Plate Tectonics

Hi. It’s Mr. Andersen. Today
I’m going to be talking about plate tectonics. What’s always interested me about plate tectonics
is it’s a fairly recent science. In other words, not until the 1950s were we sure that
the plates were actually moving. One of the first people to suggest that was Alfred Wegener.
And he actually worked right around 1920. And what he said is that if you look at the
different continents, here’s North, South America, Africa, Eurasia. They tend to fit
together almost like a puzzle. And so he was the first person to say maybe all of the plates
were connected at one point. We now know that that’s true. And that that was called Pangea.
The problem is that Wegener thought that the continental plates which are actually really,
really large and the oceanic plates, that the continental plates were somehow pushing
their way through the oceanic plates. Oceanic plates are really, really dense. And so other
scientists at that time said that they wouldn’t have the momentum to be able to push through
the rocks. And so the theory kind of fell apart. And so Alfred Wegener actually didn’t
spend most of his career studying plate tectonics. He ended up being a meteorologist for most
of his career. And ended up dying in Greenland. It’s a crazy story of how he met his end.
But that’s not what I’m going to talk about. I’m going to talk about plate tectonics. So
when did this science really take off? Well there were two things that took place. First
of all we started looking at fossils on one side of the Atlantic and on the other. And
we find that they actually matched up. So that’s a good piece of evidence. But the best
evidence came by looking at the rock in the oceanic crust. So in the floor of the ocean.
There’s rock underneath the water. And we started looking at the magnetic fields that
are stored in the rock. So as the rock is actually being formed, we can look at the
magnetic polarity in the rock itself. And our poles will actually switch back and forth
so that occasionally north will become south and south will become north. And that’s actually
recorded in the rock. And so we could look at how old the rock was and we can see how
fast it’s actually moving. And so this is a cool study because this right here, to kind
of wrap your head around this, this is right down the middle of the Atlantic ocean. These
are going to be the youngest rocks. And so in this diagram the youngest rocks are going
to be in red. And then the yellow is going to be older. And then the green is going to
be older yet. And the blue is going to be older yet. So what does that mean? Well it
means that the youngest rock are actually right at the center of the ocean. And then
it’s older the farther we go to the edges of the Atlantic. What does that mean? Well
it means that North America and Eurasia were connected and they’ve been pulled apart. But
they’re not plowing through the oceanic crust. New oceanic crust is being made in the middle.
And so it’s actually pushing them apart. Now how fast does this happen? I remember reading
once that oceanic or that plates move about as fast your fingernails grow. And so you’re
not going to see the plates moving. But over millions and millions of years it actually
ends up being quite a bit. Now another way to look at where those plates actually exist
is to look at where earthquakes occur. And so earthquakes occur where you have rock either
being pulled against itself, pushed against itself or sliding past itself. And so when
that rock gives a little bit, that’s an earthquake. And so if you look at where the earthquakes
are centered on our planet, they’re actually centered, well here would be the Atlantic,
so right along that mid-oceanic ridge. We also see them around what’s called the ring
of fire. So if this is the Pacific Ocean over here. So this would be Alaska, down the eastern
sea board, excuse me, western sea board. This would be the San Andreas fault all the way
down to South America and then all the way back to Japan again. So we have what’s called
this ring of fire. Now why do we find earthquakes in some areas but in some areas we don’t?
Well that’s where the plates actually match up. And so if we look in the US, which we’re
pretty familiar with, where would we find a lot of earthquakes? Well in California.
So why do we have so many earthquakes in California? Well, here’s the San Andreas fault. It actually
cuts California right in half. And so on one side of that we have a plate moving in one
direction. On the other it’s moving in the other direction. And so when they slip past
one another, then we actually have an earthquake. And so these are the major plates on our planet.
We have like a North American plate, a Pacific plate. The is the Wanda Fuca plate right up
here. But all of these plates as they move shape our planet. And they also give us earthquakes
and show us where there are places where it maybe not safe to live. So for example we
just had a huge earthquake in Japan which caused a great tsunami. Why is that? Well
it’s right here, located on the junction of almost four plates that are all moving past
one another. And so plate tectonics tells us that plates are moving on our planet. But
there are two types of plates that you should be familiar with. The first one is called
continental crust or continental plates. Continental plates in general are made of granite. Now
granite, the best way to think of granite, if you’ve ever been to Yosemite. This is a
giant piece of granite. Granite is going to be a lighter rock. Continents are very big,
so my fist is going to represent a continental plate. And they’re also a little less dense
than the oceanic plates. So this is oceanic plate. This is from lava forming in Hawaii.
Oceanic plates are made of basalt. And so basalt s going to be a darker rock and it’s
going to be a more dense rock. And so what does that mean? Well if you have a continental
plate hitting an oceanic plate, the one that’s more dense is going to be the oceanic plate.
And so the oceanic plate is going to be forced underneath the continental plate. And so a
great example of this would be on the west coast of the United States. We have this,
which is the Pacific plate going underneath the North American plate. And so this would
be that dense oceanic crust being pushed underneath the continental crust. And as it does that
it starts to melt as it moves farther and farther down. And it eventually causes volcanoes.
So Mt. Rainier is an active volcano. Mount St. Helens is an active volcano. Why is it
active? It’s because the oceanic crust is being forced underneath the continental crust.
It’s melting and then it’s causing that volcanism of the surface. And so almost all of the geology,
large geology on our planet can be explained by looking at what kind of a boundary it is
between the two continental plates. So first of all, or oceanic plates. First of all let’s
think about what could happen. So let’s say we have two continental plates that are coming
together. Well they’re both massive. And they both have roughly the same density. And so
when they run into each other, it’s almost like two cars running into each other. They’ll
just crumple up and get larger. So where is an example of that on our planet? The Himalayan
Mountains which are formed where India is slamming into Asia. So that’s two continental
crusts. What else could happen? Well we could have two continental plates that are actually
pulling apart. We call that a divergent boundary. What does that create? That creates a rift.
In other words when two continental plates pull across from each other, that causes a
rift. And so the Rift Valley of Africa is an example of two divergent continental crusts.
What else could happen? Well we could have two oceanic plates converging. So what happens
when we have two oceanic plates converging? Well one of them will tend to buckle under
the other. And then we get volcanism as well. And we’re going to have islands forming. And
so if you look at the Aleutian Islands, which cause that arch off of the coast of Alaska,
it forms like this. We have two oceanic plates running into each other. One’s being forced
under the other and then you get this volcanism. What happens if we have two oceanic plates
that are moving away from each other? Well, what’s that? That’s seafloor spreading. So
right here in the center of the Atlantic remember, we have two oceanic plates that are moving
past one another. And remember the Atlantic will get bigger and bigger and bigger just
as a cause of that. And then we could also have a transformed boundary. That’s when one
is sliding past it. An example of that would be the San Andreas fault in California. So
the one thing I have told is that that’s how we get all the geology on our planet or the
rock formations on our planet. But what I haven’t told you is what causes that. So what
causes that is fire . . . convection currents. In other words, this is magma down here. So
this is molten rock. And so we have convection currents. And the best way to think of that
would be like in a lava lamp where you have the oil moving up and down. So you get convection
currents where we have hot areas moving up. Cool areas moving down. And that’s actually
driving the movement of these plates. One other thing that I haven’t talked about is
what’s called a hot spot. And those are kind of pretty cool. This would be a hot spot right
here. So Hawaii is actually formed by a hot spot. That’s just an area where the magma
happens to come a little bit closer to the crust. And so Hawaii is formed by a hot spot.
But really close to home, Yellowstone Park is actually situated over a hotspot. And so
as the crust moves over it, that hot spot stays in the same location, and so Yellowstone
Park will actually move it’s location over time. We can track where it was in the past.
Just like Hawaii will move. The big island of Hawaii is where that volcanism is actually
taking place. And the reason it trails off to the edge of Hawaii is that there’s been
more erosion. And so that’s a brief introduction to plate tectonics. But I hope that’s helpful.

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