Let's think a little bit about the different clues that
have led us to conclude that we have these lithospheric plates
moving relative to each other.
Now, the first clue, and this is something
that I think many students even in elementary school
first experience when they first learn about geography,
is that it looks like the continents could kind of fit
into each other.
And the most obvious one of these
is when you look at this kind of little pointy part of South
America, and if you have a more detailed map
it really is amazing, how well it
seems to fit into the Nigerian Basin right here in Africa.
It looks like at one time this little pointy part
was nudged into this part of Africa,
that they were actually connected.
And if you're a little bit more creative,
there are other parts of the world
that you can kind of start to see how they might have fit
in with each other in the past.
And that by itself, that's just a very small clue,
but it kind of hints at well, maybe if they at one time
were fitting next to each other, if this was kind of connected,
then they've had to moved apart at some time.
Although it doesn't tell us that it's still moving
or what might have caused the movement.
And it definitely doesn't definitively
tell us that they even moved.
Maybe this is just a coincidence that this coast
of South America looks very similar to this coast
right here of Africa.
Now, the next clues truly came over
I would say about the last 60 or 70 years.
The first clue is that OK, if you
go to the mid-Atlantic ridge right here--
So if you look at the Atlantic Ocean.
Let me look at this photograph right over here.
You don't normally see the oceans highlighted like this.
So let me make it very clear to you.
This right here is South America.
This right here is Africa.
This right here is North America.
And so if you actually look at the elevations
in the middle of the ocean people noticed
in the middle of the 20th century that gee,
there's a ridge in the middle of the Atlantic Ocean.
There's kind of a mountain range that
goes straight up the middle of the Atlantic Ocean.
So that by itself doesn't tell you
that you have these plates that are moving apart,
but it is kind of a curious thing to look at.
And not only is there this ridge.
There's lot of underwater volcanic activity.
You have magma flowing out and lava flowing out
into the water, and it's kind of forming this ridge that really
goes across the whole Atlantic Ocean.
There are other ridges in the world
like that, underwater ridges.
You have one over here in the Pacific Ocean.
You have these here in the Indian Ocean.
That's just a little clue, but that by itself
doesn't tell you that these plates are actually
moving apart at the ridge.
The more conclusive-- this is just
the beginning of the clue-- but what
made this conclusive is one, the separate discovery.
And this is what's interesting is
that you have these separate discoveries
in different domains that eventually
let you come to a pretty neat conclusion.
So you've had a separate discovery that if you
look at different eras of magnetic rock,
or maybe I should say magnetic rock from different periods
in geologic time.
And you can tell where they are in geologic time
by how they're layered.
So this would be newer rock.
And then this would be a little bit older.
And then this would be even, even older.
Geologists noticed something interesting.
If I were to take magnetic rock, and if it was molten lava,
and if it were to harden, remember it's magnetic rock
so it would want to align with the poles the same way
a compass would.
So if I had a bunch of magnetic-- so
let's say this is some lava right here.
And so the molecules can align themselves.
Since it's a liquid and they can align themselves,
they are going to naturally want to align with the poles.
So they'll naturally all want to align in one direction because
of Earth's magnetic field.
And so when that lava hardens into actual rock
that alignment will kind of be frozen.
Now, if Earth's magnetic field was constant over time,
then when you look at magnetic rocks from any period,
you would expect them all to be aligned in the same direction.
So since we're taking a cross section of rock here let's
say an alignment towards the North pole looks like this.
And I draw it like that.
That's kind of an arrow pointing into our screen.
And let's say an alignment pointing to the South pole
would look like this.
This would be an arrow pointing out of our screen.
So what you would expect is the newer
rock that kind of the alignment, the field, the alignment
of the rock, would go into the screen, and then the older
rock, it would still go older into the screen.
So if I were to draw a top view--
Let me draw it like this just so I
make sure that everyone is on the same page.
So let me just draw a cross section like this
so that we know what we're talking about.
And so this is the surface up here.
This up here is the surface.
When I talk about going into the page
that means that the magnetic rock
would be aligned in that direction.
And when I talk about going out of the page
it means, so if I were to draw it
like that, that means that the magnetic rock would
be aligned in that direction.
Now, like I said, if the magnetic field of Earth
never changed, then lava that essentially cools down
into non-lava rock, or you can say freezes into rock,
it would all point into the same direction
regardless of when it hardened.
This would be the situation in a constant magnetic field.
But what we've seen is that that's not the case.
When you look at older magnetic rock,
and depending on how old you go, you
have the newer rock that's aligned
with our current magnetic field.
You go a little bit older, and right now we
think it's about 780,000 years ago roughly.
You have to find rock of that age, magnetic rock that
hardened at that time.
It's actually in the opposite direction.
So actually, the magnetic rock has hardened in a way
so it's as if the North Pole was at the South Pole now,
the magnetic North pole.
So it's aligned in the opposite direction.
So it's kind of pointing out of the page here.
And then if you get even older rock,
it's more aligned with our traditional direction.
So it's more aligned than that.
And so the only reasonable conclusion
that we can draw from this is that Earth's magnetic field
has actually fluctuated over time.
Now, you're probably thinking, Sal,
how is this relevant to plate tectonics?
Well, once you accept that magnetic fields fluctuate
over the history of the Earth, there's
another interesting observation you
can make about the rock that's kind
of at the basin of the ocean floor.
So not only do you have this mid-Atlantic ridge,
you have these volcanoes spewing kind of new rock
into the ocean, creating this kind of underwater mountain
ridge, but it also turns out that the rock that
forms the sea floor also contains
a lot of magnetite, which is magnetic.
And what's really interesting about that-- so let me draw.
So we're going to have a top view
just like we have over here.
So let's say this is the mid-Atlantic ridge right here.
Now, this is really cool.
So when they look at rocks that are
very close to the mid-Atlantic ridge, they're aligned--
and once again, we're looking at rocks
at the floor of the ocean-- they're aligned in a way
that you would expect with the current magnetic field.
They are aligned just like that, the way
that you would expect when you're
looking at the magnetic rock that's close to the ridge.
But if you go a little bit further,
and when I say a little bit I'm talking about thousands
of miles, but when you go further out from that
you have stripes of other magnetic rock
that is going in the opposite direction.
It's going like this.
And what's even cooler than the idea
that it's switched directions depending on how far you've
gone from the rift is that there's
a symmetric stripe of magnetic rock on exactly
the same distance, or roughly the same distance away
from the rift that's also pointing
in that same direction.
And you go a little bit further out
and you'll find some rock that's pointing
in the original direction.
And even better, you go on the symmetric other side
of the actual ridge and you find another set
of rocks that's doing the exact same thing.
So if you accept that Earth's magnetic field has kind of been
flip flopping over time, the only reasonable conclusion,
at least that I could think of, or the geologists can think of,
is that sure, all of this was formed
at a similar period in time.
This came out as lava, magnetic lava,
and then it all aligned with Earth's magnetic field.
And that's why it looks similar.
You fast forward in time some and the only way--
Or actually, let's not fast forward in time.
The only way that these could have formed,
and they could have been so similar.
So if we rewind in time, the only way
that these purple magnetic rocks could have aligned this way
in exactly the same way and exactly the same distance
is if at some point they were much closer to each other,
if they were actually connected.
So if we rewind in time maybe at the mid-Atlantic rift
you had all of the purple rock coming out
from those underwater volcanoes, and at that time,
Earth's magnetic field was the opposite as it is right now.
And then of course, you had this blue rock
that is looking like that.
And so this seems like a reasonable explanation.
This rock and this rock were at some point touching.
They were actually formed at the exact place
and at the same time.
And so if this is the case, if at one point
this purple rock was all together,
and they formed at the same time at the mid-Atlantic rift,
we're assuming all the rock was-- well,
we don't have to make that assumption--
but if you assume that they've formed at the same time,
and that based on the pattern it really does look like they do,
and it's a symmetric distance away from that rift,
then the only reasonable conclusion I can think of
is that the rift has had to move apart.
The rift has had to move apart from this period
to that period.
And there was a time when all this blue rock was together.
So that by itself, that frankly is the most definitive evidence
in the 1960s where it kind of became conclusive that you did
have these plates that were moving away from each other.
And obviously if the plates are moving away from each other
at some point, and that means that just based on the way
the map looks, at some point they're also
going to be moving into each other.
We could talk more about that in future videos.
But, you know, at certain points one plate
is moving under another.
And we'll talk about how that might partially explain,
and we'll talk about all of the explanations
for why we think the plates might actually be moving.
But now, if we fast forward to more present times now
that we have GPS satellites and all the rest,
we can actually measure the movement of the plates.
This is actually an image from NASA
showing the vector of the movements
at different points on the surface of the planet.
And you can see we've gotten a lot of vectors
from the United States, so it's almost hard to read
since it's so chalk full of vectors.
But you can see right over here in Hawaii.
The Pacific Plate at that point is moving in this Northwest
direction as measured by GPS satellites.
And I want to make clear, this movement is relatively slow.
It's roughly the speed at which fingernails grow,
but if you do it over millions of years,
that actually amounts to thousands of miles.
So we're talking on the order of about a centimeter a year
for most of the plates.
Some of the plates might be moving a little bit faster,
maybe close to 10 or 15 centimeters,
but most are moving about a centimeter
a year, at the same rate your fingernails are going.
But this is fascinating because we can actually measure it,
because GPS is so accurate.
Over here it looks like the North American plate
is kind of rotating generally in that direction.
The Nazca Plate right here is moving roughly
in that direction, moving into the South American Plate.
I'll leave you there right now.
And actually before I leave you there,
this is another thing that I got off
the Wikipedia that shows that same magnetic striping.
It's maybe a slightly neater drawing.
I don't know which one might be more helpful for you.
But I'll leave you there in this video.
In the next video, we'll think about some of the theories.
We know now that the plates are moving.
Let's think about some of the theories
as to why they might actually be moving.