It’s widely accepted that Earth’s plate tectonics are a key factor in life’s emergence. Plate tectonics allows heat to move from the mantle to the crust and plays a critical role in cycling nutrients. They’re also a key part of the carbon cycle that moderates Earth’s temperature.
But new research suggests that there was no plate tectonic activity when life appeared sometime around 3.9 billion years ago. Does this have implications for our search for habitable worlds?
“Our data suggests that when we’re looking for exoplanets that harbour life, the planets do not necessarily need to have plate tectonics.”
Earth’s distant past is hidden from us, and the only surviving witness to it is Earth’s ancient rock. That’s why scientists scour Earth for the oldest rock they can find, rock that has never been recycled by plate tectonics. Tiny zircons are resistant to heat, meaning they are sometimes preserved in igneous rocks. They’re also very hard and resist weathering. That means that some zircons are survivors from Earth’s ancient past.
In new research, scientists examined zircons from between 3.9 to 3.4 billion years ago. The results are in a paper titled “Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism” in the journal Nature. The lead author is John Tarduno from the Department of Earth and Environmental Sciences at the University of Rochester.
“We found there wasn’t plate tectonics when life is first thought to originate, and that there wasn’t plate tectonics for hundreds of millions of years after,” says Tarduno. “Our data suggests that when we’re looking for exoplanets that harbour life, the planets do not necessarily need to have plate tectonics.”
Zircon’s toughness means that they can be much older than the body of rock they belong to. But at some point in the past, all of Earth’s rock was molten, including what would eventually form zircons. Zircons contain magnetic particles, and as the zircons solidified, the particles are affected by Earth’s magnetic fields at the time, and the magnetic fields change in strength over time and over different parts of the Eart as the magnetic poles wander. So, the zircons that scientists study retain evidence of Earth’s ancient magnetic fields.
This image helps explain how zircons shed light on ancient Earth’s magnetic fields. The researchers collected tiny zircons from sandstone in South Africa. The tiny zircons retain the characteristics of Earth’s magnetic field at the time of their solidification, represented by the drawing of magnetic field lines. Image Credit: University of Rochester images/photo by J. Adam Fenster / illustration by Michael OsadciwIn this study, the researchers had actually set out to study Earth’s magnetic fields, and the evidence contained in zircons. “We were studying the magnetization of zircons because we were studying Earth’s magnetic field,” Tarduno says.
Scientists can date zircons through radiometric dating. The tiny rocks contain trace amounts of the radioactive elements uranium and thorium. Since scientists know the rate of decay for these elements and their decay chains, they can measure and compare the amounts of the elements and what they decay into to determine the zircons’ ages.
Once they know the ages of the zircons, and the orientation of their magnetic particles, a picture of Earth’s ancient magnetic fields emerges.
Earth’s geodynamo generates its magnetic field, but its strength and direction change with latitude and time. So if the geodynamo is consistent over time, then zircons that formed at different latitudes will have different magnetic properties, whereas zircons from the same latitude will have similar properties.
Tarduno and his fellow researchers were examining zircons from South Africa’s Barberton Greenstone Belt, the home of some of Earth’s oldest exposed rock. They found that the zircons they studied from between 3.9 billion and 3.4 billion years ago show no change in magnetic properties. The conclusion is that during that time period, latitudes didn’t change either, meaning the continents didn’t move due to plate tectonics.
Tarduno and his colleagues also studied zircons from another region of Earth’s ancient rocks, the Jack Hills region in Western Australia. They found the same patterns in those zircons as they did in the South African zircons, further strengthening their conclusion that the Earth had no plate tectonics.
“We aren’t saying the zircons formed on the same continent, but it looks like they formed at the same unchanging latitude, which strengthens our argument that there wasn’t plate tectonic motion occurring at this time,” Tarduno says.
This figure from the study shows zircon paleointensity from Neoarchaean zircons from South Africa (green circles) and from Australia (yellow circles.) The green and yellow dashed lines show the 100-Myr running average of zircon palaeointensity. The pink line and the blue field represent show the mean and the standard deviation for paleointensity. The data for both South African and Australian zircons between 3.9 and 3.4 billion years ago are basically indistinguishable. (LHB: Late Heavy Bombardment.) Image Credit: Tarduno et al. 2023These results pose another question: how did the early Earth shed heat?
The answer might be what’s known as stagnant lid tectonics. In stagnant lid tectonics, Earth sheds heat through cracks in the planet’s solid cap.
This figure shows the main difference between plate tectonics and stagnant lid tectonics. Image Credit: Karato and Barbot 2018.
“We had at least enough geochemical cycling provided by the stagnant lid processes to produce conditions suitable for the origin of life.”
About half of Earth’s heat comes from the decay of radioactive elements in the core. As things like uranium-235 and thorium-232 decay into other elements, they release heat. The other 50% is remnant heat from Earth’s formation. The heat is what drives plate tectonics and other geological activity like volcanism. Most of the heat escapes through mid-ocean ridges, while little heat escapes through the continental plates.
Most of Earth’s internal heat escapes through mid-ocean ridges. The smallest heat flux values are in the middle of the continents. Image Credit: By J. H. Davies and D. R. Davies – Davies, J. H., & Davies, D. R. (2010). Earth’s surface heat flux. Solid Earth, 1(1), 5-24. https://se.copernicus.org/articles/1/5/2010/, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=28866494With no plate tectonics, the heat still has to escape somehow. In stagnant lid tectonics, there are no continents drifting around, with ocean ridges venting the planet’s internal heat. Instead, there’s a single, monolithic lid that remains in place. Magma plumes rise up at different locations under the lid, cracking it and releasing heat. Stagnant lid tectonics don’t release heat as efficiently as plate tectonics, but they can still form continents. Despite the fact that the word ‘stagnant’ is part of the name, Earth’s surface was not exactly moribund.
“Early Earth was not a planet where everything was dead on the surface,” Tarduno says. “Things were still happening on Earth’s surface; our research indicates they just weren’t happening through plate tectonics. We had at least enough geochemical cycling provided by the stagnant lid processes to produce conditions suitable for the origin of life.”
If Earth still spawned life despite its lack of plate tectonics, then other planets can, too. And while Earth is the only planet we know that has plate tectonics, we know of others that have stagnant lid tectonics. One of them is our neighbour: Venus.
When talking about potential habitability, Venus is not a planet that often enters the conversation. It’s really hot, has crushing atmospheric pressure, and clouds of sulphuric acid. All those things seem to rule out life.
This image shows how Earth and Venus differ. On Earth, active plate tectonics sheds the planet’s heat, and mantle plumes tend to be far away from subducting plates. On Venus, the plate is stagnant, with only small local pieces of the plate being subducted in the same location as mantle plumes. Image Credit: This graphic by Fabio Crameri based on Crameri et al. (2017) is available via the open-access s-Ink repository. https://creativecommons.org/licenses/by-sa/4.0/“People have tended to think that stagnant lid tectonics would not build a habitable planet because of what is happening on Venus,” Tarduno says. “Venus is not a very nice place to live: it has a crushing carbon dioxide atmosphere and sulfuric acid clouds. This is because heat is not being removed effectively from the planet’s surface.”
But Venus is just one example. It’s possible that a planet with stagnant lid tectonics could spawn life, even though it’s not as effective at shedding heat as plate tectonics. A planet could go through a period of stagnant lid tectonics, Tarduno says, and could still host life. But plate tectonics might be necessary for life to have a nice long run like it has here on Earth.
“We think plate tectonics, in the long run, is important for removing heat, generating the magnetic field, and keeping things habitable on our planet,” Tarduno says. “But, in the beginning, and a billion years after, our data indicates that we didn’t need plate tectonics.”
Plate tectonics are also important for cycling nutrients and carbon. Carbon is released at mid-ocean ridges and sequestered through subduction when plates meet. Plate tectonics have helped Earth maintain its Goldilocks climate for billions of years. But it may not have always been this way.
Life may not have needed plate tectonics at first. It may not have been necessary for life’s emergence. “The geochemical cycling provided by plate tectonics is recognized as being a key factor in sustaining the habitability of Earth,” the authors write in the conclusion of their paper. “However, it has been less clear whether plate tectonics was required for the origin and early viability of life.”
This study shows that it’s not necessary, at least for the first billion years or so. Stagnant lid tectonics may have been enough. In fact, the similarity between the zircons also indicates that deep subduction was unlikely, meaning the poles didn’t wander, with all the rapid surface changes that can accompany that phenomenon. That could’ve helped provide the stability early life may have relied on. “The lack of large, rapid changes in environmental conditions induced by true polar wander likely fostered survival of nascent life on our planet,” the authors explain.
This isn’t the first research showing that plate tectonics may not have been active in Earth’s earliest days. Earth likely had at least one magma ocean phase, and some research shows that the crust may have formed into one more-or-less complete crust before tectonics began. A 2020 paper showed that the end of the Archaean, about 2.5 billion years ago, “marks the period in which plate tectonics became the dominant tectonic regime on Earth.”
This image shows Ordovician ophiolite in Gros Morne National Park, in Newfoundland, Canada. Ophiolites are important geological evidence, and their presence in mountain ranges like the Alps and the Himalayas constitutes one of the pillars of evidence supporting plate tectonics. Image Credit: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=377478There’s more. A 2011 study based on diamonds on the subcontinental mantle showed that modern plate tectonics started 3 billion years ago. And a 2005 research article based on ophiolites—chunks of oceanic plate thrust up at the edge of continental plates—suggested that modern subductive plate tectonics began in the Neoproterozoic, between 1 billion to 538.8 million years ago.
It’s not surprising that there are different estimates for the appearance of plate tectonics. Earth’s history is deeply concealed, and in many cases, totally erased. Different studies have regarded different pieces of evidence as pivotal, and there’s no way to figure it all out except to keep studying it.
If life arose on Earth without the benefits of plate tectonics, maybe it can arise elsewhere, too. It’s pretty widely accepted among scientists that plate tectonics are critical for life. But this study, and others like it that may follow in its footsteps, might overturn that belief.
If it’s true, then astronomers may need to redefine what habitable means when it comes to exoplanets.
