Extra-Terrestrial Impacts May Have Triggered "Bursts" of Plate
Tectonics
Boulder, Colo., USA: When—and how—the Earth's surface evolved from a hot,
primordial mush into a rocky planet continually resurfaced by plate
tectonics remain some of the biggest unanswered questions in earth science
research. Now a new study, published in Geology, suggests this
earthly transition may in fact have been triggered by extra-terrestrial
impacts.
"We tend to think of the Earth as an isolated system, where only internal
processes matter," says
Craig O'Neill
, director of Macquarie University's Planetary Research Centre.
"Increasingly, though, we're seeing the effect of solar system dynamics on
how the Earth behaves."
Modelling 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, spherule beds—distinctive layers of round particles condensed from
rock vaporized during an extra-terrestrial impact—found in South Africa and
Australia suggest the Earth experienced a period of intense bombardment
about 3.2 billion years ago, roughly the same time the first indications of
plate tectonics appear in the rock record.
This coincidence caused O'Neill and co-authors Simone Marchi,
William Bottke
, and Roger Fu to
wonder whether these circumstances could be related. "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,” says
O'Neill. 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 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 Archaean 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 Archaean, 100-kilometer-wide
impacts (about 30 km wider than the much younger
Chixculub
crater) were capable of weakening Earth's rigid, outermost layer. This,
says O'Neill, could have acted as a trigger for tectonic processes,
especially if Earth's exterior was already "primed" for subduction.
"If the lithosphere were the same thickness everywhere, such impacts would
have little effect," states O'Neill. But during the Middle Archean, he
says, 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—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," says 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," he says. "By examining the implications of
these processes, we can start exploring how the modern habitable Earth came
to be."
FEATURED ARTICLE
The role of impacts on Archaean tectonics
C. O'Neill et al.,
craig.oneill@mq.edu.au
; URL:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G46533.1/575921/The-role-of-impacts-on-Archaean-tectonics
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