Lesson video

Hello, my name is Mrs. Harking.

Today's lesson is on plate boundaries.

It's taken from the unit, "Tectonic hazards: Why is our Earth restless?" We're going to start exploring today where and why earthquakes and volcanic eruptions happen.

We're going to do this by having a look at the different types of plate boundaries that there are.

Today's outcome is to describe what happens at plate boundaries, including the tectonic hazards that occur.

Here are some keywords that are useful to know before we start the lesson today.

Our first one is tectonic plates.

The huge pieces that the earth's crust is broken into are known as tectonic plates.

Plate boundary is where two tectonic plates meet.

And dense means a lot of mass packed into a small space.

Today's lesson is in three parts.

First, we're going to look at what happens where plates move past each other.

Then what happens where plates diverge, and what happens where plates converge.

Now, that might not make a huge amount of sense just yet.

Don't worry.

That's why we're learning about it today.

Our first section, what happens where plates move past each other? We're going to look at now.

So tectonic plates move in different directions, creating different types of plate boundary.

They move alongside each other as this diagram shows.

They separate or diverge, just another word for separate that.

And then move toward each other or converge.

Again, that's just another word for moving toward each other.

Now, if we have these different types of plate movements, that creates different types of plates boundary.

So where the plates move alongside each other, we get a conservative plate boundary.

Where the plates diverge, we can get a constructive plate boundary.

And where the plates move towards each other or converge, we can get either a collision plate boundary or a destructive plate boundary.

And a little bit later in the lesson, we'll look at why there are two different types of boundary for convergent movement.

So the upper row diagrams here shows us the movement of the plates, and the lowest section here shows us the type of plates boundary itself.

Time for a quick check now.

True or false? All the tectonic plates move in the same direction so they don't bump into each other.

Is this true or false? Well done, you've been listening.

It's false.

Can you tell me why it's false? Great work.

So the tectonic plates move in different directions.

Meaning, in some places, they move away from each other or diverge.

Some places, they collide or converge.

In some places, they slide past each other.

So conservative plate boundaries are the first type of plate boundary we are going to look at.

So conservative means that tectonic plates move past each other in different directions or at different speeds in the same direction.

So if we have a look at the diagram here on the right, we can see that this plates are sliding past each other in opposite directions.

Now, the plates are locked together, and this is due to friction, yet the plates are on the move.

Stress builds up over time, and there's a way to show that with our hands, which can help you to understand it.

So if you take your fists and lock them together like this, so our knuckles are locked together.

And we move one fist down and one fist up.

You can see that, 'cause our knuckles are locked together, because it's lumpy and bumpy, that's creating friction, stopping our hands from moving.

And that's exactly what happens with the plates.

Plates are stopped from moving due to friction, but the stress builds up here.

You can see that actually, if pushing, pushing, pushing, one up, one down, one up, one down, the stress builds up, and then the stress is eventually released.

Now, when the stress is released, this means that the rock fractures or breaks, and this releases waves of energy, the ground shakes, and that is an earthquake.

So when the ground shakes, that is an earthquake happening.

So let's run that through one more time.

At any plate boundary, the plates are locked together by friction.

At conservative plate boundaries, the two plates are trying to slide past each other.

Might be because one is going faster than the other in the same direction.

It might be because the two plates are going in opposite directions.

The stress builds up and then is released.

And as it is released, that is an earthquake.

So that makes sense for our tectonic plates moving in different directions.

But let's go back to that bit at the beginning.

Tectonic plates move past each other in different directions or at different speeds in the same direction.

Right, so understanding that can be a little bit trickier.

So what I would like you to do here is to get your hands together like this, almost like a little kangaroo, or bunny, or something.

Right, what we're going to do is we're going to move both our hands forwards.

However, your left hand is going to go faster than your right hand.

And this is something you really need to think about.

You need to engage your brain.

It's like one of those things where you pat your head and rub your tummy at the same time, but you can do this, okay? So hands together, left hand is moving faster than your right hand, but both hands are moving forwards.

Let's try this together.

As they move, can you feel them sliding past each other? So even though they're not going in different directions, they are still sliding past each other.

I'm going to do that.

This, I'm gonna do it up down just so you can see.

So I'm going to move them both up, but my left hand's going to go faster.

Okay? So they're both moving up, but my left hand's moving faster.

So they're still sliding past each other.

And that's the key thing to this.

A key thing to note here is that term "fracture".

It's a better term to use than break, but it means the same thing.

Just keep that in mind, especially for the task later.

Here, Izzy is asking, "Is new crust created at a conservative plate boundary?" Now, the reason she's asking that is because at some plate boundaries, new crust is created.

Sam, he says, "No, the amount of crust here remains the same or is conserved." Now, that's interesting, isn't it? Because if you have a look at that term "conserved" stays the same.

That makes a little bit of sense.

It makes more sense for why we're calling this plate boundary a conservative plate boundary.

And that will help us to remember it.

Time for a quick check.

True or false? All plate boundaries are giant cracks between two separate tectonic plates.

Is this true or false? Well done.

It's false.

Can you tell me why? Feel free to pause the video if you need time to discuss.

Well done.

So the two tectonic plates are stuck together at conservative plate boundaries.

There's no neat crack between the two tectonic plates, and this is the same at all plate boundaries.

An example of a conservative plate boundary is a San Andreas fault in California in the USA.

You can see where it's located on the map here.

We have the North American plates and the Pacific plates meeting at the San Andreas faults.

The two plates move next to each other, but at different speeds, and you can see that represented on our map by the two arrows.

So the Pacific plate is moving faster, and therefore, has a thicker arrow.

And the North American plate is going slower, and therefore, has a thinner arrow.

That is just something I've done to show you on this map here to make it a bit clearer.

It's not something that necessarily will be the same on every map you see.

Check time now.

Which direction is the Pacific plate moving in? So I'm looking for compass points here.

Well done.

So it's north-north-west, but you may have said north or you may have said north-west.

I would accept those as well.

Time for our first practise task.

Can you complete the fact file on conservative plate boundaries? First of all, I'd like you to complete the sentence.

The plates move.

Tell me about the direction.

Then I'd like you to tell me where earthquakes happen here, where the volcanoes happen here, and then we're completing this gap fill.

So at the.

What's the line where the two plates meet called? The two plates were locked together and pressure builds.

Suddenly, the rock.

Another word for breaks, what was that word that we prefer? Releasing waves of energy.

The shaking of the ground that results is called an.

Well done, everyone.

Let's have a look at your answer compared to mine.

The plates move past each other at different speeds or in different directions.

They do have earthquakes at a conservative plate boundary, but volcanoes are not found here.

At the boundary, the two plates lock together and pressure builds.

Suddenly, the rock fractures, was another word for breaks, releasing waves of energy.

This sudden movement is called an earthquake.

Well done, excellent work.

Now, we're moving on to the second part of our lesson.

What happens where plates diverge? And remember diverge means separate, isn't it? Let's take a look.

At a constructive plate boundary, tectonic plates move apart.

This allows hot molten rock or magma from the mantle to rise up to the surface as volcanic eruptions.

Where magma erupts from the Earth's surface, we call it lava.

As we can see on the photo here.

Lava cools and hardens to form new crust.

The closer to the plate boundary, the younger the rock.

So that is because the new rock that is created is literally called lava that's hardened into rock.

So that is going to be newest, really close to these volcanoes, really close to the plate boundary.

As you move away from the plate boundary, the rock's going to be older as it has had time to cool, harden, and then the plate has moved.

Therefore, moving that rock away from the plate boundary.

Here, new crust is created or constructed, which makes sense, doesn't it? Because we've called this a constructive plate boundary, and that's because there is new crust falls or new crust constructed here.

Quick check time now.

Which diagram shows a constructive plate boundary? Is it A, B, or C? Excellent, well done, it is B.

Earthquakes occur at constructive plate boundaries as they do at all plate boundaries.

As tectonic plates move, rock fractures releasing energy and the ground shakes, creating an earthquake.

Earthquakes happen here in quite a similar way, the one that we've just discussed for conservative plate boundaries.

So once again, the two plates are locked together just like this.

This time, our two plates are trying to separate.

So pressure is building up as they are dragging themselves apart.

But because the plates are locked together and stuck together at the boundary, that pressure has to build up before actually the rock will fracture, and that earthquake will happen.

A great example of a constructive plate boundary is the Mid-Atlantic Ridge, which passes through Iceland.

The Mid-Atlantic Ridge runs the whole way down the Atlantic from north to south passing through Iceland on its way.

Here, the North American plates and the Eurasian plate separate.

The North American plate moves to the east, whereas the Eurasian plate moves to the west.

As the two plates separates, earthquakes and volcanic eruptions occur.

Time for a quick check now.

Where has the land most recently been formed? Have a look at the map here, and decide whether you think it's where A is located, B or C? Correct, it is B.

So B is because it is on the plate boundary where the new rock is forming.

The further away from the boundary, the older the rocks are.

So we'd expect to see C as slightly older than B, and then the rocks at A would be even older again, because it's the furthest from the plate boundary.

Practise task time now.

Can you answer the following questions for me? In which direction did the tectonic plates move at constructive plate boundaries? You can draw a simple diagram to help you explain this.

And don't worry about your drawing skills.

It really can be two arrows and then a few lines, and that's fine for the diagram parts.

Two, do earthquakes happen at constructive plate boundaries? Three, how do volcanic eruptions occur at constructive plate boundaries? And four, name an example of a constructive plate boundary.

Pause the video now to give yourself time to do this.

Well done, everyone.

Let's check your answers now.

So your answers may look like this.

They might be slightly different as long as they're still correct, that's fine.

For question one, in which direction do the tectonic plates move at a constructed plate boundary? Draw a simple diagram.

So my diagram looks like this.

Yours might be a bit more simple than that.

That's absolutely fine.

Key thing is that the arrows must be separating.

They must be moving away from each other.

And we should have two tectonic plates here.

So the tectonic plates move away from each other, is the little annotation that I've added there.

That's a good idea to add as well.

Number two, do earthquakes happen at constructive plate boundaries? So earthquakes do occur to constructive plate boundary as they can at all plate boundaries.

Question three now.

How do volcanic eruptions occur at constructive plate boundaries? Where tectonic plates move apart, this allows hot molten rock, also known as magma, from the mantle to rise up to the surface, creating a volcanic eruption.

When the magma erupts out, we call it lava.

The lava calls and hardens to form new crust.

Our fourth question here was name an example of a constructive plate boundary.

And our example we used was the Mid-Atlantic Ridge, which runs through Iceland.

If you've come up with any other examples, brilliant, just double check that they're correct.

Over the last section of our lesson today, what happens where plates converge? Remember, converge means moving together, moving towards each other.

In order to be able to understand convergent plate boundaries, we need to know about the two forms of tectonic plates.

Now, you might remember this from previous lessons or this might be new to you.

I'll run over it either way, just to make sure we're clear.

So oceanic plate is a section of a tectonic plate covered by ocean, and this is relatively thin and dense.

Continental plates is a section of tectonic plate covered by land, and this is thicker and less dense.

Remember that density is about how much mass is crammed into the same volume.

To converge means to come together.

A collision plate boundary is where converging plates have the same density.

So for example, they might be two continental plates coming together.

Destructive plate boundaries are where converging plates have different densities.

So here we've got the diagram, an oceanic plate and a continental plate meeting.

And obviously, we've just mentioned continental plates and oceanic plates have very different densities.

We're going to look at the collision plate boundary first.

So at collision plate boundaries, two plates of the same density collide.

Pressure builds up at the boundary where the rock is stuck together.

This is exactly what we've heard for the previous types of plate boundary, haven't we? So this pressure building up once again.

The rocks fold or crumple and fracture or break, and the pressure is released as an earthquake.

So we've talked about rock fracturing before, but we haven't talked about the rocks folding and crumpling.

So that's more unique to these collision plate boundaries.

The rocks stack up to great heights, creating a belt or line of mountains.

So here, we've got our two plates colliding into each other.

They have the same density.

Pressure builds up at the boundary, they're pushing together, and then the rocks fold or crumple, and fracture or break when that pressure is released.

And that pressure being released as we've talked about before, is an earthquake.

As those rocks stack up, they create mountains.

So the Himalayas are an example of a mountain belt.

Remember that line of mountains at a collision plate boundary? We can see here these mountains show us where the Eurasian plates and the Indian plates are converging.

The 2005 Kashmir earthquake caused the Himalayas to grow 10 metres in some areas.

That is huge.

So when these two plates come together and we get this release of pressure, this movement of the earth, that mountains actually grow.

Now, when it comes to understanding this scale, I always think about a door being about two metres high.

So if you look at a door now, try in your mind stack 10 metres worth of door up.

So we're talking five doors on top of each other.

That is how high 10 metres is.

That's a lot for a mountain to grow by in one earthquake.

So on average, the peak zone only rise about one centimetre per year due to the collision.

But in that particular earthquake, there was 10 metres worth of rise.

So you could see how significant a single earthquake can be.

True or false? Plate movement can mean that mountains grow.

You are correct, it's true.

Right, can you tell me why? Feel free to pause if you'd like to discuss.

Well done.

So earth's plates are constantly on the move if moving very slowly.

Where the two plates converge, really great word to use, remember? They may fold creating mountains such as the Himalayas, which continue to grow.

Right, we are looking at the destructive plate boundaries now.

So again, the plates are converging, but at a destructive plate boundary, the two plates are of different densities.

So the denser oceanic plate here is subducted underneath the continental plates.

Right, let's break that down.

So the oceanic plate is the denser type of plate.

We talked about that before.

And it's subducted.

Subducted means it descends down, it goes down underneath the other.

So we could see that happening in the subduction zone on the diagram just here.

I've just circled it for you.

The oceanic plate heats up, and this helps the mantle rocks above to melt and form magma.

Now, pay attention closely to which part here is creating magma.

It's not the rock that forms the oceanic plate, it is actually the rock that is in the mantle just above that oceanic plate.

So have a look at where I've labelled magma on the diagram, and you can see that there actually is a little bit of mantle just above of subducting oceanic plates.

And that little bit of mantle is the part which is creating the magma that is erupting out of our volcanoes.

So magma rises through the continental plate and erupts as a volcano.

Time for a quick check now.

Can you find Aisha's error? At destructive plate boundaries, two plates with the same density converge.

The oceanic plates is subducted underneath the continental plates.

Well done.

It's not the same density, it's different densities.

At a destructive plate boundary, continental and oceanic plates meets.

These have different densities.

The oceanic plates is more dense.

Right, what about Lucas's error? Can we find his error? And to be fair to Lucas, this is a really common error that students make.

So good one to pay attention to.

The oceanic plate heats up, helping the subducting oceanic plates to melt and form magma.

Well done.

This is a really tricky one.

So it's not the subducting oceanic plate that actually melts.

The fact that that is descending or subducting into the mantle means that the mantle rocks above can melt easier, helps them to melt, and that is what forms our magma.

It's the mantle above the subducting plate that melts to form magma, not the plate itself.

As at all plate boundaries, earthquakes occur at destructive plate boundaries.

Stress builds up where the rock of the two plates that are on the move becomes stuck.

So this is just what we've talked about before.

Stress or pressure is building up and the two plates are locked together, that friction is happening and they are locked together.

When the rock fractures or breaks, this energy is released, shaking the ground, which causes an earthquake.

The focus or the origin of some earthquakes, at destructive plate ground, boundaries, sorry, is at great depth.

So if we look at the diagram here, we can see because we have a subducting plate.

These plates, actually, they go a lot deeper than at other plate boundaries.

Meaning that the focus of that earthquake can be deeper than it can be at other plate boundaries.

Saying that, you could also have quite a shallow earthquake where the focus is quite close to the surface, just in the same way that you can at other plate boundaries as well, if that's where the two plates are moving past each other.

An example of a destructive plate boundary is the Mariana Trench, which reaches over 10,000 metres deep.

This is the deepest natural point on earth.

Deep ocean trenches are features of destructive plate boundaries.

And you can see this labelled here.

Mounts Pagan is one of the many volcanoes which have formed on the Mariana Islands.

And you can see the Mariana Islands located on the world map just at the top here as well.

And for a quick check now.

Earthquakes occur at which type of plate boundary? Is it A, all plate boundaries? Is it B, only convergent plate boundaries? Is it C, only destructive and constructive plate boundaries? Or is it D, only collision and conservative plate boundaries? Well done.

Yes, earthquakes occur at all plate boundaries.

That's because at all plate boundaries, the pressure is building up and then is released when the rock fractures, and when that happens, the earth moves, and that is an earthquake.

Volcanic eruptions are hazards at which type of plate boundary? Is it A, all plate boundaries? B, only convergent plate boundaries.

C, only destructive and constructive plate boundaries.

Or D, only collision and conservative plate boundaries.

Well done.

It's C, only destructive and constructive plate boundaries.

Task C now.

Please can you explain how collision plate boundary creates mountain belts? I like to use this diagram and the following words to help you.

We got density, collide, pressure, fold, fracture, earthquake, and mountain belt.

So that's your first task.

You might want to pause now and complete that, or you might want to hear about the other tasks before you start.

The second part of our task, I'd like you to order the statements to explain how Mounts Pagan in the Mariana Islands was formed on a destructive plate boundary.

Again, you might want to pause the video now and complete this task, or you might want to have listened to the third part first.

Part of our task is this, to add labels to identify an ocean trench, volcanoes, oceanic plates, continental plates, and rising magma.

Please pause the video now to complete your task.

Good luck.

Well done, everyone.

Let's review the first part of the task to begin with.

Feel free to pause the video at any point.

If you want to edit your own answer, that's absolutely fine.

So to explain how a collision plate boundary creates mountain belts, I said, "At collision plate boundaries, two plates of the same density collide.

Crash into each other.

Pressure builds up at the boundary where the rock is stuck together.

The rocks fold or crumple and fracture or break when the pressure is released as an earthquake.

The rock stacks up to a great height creating a mountain belt." Our second task was this one, to order these statements to explain how Mount Pagan and the Mariana Islands was formed on a destructive plate boundary.

So the first statement was B, at destructive plate boundaries, two plates of different densities converge.

E, the denser oceanic plate is subducted underneath the continental plates.

D, water and gases in the oceanic plates lower the melting points, so mantle rock above forms magma.

C, the magma rises through the continental plates and Mount Pagan erupts.

As the lava cools, it forms the Mariana Islands.

Third part of our task now is just to label this diagram.

So you can see I've labelled on the ocean trench where we've got that deep trench in the subduction zone.

Oceanic plate on the left here.

Whereas the continental plate is the plate on the right.

The rising magma is key.

Please make sure you've labelled that just above the subducting plate.

Just above that oceanic plate, that subducting down, not the actual plate itself, okay? And then volcanoes are at the top there on the continental plates.

Well done, everyone.

We've got to the summary.

There are four main types of plate boundary.

Let's run through a little summary of each one.

So at a conservative plate boundary, the tectonic plates move past each other, causing earthquakes.

Remember, that can be in the same direction or different directions.

At a constructive plate boundary, the tectonic plates move apart, causing volcanic eruptions and earthquakes.

That collision plate boundaries, plates move together, causing earthquakes.

Remember, collision, collide.

They're moving towards each other.

At destructive plate boundaries, plates move together, causing volcanoes and earthquakes.

And I would say we've got earthquakes happening at every single one of these plate boundaries.

It is just constructive plate boundaries and destructive plate boundaries though that have volcanic eruptions.

I hope you found today's lesson useful.

I certainly enjoyed learning with you.

See you next time.