Hello, my name is Mr. March and I'm here today to teach you all about the processes which are happening at destructive or convergent plate margins.

So grab everything that you need for today's lesson and let's get going.

Now, by the end of today's lesson, you'll be able to explain the processes at destructive or convergent plate margins and the causes of earthquakes and volcanoes found there.

There are three key terms for today's lesson, and those are subduction, tsunami and magnitude.

Subduction refers to the process at a destructive plate margin where a denser oceanic plate sinks beneath a less dense plate into the mantle.

A tsunami refers to a series of large ocean waves caused by underwater earthquakes, volcanic eruptions or landslides.

And finally, magnitude refers to a measure of the energy released by an earthquake.

A higher magnitude earthquake therefore refers to a stronger earthquake.

Now, a destructive plate margin is where one plate is sinking below another plate and being destroyed.

As this diagram helpfully illustrates, the illustration shows that an oceanic plate, which is a tectonic plate found beneath an ocean, is being subducted or sinking below that of a continental plate, a continental plate being that on land, there are also destructive margins between two oceanic plates where one must be of a higher density than the other, thereby causing it to subduct or sink below the other one.

Destructive plate margins are also known as convergent plate margins or plate boundaries simply because this is where two plates are literally converging.

They are coming together as once again, the diagram helpfully illustrates.

So a little learning check which shows a destructive plate margin is at A, B, or C? What I'd like you to do now is pause the video whilst you consider and then select your answer.

And the correct answer is C.

We can see that the option C is the correct answer simply because we can see those tectonic plates colliding with each other and the one tectonic plate in this example, the oceanic plate sliding beneath that continental plate.

So really, really well done if you're able to identify that as the correct option.

Now there are many different features which are found at destructive plate margins, and these include a long deep ocean trench found at the subduction zone, a long chain of mountains known as fold mountains, volcanoes known as stratovolcanoes and finally, earthquakes.

Now if we look at the picture in front of you, the diagram, we can begin to identify where these different features are found.

First of all, this ocean trench found in the subduction zone is found exactly where that oceanic crust, the denser oceanic crust is sliding or sinking beneath the continental plate in this example.

Here, we begin to find this deep ocean trench.

The Mariana Trench is just one example of this deep ocean trench.

We have this mountain chain, which also is created a destructive plate margin.

The Andes Mountains, for example, found in the Pacific Ring of Fire is an example of this chain of mountains which is created as the oceanic plate in this example collides and slides under the continental plate.

But as that happens, it also pushes slightly up the continental plate to create this chain of fall mountains near the ocean edge.

We also see examples of stratovolcanoes.

Now these are your typical type of volcano, the one that we all know about, the sort of steep sided volcano, which we'll touch on later in the lesson, but these are known as stratovolcanoes.

Now when an oceanic plate subdues under another oceanic plate, volcanic island arcs can be created.

So as we can see in the diagram, first of all, we can see once again how one oceanic crust is sliding under another oceanic crust.

And in this example, we can see how this underwater volcano event is created, which ends up creating these volcanic islands over time.

If we compare it now or overlay it with this map, we can see this volcanic island arc shown here in the Caribbean where we can see that real sort of chain or that arc of volcanic islands once again created at a destructive plate margin, which is when in this example, two oceanic plates are colliding and one which is denser sliding or subducting beneath the other.

Here we have the Caribbean plate sliding and crashing into the North American plate and one subducting in this example, the North American plate, simply because it's slightly denser or heavier.

And as a result, we then get these Windward Islands.

This is the name of the islands in this island arc formed as a result.

So a learning check.

The Mariana Trench in the Western Pacific is the deepest ocean trench in the world.

Which of the following terms best describes it? What I'd like you to do is read through the four options, then pause the video whilst you consider and then select your answer.

Best of luck.

And the correct answer is the subduction zone.

So we can begin, first of all to identify the Mariana Trench here located in the Western Pacific and the Mariana Trench is located at a subduction zone where two plates have collided and the denser oceanic plate has subducted or sunk beneath the other one to create this deep ocean trench.

So really, really well done if you're able to identify that correct answer.

So let's now walk through the process that is happening in a destructive plate margin.

At a destructive plate margin, a denser oceanic plate sinks beneath the less dense plate.

A deep ocean trench forms at the point where the plate begins to sink.

As the plate descends deeper into the mantle, it begins to melt.

Magma then rises up to the surface through the cracks in the crust and volcanic activity creates those volcanoes, those stratovolcanoes inside those mountain chains or those island arcs we looked at just previously.

So a quick learning check, true or false? Volcanoes appear directly above the subduction zone of a destructive plate margin.

What I'd like you to do right now is study the diagram in front of you, pause the video whilst you consider and then select your answer.

Best of luck.

And the correct answer is false.

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

And the reason it is false is because the melting of the oceanic plate occurs deep in the mantle, so magma rises to the surface some distance away from that plate margin.

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

So we have one practise task this first learning cycle.

It says using GIS, Aisha has measured the elevation along a transect across a destructive plate margin.

Now you need to add labels to identify the features her elevation profile shows.

So what I'd like you to do then is pause the video whilst you attempt to complete the gaps and add labels to those features on her elevation profile.

<v ->Best of luck.

</v> So some feedback to that question.

Now, if we start on the left hand side of the elevation profile, we can clearly see then it is an oceanic plate, and on the other side then we can clearly see that there's a continental plate.

Now, how do we know that those are the two plate types involved? Well, first of all, we can see evidence of subduction of an ocean trench as shown by that sudden decrease in elevation, clearly showing that ocean trench.

Then if we look up to the sudden increase in elevation, we can begin to see perhaps those fold mountains or that chain of volcanoes, that chain of stratovolcanoes, which potentially must be on a continental plate.

So really, really well done if you were able to identify those correct answers.

We're now to our second learning cycle, which is what hazards occur at destructive plate margins.

Now a range of hazards occur at destructive plate margins and these include earthquakes, tsunamis, landslides and volcanic eruptions.

In fact, 75% of volcanoes and 90% of earthquakes occur in the Pacific Ring of Fire.

The plate margins that form the Ring of Fire are destructive margins, so we get the most amount of tectonic activity and indeed hazards from destructive plate margins.

Not only do destructive margins have the most volcanoes and earthquakes, they're also associated with the most violent volcanic eruptions and the highest magnitude earthquakes as these few images really exemplify.

So a quick learning check, what percentage of all volcanoes and earthquakes occur in the Ring of Fire destructive plate margins? What I'd like you to do right now is read through the four options, pause your video whilst you consider and then select your answer.

Best of luck.

And the correct answer was 75% of volcanoes and 90% of earthquakes are found in that Ring of Fire.

Really, really well done if you're able to select the correct answer.

Now, destructive plate margins, a denser plate sinks beneath a less dense plate down into the mantle.

Remember, this is called subduction.

This though is not a smooth process.

The plates often get stuck due to friction, and as that friction and that pressure builds up in the subduction zone, it can suddenly be released as the stuck plates suddenly slip.

This violent release of seismic energy produces an earthquake and the intense pressure and friction buildup of destructive margins can produce very high magnitude earthquakes.

The destructive margin earthquakes often happen deep in the subduction zone, so they produce very deep focus earthquakes and these often tend to be very powerful earthquakes as well.

The focus, remember, is where the earthquake occurs, and this can be deep inside the subduction zone.

The epicentre is the point directly above the focus on the land surface.

So a quick learning check.

True or false? Earthquakes at destructive plate margins are often high magnitude.

What I'd like you to do right here is pause the video whilst you consider and then select your answer.

Best of luck.

And the correct answer is true.

Now, once again, what I'd like you to do is pause the video whilst you consider your reasoning as to why this statement is true.

So pause the video here.

And the reason it is true is because earthquakes at destructive plate margins are often high magnitude due to the intense pressure and friction found within the subduction zone.

And because they also have a very deep focus, they occur deep underground.

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

Now, earthquakes that occur underwater can cause tsunamis, but how does this happen? Well, earthquakes can cause the seafloor to suddenly shift upwards pushing the ocean water up with it.

As that water moves closer to the coastline, the waves can reach up to 20 to 30 metres high.

As that water builds up as it reaches shallower waters.

Earthquakes and volcanic eruptions can also trigger landslides.

Earthquakes can destabilise slopes to actually cause a landslide and volcanic eruptions can deposit thick layers of ash and other material onto those slopes which are very unstable.

Heavy rainfall after these earthquakes, all volcanic eruptions can also trigger landslides on destabilised slopes.

So when a plate is subducted or sinks into the mantle, it heats up.

This helps the rock above to melt and form magma.

It's a little bit like adding wood to a fire creates more energy, more heat, which contributes to the rock above melting.

This magma then rises up through the cracks in the crust causing volcanic eruptions.

Over time, those layers of lava and ash build up to form a volcano and volcanoes at destructive plate margins can be very different from those found at constructed margins.

When an oceanic plate is subducted, the heat and magma from the mantle can melt parts of the continental crust.

This produces magma, which is high in silica content.

Magma that is high in silica content is very viscous, which means sticky, and this then traps many gases inside that magma.

Now, what sort of eruptions do you think this combination of sticky magma and trapped gases would produce? You may like to pause a video here whilst you consider your own answer to that question.

As a result of this high silica magma at destructive margins, we find volcanic eruptions which are very explosive, not effusive at all.

We find stratovolcanoes, which are very steep sided, and eruptions which contain volcanic ash.

Stratovolcanoes are made up of layers of lava and ash.

Because silica rich lava is sticky, it doesn't really flow very far before it cools, and therefore it forms this cone shape, which is sort of synonymous with volcanoes when we think of volcanoes, these steep sides.

Explosive volcanic eruptions, produce a range of very dangerous hazards, including the following.

They can produce ash clouds, huge ash clouds that fine ash that cover the land surface.

In the atmosphere, this ash can even damage aircraft engines and can produce lava flows.

These lava flows are though, are usually slow moving due to that sticky lava that we previously mentioned.

It can produce gas emissions.

These are invisible, highly toxic gases emitted by the volcano.

It can create lahars, which is a type of mud flow that happens when an ash and debris from an eruption mixes with water and this can occur even weeks, months or in some cases even years after the volcanic eruption.

Finally, it can cause pyroclastic flows.

These are really deadly gas clouds made of hot ash.

They are clouds of gas, ash and debris heated up to 800 degrees Celsius and travelling its speeds of 700 kilometres an hour.

And actually this is one of the reasons why so many people died in that famous volcanic eruption at Pompeii all those years ago during the Roman times.

So a quick learning check.

What I'd like you to do is complete the gap in this table about hazards at destructive plate margins.

Read through the description and try to identify the name of that hazard.

One has been done for you already.

So please, pause the video whilst you attempt this question.

So, the answers.

Lahar is a type of mud flow that happens when ash and debris from an eruption mixes with water.

A tsunami are waves up to 30 metres high that are often caused by underwater earthquakes.

And finally, pyroclastic flow.

These are clouds of gas, ash and debris heated up to 800 degrees Celsius and travelling at speeds of 700 kilometres an hour.

So really, really well done if you're able to identify the those three correct answers.

So we're on now to our practise tasks, and the first one says to study this map of Japan.

It says, using your knowledge of destructive plate margins, explain why Japan is at risk of tsunamis.

The second practise task asks you to read the information about shield volcanoes and constructive plate margins.

Then explain why destructive margins can produce a very different type of volcano.

And b, suggest why a stratovolcano eruption might be more dangerous than a shield volcano eruption.

So please pause the video here whilst you attempt those two practise questions.

Best of luck.

Now some feedback.

So the first question asks you to study the map of Japan and then using your knowledge of destructive plate margins explain why Japan is at risk of tsunamis.

Now, you may have said something like this.

It says that tsunamis are often caused by underwater earthquakes, and Japan is located on a destructive margin between the continental Eurasian plate and oceanic plate.

Subduction could produce earthquakes in the sea around Japan leading to that tsunami risk.

And the second practise task asks you to explain why destructive margins can produce a different type of volcano.

Your answer may have included something like this.

When a destructive margin involves an oceanic plate subducting under a continental plate, the magma produced is high in silica.

Unlike the basalt magma of shield volcanoes, this magma is sticky and does not flow freely.

As a result, volcanoes on these destructive margins are steep sided rather than having gentle slopes.

These are called stratovolcanoes.

Unlike the effusive eruptions of shield volcanoes where the lava flows out, stratovolcanoes have much more explosive eruptions.

Now 2b asked you to suggest why strata volcano eruption might be more dangerous than a shield volcano eruption, your answer may have included any of the following.

Stratovolcanoes have a range of very dangerous hazards including in pyroclastic flows, which are clouds of gas, ash and debris heated up to 800 degrees Celsius and travelling at speeds of 700 kilometres an hour.

Lahars, which are mud flows and ash clouds.

These all relate to the explosive nature of stratovolcano eruptions in which gases get trapped within sticky silica lava.

In contrast, shield volcanoes have effusive eruptions, which means lava flows smoothly from the volcano rather than exploding out of it.

And gases aren't trapped in the lava.

While fast moving shield volcano lava is definitely dangerous, stratovolcanoes have more hazards that are all very hazardous and that makes them the most dangerous volcano type.

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

On now to our summary at a destructive or convergent plate margin, one plate is sinking below another plate and being destroyed.

A high percentage of all active volcanoes and all earthquakes occur at destructive plate margins due to the high pressure, friction and temperatures resulting from subduction.

Earthquakes at destructive plate margins are often high magnitude and tsunamis can be a feature of destructive margin earthquakes.

Volcanoes at destructive margins when an earthquake plate is subducted under a continental plate are explosive and produce a range of hazards.

This relates to the sticky magma, which is high in silica.

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

It was a pleasure teaching you today and I look forward to seeing you on the next lesson.

Goodbye.