[ ♪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 ]
