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One of the best ways to peer back into the history of our solar system
is by looking at impact craters on the Earth and Moon.
These craters, formed by massive rocks crashing into our planetary bodies,
tell a story of how the solar system and planets formed and evolved through time.
In general, Earth has very few craters from its earliest days,
because plate tectonics and surface processes usually do a really great job of erasing ancient
structures.
But one crater seems to have stood the test of time,
and promises to give us insight into the Earth about three billion years ago.
Except there’s a snag…
it might not actually be a crater after all!
On the coast of Western Greenland,
near the small town of Maniitsoq,
geologists in 2012 announced
they had found some bizarre features in the rocks;
some were melted, and others pulverized.
To figure out what was going on, scientists mapped
the different rock types across the entire area
and surveyed the local magnetic field to discover
what lay beneath the strange features.
What they discovered was like nothing else seen on Earth.
The rocks are deformed into a large circular bowl on the ground,
about 100 kilometers across.
And within that area,
there are widespread fractures and specific kinds of rock
that hint at huge stress in the crust.
These include breccias, made when rock is smashed up
and then cemented back together, and amphibolites,
which are metamorphic rocks formed under intense heat and pressure.
What’s more, in the center, there seems to be a massive area of pulverized rock,
up to 50 kilometers across.
The geologists pieced all of these clues together
and concluded that there were no natural geological processes on Earth
that could create all of these features.
The only reasonable explanation, they said,
was that the huge Maniitsoq structure was,
in fact, an impact crater.
By looking at the age of melted rocks from around the structure,
the researchers could even tell when that crater was formed:
about three billion years ago.
That makes it the oldest known impact crater on Earth,
and the only crater known from the so-called Archean period.
A crater of this age could shed a lot of light on
what Earth was like during this mysterious era,
a time that we know very little about.
For instance, asteroid impacts,
like the one that brought about the dinosaurs’ demise,
are typically associated with massive environmental changes.
Perhaps a Maniitsoq impact could have done the same,
radically changing the Archean environment just as life was starting out on Earth.
But new research from 2021 has thrown a wrench into the works.
A team of scientists took a closer look at the Maniitsoq structure,
but instead of looking at the rocks on a kilometer scale,
they examined them on the micrometer scale.
They were looking, specifically, at zircon crystals.
These are incredibly resistant and durable minerals
that can withstand heat, pressure, erosion, impact…
pretty much anything you can throw at them.
But they’re also like little time capsules,
recording what they’ve been through,
within their internal structure.
For example zircons from other,
younger impact sites around the world have tiny fractures running through them,
evidence of the massive stresses they endured during a collision.
So this new research examined more than 5,000 zircon grains
collected from around the Maniitsoq structure.
But despite careful probing with cutting-edge geochemical
and microscopic techniques,
researchers found no evidence of impact-induced stress.
So the authors concluded that the weird geology near Maniitsoq
was not in fact caused by a meteor impact,
but was instead the result of a unique time in Earth’s evolution.
The intense heat, pressure,
and fracturing could be linked to a period of massive tectonic
and volcanic activity all over the globe,
which is also thought to have happened around three billion years ago.
At this time, scientists think that the Earth’s mantle
reached incredibly high temperatures,
as radioactive decay in the core combined
with leftover heat from the planet’s formation.
All this heat caused magma to be injected upwards into the crust,
ballooning it outwards and cooking
and deforming the rocks over a period of about 200 million years.
Scientists think this process is responsible
for growing the first continental crust on Earth.
And the Maniitsoq structure might just be a scar
left behind from this turbulent time.
On the basis of the new zircon research,
the Maniitsoq structure is no longer considered an impact crater
by the Earth Impact Database.
But the structure’s original discoverers don’t believe that’s the end of the story.
Even if the zircons don’t show any record of a meteor impact,
there’s still no easy way to explain the pulverized rocks at the center of the structure.
which is much more than normal geological processes can account for.
So there’s still work to do, to get to the bottom of what created the Maniitsoq structure,
and just what it meant for our planet.
Whether it’s an impact from above, or injection from below,
it’s nothing like the Earth has seen before, or since.
Examining our own planet with innovative new techniques
will help us understand it on both the micrometer and kilometer scales.
And even if it’s not an impact,
understanding the curious structure better will help us understand
what exactly was going on on Earth during periods we know so little about.
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