Lesson video

Hi geographers.

My name's Ms. Hoggett and I'm really excited you're here to learn some geography with me today.

I'm sure if you try your best, you're going to be absolutely fantastic.

Don't worry too much if you haven't studied this topic before, I'm here to help you every step of the way.

The title for this lesson is "Mass Movement" and this sits within the unit, rocks, weathering, and soil, why is geology important? Our learning outcome for today's lesson is that I can understand mass movement and know how gravity causes sediments to move downhill.

To help us through today's lesson, we have a number of keywords that are going to be really useful to us, mass movements, weathering and erosion.

Mass movement is the downhill movement of material under the influence of gravity.

Weathering is the process of being worn down due to long exposure to the atmosphere, and erosion is the process of earth surface such as rock or soil being worn away and then transported from its original site.

So mass movement, weathering and erosion are the three key terms that are going to be really helpful to us in today's lesson.

Now today's lesson is split into three learning cycles.

First one, types of mass movement, the second one, impact of weathering and erosion on mass movement, and the last, mass movement at the coastline.

We're gonna start with our first learning cycle now, the types of mass movement.

Now mass movement is a natural process that involves the movement of rock, soil or other debris down a slope due to gravity.

It can happen suddenly or gradually.

One type of mass movement that we'll cover a bit later on is a landslide, and we can see on there in the image that took place in Rio de Janeiro, in Brazil.

Mass movement can happen anywhere in the world where there are slopes.

It's really common if these areas have also experienced heavy or prolonged rainfall.

Izzy says, "However, even light rainfall can cause mass movement if the ground is really steep or if it's already been weathered." So there are lots of circumstances where mass movement can occur.

The result of mass movement is often really dramatic.

So we could see events such as cliff collapses, landslides or slumping slopes, and all of those you can see really significant changes to the landscapes and also quite significant impacts of people.

So we can see that mass movement often has really dramatic impacts.

So let's check your understanding of mass movement so far.

I would like you to answer this question, what causes mass movement to occur? Is it A, wind, B, clouds, C, gravity, or D, sun? Pause the video and have a go answering this question.

Well done if you said C, gravity.

That is the correct answer.

We know that downhill movement occurs due to gravity.

Our next one then, true or false? Mass movement only occurs in areas where there has been heavy rainfall.

Is that true or false? Pause the video and have a go at answering this question.

Well done if you said false.

We know that mass movement can occur anywhere where there are slopes.

Whilst it can be common after heavy or prolonged rainfall, even after light rainfall, it can occur if the land is steep or previously weathered.

Well done, geographers.

Now there are three types of mass movement that we need to understand in today's lesson.

The first is rockfall, we also have slumping and we have sliding.

So we're going to look at those in a little bit more detail.

The first is rockfall and this is when rocks or debris fall freely from a rock face to the base of a rock.

It happens in areas where there are steep slopes.

Rocks will fall freely and crash to the ground.

The material that builds up at the base of the slope is called scree.

And in the diagram there we can see the rock from the top of that slope falling to the bottom building up scree.

Our next type of mass movement is slumping and this is a type of mass movement where large sections of land move downwards along a curved surface.

We can see the curved surface there in the diagram.

It often occurs after heavy rainfall, which saturates the soil, making it heavier.

So that means it's really full of water.

The cliff material, often clay, becomes unstable and then rotates downwards.

The bottom of the slump is called the toe.

And our last type of mass movement is sliding.

And this happens when there's large scale movements of rocks downhill.

When soil and water mix and form muddy, fast moving material, it slides down a slope.

This could be called a mudslide.

These are more common in areas where there is heavy rainfall or volcanic eruptions.

So we can see in the diagram, we have the flat surface and the materials sliding down that surface.

So let's check your understanding of the three different types of mass movements.

What type of mass movement involves a large section of land moving downwards down a curved surface? Is it A, rockfall B, slumping, or C, sliding? Pause the video and have a go at answering that question.

Well done if you said B, slumping.

We know that rockfall is when that debris falls off the rock surface and sliding is where it moves down a flat surface.

So well done if you've got that spot on.

I'd now like you to answer this question, what is the main difference between sliding and slumping? A, sliding involves a rotational movement along a curved surface, whereas slumping happens on a straight slope, B, slumping involves a rotational movement down a curved surface, whereas sliding happens when there is large scale movements of rocks straight down a hill, C, sliding involves rocks falling off a cliff, whereas slumping involves a rotational movement along a curved surface.

I'd like you to pause the video and have a go at selecting the right answer identifying the main differences between sliding and slumping.

Well done if you said B, slumping involves a rotational movement down a curved surface, whereas sliding happens when there is large scale movements of rocks down a straight hill.

Good work, geographers.

So mass movement is influenced by factors such as the type of rock, the slope or steepness and the presence of water.

Laura says, "The steeper the landscape, the more likely mass movement is." And Alex comes back and says, "Heavy rainfall is likely to lead to more sliding and slumping." Thank you, Laura and Alex, that's really useful information.

So we're now gonna practise what we've learned so far about the different types of mass movements.

And for task A, I would like you to use the diagrams to explain the different types of mass movement, rockfall, slumping and sliding.

Pause the video and have a go at competing this practise task.

Well done for giving that a really good go, geographers.

Let's check your answer.

So the first is rockfall.

Now when rocks or debris fall freely from a rock face to the base of a rock and then builds up material called scree, that's rockfall.

Slumping, when a large section of land moves down the slope, usually along a curved surface.

Saturated cliff material becomes unstable and rotates downwards.

The base of the slumped material is called the toe.

And lastly, we have sliding and sliding, when there is large scale movements of rocks downhill, when soil and water mix to form muddy, fast moving material, it slides down a slope and this is called a mudslide.

Brilliant work, geographers, with the first learning cycle, you are doing fantastically.

We're now ready to move on to learning cycle number two, the impact of weathering and erosion on mass movement.

Now the rate of mass movement is impacted by the extent to which erosion and or weathering have already impacted the landscape.

Erosion is the process of the earth's surface, such as rocks or soil being worn away and transported from its original site.

There are four different types of erosion, hydraulic action where air and water is trapped in the cracks of rocks causing them to break apart, abrasion, where rocks scrape along the surface, wearing that surface down, attrition, where the rocks bump into one another causing them to become smaller and rounder, and solution where minerals are dissolved in the water.

Erosion can be caused by wind, water or ice, and it creates distinctive landscapes.

We also have the process of weathering, which greatly impacts mass movement and weathering is the process of being worn down due to long exposure to the atmosphere.

There are three different types of weathering, biological, chemical, and physical.

So biological as we can see in the diagram is where we have living organisms affecting those rocks.

So in this circumstance, the tree has grown through the rocks or it could be the roots that affect it or animals that affect it too.

Chemical is where there's a chemical reaction causing those rocks to break down.

And physical weathering can be where there's changes in temperatures.

So we have three very clear different types of weathering.

So let's check your understanding.

Which of these is not a type of weathering? Is it A, chemical, B, biological, C, solution, or D, physical? Pause the video and have a go at answering this question.

Well done if you said C, solution.

We know that solution is a type of erosion but not a type of weathering because biological, chemical and physical are our three types of weathering.

Now mass movement can become more common when physical weathering such as freeze-thaw weakens the rock.

Now the process of freeze-thaw weathering is as follows, the water enters the cracks in rocks, it freezes expanding the crack, the water then thaws moving deeper into the crack, it freezes again and then the rock eventually breaks.

So we can see that water is freezing and thawing, widening that crack eventually causing the rock to break apart.

But mass movement is also really common when hydraulic action has undercut the base of a cliff.

And in the diagram here, we can see that we've got that cliff face and that body of water, so in this case it's the sea and the force of the waves is causing compressed air and water into the cracks, which is causing those cracks to become bigger and eventually they'll break apart.

So we see that undercutting of the cliff.

But it's not just those physical processes that impact mass movement because mass movement can also become more common when human activities, like construction, quarrying or deforestation, destabilise steep landscapes.

Wet weather conditions including heavy rainfall and storms are also factors the impact mass movements.

This causes the soil and rock at the surface to become saturated, making it heavier and less stable, increasing the likelihood of things like sliding.

So true or false, weathering and erosion slow down the rate of mass movement? Pause the video and have a go at answering this question.

Well done if you said false, weathering and erosion can increase the rate of mass movement.

So it's not slowing it down, it's actually increasing it and making it faster.

Well done, geographers.

So we're now ready to move onto practise task B, on the impact of weathering and erosion on mass movements.

And for this, I would like you to explain how weathering and erosion impacts the rate of mass movement.

You can write this in a paragraph if you wish to.

Well done for giving that a really good go.

Let's take a look at what we could have written within our paragraphs about how weathering and erosion impacts the rate of mass movement.

So your answer may have been similar to this, weathering, which is the process of being worn down due to the long exposure of the atmosphere weakens rocks by breaking them down, making them more likely to move.

For example, freeze-thaw weathering looses material, increasing the chance and speed of mass movement.

Erosion, which is the process of earth surface, e.

g.

rocks or soil, being worn away and transported from its original site, removes material from the base of slopes, reducing support and making the slopes more unstable.

This speeds up mass movement by making it easier for material to slide or collapse.

Brilliant work, geographers.

We're now ready to move on to learning cycle three, our last learning cycle in today's lesson, which is about mass movement at the coastline.

Now mass movement is often more visible along the coastline.

This is because we often find cliff surfaces here, which means mass movement is often more common.

Rockfalls have happened in St.

Oswald's Bay in Dorset and slumping has occurred at Barton-on-Sea in Hampshire.

Now these are just two examples of mass movement and it doesn't mean that it only happens in this place, it can happen around the whole of the UK.

But we are going to look at these two examples now.

So true or false, mass movement is more visible inland than on coastlines.

Pause the video and have a go at answering that question.

Well done if you said false, we know that mass movement is often more visible along coastlines, well done, geographers.

Now slumping is very common on some cliffs.

At the coastal cliffs at Barton-on-Sea in Hampshire, The cliffs are made of weak clay and sand layers.

When rain soaks into the top layers, they become saturated and heavy causing them to slump.

We can see that in the image on the screen.

We can see that slumping has started to occur along that coastline.

We know that that could have a really big impact on the buildings and the vehicles and the people that we can see on the top of that cliff face in Barton-on-Sea.

As a result of the mass movement, coastal paths have been lost and some houses are at greater risk of falling into the sea.

After a period of wet weather in April 2013, cracks in the chalk cliffs at St.

Oswald's Bay in Dorset widened.

They had been weakened by freeze-thaw weathering and erosion by rainwater.

This led to nearly 100 metres of the cliff collapsing in a sudden rockfall, the rockfall occurred at night, so thankfully nobody was injured.

However, 1,100 metres of the coast path there had to be closed.

So the rockfall led to an increase in geotourism, as some visitors to the area take boat trip to see the rockfall, and in other locations Dorset, this would lead to lots of fossil hunters, such as in Charmouth and Lyme Regis.

So whilst we can see that there are some negative impacts of rockfall and different types of mass movement, we've got to appreciate that it can also be positive 'cause geotourism can do lots with the local economy and it brings people to the area to appreciate its beauty.

So let's check your understanding of those two examples.

How much of the cliff collapsed following a rockfall at St.

Oswalds Bay in Dorset it in April 2013? Is it A, 100 metres, B, 10 metres, or C 1,100 metres? Pause the video and have a go at answering that question.

Well done if you said A, 100 metres.

That's absolutely spot on.

So we're now going to practise what we've learned in our final learning cycle.

And for task C, I want to start you off with some fictional headlines.

For each one, you need to identify the type of mass movement that's involved, explain what might cause it and suggest one possible impact it may have on people and the environment.

Now those impacts can be positive or negative.

So the first headline is that large boulders fall onto the beach forcing the closure of popular seaside paths.

The second is that homes were evacuated as soaked hillsides slide into narrow coastal roads.

And the last one is cliff collapses after a week of heavy rain in clay-rich coastal location.

So I'd like you to look at those headlines, identify the type of mass movement, explain what caused it, and suggest one possible impact on people and the environment.

Pause the video and have a go at completing that practise task.

Well done for having a go at that, geographer.

That was a big task, but I'm sure you've done amazing.

Let's check our answers.

So for our first headline, large boulders fall onto beach forcing closure popular seaside path, we could have said, this is suggesting that rockfall has occurred.

It could be caused by freeze-thaw weathering weakening the rock and gravity then causing the sudden fall.

If the rocks fell onto a seaside path, it could cause the path to be closed as it poses a risk to walkers.

It might also have blocked the beach access.

For our second headline which said, "Homes evacuated as soaked hillside slides into narrow coastal road.", we could have said that this is suggesting that sliding has happened.

As it says soaked, it's likely that there has been heavy rain.

This would of course the soil and weak rock to slide down the slope.

This could lead to blocked roads or damaged homes.

Homes were evacuated, which is also an impact.

And lastly, for our third headline which said, "Cliff collapses after a week of heavy rain in clay-rich coastal location", we could have said, this could be an example of slumping.

The saturation of the clay from the prolonged rainfall will have weakened the cliff and this would slump in a curved motion.

This could lead to a loss of land, damage to infrastructure and possible coastal path closures.

So well done If you really considered each of those types of mass movement correctly, identifies what might have caused them and thought about some potential impacts.

That is brilliant, geographers.

You have done such a fantastic job today, well done.

So we've now come to the end of our lesson looking at mass movement and we've learned a number of key things today.

Mass movement is the downhill movement of sediment under the influence of gravity.

The main types of mass movement are rockfall, sliding and slumping.

Slumping is common on coastlines and weathering and erosion lead to mass movement when combined with wet weather conditions.

Geographers, you've been absolutely brilliant today and I know you've done such a fantastic job, well done.

I'm really looking forward to seeing you in our next lesson.

Have a great rest of your day.