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  • 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.

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

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B1 中級

プレートテクトニクス (Plate Tectonics)

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    QAM Chen に公開 2021 年 01 月 14 日
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