geologic conundrum. For decades, geologists have speculated about mysterious blobs inside Earth’s mantle detected using seismic imaging. These dense, massive blobs appear to be made of different material than the surrounding mantle, raising questions about what they are and how they formed.
One theory — published in the journal Nature last year — offers an explanation, but also raises a few eyebrows. It suggests that after the moon-forming object slammed into our planet, bits of it ended up intact in Earth’s interior.
The new paper takes this idea a step further: Around 200 million years after the impact, these submerged blobs could have helped create hot plumes inside Earth that disrupted the nascent surface, breaching the crust and allowing circular slabs to sink — a process called subduction.
This process, the authors say, could explain why the oldest minerals on Earth are zircon crystals that appear to have undergone subduction more than 4 billion years ago. What’s more, they suggest it might have contributed to the rise of modern plate tectonics.
“The giant impact is not only the reason for our moon, if that’s the case, it also set the initial conditions of our Earth,” said Qian Yuan, a geoscientist at the California Institute of Technology and one of the authors of the paper.
Taras Gerya, a geoscientist at ETH Zurich, said in an email that the idea of an initial subduction event triggered by vigorous mantle plumes shortly after the moon-forming impact is credible and supported by models and some geochemical data. But he added that he is unsure whether this would have led to modern plate tectonics or would have resulted in rapid global recycling of the entire crust, similar to what may have happened on inhospitable Venus.
Michael Brown, a geoscientist at the University of Maryland, said that it’s unclear how a circular subduction zone would lead to the global plate boundaries and the mosaic of rocky slabs that exist in modern plate tectonics.
“We need to bear in mind there is insufficient evidence for us to ever really know what the tectonic mode was in the Archaean,” the eon that lasted between 4 and 2.5 billion years ago, Brown said. “So from a philosophical standpoint, it’s almost certainly unknown and unknowable. I think that point sometimes gets lost.”
T. Mark Harrison, a distinguished research professor at the University of California at Los Angeles, recently wrote a paper called, “We Don’t Know When Plate Tectonics Began.” He questioned assumptions built into the model, pointing to geochemical inconsistencies that cast doubt on the giant impact theory itself.
“If we didn’t have plate tectonics, you and I would not be having this conversation, because our species would not have arisen,” Harrison said. “The best thing I can tell people is that my generation … couldn’t solve arguably the most interesting question left in science, which is how and under what conditions did life emerge.”
To future scientists, he has a message: “We’ve left you a little gift-wrapped present.”
The idea, fleshed out with computer modeling in a new study in the journal Geophysical Research Letters, is an attempt to answer one of the most fundamental questions about our home world.
We live on the only planet known to have a surface made up of rocky plates that slide around and crash into each other at boundary zones as the superhot interior churns. This subterranean drama usually goes unnoticed on human time scales, except when an earthquake strikes or a volcano erupts. But most experts agree this process is absolutely essential to life as we know it because it helps the planet cycle carbon, which is important to keeping the climate habitable.
What experts don’t agree on is how plate tectonics started.
The new paper builds on a previous idea that attempts to explain a geologic conundrum. For decades, geologists have speculated about mysterious blobs inside Earth’s mantle detected using seismic imaging. These dense, massive blobs appear to be made of different material than the surrounding mantle, raising questions about what they are and how they formed.
One theory — published in the journal Nature last year — offers an explanation, but also raises a few eyebrows. It suggests that after the moon-forming object slammed into our planet, bits of it ended up intact in Earth’s interior.
The new paper takes this idea a step further: Around 200 million years after the impact, these submerged blobs could have helped create hot plumes inside Earth that disrupted the nascent surface, breaching the crust and allowing circular slabs to sink — a process called subduction.
This process, the authors say, could explain why the oldest minerals on Earth are zircon crystals that appear to have undergone subduction more than 4 billion years ago. What’s more, they suggest it might have contributed to the rise of modern plate tectonics.
“The giant impact is not only the reason for our moon, if that’s the case, it also set the initial conditions of our Earth,” said Qian Yuan, a geoscientist at the California Institute of Technology and one of the authors of the paper.
An ‘unknowable’ event?
Outside geologists said the model was intriguing but raised a host of questions.Taras Gerya, a geoscientist at ETH Zurich, said in an email that the idea of an initial subduction event triggered by vigorous mantle plumes shortly after the moon-forming impact is credible and supported by models and some geochemical data. But he added that he is unsure whether this would have led to modern plate tectonics or would have resulted in rapid global recycling of the entire crust, similar to what may have happened on inhospitable Venus.
Michael Brown, a geoscientist at the University of Maryland, said that it’s unclear how a circular subduction zone would lead to the global plate boundaries and the mosaic of rocky slabs that exist in modern plate tectonics.
“We need to bear in mind there is insufficient evidence for us to ever really know what the tectonic mode was in the Archaean,” the eon that lasted between 4 and 2.5 billion years ago, Brown said. “So from a philosophical standpoint, it’s almost certainly unknown and unknowable. I think that point sometimes gets lost.”
T. Mark Harrison, a distinguished research professor at the University of California at Los Angeles, recently wrote a paper called, “We Don’t Know When Plate Tectonics Began.” He questioned assumptions built into the model, pointing to geochemical inconsistencies that cast doubt on the giant impact theory itself.
“If we didn’t have plate tectonics, you and I would not be having this conversation, because our species would not have arisen,” Harrison said. “The best thing I can tell people is that my generation … couldn’t solve arguably the most interesting question left in science, which is how and under what conditions did life emerge.”
To future scientists, he has a message: “We’ve left you a little gift-wrapped present.”
