Lesson video

Hello, my name is Mr. March, and I'm here today to teach you all about the structure of the Earth.

So grab everything that you need for today's lesson and let's get going.

By the end of today's lesson, you'll be able to explain how Earth has different layers, each of which has different characteristics.

There are just two key terms for today's lesson, and those are lithosphere and asthenosphere.

Lithosphere refers to the rigid outer layer of Earth which includes the crust, whilst asthenosphere refers to a semi-solid layer of Earth's upper mantle.

There are just two learning cycles for today's lesson.

And we're gonna start with the first learning cycle, which is what is the structure of Earth.

Now the Earth has a layered structure, as can be seen via the diagram in front of you which you may have seen before.

This diagram though takes a rather simplistic view.

It shows the three main layers, starting with the outer layer, the layer that we are currently sat on, which is the crust.

Below this, we have the mantle.

And then finally, we have the core, which can be divided into the outer core and the inner core.

The problem with this diagram is, as I said, it's a little bit simplistic.

So we can add a little bit of complexity by introducing this diagram right here.

Now the outer layer can be referred to here as the lithosphere, and it refers to both the crust and the upper part of the mantle.

The upper part of the mantle is the solid part of that layer.

Beneath this, we have the mantle.

And then below this, we have the core.

With regards to the mantle, we also refer to this as the asthenosphere.

This layer is semi-solid.

It is made up of molten rock, molten magma.

And below this then, the core can again be divided into two.

We have the outer core, which is a liquid layer, purely molten due to the high temperatures there which has melted the rock metal found there.

And finally, at the very centre, we have the inner core, which is actually, for reasons which I'll explain later, a complete solid iron and nickel ball.

So a quick learning check.

Can you put these layers of Earth into the right order, starting from the outside of the Earth towards its centre, by numbering them from one to five, with one being the outermost layer.

So what I'd like you to do then is read through the options, pause the video here whilst you consider, and then select your answers.

And the correct answers were, so starting from the outer layers, we begin with the lithosphere, that rocky outer layer, the crust, as well as the uppermost part of the mantle.

Beneath this then, we have the asthenosphere, which comprises the sort of semi-solid part of the mantle.

Beneath this, we have the lower mantle.

Then we have the outer core, that liquid layer.

And then at the very centre, we have the inner core, that solid ball at the very centre.

And just to show exactly how that looks like again on that diagram, here we are.

So really, really well done if you were able to get those five answers correct.

Now, for centuries, scientists had clues about Earth structure, but they were lacking in evidence.

The clues were that, well, lava showed there was clearly molten rock under the Earth's solid surface.

Earth's average density was found to be far higher than the density of rocks at the surface.

So where was the extra density? And studying meteorites showed they were mostly iron and nickel.

Very dense.

Was the core also made of iron and nickel? There was also evidence from seismic waves, and seismic waves are produced when an Earthquake occurs, and they produce two different types of seismic waves, P-waves and S-waves.

And they have very different characteristics.

S-waves are only able to pass through solids.

And S-waves then can pass through the mantle.

As this diagram will now show, we are able to track and follow where the S-waves are travelling via seismometers.

And as we can see, the S-waves are able to travel only through solid material, thereby referring to the crust and the upper most part of the mantle.

So scientists found that S-waves did not pass through the outer core of Earth.

Therefore, the outer core must be liquid.

So while S-waves get stopped at a liquid barrier, P-waves are able to pass through solids, liquids, and gases.

And this was really helpful to scientists trying to understand the different composition of the Earth's structure.

Density though changes how fast these P-waves travel and in fact can even change the direction by refraction.

Now as this diagram will now helpfully show, it shows how P-waves travelling through the Earth's structure changed direction as a result of the materials it was passing through.

Scientists found that P-waves were refracted in a way that showed that the Earth has a solid inner core.

So whilst the S-waves previously mentioned in the previous slide would stop at the mantle due to its liquid composition, P-waves were able to travel all the way through the Earth's structure.

Their speed would change based on density.

The more dense the material, the faster it would travel through that part of the Earth's structure.

But also, the different structure, the different composition of that structure, would inevitably mean that the direction of those waves would change and bend.

Therefore, we were able to identify what the different layers were made of based on the nature and actions of the P and S-waves.

Time now for a quick learning check.

So true or false, before the 20th century, scientists had worked out that Earth must have a layered structure.

What I'd like you to do right now is pause the video whilst you consider, and then select your answer.

And the correct answer was true.

Now what I'd like you to do again is pause the video and consider how this statement is true.

What evidence did they have to suggest that the Earth must have a layered structure before the 20th century? Best of luck.

And the answer is that scientists before the 20th century had worked out that the Earth must have a layered structure because lava showed there was liquid rock underneath the surface of the Earth.

And calculation showed that Earth must have a very dense core.

So really, really well done if you're able to get those two answers correct.

So we're on now to our practise task.

And this one asks you to complete the gaps in this description of the structure of the Earth.

So what I'd like you to do then is pause the video whilst you attempt this practise question.

Best of luck.

And some feedback.

So these were the correct answers.

It reads, Earth's structure is made up of different layers.

At the surface is the crust, and beneath it is the mantle.

Below that is the outer core and then the inner core.

The crust and the top layer of the mantle are known as the lithosphere.

This is a solid, rigid layer.

There is a layer of the mantle beneath this called the asthenosphere.

We know about the different layers because of the way that the two kinds of seismic waves move through them.

So really, really well done if you are able to identify those correct answers.

Where on now to our second and final learning cycle, and this is, what are the characteristics of Earth's layers? Now the Earth's layers have different properties, as can be shown through this very helpful table in front of you.

It shows the four different layers on the left, the crust, the mantle, outer core, and the inner core.

We can also see the temperature, the density, the composition, and the physical state.

And it's really helpful to see how the layers change in all of those different categories the deeper into the Earth we get.

Now, if we start with the crust, we can clearly see that this is the coldest layer.

Makes sense, we're on the outermost layer of the Earth.

And it goes between 0 and 230 degrees Celsius, so clearly still a very warm layer, but clearly not as warm as the others.

In fact, if we follow the Earth's structure down through the mantle, the outer core, and the inner core, you can slowly see an increase in temperature.

Now the increase in temperature the deeper you go into the Earth is based on a number of different factors.

First of all, there's the sheer pressure from the number of layers slowly on top of one another, creating more pressure, which creates heat.

There is also the radioactive decay of some elements within the Earth's structure, such as uranium, which, as it decays, it releases heat.

Then if we go onto the density, we can slowly see, again, as we go deeper into the Earth structure, the density increases, which again makes sense.

With increasing weight of these layers, it's going to compact the material found.

That's gonna increase the pressure.

It's gonna increase that gravitational pull.

And that is going to increase the density.

And that is clearly seen if we compare the crust, between 2.

7 and 3.

0, versus the inner core at the bottom there, from 12.

6 to 13.

Now what you can see next is how the composition changes as well.

And what we see at the very centre then is much like what we saw with the asteroids and meteorites that came to Earth.

They're also made of a very similar composition at the outer and inner core.

Molten iron and nickel are the main components.

Finally, in terms of physical state, we can see also how this changes.

On the outer layer, the crust, it is a purely solid layer that we of course sit on.

Below that, it becomes semi-solid in the mantle.

Now this is due to the increase in temperature.

As we can see in the mantle, it goes between 213 and 4,200 degrees Celsius.

And this then begins to melt or partially melt the rock found there.

As we go deeper again into the outer core, we are then going into a liquid layer.

Again, that pressure and that heat that is created as a result of the pressure causes that layer to be entirely liquid, molten rock.

And finally, perhaps surprisingly, given the vast temperature found there, the high temperatures found there, between 6,000-6,700 degrees Celsius, we though have a solid ball of metal, of iron and nickel, at the very centre in the inner core.

Now this is due to the huge amount of pressure, which is essentially keeping those elements, that metal, which should be superheated and melted, into that solid iron ball.

Now the physical state of the lithosphere is solid.

We are talking about the crust and the upper part of the mantle.

It is rigid.

And under stress, when it moves, it fractures.

And this is why we have things called tectonic plates, which we'll learn about in another lesson.

The physical state of the asthenosphere, just below the lithosphere, is semi-solid and ductile.

It flows or deforms instead of fracturing.

Since it's much more versatile, it is much more semi-solid than the solid state of the lithosphere.

The boundary between the lithosphere and asthenosphere is at 1,300 degrees Celsius, and it is called the transitional state between those two layers.

What I'd like you to do then for this learning check is to match the layers to their correct physical state.

You have four different layers and three different physical states.

So what I'd like you to do right here is pause the video whilst you match those together.

Best of luck.

And the correct answers were, so we're gonna start with the lithosphere on the very outer layer of the Earth.

And this is a solid layer.

Moving below that is the asthenosphere, and this is semi-solid.

Again, we are slightly warmer in this layer, and this is causing it to melt slightly.

The rock which is found there is now slightly melted, hence the semi-solid state.

The outer core though, due to the high temperatures there, have now completely melted that layer, and that is why it's completely liquid.

At the very centre, the inner core is solid.

Even though the temperature is greatest at this layer, the huge pressure forcing those materials inwards makes it a solid layer.

So really, really well done if you're able to identify those four correct answers.

So why does density increase towards the core? There are a number of different reasons.

First is, as we mentioned before, the increasing pressure from the weight of the layers above.

Gravity.

The denser materials, the heavier materials sink towards the core and of course then contributes to the increased density into the deeper layers that we have in the Earth's structure.

And finally, phase changes.

The heat and pressure makes minerals change into denser forms. So why is Earth still hot? So although Earth was formed 4.

5 billion years ago, the heat that was created at that time, as the gravitational pull needed to sort of pull all those metals deep into the Earth's structure, and all that heat that was created as a result of that, well, that has been retained, and we call that primordial heat.

This is heat that was trapped in the core when Earth was originally formed 4.

5 billion years ago.

And remember, we have these different layers which is actually sort of insulating and keeping that heat inside the Earth.

On top of that, there's the increasing pressure which generates its own heat as well.

There's also radioactive decay.

And radioactive decay is to do with elements like uranium giving off heat energy as they decay.

So all of these different sort of factors combined is contributing to Earth remaining hot.

So a quick learning check.

True or false, the outer core is liquid, made of molten rock, while the inner core is solid, so the outer core is hotter than the inner core.

What I'd like you to do right now is read back through that statement, pause the video whilst you decide whether the statement is true or false.

And the answer is false.

Now what I'd like you to do again is pause the video whilst you consider as to why that statement is false.

And the reason it is false is that whilst half of this statement is true, it is true that the outer core is liquid and made of molten rock while the inner core is solid, but the inner core has temperatures of 6,000 to 6,700 degrees Celsius compared to 4,200 to 6,000 degrees Celsius for the outer core.

It is the pressure at the core that prevents the iron and nickel there from melting.

So really, really well done if you're able to identify those two correct answers.

So why is the crust composition different from the mantle's composition? And it all comes back to the density of the material found there.

We know that the denser materials, such as iron and nickel, sank to form the core.

The lighter materials like silicates rose to form the mantle and the crust.

Now there's a slight difference between which silicates went into the mantle and which into the crust.

In the crust we find oxygen, and we find silicon, and we find aluminium, whilst in the mantle, we again find oxygen and silicon, but we also find magnesium and iron.

Now, when the crust gets melted at plate margins, denser materials from the crust sink and lighter materials rise to form the new crust.

So there's almost an exchange of materials at some points between the crust and the mantle.

So a quick learning check.

I'd like you to match the layers to their correct composition.

You have the four layers on the left-hand side and the composition options on the right-hand side.

So pause the video here whilst you consider, and then select your answer.

And the correct answers were, crust, we find silicates such as oxygen, silicon, and aluminium.

In the mantle, we find, again, silicates, this time, again, oxygen, silicon, but now magnesium and iron.

In the outer core, we find both iron and nickel.

And also, in the inner core, we find also iron and nickel.

So really, really well done if you are able to identify those correct answers.

So a second learning check.

And this time, I'd like you to add the information that you can see on the right-hand side of the screen to the table that you can see on the left-hand side of the screen.

So please pause the video here whilst you consider, and then select the correct answers to form the information for those different layers of the Earth's structure.

Best of luck.

And the correct answers were, so we have the mantle just beneath the crust, we then have the outer core, and then finally the inner core.

Now, in terms of density, this is what you needed to show.

So in the crust, we have somewhere between 2.

7 and 3.

0.

In the outer core, it's between 9.

9 and 12.

2.

And then the inner core, 12.

6 to 13.

0.

So really, really well done if you're able to get that correct, to show really how density increases the deeper into the Earth's structure we go.

Onto our practise questions.

And I ask you to complete a table showing the characteristics of Earth's layers.

You have the four layers shown there, and you also need to try to complete the temperature, the density, the composition, and finally, the physical state.

The second question asks you to explain why the outer core has different properties to the inner core.

You could explain one difference or try for more if you can.

So please pause the video here whilst you attempt those two practise questions.

Best of luck.

And now some feedback.

So in terms of the feedback for Question 1, this is what you needed to show.

So with the crust, for example, the temperature is between 0 and 230 degrees Celsius.

It is the coolest of the four layers.

Then we have the density between 2.

7 and 3.

0.

It is the least dense of the four layers.

In terms of composition, it is made mainly of silicates such as oxygen, silicon, and aluminium.

And its physical state, it is purely solid.

Underneath this then, we have the mantle, and it's between 230 and 4,200 degrees Celsius.

Its density is between 3.

3 and 5.

7.

Again, it's made primarily of silicates such as oxygen, silicon, magnesium, and iron.

And this time it has a semi-solid physical state due to the increase in temperature.

Underneath this then, we have the outer core which is between 4,200 and 6,000 degrees Celsius.

And the density is between 9.

9 and 12.

2.

Its composition is molten iron and nickel.

And it has a physical state that is liquid due to the high temperatures found there.

And finally, in the inner core, it has the highest temperatures, between 6,000 and 6,700 degrees Celsius.

And it has the highest density as well, 12.

6 to 13.

0, due to the huge pressure and weight of the Earth found there.

Again, it is made of iron and nickel, but this time it is a solid state, again, due to that huge pressure found directly at the centre of the Earth.

So really, really well done if you're able to complete that table.

The second practise question asks you to explain why the outer core has different properties to the inner core.

And you may have said something like this.

The maximum temperature of the inner core is hotter than the maximum temperature of the outer core by about 700 degrees Celsius.

This is because the core still has heat from when the Earth was formed.

This is called primordial heat.

You may also have said that although both the inner and outer core are made of nickel and iron, the inner core is solid and the outer core is liquid.

The inner core is solid because of the immense pressure at the core prevents the iron and nickel from melting, even though the temperature is up to 6,700 degrees Celsius.

So really, really well done if you're able to include anything like that in your own answer.

On now to our summary.

So from this lesson, we need to know that the Earth's crust is made of several layers which make up the structure.

And the Earth's crust is just one of those layers.

Geographers refer to the lithosphere, which is the crust plus the solid top part of the mantle, and the asthenosphere, which is the semi-solid part of the upper mantle.

Each layer has a different range of temperatures, different densities, and different composition and physical states.

So really, really well done during today's lesson.

It was a pleasure to teach you.

And I look forward to seeing you on the next lesson.

Goodbye.