Name: プレートテクトニクス (Plate Tectonics)
Uploaded: 2021-01-14T04:46:38.000Z
Duration: 9 min 22 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

未来進行形

Hi. It's Mr. Andersen. Today I'm going to be talking about plate tectonics.

程度の副詞

What's always interested me about plate tectonics is it's a fairly recent science. In other

様態の副詞

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.

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