Lesson video

Hello, my name is Mrs. Harking.

Today, we're going to be exploring the movement of tectonic plates together.

This is taken from the unit, "Tectonic hazards: why is Earth restless?" Today, we will explore how geographers solve the mysteries of our planet.

Our outcome today is to use evidence to show that the continents have moved, and explain how tectonic plate movement happens.

Our keywords are continent.

This is a large landmass on Earth.

There are seven, so we've got Africa, Antarctica, Asia, Europe, North America, South America, and Oceania.

Are you familiar with those? We've now got tectonic plates.

So this term means large sections of the lithosphere that are on the move.

Density is our last keyword today, and this is the mass of something compared to its volume.

If you think it would be useful to note down any of those keywords and their definitions, feel free to pause the video and do that now.

Jacob asks, "Do you notice anything interesting about the shape of Africa and South America?" Let's take a look at the world map here.

So South America and Africa are labelled on.

Can you see anything interesting there? Yes, definitely.

So it looks like, as Jacob says here, "They fit together like a jigsaw puzzle," don't they? Now, actually, this is something that historically, geographers have looked at and thought, "Hmm, something funny is happening here." This is something that has raised a lot of questions for geographers.

Sofia asks, "Could the two continents have once been attached together?" Alex suggests that he needs more evidence.

So he would expect that actually the rock that the ground is made from.

That would be the same across two continents.

And also there would be the same species in both continents as well.

So you'd expect the continents if they had been joined together to be very samey, especially at the edges where these two continents meet.

You'd expect the same type of rock, same type of species of animals and plants to be in those two places before they're ripped apart.

So we've actually got some evidence here that this is the case.

So the bands of the same rock type match across some of the continents.

You can see that over here.

In addition, some types of fossils have been found across several continents.

So you can see here we've got a photograph of a fossil, and this is the lystrosaurus fossil, and it's found in Africa, India, and Antarctica, suggesting that all of these locations were once connected.

Time for a quick check now.

Which species is found across the most continents? Is it A, glossopteris? Is it B, cynognathus? C, lystrosaurus? Or D, mesosaurus? Well done, the glossopteris.

So the theory of continental drift was put forward by Alfred Wegener in 1912.

He suggested that the Earth's continents were once joined together in a super continent called Pangea and later drifted parts.

Can you see that on the animation on the left hand side? So we start with Pangaea, and those continents slowly separate.

Scientists didn't believe him to start with, and it took a really, really long time for him to be believed.

It wasn't until the 1970s that the scientific community really accepted this theory, and the reason for that was that although Alfred Wegener had actually worked this out and decided this is probably what happened, there wasn't any evidence for really how this happened and how it worked.

Alfred Wegener didn't get to see his theories accepted by the scientific community and taught in schools.

It wasn't until the 1970s that the theory was really accepted.

Unfortunately, Alfred died in 1930.

So a long, long time was required to gather the evidence and for the scientific community to actually really accept this actually could have happened.

Sofia asks, "How fast are the plates drifting or moving?" So the Earth's tectonic plates move towards or away from each other very slowly.

It varies between plates, but on average, the rate of movement is 1.

5 centimetres per year.

So that's about the speed your nails grow.

It is not fast.

Can you ask this quick check for me? Which of the following provide evidence for the theory of continental drift? Is it A, the distribution of fossils found? B, rock types found across continents? C, eye witness accounts? Or D, the shape of continents? You can pick more than one answer.

Well done.

Yes, so it is three of those options.

The distribution of fossils found, the rock types found across continents, and the shape of continents as well.

So lots of different pieces of evidence support this theory and that is why it was accepted as true by the scientific community.

Time for a task now.

Can you complete the gaps to explain why Wegener propose the theory of continental drift? Here, we've got a paragraph and some bullet points.

The theory of what type of drift was put forward by Alfred, what was his surname? In 1912, he suggested that the Earth's continents were once joined together in a super continent called.

And later drifted apart.

He thought this because some continents, for example.

And South America, which continent was it? Look like they fit together like puzzle pieces.

Bands of the same rock type match across.

And also types of something found match across several continents.

Have a look and see if you can fill in some or hopefully all of these gaps.

I'll let you pause the video now, so you've got some time to do this.

Well done, everyone.

Let's have a look at the answers.

So the theory of continental drift was put forward by Alfred Wegener in 1912.

He suggested that Earth's continents were once joined together in a super continent called Pangaea, and they later drifted apart.

He thought this was because some continents, for example, Africa and South America, looked like they fit together like puzzle pieces.

Bands of the same rock type match across continents.

And types of fossils found match across several continents.

How did you do? Excellent work.

Feel free to pause the video now, and tweak your answers if you need to.

Time to move on to the second part of our lesson now.

This is tectonic plate movements.

Sofia says, "There is a lot of evidence for continental drift, but how do the continents actually move?" This is exactly what the scientist wanted to know when Wegener proposed his theory back in 1912.

So the upper mantle and crust combined make the rigid lithosphere.

The lithosphere is not one continuous layer.

It's broken up into the large sections called tectonic plates.

Tectonic plates have two forms. We've got the oceanic plate.

This is a section of tectonic plates covered by ocean.

It is relatively thin and it's dense.

The continental plate is a section of tectonic plate covered by land.

This is thicker and less dense.

It's worth noting at this point that we are not saying the whole plate.

We're saying that oceanic plate is a section of tectonic plates and the same for continental.

This part of the South American plate is continental.

However, this part of the South American plates is oceanic.

So you can have one plate that has sections or a section that is continental, and sections or a section, which is oceanic.

Time for a quick check now.

Why is Jun's statement incorrect? "The African plate is a continental plate." Have a look at the map here to help you out.

Well done.

The African plate is made up of continental plate with land, and oceanic plate where the seabed is.

Another quick check.

True or false? South America and the South American plate are the same thing.

Well done.

That is false.

So South America is a continent, land, and the South American plate also includes part of the seabed of the Atlantic ocean.

So the continent and the plates are two separate things.

Density is the mass of something compared to its volume.

So oceanic plates are more dense.

This means that more molecules are in the same volume.

So where there are more molecules in the same volume, it is more dense, and where there are less molecules in the same volume, it is less dense.

So that would be the case for our continental plates, which are less dense as you can see on the right here.

Density affects how different materials interact, whether they sink or float in other substances.

You can see here that wood is less dense than water, so it floats.

Whereas most metals are more dense, so sink in water.

This is the same for tectonic plates.

Where oceanic plates meets continental plates, the dense oceanic plates abducts or sinks into the mantle.

This is the subduction zone I've circled here, and it's a really good key term to remember.

As the oceanic plates subducts, it pulls the rest of the plate along with it.

Gun asks, "How does that work?" Sofia says, "The weight of the oceanic plate drags the rest of the plate along, like the end of a heavy chain falling off a table." You can try this yourself with anything that you have lying around.

So if you don't have a chain, might you have a necklace or even a scarf, or something like that.

Give it a go and see if it works.

Does gravity pull the rest of that item off the table? I gave this a go myself with a scarf.

I'll show you what it looked like.

Here it is.

This process is called slab pull.

The slab part of this term refers to the slab of the tectonic plate that is sinking or subducting.

You can see circled in turquoise here.

The pull part of this term refers to the slab pulling the rest of the plate behind it so it does make sense.

I like a term that makes sense.

Quick check time.

What is slab pull? Is it A, a method of measuring plate movement speed? B, a type of volcanic eruption, which occurs in the middle of a plate? Or C, a process where dense oceanic plate sinks into the mantle? Which do you think? Well done.

So C, a process where dense oceanic plates sinks into the mantle.

Lucas says, "Okay, that makes sense for the oceanic plates on this side of the diagram, but what about the other one? What causes that to move?" And that's a really good question.

So there is another process involved, and we'll talk about that now.

At some boundaries, new rock forms pushing the tectonic plates apart.

You can see where this is occurring on the diagram here, where it's been circled.

So where this is happening? The mantle melts and forms magma.

The magma rises through cracks in the plate, and erupts as a lava.

The lava cools and hardens becoming new rock, which increases in density over time.

The new rock formed at the ocean ridges cools and becomes denser over time.

As a result, it slides away from the top of the ridge.

This makes sense, doesn't it? Where there's that ridge, as the rock gets denser, it slides down away from that ridge.

So gravity's playing a role here.

Makes sense.

The process of new rock forming, cooling, and sliding away from the ridge pushes the plate apart.

This is called ridge push.

So the ridge of this term refers to the less dense, hot magma welling up and producing that ocean ridge.

A ridge is that raised line where the two plates are separating.

The push part is dense rock slides away due to the force of gravity, and it pushes the rest of the plate along in front of it.

Ridge push takes place where plates move away from each other.

Laura has got another point she wants to make, and I wonder if any of you have this actually as well.

"I have heard it was something to do with convection currents in the mantle." Now, this is an older theory about how plate movement happens.

It's not one that scientists actually think is the main driver anymore at all.

So the main driver of plate movement is thought to be slab pull, but ridge push also plays a role as well.

Time for a quick check.

Which plate is subducting at this plate boundary? Do you remember that subducting means sinking.

Well done, it's the Nazca plate.

Which process explains why the South American plate moves? Is it A, slab pull? B, ridge push? Or C, convection currents? Well done, it's ridge push.

You're correct.

Time for our practise task now.

Can you define the following key terms? oceanic plates and continental plates.

Remember when we're talking about oceanic and continental plates, we're talking about a section of plate, not the whole plate.

Task two, can you annotate a diagram to explain how slab pull moves the plates? I've put a diagram here to help you.

And task three, annotate a diagram to explain how ridge push moves the plates.

Again, there's a diagram here to help you.

Feel free to pause the video now to give yourself time to do this task.

Well done.

Brilliant.

Let's have a look at some answers.

Your answers may look like this.

Define the following key terms. So oceanic plates is a section of plates covered by ocean, made up of relatively thin and dense material.

So here, we're really looking for that section of plate, not the whole plate.

And it's thin and dense material.

Those two elements are really good to include as well.

For the continental plate, this is a section of plates covered by land.

Again, section of plate, not the whole plate, and it's made up of relatively thin and dense material.

Feel free to pause the video to edit your own answers if needed.

For task two, you had to annotate a diagram to explain slab pull.

So our first annotation here says dense oceanic plates or the slab sinks into the mantle.

Our second annotation says that the slab pulls the rest of the plates behind it.

Feel free to tweak your answer if you need to.

Also, I would note that it's a good idea to have a title for that diagram as well, "A Slab Pull".

Do make sure you've done that.

I'm sure you have.

For your ridge push diagram, these are the sorts of annotations I would expect you to include.

So ridge push takes place where plates move away from each other.

Have you linked that to the section on the diagram where you've got those arrows moving apart? Then we've got less dense hot magma wells up and produces an ocean ridge.

So ocean ridge is a really key term to be including in this.

Do make sure you've done that.

And then denser rock slides away due to the force of gravity, which pushes the rest of the plate along in front of it.

Feel free to edit your answers if you need to at all.

Well done, everyone.

Time for our summary now.

The theory of continental drift suggests that Earth's continents were once joined together and later drifted apart.

Evidence for continental drift includes the continents' jigsaw-like shape, matching rock types, and fossils across continents.

There are two types of plates: oceanic and continental.

Oceanic plate is dense, but thin.

And continental plate is thicker and less dense.

Plates move by the processes of slab pull and ridge push.

I hope that you enjoyed the lesson today and have learned lots.

I've certainly enjoyed learning with you, and I hope to see you next time.

Bye-bye.