Lesson video

Hello and welcome.

My name is Mr. March, and I'm here today to teach you all about the plate tectonics theory.

So grab everything that you need for today's lesson.

Let's get going.

So by the end of today's lesson, then you'll be able to understand the theory of plate tectonics and the mechanisms involved in plate movement.

There are three key words that we need to know about for today's lesson, and those are tectonic plate, asthenosphere, and convection.

Tectonic plate refers to a large rigid slab of the Earth's lithosphere outer layer that sits on the semi-solid asthenosphere beneath it.

Asthenosphere then is a semi-solid layer of the earth's upper mantle.

And finally, convection is the transfer of heat through the movement of fluids, warmer, less dense material rises and cooler, denser materials sink, creating this circular flow.

There are three learning cycles for this lesson, and we're gonna start with the first, which is to look at the Earth's layered structure.

This world map in front of you has a GIS layer added called plate boundaries.

Now I'm wondering, do you know what plates are? You may like to pause the video at this point whilst you consider your answer.

Aisha gives a great answer though.

She says that the Earth's crust is split into tectonic plates.

The earth has a layered structure, and you may have come across this diagram that you can see on the screen before.

It shows the three main layers.

The crust is the outer layer, followed by the mantle, and then finally, the core at the centre.

Now, the crust is between one and 100 kilometres thick, depending on location.

And the mantle, which is the layer beneath this, is about 2,900 kilometres thick.

But realistically, this diagram is too simplistic, so perhaps we should look at it more like this, where we have the lithosphere at the very outer layer of the Earth, and this contains both the crust and the uppermost part of the mantle, which is most solid.

Beneath the stem, we have the mantle as well as the core.

And once again, the mantle can be broken down.

This part of the mantle where it is much more semi-molten state is known as the asthenosphere.

Below this, then we have the outer core, which is pure liquid.

And then at the very centre of the Earth, we have the inner core, which essentially is a solid ball of metal of iron.

Okay, time for a learning check.

And I'd like you to complete the missing labels by adding the names of the layers of the Earth that are indicated.

So please pause the video here whilst you consider and decide your answer.

Best of luck.

Okay, and now, the answers.

So in the upper outer layer we have the lithosphere, which combines both the crust and the rocky part of the mantle.

Underneath this, we have the asthenosphere, which is the semi-molten part of the mantle.

And then finally, just underneath that, then we have the mantle.

So the earth's lithosphere is split into different size tectonic plates, and these tectonic plates sit on top of the asthenosphere.

The lithosphere and the asthenosphere have different characteristics which affect how they behave.

For example, the lithosphere is 500 degrees Celsius at its most deep, and it is the same temperature as we experience at its surface.

Meanwhile, the asthenosphere, which is much closer to the centre of the earth, is between 1,300 and 1,600 degrees Celsius.

They also differ in terms of density.

The lithosphere, for example, is much less dense.

While the asthenosphere is much more dense, they also differ in terms of composition.

The lithosphere is mostly made of crust, beginning with oceanic crust.

That crust found beneath our oceans is made of dense basaltic rocks between five and 10 kilometres thick.

While continental crust, the crust which is found in our continents on our land, is less dense made of granitic rocks somewhere between 30 and 50 kilometres thick.

Meanwhile, the asthenosphere is made of the same rock as the mantle, which is high in silicate minerals.

They also different terms of physical state.

The lithosphere is rigid and brittle, and under stress breaks up into those tectonic plates that we may have heard of before.

Meanwhile, the asthenosphere is semi-solid.

It behaves like slow flowing material, little bit like the sort of slow moving treacle and is ductile.

It can bend, flow and stretch rather than break when under stress.

Time now for a learning check, and I'd like you to identify which of the following statements are true.

So please read through A, B, C, and D.

Pause the video here whilst you decide your answers.

Best of luck.

And the two correct answers you were looking for were, A, the asthenosphere is hotter than the lithosphere and D, the lithosphere is rigid and brittle.

Really well done if you managed to get those two correct.

And our next learning check is a true or false.

Plate tectonics theory says that tectonic plates float like rafts in the molten rock of the mantle.

So once again, pause your video here whilst you decide whether this statement is true or false.

Best of luck.

And the correct answer was false.

Now, once again, I'd like you to pause the video whilst you consider as to why the statement is false.

And the correct reasoning for this is that plate tectonics theory says that tectonic plates sit on top of the asthenosphere, which is a ductile part of the mantle.

It flows, but it is semi-solid rather than liquid.

It is true though that in plate tectonic theory plates are moved by movements in the mantle.

Really, really well done if you're able to get that answer.

So our practise task now for the first learning cycle, and I would like to complete this comparison of the lithosphere and the asthenosphere.

So pause your video here whilst you consider the density, composition and physical state of both the lithosphere and the asthenosphere.

Good luck.

So here's some feedback.

In terms of density, then for the lithosphere, it has a much lower density as solid rocks are not compressed as much.

Meanwhile, in terms of composition for the asthenosphere, it is made of the same rock as the mantle and very high in those silicate minerals.

Finally, in terms of physical state, when we're looking at the asthenosphere, it is semi-solid and ductile.

It bends and flows and stretches under stress.

So really well done if you are able to recall that information.

We're now to our second learning cycle, which is all to do with plate movement.

Now, we can measure the movement of tectonic plates and see the impacts of their movement now and in the past.

But why do they move? Well, you might like to pause the video at this point whilst you consider your answer, but for many years the theory was that convection currents moved plates, and this was done in the following series of steps.

They identified that heat rises hot material from the lower mantle rises up towards the lithosphere identified this because as we may know, something which is warm is generally less dense than the material around it.

And therefore, it rises.

When the material cooled, it sank back down.

This rising and falling created a circular convection current, and the currents drag on the base of tectonic plates, which moves them.

Now, convection certainly does occur in the mantle.

The source of all this heat for convection is the radioactive decay in Earth's core.

As isotopes like uranium-238 in the core decay, they release that thermal energy.

There is also heat in the core from when the Earth was first formed.

The heat from the core is transferred into the lower mantle, and that creates a temperature difference between the lower and upper mantle.

And that difference is what powers and drives that convection current.

So a learning check, I would like you to complete the missing text from each box in the flow chart to describe the older theory of plate movement.

You have the four options of text, A, B, C, and D to fit into those four boxes that you can see on the right.

So pause the video here now whilst you consider and then make your answer.

So the answers, it begins with a hot material from the lower mantle rising up towards the lithosphere, that outer layer of the earth structure.

when the material cools, its sinks back down again because now it is slightly more dense than the material around it.

This rising and falling creates circular convection currents, and these currents then drag on the base of tectonic plates, which moves them.

So the correct order was D, B, A, C.

Really, really well done if you manage to get that correct order.

Scientists now though doubt this older theory of plate movement and whether convection currents are indeed powerful enough to account for the plate movement.

Other processes must be at play and perhaps more important.

One idea is the slab pull theory.

It says that at subduction zones where two plates converge, one plate sinks or subdues under another.

Oceanic crust gets denser and thicker as it gets older and this makes it sink down into the mantle of plate margins since it is heavier than the continental crust that it is colliding into.

As the denser plate edge sinks into the mantle, it pulls the rest of the plate along with it causing a split between the plates.

Slab pull is now thought to be the main process causing plate movement.

Slab pull may also help create convection currents in the mantle too.

We also have something called ridge push.

New lithosphere is created by magma rising outta the asthenosphere between plates, creating these mid-ocean ridges.

These occur at constructive plate boundaries where we can see in the diagram two plates pulling apart and this ridge push happening in the middle.

As the magma cools and becomes denser, it slides away from the top of the ridge and becomes rock.

This process gradually pushes the plates apart.

Iceland is one example where we see this ridge push motion happening.

It's located on a mid-ocean ridge between two plates, which are being pushed away from each other.

So a learning check.

Which of these is now thought to be the main cause of plate movement? Is it A, convection currents, B, ridge push or C, slab pull? Pause the video here whilst you can consider and then make your answer.

And the correct answer was slab pull.

Really, really well done if you managed to get that correct.

So onto our practise task now for learning cycle two.

And it says to explain how the core's internal heat source generates convection.

Now, Sofia has given you a bit of a clue here and that she's saying, "I remember that radiation was involved in this process." Now, the second practise question asks you to explain the process of slab pull.

I would refer to the diagram that you can see on the screen as well as Lucas's comments here.

He states, "I remember that slab pull is connected to subduction at plate margins." So what I'd like you to do right now is pause the video whilst you attempt these two questions.

Good luck And now, some feedback.

So you were asked to explain how the core's internal heat source generates convection.

Your answer could have included, radioactive decay of isotopes such as uranium-238 generates heat in the earth's core.

This heat is then transferred to the lower mantle.

This creates a temperature difference between the hotter lower mantle and the cooler upper mantle.

Heat then rises creating a convection current up towards the lithosphere, the outer layer of the earth.

The mantle material cools as it rises and sinks down again creating a circular convection current in the mantle.

The second question asked you to explain the process of slab pull.

And again, your answer could have included the following.

Slab pull happens when the edge or margin of a plate sinks down into the mantle and drags the rest of the plate along with it.

This happens because of density.

As a plate gets older, it gets denser and thicker and this is why the older parts of it sink down.

It used to be thought that convection was the main cause of plate movement.

Now, it is thought that slab pull is the main cause and that it helps also drive convection currents in the mantle.

So really, really well done if you included any of those answers in your an in your own answer.

So onto our final learning cycle right now, and we're gonna be looking for the evidence of plate tectonics theory.

In 1912, a German geologist called Alfred Wegener published a theory that the continents had once been joined together and had since moved apart.

This theory was known as continental drift.

Now, on the screen in front of you, you can see an animation of continental drift showing a super continent known as Pangea, breaking up into the continents we know today.

Wegener had evidence to support his theory, including matching fossils of plants and animals on continents that were now separated by oceans.

The map and diagram in front of you does a great job of explaining this.

Here we can see the continents all joined together in that super continent known as Pangea.

And we can see those different bands of colour, which are showing us fossil types, different fossil types of plants and animals.

And we can see how that despite those continents ending up breaking away, for example, numbers one and two, South America and Africa, we see a common fossil type between those two continents.

So clearly, Wegener believed and thought and had evidence to show that these two continents at one period of time must have been conjoined.

He could not explain how the continents move though.

And as a result, Wegener's theory was not accepted by most geologists.

So a learning check.

Why wasn't Wegener's continental drift theory accepted by most geologists in 1912? Read through the three options and when you're ready, pause the video to consider and make your answer.

And the correct answer was B.

His theory was not accepted because it did not include a convincing explanation of how continents moved away from one another.

Really well done if you were able to select the correct answer there.

Evidence for how the continents moved came during the Second World War, whether US Navy used sonar to map the ocean floor.

And there were some interesting discoveries.

They discovered long chains of underwater mountain ranges in the middle of oceans.

They found very deep trenches at the edges of oceans.

And finally, they found the mountain chains included underwater volcanoes and this was all evidence of sea floor spreading.

On this map of the world, we can see the deepest areas of the ocean are those in the darkest shade of blue, while the shallowest areas are those in the lightest shade of blue.

So right here, we can see a shallow area in the centre of the Atlantic Ocean.

This is the Mid-Atlantic Ocean ridge.

While here, these are the ocean trenches.

The shade is much, much deeper and as previously said, these are found on the edges of the ocean.

In 1962, Harry Hess published his theory of sea floor spreading, and it was based on those World War II sonar discoveries.

He suggested that the new crust was being created at those mid-ocean ridges by magma rising up from the mantle and cooling on the surface.

He said that as this new sea floor was being created, it widened the oceans and pushed the continents apart.

He also said that at the edges of the oceans, the trenches were where sea floor was being destroyed and recycled.

So he was able to identify this perfect balance of where new crust was being created and areas where crust was being destroyed.

So a learning check, true or false.

Sea four spreading provided an explanation for Wegener's theory of continental drift.

Pause the video here whilst you consider and then make your answer.

And the correct answer was true.

And again, I would like you to pause the video whilst you consider why it is true.

And the reason it is true is Wegener had evidence that the continents had once been joined together in that super continent known as Pangaea, but he couldn't explain how continents moved apart.

The sea floor spreading provided this explanation.

New sea floor added up mid-ocean ridges pushed the continents apart.

Really well done if you were able to get those two answers correct.

In 1963, evidence was found that supported Hess's theory of ocean spreading.

Magma contains minerals that line up with the earth's magnetic field.

When the magma cools, the position of these magnetic minerals is preserved in the rock.

Earth's magnetic field reverses direction over time.

So if new ocean floor is being created and spreading away from that mid-ocean ridge, then rocks near the ridge should have a different magnetic record to those further away.

And that is exactly what the geologists found.

They found a pattern of parallel stripes either side of the mid-ocean ridge.

Now, if we just take time to look at the animated diagram in front of you, you can see that happening in motion.

We can see how the plates are being pushed apart and those magnetic parallel stripes either side of the mid-ocean ridge are being pushed apart.

And this provide geologists and it provides us with evidence of the plates moving apart like this.

So which of these statements are accurate, A, B, C or D? I would like you to pause the video here, read the options, and then decide on your answers.

And the correct answers were C and D.

Rock made from magma can preserve a record of the earth's magnetic field.

And D, a symmetrical pattern of magnetic stripes exist on either side of those mid-ocean ridges.

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

So we're on now to our practise tasks.

Now, Alfred Wegener died in 1930, while he was on an expedition in Greenland, aged 50.

At the time, no one really supported his theory of continental drift.

What I'd like you to do is write one or two paragraphs explaining the importance of Wegener for the theory of plate tectonics.

You could include a description of the theory of plate tectonics information about the theory of sea four spreading and the evidence that backed it up.

So pause the video here whilst you write your answer.

So time for some feedback.

Now, what could your answer have included? Well, plate tectonic theory says that tectonic plates sit on top of the asthenosphere and move due to convection currents in the mantle.

Wegener's theory of continental drift was important for the development of this theory 'cause he showed there was evidence that the continents had once been joined together with that super continent known as Pangea, so they must have moved apart.

Wegener though couldn't explain how the continents had drifted apart from each other.

This meant that his theory was not accepted when it was published in 1912, but it became important again in 1962 when Harry Hess provided an explanation based on sonar maps that was done during the second World War.

Hess suggested that the new sea floor was added at mid-ocean ridges and pushed those continents apart.

Now, Wegener's theory was seen as important.

It was the first step in the development of the theory of plate tectonics that still exist today.

So really, really well done if you are able to include any of that answer in your own.

Really well done.

So to summarise, according to plate tectonic theory, the lithosphere is divided into a number of tectonic plates.

These plates sit on top of the semi-solid layer of the upper mantle called the asthenosphere.

There are different theories about what causes tectonic plates to move.

Slab pull though is now thought to be the main process causing plate motion.

Evidence for plate tectonic theory came from studies of the ocean floor.

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

It's been a pleasure teaching you.

So thank you and I will see you again soon.

Goodbye.