Complete the diagram by filling in which type of rock belongs where: Sedimentary rock, Metamorphic rock, Igneous rock.
The lithosphere
Chapter overview
2 weeks
The focus for this chapter is the lithosphere and the processes involved in its formation. The lithosphere is part of a larger sphere called the geosphere. The geosphere consists of the three concentric layers of the Earth: the core, the mantle and the crust. The lithosphere refers to the outer part of the geosphere, which includes the upper part of the mantle and the crust. The lithosphere is also part of the Earth where the rock cycle is found.
The first section on 'What is the lithosphere?' gets learners to investigate their environment first, discovering that the lithosphere is found all around them. We then step back and look at the concentric layers which make up the Earth. This gives the background information need to introduce the rock cycle which involves the upper part of the mantle and the crust. The three rock types are introduced, which is followed by investigating what rocks are really made of - minerals. This sets the scene for the next chapter on mining the mineral resources.
2.1 What is the lithosphere? (2 hours)
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Tasks |
Skills |
Recommendation |
|
Activity: Investigating stones |
Observation, writing, describing |
CAPS suggested |
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Activity: The layers inside the Earth |
Classification, comprehension |
Suggested |
|
Activity: Build a 3D model of the Earth |
Design, making a model, applying knowledge, translating information |
Optional This can be used as a project to be conducted throughout the term. |
2.2 The rock cycle (4 hours)
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Tasks |
Skills |
Recommendation |
|
Activity: Summarising the rock cycle |
Recall, comprehension |
CAPS suggested |
|
Activity: Explaining the rock cycle |
Writing, comprehension |
CAPS suggested |
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Activity: Building a model of the formation of sedimentary rock |
Making a model, translating information |
CAPS suggested |
|
Activity: Comparing the properties of igneous rocks |
Comparing, observation, application |
Optional |
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Activity: Classifying rocks |
Classification, organising information, making deductions |
Optional extension |
|
Activity: What minerals are found on Earth? |
Researching, reading, writing |
CAPS suggested |
- What does the centre of the Earth look like?
- Why is it important to know about the structure of the Earth?
- Why is there so much variety in the rocks you see around you?
- How do rocks form?
- Why do we need to know about rocks?
- Why are rocks important?
The Earth is a system consisting of many parts. In the previous chapter we looked at how these parts or spheres interact. In this section we are going to look more closely at one of these spheres, namely the lithosphere.
'lithos' comes from the Ancient Greek word meaning 'stone'.
What is the lithosphere?
- geosphere
- lithosphere
- continental crust
- oceanic crust
- crust
- mantle
- core
- composition
On Earth, water, air, stone and life interact. Let's think about the stone part of this statement. Where on Earth do we find stone? What are the different forms of stone that are found on Earth? Why is it important to know about this part of the Earth? Let's investigate these questions.
Ageologist is a scientist who studies the Earth, the rocks from which which it is made, and the processes and history that have shaped it.
Investigating stones
During this activity learners will be required to observe carefully and put their observations into words. Encourage learners to look carefully and to capture as much detail as possible.
This activity can be done as an introduction to the section, before the content is discussed. Learners need to be asked before the lesson to bring these items to school. Some learners might be tempted to bring in pieces of brick and cement as examples of rock. Make sure to point out to learners that concrete and brick are made-made materials, although they have similar properties to rocks.
Alternatively learners can be sent out onto the school grounds to find the items (about 10 minutes). You could also bring items to school yourself, ensuring that there are at least 4 items per learner. Otherwise, you could place the different items at different stations (for example, 10 similar stones at station 1, 10 sand samples at station 2, etc.) and let the learners move from one station to the next. Each learner will then have his/her own stone to observe at each station. Give them 3 minutes at each station to do the observations. Learners can also attempt to crush some of the samples using a hammer and by wrapping the sample in paper towel to avoid pieces flying off in different directions. This can also be performed outside.
If learners are not able to bring stones to the classroom, the pictures provided in this section can be used. Use this as a last resort only.
The aim of the activity is for the learners to examine real artifacts and practice writing down their observations. They should realise that all the different types of stone form part of the lithosphere, including sand which is made from stone worn down by wind and water. The lithosphere is found all around us. The activity can lead into a discussion of one or more of the following questions:
- What types of stone found on Earth have we not collected in the activity? (molten rock, rock from the seafloor, etc.)
- What do we use the lithosphere for? (minerals, fuels, plant nutrients, building materials, etc.)
- What is rock or stone actually made of? Learners might say sand. If so, the follow-up question is 'What is sand made of?' Learners might not have the answer here. The idea is not to give them the answer yet, but to keep this as an open question which will be answered at the end of this chapter and the next chapter on mining the lithosphere. The activity is meant to be exploratory and not necessarily meant to provide the answers at this stage. It sets the scene for the two major topics: Different types of rocks (the rock cycle, Chapter 2) and the minerals found in rocks (what they are and how we mine them, Chapter 3).
MATERIALS:
- magnifying glasses
- hammers
- paper towel
- samples collected, as described below
When we look at thecomposition of something, we look at what it is made up of.
INSTRUCTIONS:
- Collect the following items and bring them to school: sand, pebbles, a small stone/rock, a larger rock.
- When you collect sand, stones or rock, look for the samples that look interesting and different and bring these to class.
- Find at least four different items from different locations.
- Study the different samples and complete the following table. If you have magnifying glasses available, use these to study the detail of the different samples.
- Wrap some of the samples in paper towel and see if you can crush them with a hammer. Your teacher might instruct you to do this outside.
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Location Describe where you have found your sample. |
Shape and colour Describe the size, shape and colour |
Texture Describe the texture and hardness. |
Composition Is it made of more than one material? Describe what it is made up of. |
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Sand |
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Pebble |
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Small stone/rock |
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Larger rock |
An example of what learners might write for 'Sand' is given below.
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Location Describe where you have found your sample. |
Shape and colour Describe the size, shape and colour |
Texture Describe the texture and hardness. |
Composition Can you see more than one part? Describe what it is made up of. |
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Sand |
At building site next door to our house. |
The sand has small round grains of about 1 mm in diameter on average. It is cream coloured with some darker brown grains. |
The grains are all more or less the same size. The sand is dry so it feels smooth and soft. The grains are hard, but break up even further when I hit it with a hammer. |
All the grains look the same, except for the variations in colour. The grains are not connected/stuck to each other and are free flowing. |
The strangest geological formations on Earth.
In the last investigation you would have seen a lot of variety amongst the types of stone that are found in the area around your school. There is variation in shape, colour and texture amongst the different rocks on Earth.
The lithosphere consists of all the mountains, rocks, stones, top soil and sand found on the planet. In fact, it also includes all the rocks under the sea and under the surface of the Earth. The lithosphere is found all around us and we interact quite closely with it every day.
How tall can mountains be?
Inside the Earth
A large poster that you can print and put up in your classroom for this section:
The lithosphere is considered the outer layer of the Earth. The Earth is made up of concentric layers called the crust, the mantle and the core.
Concentric objects share the same point as their centre.
Layers of the Earth.
Imagine that we cut away a slice of the Earth, as shown here:
Inside, we would then be able to see the layers of the Earth, as shown in the next diagram:
You might have already learned about this in previous grades in Social Sciences.
The core has two parts, the inner core which is solid and the outer core which is liquid. The mantle can also be divided into two parts, the lower mantle and the upper mantle. Some parts of the crust are found under the oceans. This is called the oceanic crust. Other parts of the crust form part of the continents and is called continental crust.
The brittle upper part of the mantle and the crust form the lithosphere. The lithosphere, the mantle and the core are sometimes called the geosphere. The geosphere is also one of the parts of the Earth, just like the hydrosphere, atmosphere and biosphere that you learned about in the previous chapter.
The centre of the Earth is 6371 km deep.
100 Greatest Discoveries: The centre of the Earth (video). http://science.howstuffworks.com/environmental/29266-100-greatest-discoveries-the-center-of-the-earth-video.htm
The layers inside the Earth
The aim of this activity is to provide learners with some structure to make sense of all the new terminology that is introduced in this section. Some of the terms might be familiar, but learners might not have had to link it all up. This map will be used later in the chapter again. This activity tests learner's ability to comprehend and then translate the information into a visual map. This is quite a simple activity in the sense that the basic structure of the map is provided, but later they will be required to provide their own structure. This is not a concept map, but rather a topic map to show learners how the different terms are related. The learners have been exposed to various kinds of maps over the past 3 years in Natural Sciences. These are all attempts to support them in developing different ways of organising information to enable them to learn better. The different maps could also help learners develop their own way of summarising information for studying purposes.
INSTRUCTIONS:
Use all the words in bold in the paragraph above to complete the following map:
The completed topic map.
The temperature of the Earth's inner core is about the same temperature as the surface of the Sun, more than 5000 °C.
Build a 3D-model of the Earth
This activity can be used as a project which can be completed over the course of the term. For this project learners can work in pairs. They need to build a labelled model of the Earth and do some additional reading to find out more about each of the layers. You can put the models up for display in your classroom and refer to them in further teaching. Coloured paper or painted paper mâché can be used as alternatives to other expensive materials. You can use Assessment Rubric 7 at the back of your Teacher's Guide to assess their models.
How to make paper mâché (video).
INSTRUCTIONS:
- Use recycled material and modelling clay to build a three-dimensional model of the inside of the Earth.
- All the layers of the Earth need to be included and accurately labeled on your model.
- Write a one page summary on the layers of the Earth. Read about each of the layers to be able to answer the following questions in your write-up. You can use the internet, library books or ask knowledgeable people in your community.
- Thickness of each layer
- State of matter
- Temperature
- Composition (what it is made up of)
In the first activity of this chapter we collected different rocks. Why are there different types of rocks and why do the look different? These are the questions we will answer in the next section.
The rock cycle
- cycle
- weathering
- compaction
- erosion
- deposition
- melting
- cooling
- solidify
- sedimentary rock
- metamorphic rock
- igneous rock
- sediment
- sedimentation
- cementation
The rock cycle.
In previous grades you learned about the water cycle. A cycle is a combination of processes that take place in a certain sequence and which repeat over and over again from the beginning. Processes in a cycle do not stop and are therefore said to be continuous. For example, the water cycle which is part of the biosphere describes how water forms clouds, rain, rivers and clouds again.
The carbon cycle, which takes place as part of the biosphere, describes the movement of carbon through carbon dioxide, fossil fuels and carbohydrates. The rock cycle is part of the lithosphere and it describes how rocks change from one form into another and eventually back into the first form.
How does the rock cycle work?
Watch this interactive animation to show you how the rock cycle works. http://www.classzone.com/books/earth_science/terc/content/investigations/es0602/es0602page02.cfm
Not all rock on Earth is recycled. Thousands of tons of rock fall to Earth from space every year.
The Hoba Meteorite landed in what is now Namibia around 80 000 years ago. It is the largest known meteorite still in a single piece, and the most massive naturally occurring piece of iron known at the Earth's surface. It has a mass of over 60 tons.
Rocks on Earth are divided into three broad categories:
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sedimentary rocks
-
metamorphic rock
-
igneous rock
This classification is based on where the rocks were formed. The following diagram summarizes the rock cycle.
The 3 types of rocks.
The rock cycle is a natural continuous process in which rocks form, are broken down and re-form again over long periods of time. The process can be described as follows:
Heat causes expansion of rocks and cold causes contraction.
- Wind, water, heat and cold cause the weathering of rocks on the surface of the Earth. The rocks are broken up into smaller and smaller pieces and form sand.
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Wind and water wash the sand and small stones away and deposit them as sediments into lakes and the ocean. This process is called deposition.
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The sediments settle at the bottom of the oceans, lakes and rivers. Over time they get covered with more layers of sediment. The pressure from the additional layers causes the sediments to compact and solidify to form sedimentary rock.
- The sedimentary rock may be buried deeper and deeper beneath the surface of the Earth through movement in the Earth's crust (where oceanic plates and continental plates meet). The rocks can also be pushed deeper (subducted) into the Earth. As the rocks move deeper into the Earth, temperature and pressure increase.
- Rocks become more compact as processes of compaction and cementation occur. As the chemical compounds in the rocks change, due to heat and pressure, metamorphic rock is formed.
-
Over time the metamorphic rock can move deeper into the Earth, melt and become magma.
-
Magma moves towards the surface of the Earth through volcanic pipes. The hot magma cools slowly on its way to the surface and forms igneous rock. Magma can also break through the surface as lava in volcanoes. In this case, the lava will solidify quickly on the surface to form igneous rocks. Igneous rock can form in the crust or on the surface.
- Igneous rocks get eroded by wind and water and the whole process starts again.
Magma and lava are both molten rock, but refer to different locations. Magma is molten rock that forms beneath the Earth's surface. When magma erupts out of a volcano onto the surface, it is referred to as lava.
Metamorphic rock is formed deeper under the surface and only becomes exposed to the surface when the layers above it are removed by erosion. Igneous rock, just like sedimentary rock, can move deeper into the Earth and form metamorphic rock due to the increase in pressure and temperatures.
As you can see in the previous diagram, rocks of all types may move down through the mantle, melt and mix with magma. The Earth's crust is continually recycled. This is why we refer to the process as the rock cycle.
Summarising the rock cycle
QUESTIONS:
Name the process by which igneous rock is formed.
Cooling.
Which type(s) of rock form sediment?
Igneous, metamorphic as well as sedimentary (all rock types).
What conditions are needed for metamorphic rock to form?
Increased temperature and pressure.
Explain what 'weathering and erosion' of rock mean.
It is the action of wind and water which cracks and breaks up pieces of rock.
Explain what 'compaction' means.
It is a process where the particles are compressed closer together (for example through the action of pressure).
What type of rock is formed through compaction?
Sedimentary rock.
What is magma? Explain the role of magma in the rock cycle.
Magma is molten (melted) rock. It is called lava when it flows onto the surface of the Earth. Magma and lava form igneous rock when they cool, either above or below the Earth's surface.
Explaining the rock cycle
This is an alternative activity to the preceding one. It can be used instead of the previous one, or if you feel that your learners need more practice in writing about what they have learned, then this is an ideal homework exercise. It could also be used as a quick class test, to check for understanding. Learners can assess their own writing, or swop with a friend.
INSTRUCTIONS:
Write a paragraph to explain the rock cycle in your own words. Start the explanation with the formation of igneous rock. Use full sentences and include the following key words in your write-up.
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Key words:
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The words can be used more than once, and you should add your own keywords as well. You should also include a labelled diagram in your write-up.
Learner-dependent answer. Ensure that learners use the terms correctly and that the process is explained accurately. Learners should NOT copy the text from the workbook, but should write this in their own words.
The diagram on the rock cycle can be used as a guide as to what their labelled diagrams should look like.
Use the following as a guide for the answer:
The rock cycle is the natural, continuous process in which rocks form, are broken down and re-form over long periods of time. There are three rock types: igneous, sedimentary and metamorphic rocks. The rock cycle can be explained in the following steps:
- molten rock from the mantle (magma) pushes up through the crust
- pools of magma cool down slowly in the crust to form igneous rocks, like granite
- some magma escapes to the surface as lava in the form of a volcano
- lava cools to form igneous rocks
- the rate at which the lava cools affects the properties of the rocks formed
- rocks on the surface of the Earth are weathered by heat (expansion), cold (contraction), wind and water to form smaller particles
- wind and water transport these particles to floodplains and the sea by erosion
- the particles are laid down as sediments
- the sediments are covered by more layers of sediment
- the pressure of many layers turns the lower layers into sedimentary rock like sandstone
- magma heats the surrounding rock and changes its chemical structure to form metamorphic rock like slate from shale or marble from limestone
- some rock is pushed below the crust, melts and becomes magma again
A useful website which shows all the different rock types and classifications. http://geology.com/rocks/
We are now going to take a closer look at each of the three main types of rocks.
Sedimentary rock
Learners do not need to memorise the different names for all the examples of rocks in the following three sections, but they should be able to name one or two examples for each rock type. It is important to realise that rocks are used as materials in our daily lives. Many of the words will be familiar as they are referred to in everyday language (limestone, granite, etc.) so it will be good for learners to know the origins of these stones. This section links up with what is covered in the Matter and Materials strand. Stone is a natural material and resource that humans have used in the past and are still using today. The usefulness of a particular material, stone in this case, is due to its properties.
Sedimentary rocks are formed when layers of sediment solidify over time. Sediments are layers of particles from pre-existing rock or once-living organisms, for example, shells. Rocks on the surface of the Earth are weathered by expansion and contraction due to changes in temperature, wind and water, and also by erosion due to animals. Bigger rocks break up into smaller and smaller particles through the process of erosion.
Rocks erode to form soil.
Wind and water transport the loose, smaller particles, along with debris from living organisms, and some large stones, eventually depositing them on flood plains and in the sea. This is called erosion.
The material accumulates at the bottom of oceans, rivers, lakes and swamps. The sediment settles and forms layers. These layers build up upon each other and cause the compaction of the lower layers. Over time, the bottom layers eventually solidify and form layers of sedimentary rock, as is shown in the following diagram.
An easy demonstration to show the concept of deposition is to mix some soil in water in a glass jar and then place it on the table in front of the class and allow the particles to settle at the bottom through the course of the class. You can then also pour in different colours of sand or soil to illustrate the different sedimentary layers.
Formation of sedimentary rock under the sea.
The video link provided in the Visit box on the 'Formation of sedimentary rock under the sea' provides a clear and easy to understand demonstration of how sediment is deposited on the bottom of the sea in layers. You can even construct something similar to this model in your classroom to show learners.
Although sedimentary rock is found in most places on Earth, these rocks make up only 8\% of the Earth's crust. Different layers of sedimentary rock may be seen in the mountains and rocks around us on a daily basis. In the photograph you can clearly see the layers of sediment which have solidified over millions of years to form the sedimentary rocks of the Grand Canyon.
The oldest layers of sedimentary rock visible in the Grand Canyon are believed to be nearly 2 billion years old.
You can see the layers in the sedimentary rock making up Table Mountain in Cape Town.
There are different types of sedimentary rock, including sandstone, limestone, dolomite, coal, shale and conglomerate.
Limestone is a sedimentary rock made from the mineral calcium carbonate (CaCO3), often formed from the remains of the skeletons of marine animals. We use limestone as building stone, in the manufacture of lime (calcium carbonate), and cement.
Lime is the word used for calcium-containing compounds like calcium oxide (CaO), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3).
Dolomite is a sedimentary rock made from calcium magnesium carbonate (CaMg(CO3)2). Coal is another example of sedimentary rock formed from the solidified remains of ancient plants at the bottom of swamps. Shale is a very fine-grained sedimentary rock formed from the deposition of mud and silt. It is made up of very thin layers all stuck together. Conglomerate is a sedimentary rock made up of small stones, shells and other pieces of sediment. Cementation is the process whereby sand and associated shells, pebbles and other sediment become cemented together to form sedimentary rock.
Sedimentary rock is softer than the other types of rock. It is eroded by the actions of wind, water or ice (glaciers). Fossils, especially of sea creatures, are often found in sedimentary rocks, lying cemented in the sediments in which they fell when they died. When plants or animals die, they are often covered in sand, which later becomes rock, capturing the fossil inside.
Let's take a look at how the layers of sediment are compressed and become harder over time due to pressure.
Modelling the formation of sedimentary rock
This activity can be done as a classroom demonstration. Pile books on top of a few slices of bread until they cannot be compressed further. Let the learners make observations and draw what they observe. Also show them the layers afterwards - the different layers are not distinguishable any more, they merge into one mass.
MATERIALS:
- 3 slices of white bread
- 3 slices of brown bread
- heavy books or object
INSTRUCTIONS:
-
Cut off the crust from all the sides.
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Layer the slices on top of each other, alternating the white and brown slices. Each slice represents a different layer of sediment.
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Draw a labelled diagram of what the stack looks like.
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Place a piece of plastic on top of the bread stack to protect the bottom book in your bookstack, then place a pile of books on top of the bread stack. Observe what happens to the layers. Write your observations here.
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Add more books to the pile and observe. What happens to the layers?
-
Remove the books from the bread pile. Can you distinguish the different layers now? Draw a labelled diagram of the bread layer.
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Explain how this model demonstrates the formation of sedimentary rock.
This is a good opportunity to discuss how models are used in Science to represent and explain what happens in reality. This model shows how different layers of sediment are deposited, represented by the different layers of brown and white bread. Initially the layers are quite loose, but over time as more layers are added, the bottom layers become compressed. This is represented in the model by adding on more books to increase the pressure on the layers. More books are added to represent the passing of time and more pressure. Eventually the layers of rock are squashed (cementation has taken place) and it is not as easy to recognise the different layers, as in sedimentary rock.
Metamorphic rock
A suggestion is to get samples of slate, marble, sandstone and granite from natural stone tile shops for learners to look at and handle.
Metamorphic rock makes up a large part of the Earth's crust. Metamorphic rocks are formed when sedimentary or igneous rocks are exposed to heat and pressure. Metamorphic rocks do not form on the surface of the Earth, but rather deeper underneath the surface where the temperatures and pressures are much higher. When other types of rock experience higher pressures and temperatures the rock crystals are squashed together. They undergo changes in crystal structure to form metamorphic rock.
'Metamorphic' refers to metamorphosis - a process where one thing is transformed into a completely different thing, like a pupa becoming a butterfly.
Metamorphic rock may move deeper into the Earth where they melt, forming magma. The magma may then cool and form igneous rock.
Some examples of metamorphic rocks are slate, marble, soapstone, and quartzite.
Slate is a metamorphic rock that was formed by shale (sedimentary rock) that was metamorphosed. Slate is often used for roofing or flooring. Since it can be cut into shapes and does not absorb moisture, it makes a good material for tiles.
Marble is a metamorphic rock that is produced from the metamorphosis of limestone. It is used for countertops, flooring and tombstones and is a very durable building material.
Soapstone is a relatively soft metamorphic rock. It is often used as an alternative natural stone countertop instead of granite or marble, for example in kitchens and laboratories. In laboratories it is unaffected by acids and alkalis. In kitchens it is not stained or altered by tomatoes, wine, vinegar, grape juice and other common food items. Soapstone is unaffected by heat. That means that hotpots can be placed directly on it without fear of melting, burning or other damage. Many statues and carvings are also made from soapstone.
Quartzite is formed through the actions of heat and temperature on sandstone. If you compare the texture of sandstone with quartzite in the pictures shown here, you will see that the process of metamorphism changes the texture from sandy to more glossy. The crystals in the quartzite are bigger and the layers have disappeared. Quartzite is much harder than sandstone.
Igneous rock
- extrusive rock
- intrusive rock
Why do we speak of molten rock, and not 'melted' rock?Molten specifically refers to to liquids which are extremely hot, and whose usual form is a solid, such as molten rock. However, something may be melted, but need not be hot, nor a complete liquid, such as melted butter.
Igneous rock is formed when magma cools down. Three factors play a role when igneous rocks are formed:
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Where it is formed: The rocks are formed on the surface they are called intrusive rocks. If they are formed under the surface they are called extrusive rocks.
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How quickly it cools: When magma cools quickly, small crystals are formed and the resulting rock has a fine-grained texture. When it cools slowly, larger crystals form, resulting in a more coarse-grained rock. Sometimes the individual crystals can be seen with the naked eye.
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How much gas is trapped: Magma contains molten rock and lots of gas. The gas is under pressure deep in the Earth. When the magma breaks through the surface, the gas is released. Depending on how quickly the magma cools down, the gas has more or less time to escape. When the magma cools down very quickly, lots of gas is trapped resulting in cavities and openings forming in the rock.
Extrusive comes from the word 'extrude' which means to push out.
How volcanoes are formed.
A volcano expedition.
A volcano is an opening or rupture in the surface of the Earth's crust (or another planet) which allows hot lava and volcanic ash to escape in an eruption from the magma chamber below.
Pompeii was an ancient Roman city that was completely destroyed and buried under ash and pumice in the eruption of Mount Vesuvius in 27 AD. The town and objects were preserved for thousands of years and have now been excavated. Today it is visited by millions of tourists annually.
Pompeii (full documentary).
Examples of igneous rock are basalt, granite and pumice.
Basalt is the most common igneous rock and makes up a large part of the rocks just under the surface of the Earth. Most of the oceanic crust is basalt rock. It is a dark-coloured rock and is used as building material, particularly in building stone walls.
Basalt is not only found on Earth, but also on the Moon and Mars! The highest mountain on Mars, and also the biggest, known volcano in our solar system - Olympus Mons - was formed from basaltic lava flows.
Olympus Mons is a large volcano on Mars. It is almost three times taller than Mount Everest. It is the second tallest mountain that we know of in our solar system.
Mars' largest volcano.
Granite is an igneous rock with large grains. It was formed from magma which slowly crystallised below the surface of the Earth. Granite is one of the most well-known types of rock. It is used to make numerous objects such as tabletops, floor tiles and paving stone.
Pumice rock is an example of extrusive igneous rock.It is formed from the lava emitted during volcanic explosions. Because the lava cools down very quickly, a lot of gas is trapped in the rock. As a result, pumice is a very porous rock, with lots of holes in it, making it the only rock that can float on water. Pumice stones are used in lightweight concrete and as an abrasive in industries and in homes.
Pumice rock is formed when volcanoes explode. A lot of gas is trapped in magma. The gas is under pressure when the magma is under the surface. When it breaks through the surface the pressure is released in a very short period of time. The suddenly exploding gas is forced up and out of the volcano, taking along all the molten rock around it. This is observed as an explosion of gas and molten rock that can be thrown kilometres away from the volcano. The magma cools very rapidly and can form rocks ranging from small pebbles to rocks the size of a house.
This process can very effectively (and dramatically) be demonstrated by using fizzy drinks. Fizzy drinks have gas dissolved under pressure. When the cap of a fizzy drink bottle is opened, the gas can escape very quickly. If the bottle is shaken before opening it, the effect of a volcanic explosion can be shown. This can be a very messy demonstration that should be done outside.
Once the demonstration is done, the analogy explaining the formation of pumice rock should be consolidated in class. Learners should note that the liquid is shot up and out of the bottle. They need to imagine that it is hot lava with gas dissolved in it. When the magma breaks through the crust, it explodes with a lot of force. The lava is shot up high into the air and will cool down very quickly to form rocks. Rocks are strewn over a very large area around the volcano.
WARNING: This demonstration should be done outside and learners should stay clear of the area.
Comparing the properties of igneous rocks
This is an optional activity.
INSTRUCTIONS:
Study the following igneous rocks and compare their similarities and differences in the table below.
Sample 1 is basalt, sample 2 is obsidian, sample 3 is granite and sample 4 is pumice.
|
Sample |
Where was the sample formed? Extrusively or intrusively |
How quickly did it cool? What evidence do you have for your answer? |
Was air trapped when it was formed? What evidence do you have for your answer? |
Describe the colour |
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Sample 1 |
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Sample 2 |
||||
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Sample 3 |
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Sample 4 |
|
Sample |
Where was the sample formed? Extrusively or intrusively |
How quickly did it cool? What evidence do you have for your answer? |
Was air trapped when it was formed? What evidence do you have for your answer? |
Describe the colour |
|
Sample 1 |
Extrusively |
Very fine crystals |
No, no visible holes |
Dark, green-grey |
|
Sample 2 |
Extrusively |
Fast, no visible crystals |
No, no visible holes |
Shiny black |
|
Sample 3 |
Intrusively |
Slowly, there are large interlocking crystals |
No, no visible holes |
Mottled with yellow-brown, white and black |
|
Sample 4 |
Extrusively |
Fast, there are no crystals |
Yes, there are holes in the rock where gas bubbles were trapped |
Grey-black |
Classifying rocks
This can be used as an optional, extension project.
INSTRUCTIONS:
- In this project you will be working in pairs. You need to collect rocks from your neighbourhood, or borrow some rocks from someone's rock collection.
- You will need at least 12 different samples of rock.
- Try to find as much variety as possible, applying what you now know about the three different rock types.
- You could also ask a geologist to provide you with a variety of rock samples to identify.
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Go to the website provided in the Visit margin box and follow the flow diagram to identify all your rock samples.
- You need to create a display of the rocks and how you have identified them using the flow diagram and their properties.
Resource to use for the project on classifying rocks. http://www.classzone.com/books/earth_science/terc/content/investigations/es0610/es0610page02.cfm
Identifying types of rocks.
Rocks contain minerals
- mineral
CAPS places this section directly after discussing the core, mantle and crust. We have moved it to the end of the chapter so that learners have a bit more knowledge about the different rock types. Placing it here also prepare learners for the next chapter on mining these minerals.
We started this chapter by collecting rocks and looking closely at their characteristics. We then looked at how rocks were formed. The question now is why do we need to know about rocks and why are rocks important. Let's look at what makes rocks so valuable.
Rocks contain minerals. A mineral is a chemical compound which occurs naturally, for example, in rocks. There are several thousand types of minerals which are found in different combinations in rocks. They consist of metal and non-metal atoms combined in various ratios.
Let's look at some examples. Copper is a valuable metal because it is a good conductor. It is used in electrical cabling and other electrical applications. There are about 15 different types of rock which contain copper compounds. One such compound is copper(I)sulfide or Cu2S. When this compound is found in rocks it is called a mineral named chalcocite. Copper can also be found as the compound CuFeS2 or chalcopyrite. The minerals chalcocite and chalcopyrite can be found in many different types of sedimentary, metamorphic or igneous rocks. If we would like to use the copper from these rocks, we need to find a way to get it out of the rock and into the metal form. This we will discuss in the next chapter.
Quartz and feldspar are the two most abundant minerals in the crust. Quartz is the mineral form of silicon dioxide (SiO2). Potassium feldspar has the formula KAlSi3O8. A rock may be composed almost entirely of one mineral or could be made of a combination of different minerals. Different combinations of different minerals in rock will result in a different types of rock.
A brief introduction to minerals.
What minerals are found on Earth?
This can be used as a research project. It can be presented as a written report. A useful resource giving the relevant information for this activity can be found here: http://www.rsc.org/education/teachers/resources/jesei/minerals/students.htm
This link also includes a very useful worksheet that can be used as an end of section exercise.
INSTRUCTIONS:
In this chapter you have learned about the Earth's crust and minerals that occur in rocks. Read more about the Earth's crust and answer the following questions:
- What are the most abundant elements in the Earth's crust?
- Why are these elements so abundant?
- How did the elements get into the Earth's crust?
- Why are the elements important?
- What do you think are the most important element(s) in the Earth's crust? Give a reason for your answer.
- The Earth consists of four concentric layers called the inner core, the outer core, the mantle and the crust.
- The lithosphere consists of the solid outermost part of the mantle, the crust and sediments covering it.
- The rock cycle is the natural continuous process in which rocks form, are broken down and re-form over long periods of time. The rock cycle has a number of steps.
- There are three rock types: igneous rock, sedimentary rock and metamorphic rock.
- Sedimentary rock is formed when rocks on the surface are weathered and the small particles, along with plant and animal material, are deposited in sediments at the bottom of lakes, oceans and rivers. Over time, more and more layers of sediment are deposited. The resulting increase in pressure causes compaction and the formation of hard layers of sedimentary rock.
- Fossils are often found in sedimentary rock as when some organisms die, they become incorporated into the layers of sediment..
- Hot magma is found deep below the surface of the Earth. When magma cools slowly, below the surface of the Earth, it forms intrusive igneous rock. When the magma pushes up through the crust (for example in a volcano), it cools rapidly and forms extrusive igneous rock.
- Hot magma can heat the surrounding rock and change other types of rock into metamorphic rock.
- Different combinations of elements and compounds form the minerals in the crust.
Concept map
Use the concept map on the next page to summarise what you learned about the lithosphere and rock cycle in this chapter. If you want to add more links or information into the concept map, you should do so.
Teacher's version
Revision questions
The Earth consists of different layers.
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Label the following diagram: [4 marks]
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What is the difference between parts C and D? [2 marks]
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What does part B consist of? [1 mark]
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Give three examples of things found around your school that form part of part A. [3 marks]
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C (the outer core) is liquid and D (the inner core) is solid.
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Magma or molten rock.
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sand, stone, rocks, pebbles, clay (any three).
Why are there so many different rocks found on Earth? [2 marks]
Rocks are formed through many different processes resulting in a large variety of different combinations of rock minerals.
The diagram below shows the formation of one of the rock types. Study the diagram and answer the question that follow.
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What type of rock formation is shown in the diagram? Give a reason for your answer. [2 marks]
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What processes are involved in the formation of this type of rock? [2 marks]
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What will happen if the rocks formed here move deeper into the Earth? [3 marks]
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Sedimentary rock. The layers of sediment can be seen forming at the bottom of the ocean.
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Sedimentation and cementation.
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The rocks will become hotter and more pressure will be applied. The rocks will become more compact and the chemical compounds in the rocks will change. Metamorphic rock will form.
Fossils are often found in sedimentary rocks. Explain why this is the case. [4 marks]
When animals or plants die, their remains often end up on the ground and are covered by sand over time. The sand gets compacted and eventually becomes sedimentary rock, with the fossilised remains of the plant or animal still inside the rock.
Explain the difference between the formation of igneous rock, such as granite, and igneous rock, such as pumice. [4 marks]
Granite is intrusive igneous rock. It forms as magma beneath the surface of the Earth, cools slowly and forms large crystals. Pumice is extrusive igneous rock which forms when magma pushes out of the crust to the surface of the Earth and cools very quickly, trapping bubbles of gas.
Iron is an element found abundantly on Earth, especially in the core of the Earth. Iron combines with oxygen to form haematite, the mineral form of iron (III) oxide. Haematite is present in sedimentary rocks, for example in the Sishen area in the Northern Cape.
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What is the formula for iron (III) oxide? [1 mark]
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How does iron end up in sedimentary rock? [3 marks]
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Why is hematite an important mineral? [1 mark]
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(Fe2O3)
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Sedimentary hematite crystals formed as evaporating oceans left deposits of iron in the sedimentary layers. The iron then combined with oxygen molecules created by the process of photosynthesis. Also accept an answer where learners explain part of the rock cycle, from magma to sedimentary rock.
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Iron is used to make steel, stainless steel, cars [any appropriate application]
Total [32 marks]