What Would Happen If We Just Kept Digging?
B2
We filmed this episode quite a while ago
and as we were digging into the production,
our fellow YouTuber Cleo Abram
posted one on the same subject–
which is great and you should watch it.
But it turns out her adventure to the
center of the planet turned out to be quite different
from our own, which I think is very interesting.
So, we decided to use the video we had already shot
and tell our own story.
As a kid, did you, like me, have that dream
of just digging a hole at the beach and then
making it all the way to the center of the Earth?
My friend Jeff dug a hole in his backyard
until he hit the water table.
It was a well!
But as always happens, our dreams get cut short
long before you reach bedrock.
But over the years, people have kept at it.
Scientists and engineers all over the world
have tried digging as deep as possible,
hoping to learn more about the inside of our planet.
Despite their best efforts and decades of drilling,
these projects have barely scratched the surface.
Turns out, nearly 6,400 kilometers of planet
might be a bit too much of a challenge for us right now.
But what if we could do it? What if we had the tech,
like in the terrible-brilliant 2000s film The Core,
to blast all the way to the middle?
What would we experience along the way?
I’m going to take you on a journey to the center of the Earth.
On the way, we will discover huge fields of diamonds,
solid rock flowing almost like liquid,
and a mysterious inner inner core that
scientists are only just beginning to understand.
You might want to grab a glass of water. It’s going to get hot.
The first stage of our journey takes us through the crust.
And if your objective is to get through the top layer
of the planet as fast as possible, we might start
in maybe an unexpected place: the ocean.
Oceanic crust is the thinnest kind on Earth,
maxing out at about 10 kilometers in depth,
made pretty much entirely out of basaltic igneous rock.
In the 1960s, the US chose this seemingly easy route,
with their ‘Project Mohole’ aiming to get samples from
the oceanic crust off the coast of Guadalupe, Mexico.
The problem is, before you even get to that igneous rock,
you have several kilometers of ocean to deal with.
Geologists have described drilling into the ocean floor
through the ocean as being like
drilling through the ground from a helicopter.
It was difficult and expensive, and Project Mohole only
managed to dig a hole about 180 meters deep before
the money and everyone’s patience ran out.
Even with today’s technology, honed by decades of deep
ocean drilling, researchers have only penetrated a little
more than 2 kilometers below the sea floor, less than
a third of the distance needed to make it through the crust.
Which leaves us with starting our hole on dry land,
on a nice stable bit of continental crust.
The kicker is that this crust is a lot thicker than the stuff
beneath the oceans, and we could be facing up to
70 kilometers of rock on our crustal journey alone.
And as we delve deeper into these rocks,
the conditions make our project a bit tricky.
Temperatures increase by 25 degrees Celsius for every
kilometer we descend towards the planet’s superheated interior.
And thanks to the weight of rocks above, the pressure
increases by 1 atmosphere for every 3 meters down.
Ultimately, this is what has stalled all of our real-world
attempts to dig to the center of the Earth, like
the Kola Superdeep Borehole -
the USSR’s answer to Project Mohole.
Yes, in addition to having a space race up
where we went up, we had an inside race, too,
where we tried to go down.
And the USSR beat us on that one.
We’ve talked about this amazing feat of engineering
on Scishow before, and it still holds the record for
the deepest hole on Earth, at just over 12 kilometers.
But despite more than two decades of drilling,
this distance is barely a fifth of the way through
the continental crust, and 0.2% of the way through the Earth.
The reality is, any real-world drilling effort will ultimately
falter because at some point, things will get so hot that
the metal in the drills will become too soft for them to work.
There are some people working on giant
lasers to do this though, so that's cool.
There is a reason we know more about the furthest reaches of
the solar system than we do about the inside of our planet!
There's basically nothing between us and Pluto.
Like, it's just a vacuum the whole way!
It's the opposite when you start going down.
But remember, we’re suspending our disbelief here and
conveniently setting aside silly little things like material physics.
Let’s just say we could do what the Soviets did, and more…
The science of seismology—
which uses earthquake waves as
a kind of sonar for Earth’s interior—
can give us a good idea about what we might encounter.
At depths of more than three and a half kilometers,
we will still find animals, in the form of devil worms,
which are cool as cucumbers despite the heat and pressure.
Those worms eat microscopic bacteria that
live down there in the rocks too.
And those simple lifeforms can withstand even more extreme
conditions, surviving more than 5 kilometers deep into the crust.
Of course, all life needs water to survive,
and tiny cracks from the surface can allow fresh water
to permeate as deep as 10 kilometers down.
But there are also pockets of salty water down there too,
trapped by the surrounding rock. With no way to circulate,
scientists think it could have been sitting there
for many millions of years and could contain
ancient microbes dating from the geological past.
And we’ll find ancient fossilized life here, too
All fossils were once living organisms on the Earth’s surface–
or they're products of living organisms,
you can have fossilized poop–
but over time, geological processes and plate tectonics
push them down, deep into the crust.
The Kola borehole revealed fossils of single-celled organisms
7 kilometers down, dating from 2 billion years ago.
So our journey to the center of the Earth could
take us on a journey back through time as well.
However, the deeper we go through the continental crust,
the less likely we are to find fossils.
High temperatures and pressures reshape minerals
to make metamorphic rocks with interlocking crystals
that are some of the hardest in the world.
Limestones become marble,
mudstones become schists and gneisses.
And in some places, the rocks themselves melt
and recrystallize as huge blobs of igneous rock.
After tens of kilometers of digging,
we will finally reach the mantle.
The boundary between the crust and the mantle is known
as the mohorovicic discontinuity, or the ‘moho’ for short.
It’s a place where seismic waves suddenly speed up
as they pass into a rock of a new composition.
Before we hit the moho, the walls of our hole
would be lined with huge black crystals of gabbro.
Which frankly, would make some absolutely delightful countertops.
But the view changes once we pass into mantle rocks.
Here, our drill hits peridotite, which is rich in glittery green olivine
crystals with scattered red garnets like raisins in a fruit loaf.
I bet you never read a geology textbook that told you that
the mantle was green with red polka dots, but that’s pretty
much going to be our view for the next 2,800 kilometers!
Once we’ve passed through the moho,
we’re officially in the mantle.
But we’re still slogging through the hard outer
shell of the planet known as the lithosphere.
The reason the crust and the topmost section of the mantle
are grouped together is because they stick together.
Even though average temperatures are around
500 degrees Celsius, both crustal gabbro and mantle
peridotite are still solid and rigid, so they behave as one.
This lithosphere extends 150 kilometers beneath the oceans,
and 300 kilometers beneath the oldest continents.
But our trip through the lithospheric mantle is far from dull.
Because here, scattered in amongst the green and red peridotite,
we can expect to find bright flashes from a layer of diamonds.
In fact, this is where most of the natural
diamonds on Earth are born.
Volcanic fluids are shifted around by plate tectonics,
and those fluids can contain all kinds of elements,
including carbon.
When those carbon-rich liquids are shunted towards
the bottom of the lithosphere, the high pressures and
temperatures there are just right for building diamond crystals.
Violent volcanic events have brought some of those diamonds
to the surface, in places like Russia, Botswana and South Africa.
But there’s a far bigger reservoir of the gems
lurking at the bottom of the lithosphere.
We can enjoy a few hundred kilometers of breathtaking,
crystalline scenery as we travel through the lithosphere.
But when the increasing temperatures hit 1,300 degrees Celsius,
everything changes as we enter the asthenosphere.
At this temperature, it’s hot enough for
the mantle rocks to start behaving strangely.
Now, you might be expecting to hit
something like molten magma at this point.
After all, we learn that it’s the convection of
the mantle that powers the movement of the continental plates.
And while the convection part is likely true,
I’m sorry to say that hitting the mantle isn’t
going to be like striking oil, creating a fountain of lava.
Nope, the upper mantle is a solid.
But because it’s so hot, it flows almost like a liquid,
albeit really really slowly.
In this case, the mantle convection currents involve solid rock
moving at about the same speed that your fingernails grow.
So, as far as we would see as we drilled down through it,
the upper mantle is still that glittery green solid,
encrusted with garnets.
Come back in a hundred years though,
and your hole wouldn’t be quite where you left it!
At a depth of about 410 kilometers,
the behavior and composition of the mantle changes once again.
Between here and a depth of 660 kilometers,
we’re passing through the mantle transition zone,
which is marked by two changes in
seismic wave speeds at the top and bottom.
Scientists aren’t exactly sure what’s causing these shifts,
but they think that it could be thanks
to a huge reservoir of water trapped there.
It’s likely that there’s about three times the amount
of water down here as in all the oceans on the surface.
But before you get carried away imagining a sloshing
shell of liquid down here, this is not, like, a subterranean ocean.
Instead, it’s water that’s bound up inside crystals of a
mineral called ringwoodite, which only forms at the precise
temperatures and pressures found at these depths.
So the next time you see someone on Instagram
saying, "There's a giant subterranean ocean!"
You can be like, "No! Ringwoodite!"
Like what I do! Every time!
The water in ringwoodite would make the crystals bright blue.
We might not be able to dip our toes in the mantle’s ocean,
but we can enjoy 250 kilometers of beautiful
blue rock as we descend nonetheless.
Just don't touch it! Don't touch any of this!
At the end of the mantle transition zone,
ringwoodite is no longer stable, and our drill tip might
begin to struggle a bit as it enters the lower mantle.
Here, things are hotter still, averaging
around 3000 degrees Celsius.
But the pressure of nearly 700 kilometers of
overlying rock is enough to keep the mantle solid and stiff.
Unlike the asthenosphere above, some geologists
think that the lower mantle doesn’t convect.
But slow-downs in seismic waves seems
to suggest that there is still some structure down there.
Specifically, there are two massive blobs sitting opposite
one another like earmuffs on the outer core,
roughly under Africa and the north Pacific.
And when I say massive, I mean it -
each one is twice the size of the moon.
Scientists think the seismic waves slow down here
because the blobs are iron-rich, but there’s some
disagreement about where they might have come from.
Some speculate that they’re domes of
extra-hot material that’s rising up from the core
and interacting with the colder mantle around it.
But there’s another more intriguing possibility:
that the blobs are the remnants of a cataclysmic event
in Earth’s past, like the planetary impact that created
the moon about 4.5 billion years ago!
That could leave chunks of very different
rock buried deep in the mantle.
If relics of Earth’s ancient history weren’t enough for you,
as we continue our journey down towards the bottom
of the lower mantle, we will find ourselves in
yet another reservoir of diamonds.
They form thanks to interactions at the mantle’s lower boundary,
as water molecules split apart and react with iron carbide.
This kicks carbon out of the core,
where it’s forged into diamonds under
extreme pressure and temperature.
Since they have more than 2,800 kilometers
of rock above them, most of these core-mantle
diamonds will never make it to the surface.
But some scientists believe that the famous
Hope Diamond may have come from the deep mantle.
After drilling down for 2,890 kilometers, we’ve finally
made it through the mantle, and into the outer core.
The temperature has risen gradually to nearly
4,000 degrees Celsius at the core-mantle boundary.
And for the first time, our drill tip is biting
into a completely new kind of material.
Instead of rock, we’re now drilling through metal.
Specifically, an alloy of iron and nickel,
with some other elements such as gold, platinum, and cobalt.
Because of the high temperatures,
this metal mix exists in a liquid state.
Its complex motion is affected by the planet’s rotation,
and turbulence in the outer core is responsible
for generating the Earth’s magnetic field.
Which I am a huge fan of! It's great!
However, it’s not exactly something you could go swimming in,
and not just because it would sear your skin off.
Because that liquid is thick.
High pressures at this depth mean that even at the top,
the molten metal is about the same consistency as peanut butter.
Which I have never tried to go swimming in,
but don't think I would be good at,
and it gets thicker, like thicker than tar towards the bottom.
So while the outer core’s motion is super
significant on geological timescales,
we wouldn’t exactly be swept away as we passed through.
By the time our hole is 5,150 kilometers deep,
we officially enter the inner core.
Here, the temperature is around 5,500 degrees Celsius,
similar to what you might find at the surface of the sun.
And the pressure, which has been increasing all the way down,
is now 3.6 million times that at the surface.
The material we've been drilling through for the last
few thousand kilometers hasn’t changed,
but despite the extra heat, the iron-nickel
alloy is no longer a liquid.
Instead, the intense pressure is enough
to force the metal back into a solid.
Curiously, the solid metal that forms under these
conditions is unlike anything you would find at the surface.
It exists as extremely tightly packed hexagonal iron crystals
that are all aligned north-south by the Earth’s magnetic field.
I like to imagine it looks like a metallic version of the
giant’s causeway in Ireland, or the Devil’s tower in Wyoming.
There are a little more than 1,200 kilometers of
these close-packed iron crystals before we
officially reach the center of the Earth.
But there’s one more surprise waiting for us on our way.
Inside the inner core, at a depth of some 5,720 kilometers,
there is an inner, inner core, like a pit inside a piece of fruit,
which scientists have only recently
discovered and still know very little about.
First theorized in 2002, its existence was confirmed in 2019
thanks to seismic waves from extremely strong earthquakes.
Researchers realized that seismic waves were traveling at very
different speeds depending on which direction they were going
through this part of the inner core, which is thought to be
thanks to another difference in crystal structure.
One possibility is that the iron-nickel crystals switch
to being aligned east-west instead of north-south,
as if our metallic Devil’s Tower had been turned on its side.
What’s even less clear is why there’s a 650 kilometer thick
ball of crystals on their side in the center of our planet.
It could be a fossilized record of some major event in
Earth’s past, but at that depth, we may never know.
So here we are. After 6,371 kilometers of drilling,
we’ve made it to the center of Earth.
We’ve traveled through the solid rock of the lithosphere,
solid flowing rock of the mantle, liquid metal of the outer core,
and crystalline metal of the inner and inner-inner core.
If we kept on digging now, we’d get to see it all again in reverse,
with the added challenge of gravity working against us!
It took the Soviets 24 years to drill through
12 kilometers of crust for the Kola superdeep borehole.
Even if we managed to match their pace despite
soaring heat and pressure, at 2 years per kilometer
it’d take us nearly 13,000 years to make it
all the way through the planet.
So if you’re on the beach and serious
about making the trip, better start digging!
