Science

Did giant impacts trigger plate tectonics?

Did giant impacts trigger plate tectonics?
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Artist's impression of a cratered early Earth.

One of Earth’s defining characteristics is its plate tectonics, a phenomenon that shapes the planet’s surface and produces some of its most catastrophic events, such as earthquakes, tsunamis, and volcanic eruptions. While some properties of plate tectonics were sighted Elsewhere in the solar system, Earth is the only planet we know to exhibit the full suite of processes involved in this phenomenon. And everything indicates that it started very early in the history of our planet.

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

Options and Effects

Shortly after Earth formed, its crust would have consisted of a relatively even layer of solid rock that acted as a lid over the still-molten mantle below. There was probably a global ocean above it, since plate tectonics weren’t building mountains yet. Somehow that situation morphed into what we see now: the large regions of moving, floating crust of the continental plates and the ever-expanding deep-sea crust formed from mantle materials, all driven by the heat-induced movement of material through the mantle.

The primary explanation for the origin of plate tectonics is the simple assumption that mantle circulation was also the trigger for the onset of the phenomenon. Eruptions over mantle hotspots would bring less dense material to the surface, with the added weight pushing denser material into the mantle. As these processes continued, more floating material was brought to the surface over time, expanding some areas into nascent plates. This explanation has the advantage that the process begins with the same factors that drive it today – scientists tend to hate having to rely on multiple, disparate explanations.

But they also hate coincidences, and coincidence is behind an alternative explanation. The earliest evidence of plate tectonics appeared about 3.8 billion years ago, not too long after Earth formed. This period also coincides with a series of large impacts called the Late Heavy Bombardment, which struck the bodies of the solar system.

These impacts would have added a lot of energy to the crust, both fragmenting it and melting it locally. This would allow hot material from both the molten crust and mantle to erupt to the surface through volcanism. The effect is a bit like eruptions over a hotspot, bringing lower-density materials to the surface, but it would happen in multiple locations on the planet over hundreds of millions of years.

Because of the similarities between the two theories, and the fact that much evidence has been destroyed over the past billion years, it is difficult to determine which is better supported by the evidence. But researchers in a new paper claim they found evidence the impact was likely to be critical.

Starting with a bang

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

The researchers examined the zircons for signs that they had been altered by geological processes after their formation and excluded them from further analysis. And they also got data for all crystals based on uranium decay. Then they focused on two things that tell us something about the environment in which the crystals formed. The first was to examine the type of rock in which the crystals were embedded, which was thought to reflect the environment in which they formed. The second was the fraction of oxygen that came from a particular isotope (18ON). This analysis provides an indication of the temperature at which the crystal formed, which is generally related to its depth.

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