[ ♪INTRO ]
Earth has land.
You know that — the odds are pretty good you’re on some of it right now.
But here’s a weird thing to think about: It’s possible that land didn’t always exist.
And technically-speaking, it doesn’t /have/ to.
I mean, if you were just to simply smooth out the Earth’s crust, the oceans contain enough water to
cover the planet in a sea more than two kilometers deep.
So… why does land exist?
Why is it so varied, with all those mountains and valleys and flat plains?
And — here’s a fun one: Could anything ever get rid of land on Earth?
To answer those questions, which god knows I want to do, you need to travel back more
than four billion years.
Billions of years ago, Earth started as a cloud of dust and grains left over from the Sun’s formation.
Then, over time, those pieces slowly balled together into proto-Earth.
That ball was made of all kinds of elements.
And as it aged, the denser ones, like iron, sank towards the center of the ball to become
Earth’s core, while lighter ones stayed towards the outside.
Eventually, the planet separated into the layers we know today: the inner and outer
core, the gooey mantle, and the crust.
These days, there are two main kinds of crust — continental and oceanic — and they’re
made of different ingredients.
Oceanic crust tends to be mostly a type of
rock called basalt and contains more heavy compounds.
And continental crust — which generally makes up land — tends to be mostly granite
and contains more relatively light compounds.
But the composition of the /early/ crust and how it changed in Earth’s first billion
years or so is pretty hard to pin down.
Like a lot of Earth’s early history, we just don’t have much — or in some cases /any/ — physical evidence.
It’s pretty much all been recycled and destroyed by now.
So there are a lot of interpretations.
But mostly, models seem to start with a crust that would more resemble oceanic crust today,
with continental crust slowly growing over time.
The exact date when the first continental crust appeared is one of the big questions in geoscience.
Some models say it started growing almost immediately; others say it didn’t really get
going until about 3.6 billion years ago.
But there’s something potentially really interesting hidden in there.
Because, depending on which of these models is right, early Earth might have been a /water world./
We think the oceans had to have existed by around 3.8 billion years ago.
That’s based on evidence like ancient pillow lavas dated to around that time, which only
form when lava flows into water.
So if continental crust hadn’t formed by then, there would have been a point in time
at which the Earth was, indeed, an ocean world — where land did not exist.
So, like, don’t take land for granted!
We could all be fish!
As for why continental crust started forming, there are a couple of ideas.
One of the most well-studied relies on the movement of tectonic plates, the big slabs
that make up Earth’s crust.
And it goes like this:
At some point, the idea says, the crust started to form into these giant plates, possibly
thanks to massive magma plumes from deep within the Earth.
and as the plates started pushing against each other, some of them began sliding down towards
the mantle in a process called subduction.
And as that happened, the increased heat near and in the mantle began to heat the rock.
But since rock isn’t completely homogenous, it’s not like it all melted at once.
Instead, different chemicals started to liquify at different rates, and the rock /separated/
in a process known as partial melting — with some areas being denser, and others less dense.
Over time, this process repeated, and we ended up with new oceanic crust and the first continental crust material.
That material was brought up through volcanic eruptions, which then built up into small
volcanic islands above the ocean.
The very first land!
Peeking its little head above the water!
And as more volcanoes erupted and material got scraped off subducting plates, these islands
would have grown over time into larger continents.
Of course, like I said earlier, studying the beginning of Earth’s history is hard, so
not all scientists agree that subduction was necessary to build the first continents.
Like, in 2012, one group proposed something a little more… like, ooze-y.
They got this idea while looking at rocks
from the Isua Greenstone Belt in Greenland, which are more than 3.5 billion years old.
They compared the amount of trace elements found in those rocks to amounts we’d expect
to see if they were formed by subduction And they concluded that this ancient crust may
not have needed to get /all/ the way down into the mantle via subduction to melt and reform.
Instead, it might have kind of /oozed/ up as rocks melted higher up, in the crust.
So, no subduction zone needed.
No matter how this occurred, though, eventually the Earth /did/ get its first continent.
Based on various pieces of evidence, some
researchers have proposed that this continent, which they call Vaalbara, was made of rocks
that are today found in Southern Africa and Australia.
While others favor Ur, a land mass made up of what
would today be parts of India, Madagascar, and Australia.
In any case, land happened.
And so far as we can tell, Earth has had it ever since.
Since the time of Ur and Vaalbara, plate tectonics and other forces have kept continents above
water and made them even craggier.
These days, new continental crust is still being formed and destroyed at subduction zones.
And plate collisions have also pushed up mountains, like in the Himalayas, making the Earth even less smooth.
Meanwhile, erosion and other processes have also played a part, with wind and rain carving
canyons, arches, and other amazing landscapes.
So, no matter how it got here, the land hasn’t been unchanging and still.
It’s continually shaped, changed, and even sometimes destroyed or completely hidden by
forces of nature.
And that makes you wonder: If all these forces are still at play, reshaping the landscape all the time…
Well, could those forces ever make land disappear?
Well the good news is, continental crust is usually fairly stable.
It’s mostly the oceanic stuff that subducts and is recycled when plates collide.
And today, the Earth has reached more-or-less equilibrium between the amount of crust made and the amount of crust lost.
But some models have suggested that the amount of continental crust /has/ actually decreased
from some ancient peak.
And a 2016 paper suggested that when India hit Asia, a substantial portion of the continental
crust — like, /half/ of it — ended up being forced down into the mantle.
Like, oh, bye-bye, land! Like, there you go!
So it /is/ possible to destroy continental crust on a large scale.
But even then, land will probably never disappear entirely.
Like I said, Earth seems to have reached a sort of equilibrium between crust made and crust lost.
And we also have plate tectonics working to push parts of the ground higher and higher
above sea level all the time — so even if some sort of catastrophic flooding happened,
that wouldn’t be the end of dry land.
Even if plate tectonics stopped /altogether/ — which for the record, is /really/ unlikely,
since plate tectonics is powered by heat from Earth’s core, and that’s not cooling down
any time soon — Earth still wouldn’t become perfectly spherical.
Scientists at Caltech noted that while erosion might wear the mountains down into hills,
there would still be other processes.
Things like meteorite impacts could still happen, which could create large dents in
Earth’s surface — little rings of land that could stick above water.
Volcanoes would still exist, too — because although many are powered by magma from those
all-important subduction zones, they can also exist far away from plate edges, like the
hotspot under Hawai’i.
In those places, you don’t need a subduction zone.
Instead, magma plumes in the mantle are hot enough to melt their way up through the crust.
In fact, while Earth is the only planet with active tectonic plates, volcanoes like this
have created land on other worlds, too.
Like, even though it’s dry now, Mars used
to have a huge ocean.
But it still had dry land — in part, thanks to things like Olympus Mons, its gigantic, now-extinct volcano.
So, even if Earth was a water world billions of years ago, the odds of that happening again
are pretty slim.
Which is great news!
Because while we probably haven’t always had land as we know it, the fact that it /does/
exist — well, has shaped basically everything about our species and also millions of others.
And combined with the awesome forces of plate tectonics, erosion, and other geology processes,
we’ve ended up with the vast and beautiful array of geography we have today.
Thanks for watching this episode of SciShow!
And as always, a huge thank-you to our patrons on Patreon.
We are very glad to have you, and if you haven’t already, feel free to stop by the Discord
and say hello.
If you’d like to learn more about supporting free educational content, you can head over
to Patreon.com/SciShow.
[ ♪OUTRO ]