Lesson video

Hello, my name is Mr. Norris, and this lesson is about probing planet Earth.

This is from the measuring waves topic.

This is a really interesting lesson 'cause it's about how we can use principles of physics to look inside objects which are very, very difficult to look inside, such as planet Earth, the entire planet.

So the outcome of this lesson is that by the end of the lesson, hopefully you'd be able to describe both S waves and P waves, which are produced by earthquakes, and you'll be able to describe how both of those kind of waves move through different parts of planet Earth.

Here are some keywords which we'll be focusing on this lesson: earthquake, S wave, P wave, transverse, and longitudinal.

On the next slide, I'm gonna go through examples of each keyword used in a sentence, but each word will be explained in more detail as it comes up in the lesson.

So an earthquake is caused by the sudden sliding movement of tectonic plates against each other.

S waves are transverse seismic waves.

Seismic waves just means a wave caused by an earthquake.

So S waves are transverse waves caused by earthquakes that travel through solids but not liquids.

Whereas P waves are longitudinal seismic waves, longitudinal waves caused by earthquakes that travel at different speeds through solids and liquids.

They can travel through both, but different speeds through solids compared to liquids.

A transverse wave is where the wave oscillates perpendicular to the direction of travel, and a longitudinal wave is where the wave oscillates along the direction of travel, with areas of compression and rarefaction.

You might wanna pause the video now just to review those keywords so you're prepared as possible for the rest of the lesson.

The lesson is divided into three parts.

The first part of the lesson looks at the idea of tectonic plates, so they're the different parts of the Earth's crust which all kind of move around each other and cause earthquakes when they kind of slip past each other.

And when they do slip past each other in an earthquake, that creates S waves and P waves.

That's the focus of the second part of the lesson, to look at those, each type of earthquake wave in more detail.

And then finally, we can look at how S waves and P waves tell us about what the structure of Earth is like.

So let's get started with the first section on tectonic plates.

So let's start by talking about earthquakes in general.

So you'll know that earthquakes can cause huge damage to buildings and can harm people.

It's not possible to predict when earthquakes will occur.

They usually do happen in certain geographical areas though, so more on that on the next slide.

Studying earthquakes provides scientists with information about the inside of Earth.

That's where we're going with this lesson.

Tectonic plates, then.

Earth's crust, that's the outer layer of Earth, a bit like the shell on an egg.

Earth's crust is made up of moving plates called tectonic plates.

As these plates move against each other, the friction causes earthquakes and volcanoes.

So the map shows Earth's tectonic plates, and you can see they're not all the same size.

Some are very big and some are smaller.

So they're the different sections of Earth's crust that are kind of all moving and sliding past each other, and it's when they slide past each other you'll get friction between them, and it's that friction which can cause earthquakes and volcanoes.

So this map is actually also a map of the geographic areas where volcanoes and earthquakes are most likely to occur, 'cause that's at the edges of the tectonic plates.

So anything that's close to a red line on the map, that is a place where earthquakes and volcanoes are much more likely to occur or be found, because that's the edges of the tectonic plates.

So that's where the friction from tectonic plates slipping past each other is going to potentially cause earthquakes or volcanoes.

So let's do a quick check of what we just went through.

True or false, earthquakes usually occur at the edges of tectonic plates.

Make sure you choose true or false for that.

And now, before I give you the answer, can you choose the correct justification for your answer? So whichever you chose, true or false, is that correct because friction between the moving plates causes them to vibrate as they slip, or is that correct because tectonic plates fracture in their middles? So make sure you've chosen true or false and A or B, and you can switch now if you want to.

Okay, so let's go through the correct options.

The statement is true.

Earthquakes usually occur at the edges of tectonic plates, and that's because friction between the moving plates causes them to vibrate as they slip.

It's not true that tectonic plates fracture in their middles.

That's not what happens.

So that was just to check you've got the right understanding.

So well done if you've got both of those correct.

Let's look at a bit more information now about tectonic plates.

So they move very slowly, only a few centimetres every year.

But the plates often get stuck as they slide past each other due to that friction we talked about.

So those moving plates on the right, they might just get stuck in that position.

And that could lead to a buildup of pressure, which can suddenly be released without warning as the plates slip.

And that's the earthquake, that's what causes the earthquake, the sudden slip of the plates.

And by earthquake what we really mean is S waves and P waves.

That's oscillations of the Earth's crust.

So the sudden movement of tectonic plate A against tectonic plate B as they slip past each other creates oscillations of the Earth's crust, and those oscillations are called S waves and P waves.

So waves could be caused here at that point, at the boundary between the two plates, at the moment when they slip.

And the waves will travel outwards through the Earth's crust, S waves and P waves.

More detail on those later in the lesson.

So which statement about tectonic plates is correct? Is it A, tectonic plates move relatively quickly? Is it B, tectonic plates slide past each other easily? Or is it C, it is not possible to predict when tectonic plates will slip? Which of those is the one correct statement? Choose it now.

Did you get it? It's C.

It's not possible to predict when they will slip.

A and B are false.

Tectonic plates move very slowly, not relatively quickly.

And for B, tectonic plates do not slide past each other easily.

There's a huge amount of friction, a lot of friction causing them to catch and kind of build up pressure and then they slip and that's the earthquake.

Okay, we're ready to do a task summarising what we've learned so far about tectonic plates.

There's two parts to this task.

Part one, could you describe just very briefly, a single bullet point would do, how quickly does tectonic plates move? And part two of this task, explain in writing please why earthquakes are hard to predict.

So a couple of sentences or a couple of bullet points for question two would be plenty.

So be ready to pause the video and have a go at that task now.

Okay, well done for your effort.

And here's some feedback on that task.

Tectonic plates move very, very slowly, a few centimetres a year.

So if you've got something along those lines, that's great.

And then for part two of the task, explain why earthquakes are hard to predict.

Tectonic plates can become stuck because of friction.

When enough pressure builds up, they suddenly slip.

So it's not possible to say exactly when they'll slip.

That's why they're hard to predict.

So make sure your answers are something along the lines of those and add anything to your written answers which will improve them, and then we'll move on to the next part of the lesson.

Pause the video if you need to.

Okay, so we've completed that section of the lesson on tectonic plates and how the movement of tectonic plates causes earthquakes.

But now let's look at the two kinds of wave, the two kinds of oscillation that can travel out from an earthquake kind of through the Earth's crust.

So the two kinds of earthquake wave or seismic wave is the other way of saying that.

So that's S waves and P waves.

Let's talk about S waves first.

So S waves are caused when the oscillations of the Earth's crust are perpendicular to the direction of wave travel.

So that makes S waves transverse waves, 'cause that's the definition of a transverse wave, when the direction of oscillation is perpendicular, that's 90 degrees to the direction of wave travel.

So we've got to imagine that in the diagram, the two tectonic plates, well, one of them is oscillating perhaps up and down compared to the other one, and that's gonna create a seismic wave which travels along that way.

So you've got a direction of oscillation which is 90 degrees to the direction of travel.

So there's the direction of travel and there's the direction of oscillation.

That's obviously a transverse wave where the direction of oscillation is 90 degrees to the direction of travel.

So an S wave is just the word for a transverse oscillation of the Earth's crust.

So S waves are transverse waves.

So the other type of seismic wave or earthquake wave or oscillation of the Earth's crust is called a P wave.

So P waves are caused when the oscillations are along the direction of travel.

So you might recognise that as the definition of a longitudinal wave, where the oscillations are parallel to the direction of travel.

So in the diagram this time, you've got to imagine that out of the two plates, one of them is oscillating that way and knocking into the next one and then the oscillation is passed along, transferring energy.

So there's the direction of travel, the direction of the wave travel, and there's the direction of oscillations.

And because they're parallel, the direction of oscillation is parallel to the direction of wave travel or the direction of oscillation is parallel to the direction of energy transfer.

That's the definition of a longitudinal wave.

So P waves are just longitudinal oscillations of the Earth's crust, compared to S waves, which were transverse oscillations of the Earth's crust.

So they're the two kinds of seismic wave, the two kinds of oscillation of the Earth's crust, S waves, which are transverse, and P waves, which are longitudinal.

Now, how I remember that is S waves are transverse, okay? Transverse has got an S in it.

Transverse, S waves, okay? And then P waves are longitudinal.

So I remember S waves are transverse 'cause of the S's and P waves are longitudinal.

That's how I remember it.

That might help for you.

So which statement about P waves is correct? Is it A, the oscillations of P waves are perpendicular to the direction of travel? Is it B, the oscillations are along the direction of travel? Or is it C, the oscillations are in all directions for P waves? Which one of those is correct? Make a decision now.

Okay, the correct answer is B.

For P waves, the oscillations are along the direction of travel because P waves are longitudinal.

How do I remember that? Transverse is S waves 'cause transverse has got an S in it.

So S waves are transverse, so P waves are longitudinal and B is the definition of a longitudinal wave.

The oscillations are along the direction of travel.

A is the definition of a transverse wave, which would be transverse, so S waves.

So A was about S waves and C doesn't relate to either seismic wave.

Well done if you got B.

Okay, so another important fact about S waves and P waves is that S waves can travel through solids.

So again, the S is helpful.

S waves are transverse and only travel through solids.

So the S of S waves reminds you it only travels through solids.

Whereas P waves can travel through both solids and liquids.

And P waves actually travel faster than S waves in the Earth as well.

So that is the two kind of seismic waves.

S waves and P waves are the two kinds of oscillations of the Earth's crust or the two kinds of earthquake waves.

And the other thing we need to mention is like all waves, S waves and P waves get refracted when they meet a material of a different density.

So refraction is a kind of transmission which changes the speed, 'cause you've moved to a new medium.

So when a wave goes into a new medium it changes speed, and the change of speed might cause a change of direction, and that's called refraction.

So refraction is when you get a change of direction when a wave is transmitted into a new medium.

So for example, when an S wave goes from a very dense rock to a less dense rock, it might slow down and that change of speed might also cause it to change direction.

That's called refraction.

Let's do a little check about S waves.

Which statement is incorrect? So you're looking for one statement that's false.

So two of these are correct, but the answer, what I'm looking for is the one that's false.

So which is false? A, S waves can pass through liquids, B, S waves can pass through solids, C, S waves can be refracted.

Which one of those is false? Five seconds to make a choice, choose an option.

Okay, well done if you chose A.

S waves, remember the S, S waves can only pass through solid.

S waves are transverse and can only pass through solid, 'cause they're S waves.

So S waves cannot pass through liquids.

They can pass through solids, so B was correct.

And S waves, like all waves, can be refracted in the right conditions.

So C was correct and A is incorrect.

Well done if you got that.

Okay, time for a quick task.

If an earthquake occurred in London, which type of seismic wave would be detected first in Manchester? That's 260 kilometres away.

Explain your answer.

So you just need to choose which type of seismic wave would be detected first and why.

So it should be very quick, but pause the video and write down an answer to that question.

Pause the video now.

Okay, well done for your effort in having a go at that question.

Let's see how you got on.

So which type of seismic wave? Well, there's only two types of seismic waves, so it's 50/50 for the first part of the question.

Did you choose S waves or P waves? Well, which would be detected first? The answer is P waves.

That's because P waves travel faster, and that's all the explanation you need.

Where it says explain your answer, well, why would they be detected first? Well, it's 'cause they travel faster, therefore they'll take less time to go the same distance from London to Manchester.

So P waves generally reach a destination first.

So well done if you got that.

Right, so it's now time to put it all together.

So we need to remember what we've learnt about S waves and P waves and we're going to discuss about how does that tell us what the inside of the Earth is like.

So of course, no one's been to the centre of the Earth.

People have written like novels, made up stories about journey to the centre of the Earth, but that's fiction, that's made up.

In fact, the deepest we've dug isn't very far at all into the Earth.

So how do we know, why are scientists so certain about the structure of the inside of Earth? That's what this section's about.

Okay, so I'm gonna tell you about the structure of Earth first, and then after we've got that sorted, then we'll talk about how we know that Earth has got that structure.

So the structure of Earth is that it's made up of a very, very thin crust.

And then the next layer down is called the mantle.

And then towards the centre of the Earth, the Earth's core actually has an outer core and an inner core which are slightly different.

And it's the study of S waves and P waves which has told us that the Earth has these levels and has told us that the Earth's core has two slightly different structures, the outer core and the inner core.

So we know about the Earth's crust.

In fact, the deepest we've dug is only about halfway down into the crust.

That's the deepest humans have ever been or dug, only about halfway down into the crust.

We've not gone any further than that.

Everything else we know about the structure of the Earth we know through the study of seismic waves, S waves and P waves.

And S waves and P waves measurements can tell us how thick the crust is and the different materials of the crust in different places on the Earth, and then also about the mantle below.

Now, the mantle is almost completely solid.

There's a few places where it kind of moves a little bit, but we need to think of the mantle as being almost completely solid.

And then the first part of the Earth's core, the outer part of the Earth's core is liquid, okay? It's actually molten.

Very, very hot, very, very high pressure and molten liquid.

And the reason we know that is because of seismic waves.

More on that later.

And then the inner part of the core, the very central part of the Earth's core, that's solid, okay? Very, very, very hot, very, very, very, very, very high pressure, but solid.

So the inner core is solid, but the outer core is liquid.

So that gives us the question of, well, how do we know this, and why are scientists so certain that this is really what the Earth is like? 'Cause we can't see inside.

Well the answer is we use S waves and P waves a bit like using ultrasound to see inside a pregnant woman's womb to see the baby, the foetus growing inside the womb.

The transmission of the waves, the transmission of ultrasound can help us see inside structures which are hidden to us.

And it's the same here with S waves and P waves.

The structure of the Earth is hidden to us, we can't go there, we can't see it, but the transmission of the waves through the Earth can help us understand what the structure is like.

So let's look at that.

So I'm gonna start with a blank model of the Earth where we don't know the structure yet, and then I'm gonna talk through how we know how seismic waves then tell us what the structure of the Earth is.

So imagine an earthquake happens here and S waves travel out from that earthquake through the Earth's crust, through the mantle, and the S waves reach other points on the Earth surface very far away.

And earthquake listening stations where there might be a seismometer or a seismograph, they're the names for machines which detect motion of the Earth's crust.

When the S waves arrive there, that's when the Earth crust there start shaking, so that's when the seismograph at those locations, where the S waves reach, where they'll record an oscillation.

And then people realise, oh, an earthquake has happened, but it didn't happen here.

The earthquake waves have travelled out to us.

And by tracing back where those waves look like they've travelled from, you can work out where the centre of the earthquake is.

And then there'll be other seismograph or seismic wave stations everywhere across the surface of the Earth.

So lots of countries work together to share seismic wave data across borders, because that's what scientists do.

They work together across borders and across countries.

And there'll be other seismic wave stations where the seismic waves arrive even later because they've got so much further to travel.

But then actually for S waves, that's it, okay? So seismic wave stations on the Earth's surface up to those last points where these last arrows arrive, they will all record S waves arriving, and some will record the S waves arriving earlier because they're closer and some will record the S waves arriving later 'cause they're further away.

But actually nowhere else on the Earth's surface beyond those arrows that I've showed you would record S waves arriving.

So even though S waves will be being created where that earthquake is, where it says S waves, and the S waves will be spreading out in all directions through the Earth, S waves are not detected on the opposite side of the Earth at all from an earthquake.

So the opposite side of the Earth to where the earthquake happened, no S waves are detected at all.

So why is that? Why do we think that is? That's the problem that scientists had when they first started to study seismic waves.

Why are there no S waves detected on the opposite side of Earth to the earthquake? Now, there must be something.

That suggests that there's something within the Earth, perhaps in the centre of the Earth or the core of the Earth.

There might be something at the core of the Earth which is stopping S waves passing through.

And thinking back to what we've learnt about S waves, what do we know will prevent S waves travelling? What do we know will absorb S waves because S waves cannot travel through it? See if you can have a think and remember, what will absorb an S wave? What cannot S waves travel through? Well done if you remembered a liquid.

So the hypothesis that scientists came up with is the reason that no S waves are detected the opposite side of an Earth to an earthquake is because at the centre of the Earth there must be a region of the centre of the Earth that's liquid.

And that's where we've come up with the idea that the Earth's centre, there must be a liquid section, and we now know it's the outer core of the Earth which is liquid.

So this is now what we think about S waves.

When S waves are produced at the crust, they travel through the mantle.

They get refracted as the density of the mantle changes.

That's why they follow curved paths.

You might have been wondering about that before.

But S waves are absorbed by the Earth's outer core because it's liquid.

And that idea, the idea that the Earth has a outer core that's liquid, we came up with that idea to explain why no S waves are detected on the opposite side of an Earth to the earthquake.

And by further studying all of these different seismic wave measurements, you can get an idea of the depth of the boundary between the mantle and that liquid outer core.

From all of those measurements, you can kind of put them back together to work out what the Earth's liquid outer core must look like to produce the pattern of S waves that we detect from all of the different seismic wave stations on the Earth's surface.

So that's how S wave data gives evidence for the internal structure of the Earth.

But there is something missing, 'cause the Earth's got a solid inner core.

But what tells us that? Well, S waves can't tell us that, 'cause S waves can't get through the liquid outer core.

S waves don't travel through a liquid.

S waves are solid only.

Let's just check what we've just talked about.

True or false, the outer core of the Earth absorbs S waves.

Is that true or is that false? Five seconds to decide.

That is true.

Well done if you got that.

That's exactly what we just said, it's the outer core of the Earth absorbs S waves.

But what's the reason why? So I want you to now choose between A or B.

Is that because the outer core is too dense or is that because S waves cannot travel through a liquid? Make sure you've chosen between A and B.

The answer is B.

The outer core of the Earth absorbs S waves because S waves cannot travel through a liquid and the outer core of Earth is liquid.

Very well done if you've got both of those.

Okay, so let's now talk about P waves.

So imagine P waves being produced from the same earthquake at the same time.

They're also gonna be travelling through the solid mantle.

They'll also get refracted by the different densities of the Earth mantle and therefore have curved paths.

So here's some P waves travelling out from the same point on the Earth, and here's some more P waves and they'll arrive later 'cause they've got further to travel.

So they'll arrive at their seismic stations later.

And then also to all points on the Earth, across up to those arrows as well.

P waves can travel to all of those points up to those last arrows on the Earth's surface.

But then there come points where no P waves get detected after those arrows.

But then if you go a bit further around the Earth's surface, P waves get detected again from the same earthquake.

They'll obviously arrive much later 'cause they've got much further to travel.

And P waves are detected the opposite side of the Earth to the earthquake, the direct opposite side of the Earth.

But there are those two zones that I mentioned before between that arrow and that arrow where no P waves are detected, and the technical term for that is a P wave shadow zone.

A shadow is an area space where no light hits.

But this isn't about light waves, this is about earthquake waves.

So this is a P wave shadow zone, 'cause there are points on the Earth's surface where no P waves hit.

Now, how could scientists explain that? Does our previous theory, our previous model about the Earth having a liquid outer core, does that explain the pattern of where P waves are detected and the fact that there are points on the Earth's surface where no P waves are detected, but P waves are detected the opposite side, the complete opposite side of the Earth to the earthquake? Well let's have a look.

So there's the liquid outer core that scientists suggested before by studying S waves, and you can see how that explains the pattern of P waves.

It does work, okay? Because when the P waves hit the liquid outer core, that's a new medium, they get refracted and change direction.

The P waves that only went through the mantle, they've kind of got a smooth curve.

But then there's a change of direction for the P waves that hit the liquid outer core, and that's what's gonna create those shadow zones where there's no P waves detected.

But the very opposite side to the Earth there are P waves detected, because P waves are transmitted through that liquid outer core and actually through a solid inner core.

And by studying all of that P wave data, you can piece it all together and work out, well, hold on, if we've got this pattern of where P waves are detected from an earthquake on the Earth's surface, then what structure of the Earth would give us that pattern? And this, this is the structure of the Earth that would give us exactly the pattern that we detect.

So we've got that liquid outer core and a solid inner core.

So P waves can pass through the liquid outer core, but as they go from mantle to core, they're refracted and change direction.

That gives the detected pattern of P waves that we actually see.

So that's why we're so certain that we know about the structure of Earth, because the structure of Earth that we've come up with predicts exactly where P waves and S waves will be detected after an earthquake and where they won't be detected after an earthquake as well.

So let's do a quick check about P waves.

Which statement about P waves is incorrect? So we're looking for a wrong answer.

P waves are refracted in the Earth.

P waves travel in straight lines.

P waves can pass through the outer core.

Which one is incorrect? Five seconds to decide.

The answer is this one.

P waves do not travel in straight lines.

They travel in curved lines because they're constantly being refracted by constantly changing density within the mantle.

P waves are refracted in the Earth, so A was correct.

And in C, P waves can pass through the outer core, so that's correct, but P waves don't travel in straight lines.

So well done if you got that.

Right, we're at the final task of this lesson.

So this task has three parts.

Part one, name the two types of seismic wave.

That should be very easy.

It's what we've been talking about all lesson.

Part two, list the four parts of the Earth in order, starting with the outermost part, working inwards towards the centre of the Earth.

What are the four parts of the Earth? And then part three, this one will be longer.

You'll need to spend a good amount of time on part three 'cause I want you to describe what happens to each seismic wave as it passes through the Earth and explain why that happens for both types of wave.

Okay, so you need to pause the video now and have a good go at this task.

Off you go.

Well done for your effort with the three parts of that task.

Make sure you've had a good go at all three parts, especially part three, and I'll give you some feedback now.

So the two types of seismic waves, S waves and P waves.

Everyone should have got that, fantastic.

Part two, list the four parts of the Earth in order, starting with the outermost part.

That's the crust on the outer surface of the Earth, then the mantle, then the outer core, and then the inner core at the centre.

The outer core is liquid, the inner core is solid.

Well done if you have those details as well.

And then part three, what happens to each type of seismic wave as it passes through the Earth and explain why.

So you'll need to check that your answer pretty much covers the same ideas as my answers do, but they don't have to be identical.

Add anything to your answers as we go along to improve them.

You might need to pause the video to do that.

Okay, let's go through.

So, S waves.

When S waves travel in Earth they refract as they meet material of different densities.

That's in the mantle.

That causes them to have curved paths.

They cannot pass through liquids, so they are blocked, or you might have said absorbed, by the outer core of the Earth, which is liquid.

So the outer core of the Earth is liquid, so that absorbs S waves.

Okay, onto P waves.

When P waves travel through the Earth, they also refract and have curved paths for the same reason as a above.

It's 'cause the mantle of the Earth is constantly changing, constantly changing the speed, causing a constant refraction and a curved path.

Well done if you put that level of detail.

That'd be really excellent.

P waves can pass through liquids, so they're not blocked or absorbed.

However, they change direction at the boundaries between the different parts of Earth.

And you might have added something about how that can predict the pattern of observed P waves from an earthquake across the entire Earth, including predicting where the P wave shadow zones are where no P waves are detected.

So very well done if your answer was along those lines.

So very well done for completing this lesson on probing planet Earth.

Of course, you don't actively probe planet Earth because you can't do anything to produce earthquake waves.

You have to wait the earthquake waves and then use the earthquake wave data to probe planet Earth, like work out what the structure, the internal structure of Earth is.

So, the summary is when tectonic plates slide against each other, they cause earthquakes and seismic waves, oscillation of the Earth's crust.

So S waves, which are transverse waves of the Earth's crust, they don't travel through a liquid.

S waves are solid only.

And P waves, which are longitudinal waves that can travel through liquids.

And then the way that seismic waves travel through Earth allows us to predict or come up with a model which we're now really certain about, the scientists, for Earth's structure.