Researchers from Macquarie University in Australia have collaborated with SSERVI researchers at Southwest Research Institute in Boulder, Colorado, to model how large impacts may have affected global-scale tectonics during the evolution of the early Earth.

During the Hadean era, when Earth first began to form about 4.5 billion years ago, collisions with asteroids that were nearly the size of the Moon—up to 3000 km in diameter—were fracturing the surface, vaporizing rock and creating local pools of molten rock. The most massive impacts caused upwellings deep in the mantle that may have covered the surface in a wave of molten lava, recycling the global crust. Simulations show even moderate-sized impacts could modulate tectonic activity and trigger local subduction events by thinning and spreading Earth’s crust in response to an upsurge in the mantle below. The findings were recently published in Nature Geoscience in the manuscript “Impact-driven Subduction on the Hadean Earth.”

These were the first global-scale simulations of the evolution of the Hadean Earth under a waning flux of asteroid impacts. Little geological evidence survives from the Hadean era because the Earth’s crust has been slowly reshaped over time via plate tectonics, as the solid crust on the surface was pushed back down and recycled into the hot mantle. However, geophysical models indicate that the ancient Earth was tectonically very different than today, possibly resembling a one-plate planet like Venus or Mars. Why the Earth entered into the current mode of plate tectonics during its evolution is not yet fully understood.

“Our understanding of the early Earth is poor because most of the rock record has been destroyed,” says lead author Craig O’Neill. “There is some geological evidence for an active surface, but this is at odds with geochemical and geodynamic evidence that suggests the surface plate system was locked up.”
The missing ingredient, he says, may have been impacts. “The surface of the Moon shows us how important cratering was in the early solar system,” says O’Neill. “What we didn’t appreciate was just how influential they could be on the dynamics of a planet.”

earth_impact_oneill3
Credit: O’Neill et al.

The simulations consistently show an association between the rate at which these impacts occurred, tectonic activity, and magnetic field strength. The models suggest that large impacts may have reheated the mantle, priming it for subduction, while colder, subducted crust changed the heat flow at the base of the mantle, thereby altering the planet’s magnetic field.

hadean_impacts
The image shows the time evolution effect of a large impact on mantle dynamics. The impact of a 2000 km diameter body (arrow) occurs with a velocity of 16 km/s and induces a large-scale return flow due to the positive buoyancy of the thermal anomaly. This flow results in thinning and spreading above the site of impact, and lithospheric convergence at the edges of this zone, resulting in subduction at the edges of the laterally spreading anomaly, and invigorating plume formation (4.0 Myr). Smaller impacts induce a small thermal anomaly in the mantle, and localized lithospheric thinning. Credit: O’Neill/Marchi.

“It is well established that havoc by large extraterrestrial impacts affected planetary surfaces. Less clear are the long-term effects of collisions on the internal evolution of a planet. This work shows there is a strong connection between impacts and geophysical evolution capable of drastically altering a planet’s evolution,” said coauthor Simone Marchi. “One has to wonder, how much of the current Earth, and other terrestrial planets, is the result of collisions that took place eons ago?” Marchi added.

Posted by: Soderman/SSERVI Staff
Source: S. Marchi/SSERVI Team

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