Lesson video

Hello, my name's Mrs. Harking.

Today's lesson is on types of volcanoes taken from the unit Plate tectonics: Why is the Earth restless? Today we're going to be exploring different types of volcanoes.

Not all volcanoes are the same.

They actually have very different shapes and we're going to find out why and how the lava reflects that.

Our outcome today is to name the key features of volcanoes and to explain how composite and shield volcanoes are formed.

Here are some keywords that are useful to know before we start the lesson.

First we have crust.

This is the outer layer of Earth where our continents and oceans are, and it's the layer on which we live.

Magma is the molten or semi-molten rock underground.

Mantle is the layer between the Earth's outer core and crust.

It is the thickest layer of Earth and is solid, though 1% of it can flow like a liquid.

Destructive plate boundary is a boundary where the tectonic plates move together, causing volcanic eruptions and earthquakes.

Constructive plate boundary: a boundary where tectonic plates move apart, causing volcanic eruptions and earthquakes.

If there's any of those keywords that you feel you'd like to jot down before we start, that's absolutely fine.

Pause the video now and write them down.

Our lesson today is in three parts.

First of all: What are the key features of volcanoes? Then we'll look at: What is a composite volcano? and What is a shield volcano? So first of all, what are the key features of a volcano? So right now it's likely that 20 to 30 volcanoes are erupting somewhere in the world.

That's quite staggering, really isn't it? We don't really think that's happening all the time, but it is.

But Jacob asks, "What actually is a volcano?" A volcano is an opening in the Earth's crust through which lava, gases and ash erupts.

Time for a quick check.

Select the best definition of a volcano.

Is it A: an opening the Earth's crust through which lava, gases and ash erupt? Is it B: the Earth shaking violently at a plate boundary? Is it C: a series of large ocean waves caused by an earthquake? Well done.

Yes, it's A.

An opening in the Earth's crust through which lava, gases and ash erupt.

Brilliant.

Key features of a volcano now.

First of all, we have the magma chamber.

This is where the magma is stored.

And magma is literally molten rock or melted rock, and that's where it comes from.

That's where it's stored before the volcano.

The magma travels through the vent in order to reach the surface where it erupts.

The crater at the top is like the mouth of the volcano.

This is where one or two vents emerge, and this is where the eruption will happen from.

The lava flow is the molten rock erupting from the volcano and it flows down.

Now notice we started using the term lava instead of magma.

Magma is molten rock that is under the ground and lava is molten rock after it has exited the volcano.

When lava cools down, it forms new layers of igneous rock.

Igneous comes from the Latin word ignis, which means fire.

So igneous rock is actually just cooled melted rock.

Now volcanoes are literally layers and layers of the igneous rock on top of each other.

So every time a volcano erupts, the lava pours down the slopes, cools and hardens, forming a new layer on the volcano.

This makes a volcano higher than the surrounding slopes.

The rock surrounding the volcano that forms the volcano is also made from ash and I'll explain how that happens.

So ash clouds are clouds of tiny rock particles ejected from the volcano.

So these are so tiny, like dust.

Ash settles to form new layers of the volcano that alternate with layers of cooled lava.

So the lava will cool first, probably, and form a layer of solid rock.

And then on top of that, that ash cloud will then drop and a layer of ash will form on top, which will be the next layer of the volcano.

So you can imagine actually two layers of rock are added every time the volcano erupts, first layer being the lava cooling, the second layer being the ash cooling.

Right, time for a quick check now.

Can you name the feature x? Well done, it's an ash cloud.

Time for a practise task now.

Here's Task A.

I'd like you to add the name of the features labelled.

So I've actually labelled lots of features on here but I haven't named them.

I described them but not named them.

Could you please just add the name to each one? Feel free to pause the video now to give yourself time.

Well done, everyone.

Great effort.

Your answers should look like this.

So we've got a crater is the bowl shape feature where one or more vents emerge.

We've got vent, which is the passage that magma travels through to reach the surface.

We've got magma chamber where molten rock is stored before a volcano erupts.

Ash cloud, which is tiny rock particles ejected from the volcano.

And lava flow, molten rock that has erupted from a volcano.

We're onto the second section of our lesson now, what is a composite volcano? Volcanoes come in all shapes and sizes.

Two common types of volcano are composite volcanoes and shield volcanoes.

And you can see here they do have a different shape.

Composite volcanoes are commonly found at destructive plate boundaries.

They also occur in a few locations far from a plate boundary.

Mount Teide is a composite volcano in the Canary Islands, formed by a mantle plume.

At a destructive plate boundary, the subducting oceanic plates allows the mantle above it to heat enough to melt the rock.

This molten rock then rises up through the continental crust, forming a volcano.

They also occur in a few locations far from a plate boundary.

Mount Teide is a composite volcano in the Canary Islands, formed by a mantle plume.

A mantle plume is where we have rock rising in the mantle.

This can then pierce through the crust, forming a volcano.

Composite volcanoes' viscous lava does not travel far before cooling and hardening.

That word viscous means that it's not very runny.

So if we compared water and honey, for example, honey would be more viscous, more thick.

So if we poured water and we poured honey on the table, the water would run off the table a lot faster because it's less viscous.

Whereas the honey would take a long time to travel away from the points at which pour it.

Therefore the layers of new rock fall close to the vent creating steep-sided volcano.

Let's run through that one more time.

So because the lava is viscous, it is thick and slow flowing, it flows away from the vent quite slowly.

It doesn't get very far away before it cools and hardens.

So the layers that are added are added quite close to the vents and that means that we get a tall shape to our volcano.

Time for a check for understanding.

True or false? The steep sides of a composite volcano are caused by the type of lava it erupts.

True or false? Well done.

And can you tell me why that is true? Well done.

So thick lava flows slowly so it cools closest to the volcano.

Correct.

Slow-flowing viscous lava can harden in the vent, acting like a cork.

Gas and lava build up below the blockage until the pressure forces that blockage to move, leading to an explosive eruption.

Boom! So eruptions at composite volcanoes can be quite explosive.

It's like popping a cork off a champagne bottle or something like that.

Mount Agung is one of the several composite volcanoes found on Bali in Indonesia.

It's on the destructive plate boundary here.

So we can see we've got Eurasian plate, Pacific plates, and the Philippine plates meeting.

And the Eurasian plates and the Pacific plate are moving towards each other.

And this causes the destructive plate boundary.

Which statement describing Mount Agung is correct? It is A: is likely to experience gentle, effusive eruptions as it is a composite volcano on a destructive peak boundary? Is it B: is likely to experience explosive eruptions as it is a shield volcano on a constructive plate boundary? Or is it C: is likely to experience explosive eruptions as it is a composite volcano on a destructive plate boundary? Well done.

So it's not A Mount Agung experiences explosive, not effusive eruptions.

It's not B because it is a composite volcano on a destructive plate boundary.

So that means it is C.

C is the correct answer, it is likely to experience explosive eruptions as it is a composite volcano on a destructive plate boundary.

Time for our practise task now.

Can you complete the table for me? On the left hand side I've got a diagram, composite volcano.

Then we have got a series of categories, so boundary type, magma type, shape and explosivity of eruptions.

I would like you to complete the information about each of these categories using the gap fills on the right hand side, please.

Please feel free to pause the video to give yourself time to do this.

Well done, everyone.

Your table should look like this.

So composite volcanoes boundary should say: Common destructive plate boundaries or over mantle plumes.

For magma it should say that it's viscous, so thick and slow flow.

The shape, it should be steep-sided volcano as the viscous lava does not travel far before cooling and hardening.

Therefore the layers of new rock form close to the vent.

Lastly, the explosivity of eruptions.

So the explosive eruptions are there because lava can cool and harden within the vents, acting like a cork.

Lava and gases build up behind the blockage until the pressure forces the blockage to move.

Well done, everyone.

Feel free to pause if you need to add a timeframe.

Now is your time for shield volcanoes.

So what is a shield volcano? Shield volcanoes are found on constructive plate boundaries or over mantle plumes.

If you remember, composite volcanoes are also found over mantle plumes.

So a mantle plume can create either shield or composite volcano.

Lava from a shield volcano is not normally very viscous.

It is runny.

The lava flows quickly and travels further from the vent before it cools and hardens.

And that means that we end up with a shield shape.

Low, shallow slopes, like this, as opposed to the steep-sided slopes of the composite.

And that is because the lava is cooling and hardening further from the vent than with a composite volcano.

It's runny as it travels further away from the volcano vent before it cools and hardens and turns into rock.

Time for a true or false check now.

Composite volcanoes have gently-sloping sides.

Is this true or false? Well done.

Yes, it's false.

And can you tell me why? Well done So shield volcanoes have gently sloping sides as a result of the runny nature of the lava they produce.

Eruptions from shield volcanoes tend to be effusive.

The word effusive just means less violent.

For our composite volcanoes, we said that they were really explosive, didn't we? This is the opposite.

They're far less explosive.

This is because the lava is less viscous.

So more runny.

And magma allows gas bubbles to escape before it emerges from the volcano's vents.

Lava flows rather than explodes in all directions from the vent, depending on the shape of the land.

So lava flows from a Mauna Loa in Hawaii, appearing black in this enhanced satellite image.

You may have heard of Eyjafjallajokull I'll repeat that one more time, as it's a mouthful.

Eyjafjallajokull, which is one of the shield volcanoes, many shield volcanoes found in Iceland on the constructive plate boundary.

This particular one has very a interesting eruption, which actually prevented a lot of flights.

And it might be one that you've heard of.

Time for our last practise task now.

Can you complete the table to explain the features of shield volcanoes? So here we have got a shield volcano table, but you notice that this time there is no gap fill.

So I've just put the categories in: boundary, magma, shape, and explosivity of eruptions.

Can you fill in the gaps for me now that you've done this for our composite volcanoes? Pause the video now to give yourself time.

Well done.

Great effort.

Let's have a look at your answers.

Your table might include something like this.

I doubt it's word for word but feel free to edit to improve your answer now.

So shield volcanoes, for their boundary, they can be on constructive plate boundaries or over mantle plumes.

Magma is not very viscous, it is runny.

For the shape, shield volcanoes have lava that flows quickly and travels further from the vent before cooling and hardening.

This results in shallow slopes.

The explosivity of eruptions.

We have effusive or less violent volcanoes because there's less viscous, more runny lava, allowing gas bubbles to escape before it emerges from the volcano's vents.

For the explosivity of eruptions, shield volcanoes have effusive or less violent volcanic eruptions.

This is because the lava, or the magma, is less viscous or more runny.

So the magma, when the lava is below the ground, remember it's called magma, allows gas bubbles to escape before it emerges from the volcanoes vents.

When it then emerges, the lava flows, rather than explodes, in all directions from the vent, depending on the shape of the land.

Did you get all of that? Feel free to pause the video now and edit anything you'd like to.

Time for a quick summary now.

There are two main types of volcano.

You have composite volcanoes, which are usually found at destructive plate boundaries.

They are highly explosive volcanoes.

And we have shield volcanoes, which are usually found at constructive plate boundary.

We mentioned also mantle plumes and they often produce effusive rather than explosive eruptions.

I hope you enjoyed learning with me today.

I certainly enjoyed exploring this with you, and I hope to see you next time.

Thank you very much.

Bye.