Why Does The Earth Have Layers?


[MUSIC] Hey! I’m just making a planet. Well, this
is how planets like Earth get their structure, anyway. Because of different layers of density! Even today, with all of our modern technology,
we’ve only been able to drill about a third of the way through Earth’s crust, so how
do we really know if it’s solid, liquid, hollow? Luckily, Earth has this tendency to
violently shake and occasionally burp up some of its insides, and that’s taught us a lot
about our planet’s guts without having to go down there. We’re used to seeing density at work. That’s the same reason that the atmosphere,
the least dense part of our planet is on the outside, and the crust, the second least dense
part of Earth, is beneath our feet. Because I’m less dense than the dirt, I
don’t sink into the ground. And even though there’s about one ton of atmosphere above
my head, it’s not dense enough to send me floating. The main layers of Earth are organized in
the same way. Depending on whether they’re divided up by how they squish around or what
they are made of, geologists give different names to the different layers of the Earth. So that’s how it is now. But to really understand
why Earth is organized the way it is, we need to go back to before our planet even existed. In the very young universe, hydrogen and helium
were pretty much the only elements around. They condensed into stars, began the process
of nuclear fusion, and eventually died, spitting heavier elements, from carbon and oxygen to
things like nickel and gold, back out into the universe. One of those heavy elements,
iron, is the most stable element produced outside of a supernova. The early universe
produced a lot of iron, that’ll be important in a second. Fresh hydrogen and helium went on to form
new stars like our sun, and the heavier elements collided to form the dust and debris that
would become our solar system’s planets, moons, asteroids and everything else. High temperatures in the early inner solar
system meant that light, volatile elements could only condense further out, which is
why the four inner planets of our solar system are dense and rocky, while outer gas giants
like Saturn could hypothetically float in a really, really, really big swimming pool. After proto-Earth grew larger, radioactivity,
gravity and violent boom booms melted the messy mixture of rocks and minerals. And this
is where things started to get organized. Just like that tower of density, the heaviest
materials like iron and nickel worked their way to the core, and the lighter materials
like aluminum and silicon stayed near the surface. The inner core experiences pressures more
than three million times what we do on Earth’s surface, which means that despite being as
hot as the surface of the sun, the iron in our planets inner sphere is likely solid,
not liquid. The outer core is most likely liquid because
it’s hot, but not under as much pressure as the inner core. We know that’s the case
because of earthquakes up here on the surface. As certain kinds of seismic waves travel through
the Earth, the liquid outer core either refracts them, or blocks them altogether, creating
seismic shadows on the opposite side of the planet. If you want to use pressure to melt metal
at home, just stack up 16 million pennies. The one on the bottom should liquify in no
time! Mercury is so close to the sun that its atmosphere
has long since blown away, but luckily for you and me, our liquid metal outer core lets
us have an atmosphere. Deep metallic convection currents create a magnetic field that shields
Earth from the solar wind. Otherwise we’d be pounded with deadly radiation, our atmosphere
would be blown away, and Earth wouldn’t be a very “lifey” place. Over time, Earth continues to cool, so more
and more of its liquid outer core is turning solid, and we’re shrinking little by little.
Every earthquake that we feel is Earth taking one step closer to cooling off, and becoming
a real third rock from the sun. So there you have it. Like Carl Sagan said:
“We are star stuff who has taken its density into its own hands.” It’s density, right? “I’m your density!” Stay curious.

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