Lesson video

Hello, my name's Mrs. Niven.

And today, we're going to be talking about tectonic plates as part of our unit on Earth's resources.

Now, what we talk about today, you may have some experience of from your previous learning, but what we learned from today's lesson will help us to better understand and answer that big question of how can we explain changes that we see in the air, the oceans, and the land? By the end of today's lesson then, you should feel more comfortable and confident describing evidence of how we know tectonic plates move across the surface of Earth.

Throughout the lesson, I'll be referring to some key words, and these include continental drift, continental crust, oceanic crust, tectonic plate, and plate tectonics.

Now, you can see that these are very similar to each other, but their definitions have subtle differences.

Now, these definitions are given in sentence form on the next slide, and you may wish to pause the video here so you can read through them.

I also recommend you jot down a note of what each represents so that you can refer back to these definitions later on in the lesson or later on in your learning.

Now, today's lesson is broken into three parts.

We'll start by looking at what we mean by theories and evidence before defining what a tectonic plate is, and then looking at how these tectonic plates move.

So let's get started by looking at theories and evidence that's related to our lesson about tectonic plates.

What we need to remember is that at their heart, scientists are observers, and the observations that scientists make can lead to the development of theories.

Now, a theory is simply a suggested explanation for an observation that's been made.

So let's look at an example.

If I was to put some metal into some acid, I might observe bubbles forming, and a theory might be that the acid and the metal have reacted together.

Now, what we need to remember is that scientific understanding is dynamic.

It's ever-changing.

And what that means is that how we explain these observations might alter or change with new observations that are made or new evidence that's collected.

Now an early theory about the features of Earth's surface, so we're talking about things that could be easily seen like mountain ranges, was that they had always existed, and that these features on Earth's surface were formed as the Earth cooled and shrank, which seems a bit odd to us at this day and age, but this theory matched similar observations that had been made with ageing fruit.

So a young apple might have very smooth skin, but as it ages and gets old over time, that older apple becomes a little bit more wrinkled.

Now, this theory didn't explain though other observations on Earth's surface like earthquakes occurring or volcanoes erupting.

Now, over time, other observations were made.

For instance, in the late 1500s, a Dutch cartographer, so that's a map drawer, observed that some countries' coastlines looked like they fit together almost like pieces of a jigsaw puzzle.

Now in 1912 then, meteorologist Alfred Wegener proposed a theory of continental drift.

Now, this theory suggested that millions of years ago, all the continents on Earth were once joined together in a super continent called Pangea, and that Pangea then broke apart and the pieces drifted like icebergs on an ocean across Earth's surface.

Wegener supported his theory of continental drift with several pieces of evidence.

One piece of evidence was back to what this Dutch cartographer had noticed that the coastlines of South America and Africa fit together like pieces of a jigsaw puzzle.

He also noticed though is that when these pieces were put together, that similar rock formations were found across several of those matched up continents.

He also found that the location of some plant fossils also matched across these several continents that were matched up, and the location of several animal fossils also matched up across these matched up continents.

Let's stop here for a quick check.

Which of the following were supporting evidence for the theory of continental drift? Well done if you said all of them.

Every single one of these was used as supporting evidence for that theory of continental drift.

Now, initially the theory of continental drift was not widely accepted by other scientists, and there's a variety of reasons why, but one of the main ones is that despite all that supporting evidence, Wegener's theory did not include an explanation for how those solid pieces of Earth were moving across its surface.

Now from the 1950s, there were some amazing advances occurring in marine geology.

Now, marine geology includes the study of the history and the structure of the ocean floor.

Now, the observations that were made at this time helped provide the evidence that was needed to explain the forces that might be moving Earth's continents, so that missing explanation for continental drift.

The evidence was now being collected.

That by 1965, continental drift then was accepted as a possible theory.

Let's stop for another quick check.

True or false, other scientists readily accepted the theory of continental drift.

Well done if you said false, but which of these statements best justifies your answer? Well done if you said B, continental drift did not explain how those continents were moving across Earth's surface.

So very well done if you managed to get those correct.

You guys are off to a flying start today, keep it up.

Let's move on now then to the first task of today's lesson.

What I'd like you to do is to use the words from the box to complete the summary about the changing theories concerning Earth's surface.

So you may wish to pause the video here and then come back when you're ready to check your answers.

Okay, let's see how you got on.

Now, as I go through the answers, I highly recommend that you tick any answers that you got correct, fix any that you got wrong, and definitely, fill in the blanks for any that you may have left because this is an excellent summary about the changing theories on Earth's surface.

So let's read it out, "Early theories suggested mountains formed when Earth cooled and shrank.

However, these could not explain why earthquakes or volcanoes occurred.

Alfred Wegener suggested Earth's continents were once all connected but broke apart and drifted across Earth's surface.

He thought this because continental coastlines appeared to fit together like pieces of a jigsaw puzzle.

When joined up, the locations of rock formations and plant and animal fossils matched across several continents.

However, continental drift was not accepted as a theory because it did not include an explanation for how the pieces moved." A great start to today's lesson, guys.

Very, very well done.

Now that we're feeling more comfortable discussing the theories and evidence about ideas regarding Earth's structure, let's move on to discuss tectonic plates.

Anyone who's watched the news understands that earthquakes and volcanic eruptions are very destructive, but these have been happening throughout history, and scientists observing that destructive power theorised that these events occurred because there was a large release of energy at the weakest points in Earth's surface, in its crust.

So let's take a closer look at Earth's surface.

Now, you may recall that Earth is a rocky spherical planet and its structure is divided into different layers including the crust, mantle, outer and inner core.

The crust is Earth's thinnest and most brittle layer.

It also varies in thickness.

Now, there are two types of crust.

There's continental crust, which is found under the continent, and this is where the crust is thickest.

There's also oceanic crust, and this is found under the oceans.

The crust here is where it's thinnest.

A quick check then.

True or false, oceanic crust is the thinnest.

Well done if you said true, but which of these statements best justifies that answer? Well done if you said B, oceanic crust is found under oceans.

Very well done.

So we know that Earth's crust will vary in thickness, but it also is not one continuous layer.

Its rocky surface is broken into large pieces, which is known as a tectonic plate, and we can see one outlined here.

Here's another tectonic plate and another one.

Now, each tectonic plate is about 125 kilometres thick, and it's composed of a piece of Earth's crust and the uppermost mantle that lies directly beneath that pot of crust.

So if we were to zoom in on a piece of tectonic plate, its outline or cross section might look a little bit like this.

Now, there are approximately 15 major tectonic plates, but each of these is composed of both oceanic and continental crust.

Remember, the oceanic crust is the crust that lies below the oceans, and the continental crust is that crust that lies below the continents.

Now, to distinguish between each of these plates, 'cause there are quite a lot of them, each tectonic plate is named broadly for the geographic region in which it's found.

Let's stop for another quick check.

What are tectonic plates made of? Well done if you said C, all of the crust and some of the upper mantle.

Very well done if you managed to get that correct.

Great job, guys.

What's really interesting is if you were to map the location of earthquakes and volcanic eruptions, they would provide a visible map of the edges of tectonic plates on Earth's surface.

Let's move on now to the next task in today's lesson.

For this first part, I'd like you to use this map that is showing Earth's tectonic plate boundaries marked in red.

What I'd like you to do then is to decide which letters are resting on a tectonic plate's continental crust, and which letters are showing the tectonic plates oceanic crust.

So you may wish to discuss this ideas with the people around you but pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

So the letters representing tectonic plates continental crust were B, C, F, and H because the letters were resting on continental land masses.

And for B then, showing the oceanic crust would've been A D, E, and G because those letters are resting on oceans.

So very well done if you managed to get those correct.

Great job, guys.

For this next part, I'd like you to consider this map and decide which country listed is least likely to experience an earthquake or volcano.

And I'd like you to explain your answer, and for this, you need to then include a because clause.

So again, you may wish to pause the video here so you can discuss your ideas with your neighbours and come back when you're ready to check your work.

Okay, let's see how you got on.

Well done if you chose Poland as being the country least likely to experience an earthquake or a volcano, and the reason why is because it's furthest from a tectonic plate boundary, which is where most earthquakes and volcanic eruptions actually occur.

So very well done if you managed to choose the correct country, and incredibly well done, if you were able to correctly explain why.

Great job.

So now that we know more about what a tectonic plate actually is, let's look at some of the theories and evidence of how these tectonic plates might move.

So in the late '60s, early '70s, a theory called plate tectonics was developed that extended that theory of continental drift, and it extended that theory because now it described how those continents, those tectonic plates, move and interact with each other on Earth's surface.

One thing we need to keep in the back of our minds is that the mantle that's located directly below a tectonic plate is under a very high pressure and as a consequence is very, very hot.

Now, under these conditions, the mantle's dense rock can behave like a fluid over time, and it's these fluid properties in this layer, the mantle, that can help move those tectonic plates that are resting on it.

Let's stop for a quick check.

What is rock like at the bottom of a tectonic plate? Well done if you said A and D.

The rock here is dense, but it's also under a huge amount of pressure and very hot.

It's not fluid.

It only appears to behave like a fluid, and that's over time, but very well done if you manage to get A and D.

Great job, guys.

Another thing we need to be aware of in order to help us explain how these tectonic plates move is that radioactive processes in Earth's inner core generates energy, and this energy then can transfer through Earth's layers until it reaches that mantle just below those tectonic plates.

Now, three theories have been suggested to explain the forces that drive plate tectonics.

This theory explaining how tectonic plates interact and move.

Now, these theories include slab pull, ridge push, and convection currents.

So let's take a moment and look at each of these three theories.

Now, this first theory of slab pull concerns tectonic plates that are moving very slowly towards each other, and they're colliding with each other.

Now, if we take a closer look at these plates, we can see that there's a warmer, less dense plate part of it and a colder, more dense plate.

What happens here is that as that colder, more dense plate sinks into the mantle, it's pulling the rest of that warmer plate, the slab part of it, pulls it along with it as it sinks into the mantle.

So it's pulling along that slab and then as it sinks, it's just pulling along the bits behind it.

Now, the second theory of ridge push concerns oceanic ridges.

So these are places where new tectonic plates are being formed.

Now, here, we have those new plates that are warmer, and because they're less dense, they sit relatively higher than the older plates further away.

Now gravity then at these new plates, because they're higher, are going to push those older plates further away from the ridge.

So it takes place at a ridge, and it's pushing those older ridges.

Those older bits of tectonic plate further away purely due to gravity.

Now, this idea of convection currents relies on the understanding of energy from Earth's core having been transferred through the different layers of Earth, and eventually, we get to where a point where there's warmer mantle that is less dense and rises, and there are areas then in this mantle, where it is cooler near the surface, and it's going to be more dense and starts to fall or sink, okay? Now, this happens over a very, very long time period, but essentially, these convection currents then are driving the mantle, and it's kind of carrying that tectonic plate, resting on it a bit like a conveyor belt that is moving an incredibly slow speed.

Another quick check then, which of the following is not a theory to help explain the forces that drive plate tectonics? Well done if you said B, density differences is not the name of a theory that is used to explain these forces.

Instead, we discuss convection currents, ridge push, and slab pulse.

Well done if you manage to get that correct.

Now, I mentioned earlier that these tectonic plates are moving very, very slowly, and it's all three of these different theories working in combination to move these tectonic plates, but to an average of only about one and a half centimetres each year.

To put that into context, that's about how much your fingernails may grow each year.

That's how far these tectonic plates are moving on average.

However, there is some evidence to suggest that the theory of slab pull is the largest driving force because measurements have been made that these plates that are experiencing slab pull are actually moving the fastest out of all of them.

Now, these measurements and observations are constantly being made to see if the theory of plate tectonics remains possible or to see if it needs to be adjusted or altered in some way.

Remember, this theory only came about and was accepted in the mid to late '60s.

So in the terms of our scientific understanding, it's a fairly recent theory, and we're still gathering that evidence to support it.

Let's stop here for a quick check.

About how quickly on average are tectonic plates moving? Well done if you said one and a half centimetres each year.

Incredibly well done, guys.

Great job.

Now, overall plate tectonics helps us to explain observations concerning Earth's surface.

For instance, the Himalayan Mountains lie between India and Asia, and measurements made there suggest that these mountains are still growing in height.

And an explanation for this observation could be that the two tectonic plates are continuing to collide and pushing crust upwards as a result of that collision.

Plate tectonics also goes further than continental drift because it can help to explain what earthquakes and volcanoes occur because tectonic plates sometimes stick as they're moving slowly and forces can build up along these sticking points.

Now, eventually, the force is so large that the tectonic plate breaks at a weak point, usually deep underground.

Now, this can cause the surrounding rock to shake quite violently, and the shaking then is felt on Earth's surface as an earthquake.

Let's stop for another quick check.

Fossils of sea creatures have actually been found on Mount Everest, but why do you think that's possible? Now, you may wish to pause the video here so you can discuss your ideas with the people nearest you and then come back to check your answer.

Well done if you said A, moving sheets of rock push up into the mountains and what was at the bottom might then end up at the top of our mountains.

So very well done if you got that correct.

Great job, guys.

Let's move on then to the last task in today's lesson.

For this first part, I'd like you to match the key terms to the best description.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

So a tectonic plate is about 125 kilometres thick, and it's composed of both crust and the upper mantle.

Ridge push is as a result of gravity causing new plate to push the older plate further away.

Slab pull is when colder, more dense plate sinks and it drags along that warmer, less dense plate with it.

A convection current is a force that carries tectonic plates along like they're on a conveyor belt, which means that plate tectonics then is the theory explaining how tectonic plates move and how they interact with each other.

So very well done if you manage to get those correct, guys.

Great job.

For this next part, I would like you to consider models.

Some students have decided to use a sponge cake to model plate tectonics, and in order to demonstrate the movement of tectonic plates, they push the top layer of their sponge cake over the bottom layer.

Considering this model, I'd like you to suggest three ways in which this is a good model of tectonic plates, and three reasons why this may not be a very accurate representation of how tectonic plates move.

So pause the video, discuss your ideas with the people nearest to you and come back when you're ready to check your answers.

Okay, let's see how you got on.

Now, there were a variety of things that you could have said.

So your answers may have included one or a few of the following.

The icing is like the crust and the sponge then can represent the mantle.

The crust, and some of the mantle, so some of the sponge, is moving when the top layer is pushed.

The icing and the top layer are like then a tectonic plate.

The chocolate buttons might represent mountains showing the uneven nature of Earth's crust.

The cream between the sponge layers is a solid that looks like it can flow, and as the top layer is pushed over the lower layer, the cream flows to allow that movement.

So any three of these is a reason why this model is a good representation of how a tectonic plate moves.

So well done if you got even a few of those, even if you didn't get all three.

Incredibly well done.

I also asked you to consider ways in which this may be an inaccurate representation of a tectonic plate movement.

So your answers may have included any of the following.

For instance, the icing is too thin to represent the crust in this particular model.

And the cream or that fluid like area that we're using to represent the mantle is not made of the same material as what represents the mantle, which is the sponge.

In Earth's layers, the mantle is all one material, and the fluid nature of it is made of the same material of that rock.

The other thing you could have said is that the cake is the same temperature throughout, but temperature actually increases towards Earth's core, and that's something that's not very accurately done with this particular model, and that the main driving force in this model is something like a push, like ridge push.

However, a lot of the evidence that we have suggests that slab pull is the major force of plate tectonics.

So the way that this model is used is not representing the main driving force of how tectonic plates move that we know of from the measurements and observations that have been made.

So this was an incredibly tricky task to evaluate models to see what makes it a very good model and what makes it an inaccurate model is a tricky, tricky thing to do.

So please be kind to yourself.

If you got even just one or two ideas, then you are on the right track.

And incredibly well done, if you got three ways in which it's good and an inaccurate representation.

Fantastic work, guys.

I am so impressed.

For the last part of this task that I want you to think about earthquakes.

We already know that earthquakes shake the ground and they can cause a lot of damage, but what I'd like you to do is to put these statements into the correct order to describe how an earthquake occurs.

Now, to help you get started, I have provided you with that first statement.

So pause the video, discuss your ideas with those around you, and then come back when you're ready to check your answers.

Okay, let's see how you got on.

So I already told you that we start with huge tectonic plates move across Earth's surface.

The second statement then should be that they move a few centimetres per year, and that the edges of tectonic plates do not slide smoothly against each other, and they can stick.

Forces then build up along the edges of tectonic plates.

And when the force becomes too large, the tectonic plate breaks at a weak point deep underground.

The surrounding rock is made to shake violently, and the shaking then spreads out from where that rock broke.

And finally, on Earth's surface, the shaking is felt as an earthquake.

So very well done if you manage to put these statements in the correct order.

You've got a very firm grasp of how tectonic plates move, and how they can affect change on Earth's surface.

Fantastic work, guys.

A great job.

Wow, we have gone through a lot in today's lesson.

So let's take a moment to summarise what we've learned.

Well, we've learned that the occurrences of earthquakes and volcanic eruptions suggest that Earth layers are not static.

They actually interact with each other and are moving.

We've also learned that Earth's crust is broken into pieces known as tectonic plates, and these plates move very, very slowly over thousands to millions of years.

And evidence such as rock formations and fossils supports the idea that the continents were once all joined up.

Plate tectonics then is a scientific theory that explains how these tectonic plates move and interact with each other, and that this theory also helps scientists better understand and predict changes to Earth's surface.

I hope you had a good time learning with me today.

I certainly had a good time learning with you, and I hope to see you again soon.

Bye for now.