Extra-terrestrial impacts may have shaped Earth 3.2 billion years ago, study finds

Researchers suggest the rocky Earth we know today is a result of extra-terrestrial impacts from billions of years ago.

A new study has found that plate tectonics were triggered by intense bombardment of meteors – transforming the hot, primordial mushy surface into the present rugged landscape.

The team discovered distinctive layers of round particles condensed from rock vaporized during an impact suggest the Earth experienced a period of intense bombardment about 3.2 billion years ago – similar to when first plate tectonics appeared.

Craig O’Neill, director of Macquarie University’s Planetary Research Center, said: ‘We tend to think of the Earth as an isolated system, where only internal processes matter.’

‘Increasingly, though, we’re seeing the effect of solar system dynamics on how the Earth behaves.’

Using modelling simulations and comparisons with lunar impact studies, the team found that following Earth’s accretion about 4.6 billion years ago, impacts continued to shape the planet for hundreds of millions of years.

The data suggests that the impacts tapered off over time, the team was able to find evidence of distinctive layers of round particles condensed from rock vaporized during an extra-terrestrial impact found in South Africa and Australia

These findings suggest the Earth experienced a period of intense bombardment about 3.2 billion years ago, which is the first indications of plate tectonics appear in the rock record.

And O’Neil and co-authors of the study wondered if this could all be linked.

‘Modelling studies of the earliest Earth suggest that very large impacts– more than 300 km in diameter — could generate a significant thermal anomaly in the mantle,’ O’Neill said.

This appears to have altered the mantle’s buoyancy enough to create upwellings that, according to O’Neill, ‘could directly drive tectonics.’

But the sparse evidence found to date from the Archaean, the period of time spanning 4.0 to 2.5 billion years ago, suggests that mostly smaller impacts less than 62 miles in diameter occurred during this interval.

To determine if these impacts were worthy of triggering global tectonics, the team used existing techniques to expand the Middle Archaean impact record and then developed numerical simulations to model the thermal effects of these impacts on Earth’s mantle.

They found that  during the Middle Archaean, 62-mile-wide impacts were capable of weakening Earth’s rigid, outermost layer.

This, O’Neill explained, could have acted as a trigger for tectonic processes, especially if Earth’s exterior was already ‘primed’ for subduction.

The modelling showed that if an impact were to happen in an area where these differences existed, it would create a point of weakness in a system that already had a large contrast in buoyancy -ultimately triggering modern tectonic processes (artist impression)

‘If the lithosphere were the same thickness everywhere, such impacts would have little effect,’ states O’Neill. 

But during the Middle Archean, he said, the planet had cooled enough for the mantle to thicken in some spots and thin in others.

The modelling showed that if an impact were to happen in an area where these differences existed, it would create a point of weakness in a system that already had a large contrast in buoyancy -ultimately triggering modern tectonic processes.

‘Our work shows there is a physical link between impact history and tectonic response at around the time when plate tectonics was suggested to have started,’ said O’Neill.

‘Processes that are fairly marginal today — such as impacting, or, to a lesser extent, volcanism — actively drove tectonic systems on the early Earth.’

‘By examining the implications of these processes, we can start exploring how the modern habitable Earth came to be.’

WHAT ARE TECTONIC PLATES?

Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle. 

Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

The Earth has fifteen tectonic plates (pictured) that together have molded the shape of the landscape we see around us today

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other. 

Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate. 

These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.