Andrew Masterson
Andrew Masterson is a former editor of Cosmos.
Meteorite impacts might have kick-started the Earth’s tectonic plates and boosted the planet’s magnetic field, according to a study from Australia’s Macquarie University.
The research, led by Craig O’Neill from the university’s Planetary Research Centre, and published in the journal Nature Geoscience, offers a scenario to illuminate what happened during the first 500 million years of the Earth’s existence – a period known as the Hadean, or, more poetically, the geologic dark ages.
To date, the question of whether the young planet featured moving tectonic plates has been moot, primarily because almost nothing of its early crust remains.
Some scientists have proposed that grains of zircon, dating to before 4.1 billion years ago, are evidence of early, active tectonics. Others, however, are more convinced by geochemical data indicating that in its formative years the Earth was encased in a motionless “lid”, with moving tectonic plates emerging later.
Tectonic plates were until recently thought to be unique to Earth, at least within the solar system. However, research by scientists at the University of California, Los Angeles, in 2012, using satellite imagery, established that Mars also experiences plate movement, although on a smaller scale.
It is safe to say, though, that on the evidence plates tectonics is a rare phenomenon in local space, which raises the question of how it started.
Modelling by O’Neill and his colleagues has thrown up a possible answer.
“Our results indicate that giant meteorite impacts in the past could have triggered events where the solid outer section of the Earth sinks into the deeper mantle at ocean trenches – a process known as subduction,” he says.
“This would have effectively recycled large portions of the Earth’s surface, drastically changing the geography of the planet.”
Positing meteorite collisions during the Hadean fits well with evidence from other planets in the inner solar system, as well as with well-established observations of the moon, which is scarred by impacts from that period. The modelling potentially reconciles the conflicting geochemical findings that have left the question of early tectonic movement open.
“We’ve seen evidence of some geological activity that suggests something like subduction acted on the early Earth – but this is hard to reconcile with other geodynamic simulations,” adds O’Neill.
“But if we consider Earth as part of an evolving early solar system, as opposed to only looking at the planet in isolation, then this evolution starts to make more sense.”
The modelling also finds support in research that indicates the strength of Earth’s magnetic field moved suddenly from a low value to a higher one closer to that found today around four billion years ago.
Meteorite impacts, the team suggests, would have caused the planet’s cold outer crust to move comparatively rapidly downwards towards the planetary core. This process would have changed the intensity of convection within the core, thus affecting the “geodynamo” – the conductive layer of liquid iron that surrounds the solid inner core and that generates a magnetic field.
“This is a really important age in the inner solar system,” says O’Neill.
“Impacting studies have suggested a big disturbance in the asteroid populations at this time, with perhaps a big upswing in impacts on the Earth. Our simulations show that larger amounts of meteorite collisions with the planet around this time could have driven the subduction process, explaining the formation of many zircons around this period, as well as the increase in magnetic field strength.”
Coauthor Simone Marchi from the Southwest Research Institute, in the US, adds that the study throws up the question of how much the structure of Earth and other rocky planets is the result of collisions that took place billions of years ago.
“This work shows there is a strong connection between impacts and geophysical evolution capable of drastically altering a planet’s evolution,” she says.
