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Did giant impacts start plate tectonics?

Plate tectonics may have its origins in impacts, based on new data from Australia.

John Timmer | 98

One of Earth's defining features is its plate tectonics, a phenomenon that shapes the planet's surface and creates some of its most catastrophic events, like earthquakes, tsunamis, and volcanic eruptions. While some features of plate tectonics have been spotted elsewhere in the Solar System, the Earth is the only planet we know of with the full suite of processes involved in this phenomenon. And all indications are that it started very early in our planet's history.

So what started it? Currently, two leading ideas are difficult to distinguish based on our limited evidence of the early Earth. A new study of a piece of Australia, however, argues strongly for one of them: the heavy impacts that also occurred early in the planet's history.

Options and impacts

Shortly after the Earth formed, its crust would have been composed of a relatively even layer of solid rock that acted as a lid over the still-molten mantle below. Above that, there was likely a global ocean since plate tectonics wasn't building mountains yet. Somehow, this situation was transformed into what we see now: The large regions of moving, buoyant crust of the continental plates and the constantly spreading deep ocean crust formed from mantle materials, all driven by the heat-induced motion of material through the mantle.

The primary explanation for the origin of plate tectonics is to simply assume that mantle circulation was also what triggered the phenomenon's onset. Eruptions over mantle hotspots would bring less dense material to the surface, with the added weight forcing more dense material down into the mantle. As these processes continued, more buoyant material would be brought to the surface over time, expanding some areas into nascent plates. This explanation has the advantage of showing the process starting with the same factors that drive it today—scientists tend to hate having to rely on multiple, distinct explanations.

But they also hate coincidences, and a coincidence is what's behind an alternative explanation. The earliest indications of plate tectonics appeared about 3.8 billion years ago, not too long after the Earth's formation. That period also overlaps with a series of large impacts, called the Late Heavy Bombardment, that struck the bodies of the Solar System.

These impacts would have delivered a lot of energy to the crust, both fragmenting it and causing local melting. This would allow hot material from both the melted crust and the mantle to break through to the surface through volcanism. The effect is a bit like eruptions above a hotspot, with lower density materials being brought to the surface, but it would happen at multiple locations across the planet over hundreds of millions of years.

Because of the similarities between the two theories and the fact that a lot of evidence has been destroyed over the last several billion years, it's difficult to determine which is better supported by the evidence. But researchers in a new paper claim they have found evidence that impacts were likely to be critical.

Starting with a bang

The work relies on zircon crystals, extremely stable structures that include the oldest confirmed pieces of Earth. The authors focused on crystals originating in a part of Australia called the Pilbara Craton. Cratons are the oldest, most stable pieces of continental crust, and they tend to form the cores of modern-day continents. Pilbara is one of the two oldest known cratons on Earth.

The researchers screened the zircons for indications that they had been modified by geological processes after their formation, eliminating those from further analysis. And they also obtained dates for all the crystals based on uranium decay. They then focused on two things that tell us something about the environment the crystals formed in. The first involved looking at the type of rock that the crystals were embedded in, which was assumed to reflect the environment they formed in. The second was the fraction of oxygen that was from a specific isotope (18O). This analysis provides some indication of the temperature that the crystal formed at, which is generally related to its depth.

Image of a graph with large clusters of data points.
The oxygen isotope ratios of the samples go through three distinct phases. (Youngest is to the left.)
The oxygen isotope ratios of the samples go through three distinct phases. (Youngest is to the left.) Credit: Johnson, et. al.

Strikingly, the oxygen isotopes clustered strongly in time. There's a lot of variation at a given time, and each measurement has a fair amount of uncertainty associated with it. But the mean value pretty clearly changes, which provides some indication that the sort of temperature that these crystals were forming at was changing over time.

After analyzing differences in the surrounding rock, the researchers divided things up into three distinct phases of zircon formation. The first marks a transitional period in which early rocks were dominated by two overlapping populations. Based on the oxygen isotopes, one of these populations formed in mantle-like conditions, while another formed with much less 18O. Over time, the surrounding rocks shifted from being mostly similar to basalts at the earliest times to rocks that are more similar to granites.

This later group overlaps with a large impact in the area of the Pilbara Craton, and its properties are consistent with formation following impact-driven melting.

The next period appears to have mostly mantle-like conditions, based on the oxygen isotopes, and it also covers a second large impact in the area. The researchers argue that this represents a post-impact stabilization of the nascent craton, with the later zircons in this period being formed at the base of the craton where it meets the mantle.

The latest period, stage 3, represents zircons that formed after plate tectonic forces became active in the region. That's because the changes in oxygen isotopes require a significant contribution from rocks that remain at the top layers of the crust.

Why impacts?

This general outline of events is consistent with both of the models. But the researchers argue that two things point to impacts being the culprit. The first is the impact debris itself, which indicates that the Pilbara Craton was struck right at the onset of a transition that ultimately resulted in the formation of a craton. The second is the oxygen isotope ratios themselves. These tend to drop over time at sites of hotspot eruptions. In contrast, they appear to rise over time in these zircons. The two impacts also coincide with the formation of much of the granite in the Pilbara Craton, and granite is a low-density rock typical of continental crust.

Of course, other cratons may have very different histories, so we should be cautious about inferring too much from a single example. But the researchers show that at least two other cratons appear to have a similar pattern of changes in oxygen ratios over time. There's not much associated data regarding rock types and impacts here, but the data is at least consistent with this being a more general mechanism.

While the work doesn't give us a definitive answer on how plate tectonics started, it provides a nice illustration of how researchers try to build a case for an explanation. The next step, of course, is criticism from their peers and carefully evaluating similar data from elsewhere. As further results come in, it's important to remember that both models could be right and just apply to different locations.

Nature, 2022. DOI: 10.1038/s41586-022-04956-y  (About DOIs)

Listing image: Simone Marchi/SwRI

Photo of John Timmer
John Timmer Senior Science Editor
John is Ars Technica's science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.
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