December 1, 2019 37

Plate tectonics: Evidence of plate movement | Cosmology & Astronomy | Khan Academy

Plate tectonics: Evidence of plate movement | Cosmology & Astronomy | Khan Academy

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.

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