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0:02
- Hi students, this your instructor, I'm Todd Yurchism. PSC 2121 Fundamentals of Physical Science and we're gonna start the next section here it's on plate tectonics. And plate tectonics is about the continents moving and why they move. And remember, this class is based on the fact that we're looking at general physical principles. What I want you to keep in mind here, is the physical principles that are involved in making this happen. This happens for a reason.
0:31
So we're gonna go through some of the background as to why it happens.
0:35
So plate tectonics, let's look at the history of how the earth was formed because a lot of what happens is because of the way the earth is shaped and how it started.
0:48
4.6 billion years ago, in the solar nebula, the material around the sun was a creating, meaning that leftover material that didn't go into the sun, there's lots of material going around in this accretion disk. And in the orbit of the Earth, a lot of it gathered together by gravity small bits as they got larger ran and other bits as they got larger and larger attracted other pieces. And these bits came closer together because of gravity, and it got larger and larger.
1:20
And as this proto Earth got larger and larger because of gravity would pull in other smaller bits. And this happened over millions of years it's quite some time to do this. And as these bits hit the Earth, there's lots of heat generated, lots of kinetic energy has to be converted into heat. So the earth had molten surface from all these bombarding materials. The surface cools, but the interior doesn't cool. It stays warm, stays very hot, and there's also radioactive material in there,
1:52
uranium and other radioactive materials, and there's a lot of heat generated on the inside. So the surface cools on the outside and forums crusty material.
2:05
The gravitational settling what happens is this called differentiation. What happens is that the lighter material stay on the outside of the sphere and as we go towards the center,
2:22
the lighter weight materials, the igneous rocks and granites and such, you don't need to know that for the test, but it's lighter materials less dense.
2:31
If it's less dense,
2:33
then it doesn't sink as much the heavier materials mostly iron sinks towards the center. Why does it sink towards the center? Because of gravity. As you know if you have a chocolate milkshake or something and you put some powdery stuff in there and stir it up, what happens if you were to let it go for a while, some of the heavier stuff is gonna accumulate towards the bottom and the lighter stuff will sit on top. You have to start back up again. Same things happening with the earth.
2:59
And most of the stuff that's in the center mostly of it is iron and nickel, it's magnetic. The Earth is spinning 24 hours, once every 24 hours. And it sets up a magnetic field, and it's warm, and it's a liquid, so it moves. And so we generate this magnetic field that the earth needs to protect us from the sun. More on that later.
3:24
Internal structure, let's start at the outside. The crust, it's a thin outer shell. And it's rigid, it's very thin compared to the mantle and core. When I say thin is sort of like an eggshell or even less, it's very thin relative to the size of the Earth.
3:40
And it's made up of mostly igneous rocks and granites and such. Beneath the oceans, the crust is thinner. And there's reasons for that, and I'll get to that a little bit. The continents are a bit thicker, and they're less dense than the ocean crossing, I'll get to that. It averages about 30 km in thickness. Under large mountain ranges, it's sort of like an iceberg, where under large mountain ranges, it has to be a little bit thicker to offset. So here you have a mountain range here,
4:13
the cross would tend to thicken out underneath the mountain range.
4:21
And like the shell of an egg, the Earth's crust is brittle, and it can break and it does break and it does so fairly easily. The crust it covers the entire Earth. The oceanic crust is much thinner. It's made of basalt it's a much denser than the continental crust. And this particular rock the reason it's made this way is because when it comes out, we're gonna talk about this, as it comes up through the cracks in the ocean, when it hits the water cools in such a fashion
4:54
that it's quite a bit denser than the continental crust. The continental crust is made of mostly granite that had dried up pretty much since the beginning of time. And so it's sitting on top and floating on top of the liquid mantle. So but what I want you to know is that the oceanic crust is denser than the continental crust. And you'll see a little bit later why that makes a difference.
5:20
The mantle is made up of dense hot layer, semi solid rock. It's about this thickness. I'm not gonna test you on the thicknesses. It's here for your information, but I'm not gonna ask you it's 29,000 kilometers thick. Don't worry about that, I just want you to know what it is I want you to know what the mantle is. It contains more iron and magnesium, calcium than than crust so it is denser that's why it's further in.
5:49
The tendency as you dig deeper into the earth, the temperature starts to rise and the pressure starts arising which should be obvious because as you go deeper and deeper into the into the earth it's getting hotter and hotter because there's a lot of heat that's still there since the beginning of the formation of the earth.
6:07
So you can imagine what it would be like at the core of it, the inner core, the temperature is the temperature of the surface of the sun, believe it or not.
6:15
The mantle makes up by eight percent of the volume. Again, I'm not gonna test you on that I just want you to know, I want you to be aware that it's there because we're gonna talk about how the continents do what they do because of the mantle.
6:28
The core is two pieces. It's mostly metallic iron, and nickel. It's mostly iron. And it's very dense has two distinct parts. There's a liquid outer core, and a solid inner core.
6:44
As the Earth rotates, this liquid outer core is spinning, and it generates the Earth's magnetic field. What is the Earth's magnetic field do for us?
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It protects us from the sun, magnetic field deflects ionic particles the sun gives off all kinds of nasty particles charged particles and as they come towards the earth,
7:10
any good strong magnet will deflect these charged particles and instead of hitting his head on what happens is it comes down and comes in at the poles. You've heard of the aurora Borealis and the aurora australis, the Northern Lights and the southern lights. That's the interaction of these charged particles coming in, in the northern latitudes, and spiraling down and generating light. So the Earth's magnetic field is very important to us. If we didn't have it, we probably wouldn't be here.
7:38
In fact, if you look at a lot of the things of the earth, the way it's put together, there's a lot of things about it that if we didn't have certain things happening, we wouldn't be here. And as we talked about another time without the moon we probably wouldn't be here. I mentioned that before.
8:00
So let's look at the layers again as to how we want to define and look at plate tectonics.
8:12
So our first, by the way, your vocabulary words we had differentiation
8:20
back to that differentiation,
8:23
sometimes called internal differentiation. And what is that again? That's the different layers forming and separating out due to differences in density of the materials from the outer layers of the earth that are much less dense. and as you go towards the center, the heavier material sank to the inside and so it gets denser and denser as you go further in.
8:47
That's one of our vocabulary words. So the lithosphere.
8:54
For those that are into root words, anybody knows what "lith" means? That means stone, rocky and of course sphere it's a sphere.
9:05
The lithosphere is made up of the crust and the upper layer of the mantle. Yes, this is testable material. Crust and upper layer of the mantle. We talked about the crust and then came the mantle. Well, just below the crust, there's an upper layer of it, and it's nearly stony itself. So the crust in the upper layer, you can see the brackets here and closing that, that is the lithosphere, the stony outer sphere of the earth.
9:35
It's thinner under the oceans, you can see that it's thinner under the oceans and thicker under the continents.
9:42
It averages at about 80 kilometers and thickness over much of the earth. We've never broken through the lithosphere we've never dug that deep, in fact, nowhere near 80 kilometers.
9:58
Next comes the asthenosphere. Asthenosphere coming from the Greek for weak. And it's the thin hot elastic semi solid material in the upper part of the mantle. Remember, we're still in the mantle here. And underneath the upper part is what we call the asthenosphere. And it's essentially identified by the fact that it flows. It's semi plastic, it can flow it can push, it moves.
10:26
So, the lithosphere, the point here is that the lithosphere rides on top of this semi-liquid, semi-elastic asthenosphere.
10:40
So another vocabulary word.
10:51
And as you know, I like to have pictures of the same thing but different pictures showing because some pictures are clearer than others sometimes. So again, we have crust, here's the crust, the continental crust, in the brownish color here in the green, here's the oceanic crust. And then under here we have the upper mantle. And those two together make up the lithosphere.
11:13
Remember the test I'll be asking what is the lithosphere. It's the crust and the upper part of the mantle. And underneath that is the authentic sphere. What distinguishes that is semi plastic compared to the lithosphere and these bits rides on the asthenosphere. Underneath that you have the lower mantle, the core, the inner and the outer core.
11:37
So you can see here that the oceanic crust is relatively thin. There's a reason for that, I'll talk about that. And the Continental crust which is made up of mostly those lightweight rocks that went in the internal differentiation long ago, when the earth was cooling when it was still pretty warm on the outside. When it cooled it's this much less dense rock. Now I know you pick up a rock it seems pretty dense to you. But it's much less dense than say iron,
12:05
and some of the other materials that have sunk towards the earth due to gravity. So, again, this lithosphere here, I mean that the crust here, and the upper part of the mantle, this is the lithosphere and it's floating on this asthenosphere.
12:25
Well, so what is the flowing, where's the flowing coming from? The flow comes from again, the core is still hot, very hot. And, as you can imagine, it's not always hot in the same places that some places are hotter than others. And so there are hotspots. So it's very hot here and what happens with hot things is that heat tends to make things rise, the material flows up, as it encounters the crust, which I said was rather brittle. Then it could push on this area here
12:59
and it could break take it apart. And as it comes up and hits, it's gonna come up and it's gonna circulate. So this is a convection cell. And this is what's driving continental drift. This is what's driving plate tectonics.
13:14
So the middle mantle flows because of convection currents. The convection currents are caused by very hot material at the deepest part.
13:22
And then they cool in sync again repeating the cycle. And the lithosphere, the crust in the upper mantle are riding on these currents. In other words, they're being influenced by this as this stuff comes up. It's gonna tug on these plates and move them around.
13:38
So let's put that as another one of our words.
13:45
Convection currents
13:48
drive Plate tectonics.
14:05
And as you can see,
14:07
there's different zones here that this is breaking through and over here we have some interaction we're gonna get to that.
14:14
So the earth itself is broken up into lots of different lithosphere plates. The red lines here are showing you the some of the major plates, the areas of the major plates.
14:28
You can see that say this plate here, South American plate, it's mostly content, but it's also got a lot of oceanic plate here. These are all separate ones. There's some minor ones in there, but these are all the major plates of the earth.
14:45
And that's where the action is, is all along those lines where these plates interact with each other is where things happen. Here's another picture the same thing.
14:56
And we're in the Caribbean plate here, North American plate, I'm sorry, there's Caribbean plate here, we're on the North American plate, South American plate. And these are all moving, in India here's the India plate, it's moving this way, creating the Himalayan Mountains.
15:13
And all these are riding on top of this
15:16
on top of the stratosphere.
15:19
So the theory of plate tectonics. What's a theory again? What is the theory? Is the encompassing idea that when we think of, when we think of a subject in this way, this is when we put of all of our ideas in one place and we do experiment after experiment it stands up over a period of time. This is our way of thinking about this particular thing. And when we look at the earth, and its crust and the interactions, of volcanoes and all this stuff, we call it the theory of plate tectonics because
15:53
this is how when we look at
15:58
why it works the way it does with earthquakes and volcanoes and continents, the way they're shaped and everything when we look at it this way, this is the theory. And the theory, remember, it cannot be proven, it can only be disproven.
16:14
So the theory of plate tectonics states that the Earth's outermost layer is fragment into a dozen or more large and small solid slabs, lithosphere plates or tectonic plates. Now, this used to be, I want to point this out, this used to be called the theory of continental drift.
16:35
And that was because with the way this came about was you can look at certain maps and you can see, for instance, in a lot of places, you can see that South America fits right neatly into here into Africa here. And they realized if you dig deep enough when you dig deep enough, it's an axiom of geology that when you dig deep enough, then the further you dig the further back in time that you go. And if you dig deep enough, in South American Africa, you come upon identical rocks.
17:09
And anything below that the rocks are the same. And for that matter, the fossils are the same. Above that there at some point that they broke apart, had to be broken apart because after that the rock layers are different and the fossils look different.
17:25
And because of this, the geologists realize that well,
17:32
they are drifting apart. But they weren't exactly sure why and how yet, so it was called the theory of continental drift, that the continents were drifting around.
17:45
But since then, we have now come up with a mechanism as to why they're drifting. And so the term plate tectonics is more encompassing. It's actually a little more general term and your theory wants to be all encompassing. So the term the theory of continental drift doesn't apply anymore. More properly called the theory of plate tectonics. The plates are rigid spherical caps. A spherical cap is like a plate, but remember, this is a curved Earth.
18:15
So it's kind of roundish and thing but it's limited in size. And they move relative to one another as they ride on top of this hot, more elastic asthenosphere, they ride around on this. The plates themselves are relatively rigid. They don't generally move very much, I mean within themselves. And a plate can may be covered with only oceanic crust, continental crust or combination of both. You can see that for instance, the Pacific plate is virtually all oceanic crust.
18:49
It's one very large spherical cap, but it's a fairly thin member, its oceanic crust, so it's relatively thin. Here the South American plate you have continental,
19:00
you have continental lithosphere. But you also have oceanic material here too and this is one large plate moving. And in fact, we're gonna get to this but in fact, along this line, this is where material is being created and pushing these apart.
19:18
The theory states that within a plate, all of the parts move together, more or less.
19:25
Not exactly, but more or less for our purposes, for our class, just think of a plate as being a rigid plate and it stays, it doesn't move within itself.
19:35
The defamation of plates occurs only at their boundaries. In other words, that's where the action is, where these plates interact with each other, where these plates hit each other or pull away from each other that's where things happen.
19:52
The average rates of motion of these restless plates in the past while the present, the average rate is anywhere from one to more than 15 centimeters a year. And that's about a centimeter. That's not very much in a year but you can imagine in a million years, that's a million centimeters. What's a million centimeters? It's divided by that by 100, that's 10,000 meters, divide that by 1000, that's 10 kilometers. So in a million years, that's 10 kilometers.
20:23
And nearly all of the world's earthquake and volcanic activity occur along these lines. That's where things happen as I'm gonna show you.
20:34
Now, long ago, if the continents were
20:39
have broken apart, then they must have formed at some point one large content, they must have been part of one larger continent. And here is a map showing of how it probably look like at one time, you can see India's way down here and it's gonna end up smacking into Eurasia up here, but it's way down here. And how do we know this? How do we know this? Remember the theory? How can we come up with this stuff? Well, again, if we dig deep enough,
21:06
we can find that the rocks for instance, here in Africa, we can find that the rocks right here and right here in Antarctica, if we dig deep enough at some point are identical. And that's how we know that Antarctica and Africa we're joined together at some point. Same with Indian, this part of Antarctica, same with Australia all along that side. That they've put this together over a long period of time, people have made these maps. So you can see that this is
21:31
what they originally were, and they've moved since then broken apart move since then, to their current locations. Here you can see the equator the earth. Looks like New York City was on the equator at one point.
21:44
Anyway, they were all part of what was called a single large mass what we call Pangaea. So anybody know what Pangaea stands for, where that comes from, for those that are into root words.
22:00
Pangaea is "Pan" comes to Greek meaning cross like Pan American or something, it means to go across you're covering the whole thing. And "Gaea" of course comes from Gaia the earth, it means the earth. So this this thing is spanning the earth is what they mean, this this continent. And apparently, though that Pangaea was actually made up of previous continents that have broken up and had come from a previous super continent and broken up and moved around, went together into Pangaea and such.
22:32
But the further back you in time you go, the harder and harder it is to recreate that type of map. So this is the one that's most common is the one of Pangaea.
22:44
So here we have 225 million years ago, roughly Pangaea here, 200 million years ago, you can see that the earth is starting to break up.
22:57
I'm sorry the comments starting break up. North America is going further, here's larger Asia, they call it, there's India way down here it's got a long way to go, doesn't it? And here Africa and South America are still together. They didn't break up until later, much later. You can see about 150 million years ago, they're still together, but by 65 million years ago, they've broken apart. And by the way, look, you can see here, there is no ocean here.
23:24
There's a inland jetty jetty of the Pacific Ocean here but as this starts to break up, it's forming the Atlantic Ocean and others the Atlantic Ocean was formed, because these continents were spreading apart. So 65 million years ago, at the end of the dinosaur era, remember we talked about that.
23:43
Roughly when the supposedly the theory is that an asteroid hit the Earth and destroyed the dinosaurs by kicking up all kinds of dust into the atmosphere and causing a block out of the sun, a lot of the plants died off
23:59
a lot of dinosaurs eat the plants. There's other dinosaurs that are much larger, and they eat the smaller dinosaurs and without them with the food chain disrupted like that, then they're gonna die off and die off fairly quickly. And all the little animals like the mammals and such, which were there all along, by the way, the mammals were there. They weren't that dominant because the dinosaurs were running around grabbing everything. So it help give rise for the mammals.
24:27
Anyway, this is roughly what the earth looked like, about 65 million years ago when that happened.
24:38
So Pangaea, present day continents are fragments of this super kind of Pangaea that broke up about 225 million years ago. Pangaea itself the product of accretion of fragments of a pre-Pangaea supercontinent,
24:54
and several pre-Pangaea super continents, but there were several of them but evidence becomes more difficult because the further back you go in geologic time.
25:05
So you can see they've got some arrows here the one that fascinates me always is India. India has a long way to go. It comes from way south, and it just travels very far north, and it smashes into Asia here. So India is not really part of Asia originally.
25:25
Anyway, you can see on this I've got an animation here. Can see this animation. I can see it breaking apart. North America breaks off and Europe breaks off and then South American Africa break apart
25:38
and forming the Atlantic Ocean. And here's India, you can see it moving fairly quickly,
25:46
fairly quickly going North.
25:48
And as it does so, and it smashes into Asia, then it pushes up lots of mountains and forms of Himalayan.
25:57
And this is a map of projected positions, about 100 million years from now. Remember, it's probably not gonna be exactly like this, but roughly based on current movements of the continents and how they're moving now, the brown is what it might look like 150 million years ago, the black outline is the map of today. So you can see that the Atlantic Ocean will be wider, be further apart. Greenland is moved over quite a bit. This part of East Africa has broken off more on that later we're gonna talk
26:30
about the Rift Valley there. And but that has broken off and that's essentially how Madagascar was formed. It was part of East African it broke off, it was a divergent boundary there and broke off. Australia becomes more equatorial
26:45
and India goes further into Asia, Saudi Arabia, that sea disappears there. Looks like the Mediterranean is filling in.
26:56
Anyway, so what I'm gonna do then is
27:01
I'm gonna stop here for today for this particular one. I'm gonna pick up on the next session about the boundaries, the specific characteristics. Lots of good test questions, lots of fun on this. And we'll talk about where does earthquakes come from and volcanoes and such. And we'll pick it up on the next session. See you next time.
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