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The fact that Earth has land seems like it should be a given.
Like, the sky is blue, cats puke on the carpet, and Earth has land.
But that hasn’t always been the case.
And how all this land formed in the first place…
that’s a bit of a mystery.
Geologists are still working on the answer,
but a study published in 2022 suggests we might have rocks
from space to thank for the land beneath our feet!
If you drained all the water off the Earth,
you’d see that the outer layer of our planet, the crust,
is like a collection of puzzle pieces.
They’re called tectonic plates, and they sit on a layer called the mantle.
Generally, tectonic plates fall into one of two categories.
You have oceanic pslates, which are usually underwater,
and are mostly made of a rock called basalt.
And you have continental plates, which make up the majority of the land on Earth,
and are mostly made of granite.
The critical piece here is that granite is less dense than basalt.
This means continental plates are more buoyant than oceanic plates.
As a result, they’re able to poke up above sea level and give us land,
which is important for our mostly non-aquatic lifestyles!
But about 4.5 billion years ago,
shortly after the planet formed, continental plates didn’t exist.
The Earth’s crust was a single piece of rock, like a shell.
And that shell looked more like an oceanic plate than a continental one.
So, for us to get the world we see today, continental plates needed to form.
And for that, we needed granite.
Today, you can find the right conditions for making granite at the base of continents,
where an oceanic plate slips underneath a continental one.
There, it melts and forms granite.
But you can probably see the chicken and egg problem coming here.
If you need granite to make a continental plate,
and granite is formed at the base of continents…
how did the first continental plate form?
Where did land come from?
Generally, researchers believe that if something caused one spot on an oceanic plate
to grow and thicken into a huge mass of basalt,
that mass would sit so deep in the mantle that temperatures
would be high enough for its base to melt.
And from there, you’d have the right conditions to form granite.
But what was that something?
The standard idea is that plumes of magma came up
from the mantle and caused an area of the crust to grow and thicken from below.
Basically, some of the rocks that had been on Team Mantle joined Team Crust.
But another idea, supported by a 2022 research paper,
is that the culprit may have come from above.
The idea is that a series of enormous asteroid or comet impacts
could have fractured Earth’s shell
and excavated a huge amount of material from craters.
Removing all this material would make the pieces of that shell lighter and more buoyant,
relieving pressure on the mantle below.
Rock in the mantle would have then risen up, melted,
and thickened the crust.
Conveniently, geologists do have evidence that Earth
was hit with a barrage of asteroids at roughly the same time that the continents emerged.
lBut solid evidence that these impacts started continent formation has been hard to find.
In the last four-plus billion years,
much of Earth’s rock has melted and been recycled into something else.
But luckily, a few tough crystals are still around that preserve a record
of Earth’s earliest days. They’re called zircons.
In their 2022 paper,
researchers looked at zircons
from a piece of ancient continental crust in western Australia.
Specifically, they looked at how the amounts of different oxygen isotopes
within the zircons changed over time.
An isotope is a lighter or heavier version of an element.
In this case, a tiny percentage of Earth’s oxygen
is a little heavier than normal because the atoms have an extra two neutrons.
Heavy oxygen behaves differently in many reactions and processes
, so different kinds of crust or the mantle have different amounts of these isotopes.
That means that measuring the amount of heavy oxygen in zircons
can give you a sense of what kind of material that crystal formed from.
And the oxygen in these zircons told a surprising story.
Between 3.6 and 3.4 billion years ago,
there were sudden drops in how much heavy oxygen the zircons of the time had inside them.
And usually, you see that when surface water has reacted with rock,
not when material in the mantle is rising up.
So, the scientists interpreted this to mean that around this time,
impacts cracked and melted Earth’s crust and allowed water to flow through it.
Then, between 3.4 and 3 billion years ago,
zircon crystals formed with moderate amounts of heavy oxygen.
That matches the heavy oxygen amounts
that crust has when it forms from melted material in the mantle.
So, maybe the mantle had risen up by this point,
but after an impact got the ball rolling.
Finally, 3 billion years ago,
the amount of heavy oxygen in the zircons increased to even higher levels.
This indicated that the crust that already existed was melting and being recycled.
This might mean that a continent had finally formed by this point,
but the team isn’t absolutely sure.
In any case, the key detail in these zircons
is that the crust seemed to break before the mantle rose up.
And researchers took this as evidence that impacts from space likely kickstarted continent formation!
In general, impacts often get a bad rep here on Earth.
I mean, look what happened to the non-avian dinosaurs.
But this study provides evidence that without impacts, those dinos
— and all of us —
might not have had any land to live on.
That said, it often takes more than one paper
to convince every researcher that a new hypothesis is the best one.
So, the team plans to investigate samples from other parts of the world,
to see if there are similar patterns in ancient continental crust beyond Australia.
With every new study, we’ll get closer to the answer,
and to a better understanding of how Earth became the place we call home, land and all.
Thanks for watching this episode of SciShow Space.
If you’re interested in absorbing even more awesome geology knowledge
into your brain sponge
you can check out this video on that period in Earth’s history
when the North America tried to rip itself apart.
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