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 today's lesson is "Weathering and rocks" and this sits within the unit "Rocks, weathering, and soil: why is geology important?" Our learning outcome for today is that I can explain the different types of weathering.

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

Biological, relating to living organisms. Chemical, relating to the interaction of substances.

And weathering, the process of being worn down by long exposure to the atmosphere.

So biological, chemical, and weathering are going to be really helpful to us in today's lesson.

Today's lesson is split into two learning cycles, the different types of weathering and landscapes impacted by weathering.

We're going to start with our first one now, the different types of weathering.

Rocks can be broken down into smaller fragments by different processes.

Lucas says, "This can be happening even when it looks like nothing is affecting the rock!" Weathering is the process of being worn down by long exposure to the atmosphere.

And there are three different types of weathering.

They include biological weathering, chemical weathering, and physical weathering.

And we're going to spend a bit of time today unpicking what each of those different types of weathering are.

Let's check your understanding so far.

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

Let's check your answers then, geographers.

So well done if you said A, environmental.

We know that environmental is not a type of weathering because we've just seen that chemical, biological, and physical are all examples of weathering.

You're doing a brilliant job.

Well done.

So now we're going to take a look at our three types of weathering.

And our first is biological weathering, which occurs when living organisms break down rock.

Now, this can happen for a number of different reasons.

Firstly, when plant roots grow into cracks in rock and break them apart, they contribute to biological weathering.

As seeds settle in small cracks and begin to grow, the roots expand, putting pressure on the rock.

Over time, this pressure forces the cracks to widen, eventually causing pieces of rock to break off.

This process is especially common in areas with trees and shrubs where roots can penetrate deep into rock surfaces, gradually weakening them and contributing to rockfalls and soil formation.

Secondly, when animals burrow into rocks, they speed up the weathering process.

Burrowing animals, such as rabbits, moles, and even some insects, create tunnels in soft rock, breaking it apart and exposing fresh surfaces to further weathering.

In coastal areas, marine organisms, such as shellfish and worms, can also burrow into rocks, weakening cliffs and causing erosion.

This movement of animals through soil and rock helps to mix materials, allowing air and water to reach deeper layers, which increases weathering.

And lastly, when organisms such as lichens and mosses grow onto rocks and make them brittle, biological weathering continues to occur.

Lichens, a combination of fungi and algae, produce acids that slowly dissolve minerals in rock surfaces.

This weakens the rock structure over time, making it more vulnerable to breaking apart.

Mosses also retain moisture, keeping rocks damp and encouraging further breakdown through chemical and physical weathering.

These organisms are found on old stone walls, rooftops, and exposed rock surfaces, gradually wearing them away.

These rocks in these images have all been impacted by biological weathering, so we can see that living organisms have affected that rock and caused them to break down.

Our next example is chemical weathering, and this occurs when rocks break down due to chemical reactions, altering their minerals and weakening their structure.

This process is caused by interactions between rock surfaces and substances such as water, acids, and gases in the atmosphere.

There are different things that cause chemical weathering, and one cause is acids in rain, which speed up the breakdown of certain types of rock.

Rainwater is naturally slightly acidic because it absorbs carbon dioxide from the air, forming a weak acid called carbonic acid.

However, when pollutants like sulphur dioxide and nitrogen oxides from burning fossil fuels mix with rainwater, they create acid rain, which is much stronger.

Acid rain reacts with minerals in rocks, such as limestone and chalk, dissolving them over time and causing visible damage to buildings, statues, and natural landscapes.

Another cause of chemical weathering is exposure to the air containing chemical elements.

One important reaction is oxidation, which happens when rocks containing iron are exposed to oxygen and water.

This reaction is similar to rusting in metals, causing rocks to weaken, change colour, and eventually break apart.

For example, some rocks turn a reddish brown as iron reacts with oxygen, forming iron oxide.

Carbon dioxide in the air can also react with certain minerals, creating weak acids that slowly dissolve rock surfaces.

And lastly, a third cause is chemicals produced by living organisms. Plants, fungi, and bacteria can release acids that break down rocks over time.

For example, lichens which grown on rocks produce acid that dissolve minerals, gradually wearing rocks away.

Tree roots can also contribute by releasing organic acids as they grow into cracks, further weakening the rock structure.

Over long periods, these biological processes play significant roles in shaping landscapes by breaking down rock surfaces and forming soil.

These rocks here in these images have both been impacted by chemical weathering, and you can see how their structure has changed as a result of that.

Our last type of weathering then is physical weathering, and this happens when there's changes in the temperature causing rocks to expand and contract.

Exfoliation is a type of physical weathering.

The sun heats up the rock surface, and the rock expands.

When temperatures drop, the rock cools and contracts.

Then joints, so cracks, form on the outer part of the rock.

And the outside layer finally breaks off, exposing the rock, as you can see in the image.

These rocks here show evidence of exfoliation.

Some people refer to exfoliation as onion-skin weathering because it's the outer layer of that rock peeling away, a little bit like it does on an onion.

But there's also another type of physical weathering, and this is called freeze-thaw weathering.

The water enters the cracks in the rock, as you can see in the first image.

Then that water freezes, expanding the crack.

The water then thaws and moves deeper into the crack, as you can see in the third image.

Then that water freezes again, and eventually, the rock might break in half.

So rather than it being the hot temperature this time, we're actually looking at the cold temperatures.

And we know that the water in the rock is freezing and thawing, freezing and thawing, causing that rock to break apart.

As you can see in the image, we have two photos of rocks being affected by freeze-thaw weathering, and they've both split directly in half.

So let's check your understanding now, and I'd like you to match up the definition to the type of weathering that we've discussed in today's lesson.

So the first one, when changes in temperature cause rocks to expand and contract.

The second, when rainwater contains weak acids that react with minerals in rocks.

And the third, when living organisms break down rocks.

So which of those statements are chemical? Which of those statements are biological? And which of those statements are physical? Pause the video and have a go at completing that question.

Let's check your answers.

So when changes in temperature cause rocks to expand and contract, that's physical weathering.

When rainwater contains weak acids that react with minerals in rocks, that's chemical weathering.

And lastly, when living organisms break down rocks, that's biological weathering.

Well done if you got all of those spot on.

You're doing a brilliant job.

So we're going to practise what you've learned so far, and I'd like you to complete the table by explaining each process of weathering.

So you've got the type of weathering on the left-hand side, biological, chemical, and physical, and then you have a box for explanation.

That's part one.

Part two is to explain what type of weathering you think has impacted these rocks.

So underneath the images, you're going to explain which type of weathering you think has impacted the rocks in the photo.

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

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

Let's check your answers.

So for part one, you needed to complete the table explaining each process of weathering.

So here your explanations might have included, for biological: When living organisms break down rocks, this could be due to the roots growing through rocks, animals burrowing, or organisms such as lichens or mosses growing on rocks.

Well done if you got that correct.

For chemical, we could have said: When there is a chemical reaction that causes rocks to break down, this could happen from exposure to the air, exposure to acid rain, or through chemicals released from organisms. Again, well done if you got that spot on.

And then lastly, for physical weathering: When changes in temperature cause rocks to expand and contract, this could be freeze-thaw weathering or exfoliation.

If you put onion-skin weathering, that's also absolutely spot on, but we know it could be when there's really cold temperatures or when there's really hot temperatures.

So well done for giving part one a really good go, geographers.

For part two, we needed to explain which type of weathering you think has impacted these rocks.

Now, for the first image, you might have said something like this: This rock looks like it's been impacted by physical weathering, specifically freeze-thaw.

We know this because in the back of the image, there is ice in the water, suggesting the temperature is cold.

Water would have collected on the rock, frozen, added pressure to the rock, and over time caused it to break apart.

So well done if you got that one accurate.

For our next image, you could have said: This rock has been impacted by biological weathering.

The surrounding trees and plants have rooted themselves in and around the rock, causing it to split open.

And we're using evidence from the back of the photos to see what could have affected it, including the trees and living organisms we can see in that image.

And for our third photo, you could have said: The rock in this image has been impacted by chemical weathering.

The minerals in the rock, which could be chalk, have reacted with the acids in rain, which have caused it to break down.

Its coastal location also suggests that salt crystals from the sea spray and salt water may have also been a factor in the pitting of the surface.

So you're taking a look at the images, you're looking at the surroundings, and you're using that to influence what type of weathering you think's affected the rocks.

You're doing fantastically, geographers.

I'm really impressed with your hard work so far.

We're now at the end of learning cycle one, and we're ready to move on to learning cycle two, landscapes impacted by weathering.

Weathering plays a significant role in shaping landscapes all around the world.

Over time, it breaks down rocks into smaller pieces, influencing the formation of valleys, cliffs, and even soil.

It also plays a key role in the rock cycle by helping to create sediments that can later form new rocks.

To start, biological weathering can occur in many different environments, as it happens anywhere where there are living organisms. In forests, tree roots grow into cracks in rocks, gradually breaking them apart.

In coastal areas, burrowing animals weaken cliffs, making them more vulnerable to erosion by waves.

Even in harsh environments like deserts and mountain regions, small organisms like lichens and mosses break down rocks by producing acids that dissolve minerals.

This process is slow but essential in shaping landscapes over long periods, contributing to soil formation and the changing appearance of rock formations.

Some examples of this can include the Rocky Mountains in the USA are affected by biological weathering because tree roots grow into the rocks and cause them to expand, gradually weakening the mountain slopes.

Another example could be the Amazon Rainforest in Brazil, where fungi, bacteria, and plant roots grow in and around rocks.

There's also the Purnululu National Park in Western Australia, where plant roots, lichens, and burrowing animals break down the rock structures.

And then lastly, another example could be the White Cliffs of Dover in the UK, where animals, such as seabirds and rabbits, and plant roots weaken the chalk cliffs.

Now, this is by no means an exhaustive list, but these are just some examples of where biological weathering has affected different landscapes.

Let's take a moment to check your understanding.

So what type of weathering can occur anywhere where there are living organisms? Is it A, chemical, B, biological, or C, physical? Pause the video and have a go at answering this question.

Well done for giving that a go, geographers.

If you got biological, you're absolutely spot on.

We know that biological weathering relates to living organisms, and therefore it can occur anywhere where living organisms are found.

Good work.

Now, chemical weathering often requires water and occurs more rapidly in places that have higher temperatures.

Therefore, places that have warm and damp climates experience faster rates of chemical weathering.

On the screen, you can see there's a world map which shows our climate zones, and the equator is circled.

Along the equator, we know that it's very hot and it's very wet, and therefore chemical weathering is more common here.

The Postojna Cave in Slovenia is an example of a limestone cave.

It has formed due to acidic rain dissolving the rock over thousands of years.

Whilst it doesn't rain in the cave, the water seeps through the soil and the chemicals are carried through the atmosphere.

So this has caused the cave to become weathered chemically over time.

Let's check your understanding.

So chemical weathering is common in areas along the Tropics of Cancer and Capricorn.

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

Well done if you said false.

We know that chemical weathering is more common closer to the equator as it happens quicker in warm and damp climates.

When we looked at our map, we saw that the climates along the Tropics of Cancer and Capricorn, whilst warm, was quite dry, and therefore the equator is where we more commonly see chemical weathering.

Well done, geographers.

Lastly, we have physical weathering, and this is common in places where there is little soil and few plants.

This makes it common in mountain regions and in hot deserts.

The cold temperatures found at high altitudes mean that freeze-thaw weathering plays a major role in breaking down rocks in these areas.

This type of weathering occurs when water seeps into cracks in rocks, freezes, and expands.

As the ice expands, it puts pressure on the surrounding rock, gradually widening the crack.

When the ice melts, the water can seep further into the rock, repeating the process.

Over time, this causes pieces of rock to break off, leading to jagged peaks, scree slopes, and rocky debris fields.

Freeze-thaw weathering is particularly common in mountain regions, such as the Rocky Mountains in North America, where temperatures frequently fluctuate above and below freezing.

This process contributes to the rugged, sharped-edged appearance of many mountain peaks and can trigger rockfalls and landslides.

The photo shows the fourth highest peak in the Rockies, demonstrating how freeze-thaw weathering has shaped the dramatic landscape.

But likewise, the Sahara Desert in North Africa experiences large temperature changes.

It's over 40 degrees Celsius in the day but falls below freezing at night.

This means that exfoliation, or what we earlier referred to as onion-skin weathering, is also very common.

We're now going to practise what you've learned so far, and I'd like you to imagine that you are a rock.

I'd like you to write a short story about how you experience different types of weathering over thousands of years.

When you're doing this, you should discuss how your shape and appearance change over time, and try to include these words, physical, chemical, biological, and weathering.

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

Good luck.

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

Now, there's lots of different ways you could have approached this, but your answer may have included something similar to the below: For thousands of years, I have stood tall as part of a great mountain, unchanged and solid.

But over time, nature has slowly broken me down through weathering.

It started with physical weathering.

The days were warm, but the nights were freezing.

Rainwater trickled into tiny cracks on my surface, and as temperatures dropped, the water froze and expanded.

Over many years, these cycles of freezing and thawing widened the cracks until I finally broke in two.

As I settled lower down the mountain, I found myself near a tree.

At first, I barely noticed it, but as the tree grew, its roots grew straight through me.

This was biological weathering, the power of plants breaking down rocks.

Then, the rain came.

At first, it felt refreshing, but it carried weak acids that reacted with my minerals, slowly dissolving parts of me.

This was chemical weathering, gradually changing my shape and making my surface rough and worn.

Now, I am smaller and in more pieces than before.

Over time, I will continue to change, breaking down into smaller pieces and becoming part of the landscape around me.

So notice within that answer, I used those keywords, and I also talked about how my size and appearance changed over time.

So this was an example of a good answer, and yours was probably different, but I'm sure you did an absolutely fantastic job.

Well done, geographers.

So that brings us to the end of today's lesson looking at weathering and rocks.

And to summarise what we've learned today, there are three types of weathering, biological, chemical, and physical weathering.

Biological weathering is when rocks are broken up by living organisms. Chemical weathering occurs when there is a chemical reaction that causes rocks to break down.

And physical weathering is when rocks are broken up by changes in temperature, such as when ice forces apart cracks in rock.

You've been brilliant today, geographers, and you've done such a fantastic job.

I hope you have a brilliant day, and I look forward to seeing you in our next lesson.