Hello, and welcome, my name is Mr. Marsh and I'm here today to teach you all about the processes happening at constructive or divergent plate margins.

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

So by the end of today's lesson, you will be able to explain the processes at constructive or divergent plate margins and the causes of earthquakes and volcanoes found there.

Now there are three key terms that you need to know for today's lesson and those are rift valley, shield volcano and magnitude.

A rift valley refers to a long narrow depression formed to in a tectonic plate, stretches and fractures at a constructive margin causing the land to sink between parallel fault lines.

A shield volcano refers to a broad gently sloping volcano formed by basaltic lava that is runny and flows easily.

Eruptions tend to be effusive rather than explosive and lava flows out, magnitude refers to a measure of the energy released by an earthquake.

So a higher magnitude means a stronger earthquake.

Now in today's lesson there are two learning cycles and we shall begin with the first learning cycle, which is what are constructive plate margins? At a constructive plate margin, the two tectonic plates are moving apart.

This can happen between two oceanic plates where they move apart and it can happen on land between two continental plates again where they're moving apart, with the case of the two oceanic plates where they pull apart.

this is where a mid-ocean ridge forms, whilst with the example of where two continental plates are pulling apart, this is where a rift valley can be formed.

Constructive margins are also known as divergent plate boundaries.

Now some of the features found a constructive plate margin include underwater mountain chains or otherwise known as a mid-ocean ridge.

We find riff valleys, these are found on land, we find volcanic activity.

We find examples of shield volcanoes.

We find seismic activity through earthquakes and we find the formation of new crust, of new land, hence the name constructed plate margin 'cause this is where new land is literally being constructed.

Now if we take a look at it from a GIS perspective as shown with a map in front of us, we can see this mid-ocean ridge with a Mid-Atlantic ridge here dividing the Atlantic ocean almost in two here we can see that this Mid-Atlantic ridge is much shallower and this is from the the land which is being formed as those two tectonic plates are pulling apart from one another.

So we can use GIS to produce an elevation profile of a transect across this feature.

And by looking at that graph, you can see on the screen in front of you, you can see just how much more elevated the Mid-Atlantic ridge is due to the pulling motion of the constructive plate margin.

So a quick learn check, which of these diagrams illustrates a constructive plate margin, A, B, or C? Pause your video here whilst you consider and then select the answer.

And the correct answer was B.

Really, really well done.

If you were a able to identify the constructive plate margin where the two plates are pulling apart from one another.

So constructive margins form as a result of the slab pull motion.

A ridge push also occurs at the constructive margins as we can see in this diagram in front of you.

Now, if we were to break this down a little bit further at destructive margins, the older and denser edges of the oceanic crust sinks down into the mantle.

What does this mean for constructive plate margins? Well, the slab pull, the rest of the plate is pulled to towards the destructive margin.

The plate splits in the middle, forming this constructive margin, magma rises up through the split cools and creates new crust.

This ridge push motion then happens as more and more new crust is added, pushing through that split in the crust, it pushes the plates further apart.

So we can see how both slab pull and ridge push are contributing to causing this spreading motion of the at the constructed plate margin and how it contributes to creating this new land.

So what we can see then is for this new land to be constructed at constructive plate margins, we need to see this sea floor spreading action because as the two plates pull apart, that magma rise up through the crust, it cools and it causes that new land to be created.

It can even create those volcanoes, those shield volcanoes, sometimes beneath the ocean to be formed.

So true or false, volcanoes are not a feature of constructive plate margins that form underwater.

What I'd like you to do right now is pause the video whilst you consider whether this statement is true or false? And the correct answer is false.

So what I'd like you to do once again is pause the video whilst you consider as to why this statement is false? Now the reason why this statement is false is that first of all, there are volcanoes found at constructive plate margins.

And this happens because magma rises up at constructive plate margins and this means that volcanoes are a feature of these margins both on land and underwater.

Really, really well done if you are able to identify those two correct answers.

Time for some practise questions.

Now, Jacob has been exploring plate margins using GIS.

He has seen that a constructive plate margin goes through Iceland.

Now what features associated with constructive plate margins might Jacob be able to spot in Iceland? Try to name any two.

Jacob also created an elevation profile for this transect to cross southwest Iceland.

He thinks the feature along the transect is a shield volcano.

Do you agree and could you explain his thinking? Once again, I would like you to pause the video now whilst you study the maps and study that elevation profile in front of you as well as Jacob's comment here.

So pause the video whilst you consider and then attempt these two questions.

Best of luck.

And now some feedback.

So in terms of feedback for question one, you were asked to try to name any two features associated with constructive plate margins that Jacob might have been able to spot in Iceland.

Now these are just some that you may have suggested.

The first of which is a volcano, especially shield volcanoes are found at constructive plate margins.

A rift valley is also found at a constructed plate margin, signs of new crust being created and other types of volcanic activity.

For example, volcanic lakes.

Really, really well done if you are able to identify or think of any of those features of a constructed plate margin.

Now the second question asks you to consider the elevation profile that Jacob had created as well as the satellite view of Iceland and that you can see in front of you.

Also, you needed to think about whether you agreed or not with Jacob's assessment that he could see a shield volcano and explain your thinking.

Now, you may have written something like this.

It is more likely to be a rift valley than a shield volcano because the satellite image view shows it as a long valley with high sites, a volcano would be much more rounded.

It might mark the location of a constructive margin.

So really, really well done if you are able to identify that reasoning in your own answer.

Now the second and final learning cycle is what hazards actually occur at constructive plate margins.

Now a range of hazards occur at constructive plate margins and these include volcanic eruptions, earthquakes, landslides hydrothermal activity, for example, geezers as well as flooding.

If we think specifically about volcanic eruptions for a moment, as plates move apart, magma rises from the mantle to fill the gap.

This process often creates shield volcanoes.

The magma at a constructive margin is very high in basalt and therefore flows easily and rapidly because of the runny magma that you find at a constructive margin shield volcanoes are broad with gentle slopes since the lava is able to flow much much further before it cools.

Hence, it's able to create this very broad but gently sloping shape that is synonymous with a shield volcano.

Gases don't get trapped in this runny magma and therefore the eruptions we find here are not explosive.

Now, just to give you a few different examples, then we have two videos here showing two different things.

The first on the left is a lava eruption at sea in Hawaii, which is shield volcano.

So we can clearly see how constructive plate margins sometimes give rise to underwater volcanoes and underwater volcanic eruptions.

One on the right is really interesting.

This is showing the movement of lava and this type of lava that we find at constructive plate margins is very high in basaltic content and this is called pahoehoe and it can flow quite quickly.

It may not look significantly fast on the video, but in terms of lava movement, this is significantly fast and it's very much synonymous with a constructive plate margin.

So a quick learning check, true or false, volcanic eruptions on constructive plate margins are usually highly explosive due to the magma being high in silica.

So I'd like you to do that is pause the video here whilst you consider and then make your answer.

And the correct answer was false.

Now, once again, I'd like you to pause the video here whilst you consider your reasoning as to why this statement was false.

And the reason is that volcanic eruptions at constructive plate margins are usually non-explosive because the magma is actually high in basalt, not silica, but basalt, which makes it runny as a result, gases do not get trapped in this magma and this reduces how explosive these volcanic eruptions are.

So once again, really, really well done if you are able to identify those two correct answers.

When it comes to earthquakes, earthquakes occur at constructive plate margins and they occur when two plates move away from each other and the crust becomes under stress as it stretches and pulls or pushes apart because the crust is rigid, a point comes when the stress causes the crust to split along those fault lines and this releases seismic energy, which is an earthquake and the focus is usually quite shallow at constructive margins, usually less than 70 kilometres.

Now earthquakes at constructive plate margins are usually low to moderate in magnitude by which I mean their strength, whereas the higher magnitude earthquakes tend to happen at destructive plate margins or conservative margins.

And this makes sense because at destructive or conservative plate margins, there's much more collision with the two plates involved.

They may be grinding next to each other or they may be smashing or colliding into each other.

At conservative plate margins, we don't have that.

This is where they are moving away from each other.

So there's a lot less stress happening at these plate margins.

So the stress is lower at constructive margins.

At destructive and conservative plate margins, there's much more pressure which builds up as those plates get stuck on one another.

The crust is thin and weak at constructive margins and fractures much, much easier, which releases that stress much more quickly.

Whereas at the other plate margins like destructive and conservative, it's much thicker and this causes a much more higher buildup of energy which releases itself through much stronger magnitude, much higher magnitude earthquakes.

This GIS layer map is really interesting, and it actually shows us the earthquakes happening from 1900 to 2022 and we can even see their magnitude right there using the legend in the top left corner.

My question to you is, what do you actually notice about the earthquakes, their location and their magnitude? Now you might like to pause the video here whilst you study the map and consider your own answer to that question.

Now what we can see then is the majority of earthquakes are happening along this Mid-Atlantic ridge, that conservative plate margin, which we identified earlier in fact, there are lots of earthquakes happening along this Mid-Atlantic bridge because it's a constructive margin, but their magnitude is quite low.

If we look at the colour, we can see that it's blue, and if we cross reference it with the legend, we can see that these earthquakes are usually somewhere between three and 4.

5.

So there are very low intensity, low magnitude earthquake.

So a quick learning check Iceland is located on a constructive plate margin.

Which one of these statements about Iceland is most accurate? I would like you to read through the three options, pause your video, consider your answer, and then make your answer.

And the correct answer to this question was B.

Yes, Iceland's shield volcanoes are generally less explosive than volcanoes on destructive margins.

Really, really well done if you are able to identify that correct answer.

So with regards to landslides, the riff valleys which are formed at constructive plate margins but on land, these have very, very steep sides.

Now these steep sides can become unstable and when perhaps a small earthquake happens, this can trigger landslides, with regards to hydrothermal activity, super heated water under the ground and steam can erupt unpredictably and these are known as geezers in this example.

With regards to flooding, Iceland has volcanoes that are covered by glaciers.

Those big sheets of ice, volcanic eruptions can cause floods when the eruption melts glacier ice.

These floods can involve huge volumes of melt water, which can make them very dangerous indeed.

So a quick learning check.

I'd like you to try to complete this spider diagram of hazards that we've mentioned at constructive plate margins.

Now, two have already been done for you.

Can you now pause the video whilst you consider the correct answers to the other three? Best of luck.

And the three correct answers you needed were volcanic eruptions, particularly from shield volcanoes, earthquakes typically quite low level magnitude earthquakes.

And finally, landslide often triggered as a result of those small earthquakes.

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

So we're on now to our practise questions and the first one says to describe what volcanic eruptions on constructive plate margins are like, and the second practise question asks you to explain the pattern of earthquakes and their magnitude shown by this GIS layer.

Now, I would like you not just to describe what is shown in the map, but do your best to actually explain this pattern.

So what I'd like you to do at this point is pause the video whilst you attempt these two practise questions.

Best of luck.

And now some feedback.

So in terms of the feedback for question one, you were asked to describe what volcanic eruptions that constructed plate margins are like.

Now your answer may have included something like the magma constructed margins is high in basalt and flows easily and rapidly.

Gases don't get trapped in the runny magma and so eruptions are non-explosive.

For the feedback for the second question, you needed to explain that pattern of earthquakes and their magnitude shown by that GIS layer.

Now once again, your answer may have included something like this, although there are earthquakes dotted around in quite a spread out way in parts of the map.

Almost all of the earthquakes here are following a long line.

This is because they are occurring along the constructive plate margin.

The crust comes under stress when the plates move apart and earthquakes occur when the stress is released.

The magnitude of the earthquakes is quite low, almost all of them are under magnitude 4.

5.

This may be because the stresses from plate movement are not huge and 'cause the epicentre is quite shallow, usually less than 70 kilometres below the surface.

So really, really well done if you are able to get any of those correct points in your own answer.

Now time to sum up.

So at a constructive or divergent margin, plates are moving apart.

Features of constructive margins include mid-ocean ridges, riff valleys, shield volcanoes, and lower magnitude earthquakes.

Slab pull is thought to be the main process moving tectonic plates and it has the effect of putting plates apart.

The process operating at constructive margins produces less explosive volcanic eruptions than at destructive margins and lower magnitude earthquakes than at destructive or conservative margins.

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

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

Goodbye.