The first realization that there were actually
distinct layers of the earth came from this guy right
over here, Andrija Mohorovicic.
And I apologize ahead of time to any Croatians
for butchering any of the pronunciation.
And he was a meteorologist and a seismologist.
And he was the first one to notice,
in 1909, when there was an earthquake.
There was an earthquake in Croatia,
a little bit southeast of Zagreb.
So the earthquake was roughly over here.
And lucky for him and lucky for us, before that
earthquake there was actually a bunch
of seismographic stations already in the area.
And all these seismographic stations are
are, essentially, instruments were installed
so that if there was any essentially
seismic waves passing, they would
be able to measure it when the waves got there.
And what was interesting about this,
Andrija realized that if the entire earth was just
kind of a uniform materials-- let's draw that scenario--
it would get denser as you go down.
And so you would have kind of this refraction,
this continuous refraction, or these curved pats, happening.
But he realized that, let's say we had an earthquake
right over here, so this is the uniform case.
Uniform.
Uniform layer, only one layer, although it does get denser.
Then the closer you are to the earthquake--
so waves would get there first, then waves
would get over there, then waves would you get over there--
and these are the body waves.
These are the ones that are traveling
through the earth's crust.
But in general, the further you are away from the earthquake,
or the time it takes for the waves to get to a point,
is going to be proportional to the distance
that point is away from the earthquake.
So you would expect to see something like this.
So if you were to plot on the horizontal axis,
if you were to plot distance, and on the vertical axis
you were to plot time, you should see something like this.
You should see a straight line.
And that's just because it's traveling
roughly the same velocity along any of these arcs.
It's maybe getting a little bit faster as it's getting deeper.
But roughly the same velocity as it's
traveling along these arcs.
And the distance of these arcs are
proportional to the distance along the surface,
along the distance of the surface.
So essentially, the time is, they're all traveling roughly
at the same velocity, and their just traveling
different distances, so the time it takes
is just going to be proportional to the distance.
But he noticed something interesting.
When he actually measured when the waves from that earthquake
reached different seismographic stations,
he saw something interesting.
So this is in the theoretical, if we
had a kind of this uniform one-layered earth.
But he saw something interesting.
So once again, this is the distance,
and this right over here is time.
And at 200 kilometers, at 200 kilometers away
from the earthquake-- so until 200 kilometers,
he saw exactly what you would expect from a uniform earth.
It was just the time took was proportional to the distance.
But at 200 kilometers, he saw something interesting.
All of a sudden, the waves were reaching there faster.
The slope of this line changed.
It took less time for each incremental distance.
So for some reason, the waves that we're
going at these farther stations, the stations that
were more than 200 kilometers away, somehow
they were accelerated.
Somehow they were able to move faster.
And he's the one that realized that this
was because the waves that were getting
to these further stations must have traveled through a more
dense layer of the earth.
So let's just think about it.
So if we have a more dense layer,
it will fit this information right over here.
So if we have a layer like this, which we now
know to be the crust, and then you have a denser layer, which
we now know to be the mantle, then what you would have is--
so you have your earthquake right over here, closer
by, while you're still within the crust,
it would be proportional.
It would be proportional.
And then let's say that this is exactly,
this right here is 200 kilometers away.
But then if you go any further, the waves would have to travel.
They would travel, so they would go like this.
And then they would get refracted even harder.
So they would get refracted.
So they would be a little bit curved ahead of time.
But then they're going to a much denser material.
Or it's not gradually dense, it's actually kind of a all
of a sudden a considerably more dense material,
so it will get refracted even more.
And then it'll go over here.
And since it was able to travel all
of this distance in a denser material,
it would have traveled faster along this path.
And so it would get to this distance
on the surface that's more than 200 kilometers away,
it would get there faster.
And so he said that there must be a denser layer
that those waves are traveling through,
which we now know to be the mantle.
And the boundary between what we now
know to be the crust and this denser layer,
which we now to be the mantle, is actually named after him.
It's called the Mohorovicic discontinuity.
And sometimes this is called the Moho for short.
So that boundary between the crust and the mantle
is now named for him.
But this was a huge discovery, because not only was he
able to tell us, based on the data--
based on, kind of, indirect data, just based on earthquakes
happening, and measuring when the earthquakes reach
different points of the earth-- that there probably
is a denser layer.
And if you do the math, under continental crust
that denser layer is about 35 kilometers down.
He was able to tell us that there is that layer.
But even more importantly, he was
able to give the clue that just using information
from earthquakes, we could essentially
figure out the actual composition of the earth.
Because no one has ever dug that deep.
No one has ever dug into the mantle, much less
the outer core or the inner core.
In the next few videos, we're going
to kind of take this insight, that we can use information
from earthquakes, to actually think
about how we know that there is an outer liquid core
and that there's an inner core, as well.
And then, obviously, you could keep going
and think about all the different densities
within the mantle and all of that.
I won't go into that much detail,
but I'll see you in the next video.