When and how the Earth evolved from a molten mass into a rocky planetary body continually resurfaced by plate tectonics remain some of the biggest questions in earth sciences. According to new research by scientists from Macquarie University, Southwest Research Institute and Harvard University, this transition may have been triggered by intense extraterrestrial bombardment.
An artist’s concept of meteors impacting the ancient Earth. Image credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.
Modeling simulations and comparisons with lunar impact studies have revealed that following Earth’s accretion about 4.6 billion years ago, Earth-shattering impacts continued to shape the planet for hundreds of millions of years.
Although these events appear to have tapered off over time, Pilbara craton in Australia and Kaapvaal craton in South Africa suggest the Earth experienced a period of intense bombardment about 3.2 billion years ago (Archean Eon), roughly the same time the first indications of plate tectonics appear in the rock record.
This coincidence caused Macquarie University’s Dr. Craig O’Neill and colleagues to wonder whether these circumstances could be related.
“Modeling studies of the earliest Earth suggest that very large impacts — more than 186 miles (300 km) in diameter — could generate a significant thermal anomaly in the mantle,” Dr. O’Neill said.
“This appears to have altered the mantle’s buoyancy enough to create upwellings that could directly drive tectonics.”
But the sparse evidence found to date from the Archean suggests that mostly smaller impacts less than 62 miles (100 km) in diameter occurred during this interval.
To determine whether these more modest collisions were still large and frequent enough to initiate global tectonics, the researchers used existing techniques to expand the Middle Archean impact record and then developed numerical simulations to model the thermal effects of these impacts on Earth’s mantle.
The results indicate that during the Middle Archean, 62-mile-wide impacts were capable of weakening Earth’s rigid, outermost layer.
“This could have acted as a trigger for tectonic processes, especially if Earth’s exterior was already ‘primed’ for subduction,” Dr. O’Neill said.
“If the lithosphere were the same thickness everywhere, such impacts would have little effect. But during the Middle Archean, the planet had cooled enough for the mantle to thicken in some spots and thin in others.”
“The modeling 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 — and ultimately trigger 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,” he said.
“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.”
The study was published in the journal Geology.
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C. O’Neill et al. The role of impacts on Archaean tectonics. Geology, published online November 22, 2019; doi: 10.1130/G46533.1
