An artist's rendering of the early Earth covered in craters.

Did the giant impact start plate tectonics?

A defining feature of Earth is its plate tectonics, a phenomenon that shapes the planet’s surface and causes some of the most catastrophic events such as earthquakes, tsunamis and volcanic eruptions. While some features of plate tectonics have been discovered elsewhere in the solar system, Earth is the only planet we know of that possesses the full set of processes involved in this phenomenon. All indications are that it began very early in our planet’s history.

So what started? Currently, based on the limited evidence we have on the early Earth, it is difficult to distinguish between the two main views. However, a new study of a piece of land in Australia strongly supports one of these: a severe impact that also occurred early in Earth’s history.

Options and Impact

Soon after Earth formed, its crust would consist of a relatively uniform layer of solid rock that acted as a lid over the still-melting mantle below. On top of this, since plate tectonics have not yet made mountains, there may be a global ocean. Somehow, this became what we see now: large-scale motion of continental plates, a buoyant crust, and an expanding deep-ocean crust formed by mantle material, all driven by the thermally induced movement of material through the mantle .

The main explanation for the origin of plate tectonics is to simply assume that mantle circulation is also what triggers this phenomenon. Eruptions on mantle hotspots bring less dense material to the surface, and the added weight forces denser material into the mantle. As these processes continue, more buoyant material will be brought to the surface over time, expanding some areas into nascent plates. The advantage of this explanation is that it shows a process that starts with the same factors driving it today—scientists often hate having to rely on multiple different explanations.

But they also hate coincidences, which are the reason behind another explanation. The earliest signs of plate tectonics appeared about 3.8 billion years ago, shortly after the Earth formed. This period also overlapped with a series of large impacts, known as the Late Heavy Bombardment, hitting objects in the solar system.

These effects transmit a lot of energy to the crust, breaking it up and causing localized melting. This would allow hot material from the melting crust and mantle to break through to the surface through volcanism. The effect is a bit like an eruption above a hotspot, bringing lower-density material to the surface, but it occurs in multiple locations on Earth over hundreds of millions of years.

Because of the similarities between the two theories, and the fact that vast amounts of evidence have been destroyed over the past billions of years, it is difficult to determine which is more supported by the evidence. But researchers claim in a new paper that they have found evidence that the impact could be crucial.

Start with a bang

The work relied on zircon crystals, an extremely stable structure that includes the oldest known fragments of Earth. The authors focus on crystals originating in the Pilbara Craton region of Australia. Cratons are the oldest and most stable parts of continental crust, and they tend to form the core of modern continents. The Pilbara is one of the two oldest known cratons on Earth.

The researchers screened zircon for signs that they were altered by geological processes after they formed, which were ruled out from further analysis. They also obtained dates for all crystals based on uranium decay. They then focused on two things that tell us about the environment in which the crystals formed. The first involved looking at the type of rock in which the crystals were embedded, which is thought to reflect the environment in which they formed. The second is the fraction of oxygen from a specific isotope (18○). This analysis provides some indication of the crystal formation temperature, which is usually related to its depth.

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