Lesson video

Hello, my name is Mr. Conway.

I'm very pleased to have this opportunity to guide you through this geography lesson.

The emphasis is going to be very much on geographic information systems, also known as GIS.

So let's get started.

This lesson is part of the unit called Natural and Tectonic Hazards.

By the end of today's lesson, the intended outcome is that you'll be able to use GIS Elevation Profiles to visualise physical processes at different plate margins.

Some of the learning about GIS may be new to you, but I'm here to help you along the way.

To help us to achieve the outcome, we need to learn or remind ourselves about certain keywords.

The keywords for today's lesson are bathymetry, which is the measurement of elevation for bodies of water.

That's the depth of the oceans, seas, or lakes.

Topography is the shape of the earth's surface, including land areas and the floor of seas and oceans.

And Elevation Profile is a very useful GIS analysis tool, which visualises changes in height above or below sea level along any transect line on the Earth's surface.

There are three learning cycles for this lesson, looking at the three main categories of plate margin.

So we're going to look at the first of these learning cycles now about constructive margins, also known as divergent margins.

Here's a world map showing information that you may well be familiar with.

Tectonic plates with their names and the places where the tectonic plates meet, which are known as plate margins or plate boundaries.

In this case, they're shown as red lines.

The plate margins are not actually all the same.

There are three main categories of plate margin according to the processes that take place.

In this lesson, we're going to find out how we can use GIS to visualise these three different types of plate margins.

To do this, we need to think of the whole of the Earth's surface, which must include the floor of the oceans and seas.

How much do we actually know about that? It may surprise you to know that by 2023, only about 1/4 of the ocean floors have been mapped in any detail.

So an international project, endorsed by the United Nations, called Seabed 2030 is aiming to map 100% of the ocean floor by 2030.

A lot of what we do know so far about the ocean floor is thanks to a geologist and oceanographic cartographer called Marie Tharp, who pioneered the mapping of the ocean floors during the middle of the 20th century.

She was using bathymetry data, and that's measurements of elevation for bodies of water, so the varying depths of the oceans.

Why is bathymetry so important in geography? Well, it means that we can map the ocean floor.

It helps us to understand the topography of the Earth underwater, not just on land.

And as things turned out, it's been absolutely crucial to help us to understand what the tectonic plates are up to.

Also, as we will see in this lesson, it provides us the data we need to create GIS visualisations of the ocean floors.

Marie Tharp's work on ocean floor mapping helped to solve the mystery about what's happening along divergent or constructive margins, where plates are moving apart on the ocean floors, such as at the Mid-Atlantic Ridge.

So here's a simplified theoretical model of what scientists think is probably happening at divergent or constructive margins, showing the interaction of processes such as ridge push and then slab pull along distant convergent margins.

Marie Tharp had to analyse the bathymetry data mainly by hand, but now the data has been digitised and geo-referenced, we're able to use GIS tools, such as Elevation Profiles, to analyse the plate margins and identify key features a lot more easily.

Here's a quick word about the source of the bathymetric data we're going to use.

The Seabed 2030 project is already well underway and releasing some of its data, which is provided in collaboration between the Nippon Foundation, and it's a bit of a mouthful, but the General Bathymetric Chart of the Oceans, fortunately it's known by an acronym called GEBCO, much easier to pronounce.

The project will continue to collect bathymetric data to eventually produce this definitive map of the world ocean floor, and it's going to be freely available to everybody.

The first thing we're going to do is to see how we can use ArcGIS Online Scene Viewer to visualise constructive or divergent plate margins.

And we're going to use that bathroom metric data by GEBCO to do so.

This video guide is going to demonstrate how we can use 3D Scene to visualise different plate margins.

This typical working view of Scene enables us to explore the elevation detail of all the land masses, because the data is built in to do that.

But what we've added to this is a new layer, which we can see in Layer Manager, of bathymetry data, and it's organised by an international collaboration called Seabed 2030, including a group called GEBCO.

And if we float over the layer, we can see it says, it's a bit of a mouthful, really, Global TopoBathy Elevation, which means it's bathymetry data showing the elevation of the sea floor.

And we've chose to add the layer that's got the 3.

5 times vertical exaggeration, because it just makes the profiles in the bathymetry easier for us to see.

However, this does mean that the elevation measures are multiplied by 3.

5, so we need to bear that in mind if we're at the elevation numbers for heights above or below sea level.

So let's switch to presentation mode to have a look at these slides.

So we start with a global view and you can see that the plates are labelled.

There's a layer for plates and a layer for the margins in bluey/purple colour.

And as we turn the world around, you'll see others labelled, and there's one anomaly, which is the 180 degree line.

Don't think that's a plate margin, it isn't.

So we're bringing the world back around to the Atlantic, because that's where we're gonna look at an exemplar of a constructive plate margin/diverging plates.

And we're looking in particular at the line down the middle of the Atlantic, which is the Mid-Atlantic Ridge, a divergent plate margin or a constructive plate margin where plates are moving apart.

So to the west, the North American plate, for example, is moving away from the Eurasian Plate to the east.

And as we zoom in, we can see how the Mid-Atlantic Ridge is labelled along most of its course on the ocean's layer.

The red line you can see is the first of four transects we'll take across different plate margins.

So that will help us to generate an Elevation Profile across the Earth's crust at that place.

And when you zoom down to the Mid-Atlantic Ridge, you see that the sea floor isn't smooth at all.

It's actually a whole lot of, basically, underwater mounted, many of which are volcanoes or ex-volcanoes.

And the bathymetry layer shows that particularly well.

So let's generate a transect across this ridge.

To do that, we need to dip out of presentation mode and go back to the original format, and we'll just close the Slide Manager.

And then we go to the Scene tools, which is this spanner symbol over here on the right hand side.

We click that once and then we look for the Elevation Profile tool, which is the fourth one along, it looks like this.

If we click that, it opens a panel, and it's asking us to generate a Elevation Profile.

If you just switch off these two, line and layers, and then we're gonna select the line, we're gonna select that transect.

So we say, we're gonna select the line, it knows we wanna select the line, you'll see a cross appears here, that's a signal that it knows, and it's as simple as this, you just literally click on the line and an Elevation Profile is generated across that transect.

So we need to think about what's happening along this transect.

And we can do so by moving the cursor across the Elevation Profile.

As soon as I go onto it, you can see it's showing me where I'm along the transect.

And notice as well, that the height is actually negative territory.

We're dealing with the depths of the ocean here.

And what we see if we move across the transit, you watch that little orange dot, the sea floor rises, so the sea is a little bit shallower or quite a lot shallower as you move toward the Mid-Atlantic Ridge.

And that's because there are these mountains around the Mid-Atlantic Ridge, many of which are volcanoes or ex-volcanoes.

And what you'll notice right in the middle of the ridge, if I just move the cursor to the end, there's a kind of gash in the middle, and that's where the two plates are moving apart.

And there's a rift right down the middle of the Atlantic Ocean all the way down the -Mid-Atlantic Ridge.

What this means, of course, is that what we see to the west of the ridge is the North American Plate actually being formed and moving off towards the west.

And what we see on the eastern side of the ridge is the Eurasian Plate, with material being added to it from the mantle in the form of magma that reaches the surface of the crust, which, of course, is the sea floor, and that becomes new crust and moves off with the Eurasian Plate.

Soon you'll have the opportunity to use the Elevation Profile tool to analyse the bathymetry data yourself.

But let's just check up on some of the points from the video demonstration.

Here's our first check.

Look at the icons labelled A, B, C, and D, which icon is used to access the Elevation Profile tool in Scene Viewer? If you wish to pause the video, do so here, and restart it when you've selected your answer.

Well done if you selected D.

The other options are analysis tools.

But the only icon showing mountains with a vertical double arrow is the one for the Elevation Profile tool.

Now for our second check, which is a true or false.

Is it true or false to say that there are no tectonic plate margins on the ocean floor? Pause the video here if you wish and restart it when you've decided if that statement is true or false.

Okay, this is actually false.

Why would that be the case? You may wish to pause the video again and restart it when you've decided why.

The reason for the statement being false is that many tectonic plate margins are on the ocean floor, especially constructive margins, also known as divergent boundaries.

That's why it's so important that research is carried out to find out what's going on on the ocean floor, including the mapping of it using bathymetry.

This is, of course, in the tradition established by the great Marie Tharp.

Now for the task, which will help you to use the Elevation Profile tool you've seen in action for constructive or divergent margins.

For these tasks, you'll need to open the link provided, which takes you to the ArcGIS Online Scene Viewer 'Tectonic plates 3D Scene'.

In task one, you're going to create an Elevation Profile for a plate margin.

In this case, the constructive or divergent plate margin.

As a follow-up, task two is going to ask you to analyse your Elevation Profile by identifying key features which have been visualised.

To do this, you need to match the numbers on the Elevation Profile with the feature descriptions in the table.

So pause the video now to take some time to undertake task one and two.

And when you're ready, press play to get some feedback.

Hopefully you were able to undertake those tasks effectively.

For task one, your Elevation Profile for a constructive or divergent plate margin should look something a bit like this.

For task two, here's a check to see if you then analyse the features of the Elevation Profile by matching the numbers correctly.

So the Mid-Atlantic Ridge is number three.

The North American Plate moves west is number one.

Then magma from mantle creates new crust is number four.

And the Eurasian Plate moves east is number two.

So the order is three, one, four, two, going down the table.

If your answers were very different or you recognise some errors, take another look back at the video demonstration.

Our second learning cycle we'll explore the use of GIS to visualise destructive margins, also known as convergent margins.

Convergent margins are where two tectonic plates move together.

We might say they're crashing into each other.

And there are two ways this tends to happen, as shown by a couple of simplified theoretical models.

And these are very simple.

In a destructive convergent margin, this is known also as a subduction zone, what typically happens is that an oceanic plate is converging with a continental plate.

The oceanic plates are more dense than continental plates, so the more dense oceanic plate subduct beneath the less dense continental plate.

We can see the oceanic plate on the left of this diagram and the continental plate on the right.

The subducting oceanic plate dives downward into the mantle.

It used to be thought that the oceanic plate would be completely destroyed, hence the name destructive plate margin.

But it's now thought that the plate may dive down, remaining more or less intact, creating a significant slab pull effect.

Along such margins, we often find very deep ocean trenches.

So bathymetry data provides us with a very good opportunity to visualise these.

With a collision convergent margin, two plates doing the converging are of very similar densities.

For example, two continental plates.

So what happens here is a crumpling effect where they collide and the crust is forced together, and anything caught in between moves upwards and downwards by this pressure.

So the crust becomes very much thicker here and can create fold mountains, such as the Himalaya.

We can use GIS Elevation Profiles to go somewhere to assessing the extent to which these models correspond to what we see in reality.

So we're now gonna see a second video demonstration clip using ArcGIS Online Scene Viewer to visualise converging plate boundaries, in particular the so-called destructive plate margins and collision zones.

Next, we're going to look at a very different situation at a plate margin.

And this is where two plates are converging.

So the area we're looking at is on the western side of South America.

And the two plates involved are the Nazca Plate and the South American Plate.

They're actually moving towards each other.

So we can draw a line, as we did before, at right angles or perpendicular to that fault line and see what's happening there.

If we zoom in a bit, we see this, it's a very different situation altogether.

On the left of the screen, to the west, we have the Nazca Plate, which finishes roundabout here where you can see this very, very big valley, and that's actually an ocean trench.

On the right hand side of the screen, to the east, we have the South American Plate.

One of the reasons that this is such a different situation is because the Nazca Plate to the west is an oceanic plate and it's more dense than the continental plate of South America that it's meeting over here.

So where you have two plates like this and one is much more dense than the other, the more dense plate tends to sink deeper into the mantle underneath.

And when they meet, this plate, the Nazca Plate, is actually diving down underneath the South American Plate.

And that's one of the reasons for this trench.

So at this point, this plate is being subducted, it's going down underneath the South American Plate.

Some of it will be recycled into the mantle, some of it will be melted down, some of it will probably reemerge as volcanoes along here in the Andes Mountains.

So what's interesting for us is how can we visualise what's going on at this plate margin along the transect line? So we're gonna use the transect line to draw an Elevation Profile of the crust.

So here we have the transect across that boundary, that fault line in between the Nazca Plate and the South American Plate.

What we're gonna do is find an Elevation Profile tool.

So we go to the Scene tools, we click Elevation Profile, we make sure that ground is ticked and not the other two.

We then say that we want to select the line, and we're going to click on the transect line across this ocean trench and see what happens.

So what can help us here is if we move the cursor along the Elevation profile and watch where that is on the transect line.

So we moving from the west, and we can observe that the sea floor above the Nazca Plate is fairly flat.

There are a few bumps along the way, they're probably seamounts.

It's also extremely deep.

And then what happens is we reach the ocean trench and the depth of the ocean increases dramatically very suddenly.

And that's happening because the Nazca Plate is diving down underneath the South American Plate.

The Nazca Plate is more dense, so it goes underneath the South American Plate, and subduction takes place, which means the plate breaks up and is partially destroyed.

And that's why we sometimes call this a destructive margin.

And as the leading edge of the Nazca Plate goes down, we say that's a process called slab pull.

It's trying to pull the rest of its plate with it, and it's also trying to pull the South American Plate down as well.

But that can't really happen, because the South American Plate isn't sufficiently dense.

So it tries to pull it down, and then what happens is part of the South American Plate will bounce back up, and that's when you get very severe earthquakes and tsunamis happening along this coastline.

And in fact, the biggest earthquake ever recorded happened along this coastline.

So these kind of margins tend to experience the most extreme tectonic events that we see on our planet.

So if we move east away from the trench, we're then moving onto the South American Plate, and it's less dense and you can see a lot of crumpling going on 'cause there are fold mountains there due to the fact that the rocks there are squashed and crumpled in an upwards direction, creating, of course, the second highest mountains in the world, the Andes.

And the highest mountain in that range is Aconcagua, which is actually an extinct volcano.

So there are quite a few volcanoes in that range as well.

The next type of margin we're looking at is gonna be another type of convergent margin, and this time we're heading to Asia, and we're gonna be looking at what's happening where the Indian Plate meets the Eurasian Plate.

This is a convergent plate margin where two continental plates meet, with the Indian Plate moving northeast and colliding with the Eurasian Plate, which is moving in a broadly easterly direction.

That's why such an area is known as a collision zone.

Let's take a closer look at that and use much the same technique as we have done before where we're drawing a transect line, the line in red, and it's going perpendicular to the plate margin.

And the plate margin in this case is between the Indian Plate and the Eurasian Plate.

So just to the north of India, running right through Nepal.

What we have there where these two plates are meeting, these are two continental plates, and because they're of a similar density, what happens when they meet at the collision zone is that the crust crumples so significantly that we have the highest mountains in the world, the Himalaya and the Tibetan Plateau, because the crust is so thick due to the crumbling, there are no volcanoes here at all, but there are some very severe earthquakes.

So let's use GIS to visualise that margin.

As before, we go to the Scene tools, we find the Elevation Profile, and then we make sure that the ground is ticked, layers and line are not, click select line.

And when the cross appears, we can click the line to produce the Elevation Profile.

And the Elevation Profile this time is quite spectacular.

We're looking from the east, and that means south is to the left of this image and north is to the right.

And if we follow the transect in the way we did before, what we can see is that India itself is largely quite flat, and you will see it rising up just a little bit as you go off the continental shelf.

If we follow the cursor, moving through the shallow waters of the continental shelf, you can see the numbers going from negative through to positive as you reach the shore, and moving on through India in a northly direction.

And eventually we reach northern India where we have the Ganges Plain.

You can see the land dips down just a little bit there.

And then we have this amazing wall of mountains which is created by the collision zone.

You can see just after the collision zone is passed, you get the foothills of the Himalayas going up to the highest mountains in the world, the Himalaya.

And beyond that, you've got the Tibetan Plateau.

So that's another very significant area of high ground in the world.

Soon you'll have the opportunity to use the Elevation Profile tool to use bathymetry data to analyse convergent margins yourself.

So let's just check up on a couple of points from the demonstration first.

Which two of the following would typically be found at a destructive margin? You may wish to pause the video here and restart it when you selected your two answers.

Okay, the correct choices are C, a continental plate and an oceanic plate, and D, and ocean trench.

Here's our second check, which two of the following would typically be found at a collision margin? Again, pause the video here and restart it when you've selected your two answers.

The correct choices are B, two continental plates, and C, fold mountains.

Well done if you've got those.

Now for the task, which will help you to use the Elevation Profile tool that you've seen in action for convergent or destructive plate margins.

As for the task in learning cycle one, you'll need to access the same ArcGIS Online Scene Viewer 'Tectonic plates 3D Scene'.

The link is here in case you don't have it open already.

Because the task for learning cycles two follow very similar pattern, you should be able to tackle these a bit more quickly.

So there are actually four tasks to follow just here.

In task one, you're going to create an Elevation Profile for a plate margin, in this case a convergent or destructive plate margin.

As before, for task two, you're challenged to analyse your Elevation Profile by identifying key features which have been visualised.

In a similar way to the earlier task, you need to match the numbers on the Elevation Profile with the feature descriptions in the table.

In task three, you're going to create an Elevation Profile for a collision plate margin, also convergent, but with two continental plates involved.

As previously, the follow-up task four, asks you to analyse your Elevation Profile by identifying key features which have been visualised, putting them correctly matched in the table with the numbers accordingly.

So pause the video now to take some time to undertake task one to four.

And when you're ready, press play to obtain some feedback.

Hopefully you were able to undertake the tasks effectively.

For task one, your Elevation Profile for a convergent destructive plate margin should look something like this.

For task two, here's a check to see if you then analysed the features and matched them correctly.

So, the Nazca Plate moves east and subducts goes with number one.

Deep ocean trench is number three.

Then Andes mountains, including volcanoes, is number four.

And the South American Plate moves west is number two.

For task three, your Elevation Profile for a convergent collision plate margin should look rather this.

For task four, the matching exercise should looks like this.

So the Indian Plate moves northeast is number one.

Collision plate margin is number three.

Then Himalaya and Tibetan Plateau is number two.

And the Eurasian Plate is number four.

If your answers were very different or you recognise any errors, take a look back at the video presentation to see if you can follow that through again.

Our third learning cycle, we'll look at the GIS visualisation of conservative plate margins.

These are sometimes known as transform margins.

Arguably the most famous plate margin anywhere in the world is the San Andreas Fault in California, USA.

But it's not like the other plate margins we've been looking at.

The San Andreas Fault is what's known as a conservative or transform plate margin.

Here's a theoretical model of a conservative or transform plate margin.

What happens is the plates don't move towards each other or away from each other.

Instead, the plates move sideways past each other, rather like two big trucks on a road.

There are two ways this can happen.

They can move sideways past each other in opposite directions or in the same direction, but at different speeds.

This doesn't happen smoothly, because the plates are gigantic pieces of the Earth's crust, so there is a lot of friction between them, and they can get stuck in places, sometimes for many years, and then suddenly move forward with a great jolt, creating significant earthquakes.

Just as with the other plate margins, we can use a GIS Elevation Profile tool to assess the extent to which a particular plate margin corresponds to the theoretical model of a conservative plate margin.

The following video demonstration is gonna demonstrate how the Elevation GIS tool can be used with Scene Viewer to visualise conservative or transform plate margins.

We're now gonna look at a transect for a fourth type of plate, which is where plates are not moving apart from each other, they're not moving towards each other, but they're moving sideways past each other.

So one place where that's happening is the West Coast of North America, and it's the very, very famous San Andreas Fault.

We've placed the transect at right angles, perpendicular to the margin.

There's another fault line over there, but we're not looking at that one today, we're looking at the San Andreas Fault, which follows this line of mountains just to the west of the city of San Francisco.

If we just zoom in, we can see the city of San Francisco and its surrounding suburbs all around San Francisco Bay.

And the San Andreas Fault actually runs right down this valley just here.

But it's not the same as the other faults.

So we can have a look at that by checking out its cross section, you see the red line across that, as usual, perpendicular to the fault line, at 90 degrees to it.

So we go to Scene tools and Elevation Profile.

We check that we've got ground ticked, but not layers and line.

We select the line, wait for the cross to appear, click on the line, and we get the Elevation Profile across the Andreas Fault.

Now, as we move from west to east, we do see some interesting things here.

First of all, moving actually from the sea, because the numbers are negative until we reach the coast.

You can see reach the coast roundabout there.

And then we see several sort of peaks with a big valley in between.

So what's happening here is that this side is moving faster than this side.

So they're moving sideways past each other in the same direction, but at different speeds, and that's caused this rift between the two.

So this type of fault is called a conservative plate margin or a transform margin, where the plates are effectively sliding past each other.

No crust is destroyed and no crust is created, hence the use of the term conservative with a little C to describe this kind of margin.

And it can produce some quite severe earthquakes.

On the other side, you can see you've got the North American Plate.

So this is the Pacific Plate over here, North American Plate over here.

And that area there is San Francisco Bay, the low lying area, you can see where there's a lot of water just here.

So this whole area is very vulnerable to a future earthquake, and in the past there's been an earthquake which actually destroyed the city of San Francisco.

Before you use the Elevation Profile tool to analyse the bathymetry data for a conservative or transform plate margin yourself, let's check on a couple of points in the video demonstration.

What can be moved on an Elevation Profile to show the corresponding location on the 3D map? Select one answer from the three options, A, B, or C? You may wish to pause the video here and restart it when you've selected your answer.

Hopefully you chose B, the cursor.

That's the item on the screen that moves around as we use a mouse or a mouse pad.

Now for a second check.

Which two of the following types of plate movement occur at a conservative margin? Again, pause the video if you wish, and restart it when you selected your answer.

Well done if you chose A and C.

Yes, at such margins, the plates may be moving sideways past each other in opposite directions or the plates may be moving the same direction, but at different speeds.

And this can lead to similar tectonic hazards, mainly severe earthquakes.

Now, for the task which will help you to use the Elevation Profile tool that you've seen in action for conservative or transform margins.

As for the previous task in learning cycles one and two, you'll need to access the very same ArcGIS Online Scene Viewer, called 'Tectonic plates 3D Scene'.

So the link is here in case it's needed.

In task one, you're going to create an Elevation Profile for a plate margin, in this case a conservative transform plate margin.

Then task two challenges you to analyse that Elevation Profile by identifying key features which have been visualised, putting the correct match numbers in the table.

Now, this one is a bit more tricky than the previous one, so you'll need to make sure you look at the big picture geography carefully.

Pause the video now to take some time to undertake task one and two, and when you're ready, press play to obtain some feedback.

Hopefully you're able to undertake the tasks effectively.

For task one, your Elevation Profile for a conservative margin should look something like this.

For task two, here's a check to see if you then analysed the features correctly.

So, in this case, the Pacific Plate moving northeast fast is number one.

The North American Plate moving northeast slowly will be number two.

Then San Francisco and San Francisco Bay is number four.

And the San Andreas Fault rift valley is number three.

So if you made any errors, go back and check the video demonstration to see if you can follow that through.

Very well done.

What we've been doing in this lesson is using some GIS skills and then applying them in quite different situations.

It's this kind of deliberate practise that can really help us to make our GIS capabilities more fluent.

Let's summarise that learning with these key points.

3D GIS provides us with powerful ways to visualise what's happening at the different plate margins.

GIS transects, that is, the lines we saw drawn on the web maps, can then be used to create Elevation Profiles so we can visualise changes in the relative height of the land and ocean floor.

GIS Elevation Profiles can, if you like, help us to see geography happening.

Because what they're doing is visualising physical processes at different plate margins.

That's divergence, convergence, subduction, and mountain building.

All of these are using that very useful bathymetry data.

So, excellent work.

And a really good way to follow this up is to use the same methods to investigate other plate margins in the world, or perhaps the same plate margins but at slightly different locations.

Hopefully you've found this learning interesting and useful, and I very much look forward to learning with you again in the future.

Bye for now.