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Study Commentary for Unit 27 Study Commentary for Units 28-29

PS548 SCIENCE FOR PRIMARY TEACHERS

TheOpen University

STUDY COMMENTARY FOR UNlT 27: EARTH MATERIALS AND PROCESSES

ATTAINMENT TARGETS ADDRESSED IN UNlT 22: AT6 AND AT9

COMMENTARY GUIDE

1 INTRODUCTION AT6: levels 2 and 4; AT9: level 5 Study notes Teaching notes Key points

2 ROCK MATERIALS AT6: levels 1 to 4 Study notes Teaching notes

lnvestigation 1 : Making a collection of rocks

Key points

3 IGNEOUS PROCESSES AT6: levels 1 to 4; AT9: level 5 Study notes Teaching notes Key points

4 SEDIMENTS AND SEDIMENTARY ROCKS

, AT6: levels 1 to 4; AT9: levels 2.3 and 5 Study notes Teaching notes

lnvestigation 2: Eroding soap lnvestigation 3: Can you feel the difference?

Key points

5 TECTONIC AND METAMORPHIC PROCESSES AT6: levels 1 to 4; AT9: level 5

Study notes Teaching notes Key point

SUPPLEMENT: SOIL AT9: levels 2 and 3 Study notes Teaching notes

lnvestigation 4: Are all soils the same? lnvestigation 5: Do all soils hold the same amount of water?

Key points

RESOURCES

QUESTIONS

NOTES

CENTRE FOR SCIENCE EDUCATION

SCIENCE FOR PRIMARY TEACHERS: CONTRIBUTORS

Barry Alcock (human biology, Nene College, Northamptonshire) Fiona Allen (reader, Hillside Infants School, Northwood, Middlesex) Bob Allgrove (chemistry, Chichester College of Technology) Matthew Baird (advisory teacher, London Borough of Enfield) Steven Baker (Earth sciences, Droitwich High School) Chris Brown (Earth sciences, consultant author) Sue Browning (advisory teacher, EPSAT, Essex) Andrew Coleman (editor) Hazel Coleman (editor) Chris Culham (advisory teacher, EPSAT, Essex) Carolyn Dale (advisory teacher, Buckinghamshire) Myra Ellis (secretary, electronic publishing, The Open University) Graham Farmelo (physics, The Open University) Stuart Freake (physics, The Open University) David Gamble (county adviser, science, Buckinghamshire) Jack Gill (senior science inspector, Essex) Jackie Hardie (adviser, London Borough of Enfield) Linda Hodgkinson (CO-director, Science for Primary Teachers, The Open University) Barbara Hodgson (IET, The Open University) Anne Jones (deputy headteacher, Simpson Combined School, Milton Keynes) Hilary MacQueen (biology, consultant author) Baird McClellan (consultant author) Catherine Millett (chemistry, consultant author) Peter Morrod (chemistry, The Open University) Shelley Nott (illustrator, En-igma Design) Katharine Pindar (information officer, The Open University) Jane Savage (Institute of Education, University of London) David Sayers (Science INSET co-ordinator, North London Science Centre) Freda Solomon (advisory teacher, London Borough of Enfield) Valda Stevens (biology, consultant author) David Sumner (physics, Tarragon Press) Liz Swinbank (physics, consultant author) Margaret Swithenby (biology, The Open University) Peter Taylor (chemistry, The Open University) Jeff Thomas (biology, The Open University) Susan Tresman (CO-director, Science for Primary Teachers, The Open University) Liz Whitelegg (academic liaison adviser, The Open University) Margaret Williams (advisory teacher, Buckinghamshire) Geoff Yarwood (electronic publishing, The Open University)

The Pilot Project for Science for Primary Teachers was made possible by funding from the Department of Education and Science and from National Power plc and Nuclear Electric plc.

The Open University, Walton Hall, Milton Keynes MK7 6AA. First published 1991. Copyright 63 1991 The Open University. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, without permission in writing from the publisher or a licence from the Copyright Licensing Agency Limited. Details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Ltd., 33-34 Alfred Place, London WClE 7DP. printed in the United Kingdom by H. Charlesworth & Co. Ltd, Huddersfield.

Further information on this and other Open University courses may be obtained from the Learning Materials Sales Office, The Open University, P.O. Box 188, Walton Hall, Milton Keynes, MK7 6DH. ISBN 0 7492 5028 3

1.1

STUDY COMMENTARY FOR UNlT 27

ATTAINMENT TARGETS ADDRESSED IN UNlT 27: AT6 AND AT9

ATTAINMENT TARGET 6: TYPES AND USES OF MATERIALS Pupils should develop their knowledge and understanding of the properties of materials and the way properties of materials determine their uses and form a basis for their classification.

Key Programme of study Level Statement of attainment stage

1 Children should collect, and find similarities Pupils should and differences in, a of everyday 1 a be able to describe familiar and unfamiliar materials, natural and manufactured, including objects in terms of simple properties, for cooking ingredients, rocks, air, water and example shape, colour, texture, and describe other liquids. They should work with and how they behave when they are, for example, change some of these materials by simple squashed and stretched processes such as dissolving, squashing, pouring, bending, twisting and treating 2 a be able to recognize important similarities

surfaces. and differences, including hardness, flexibility and transparency, in the characteristics of materials

2 Children should work with a number of different everyday materials, grouping them according to their characteristics, similarities and differences. Using secondary sources, they should explore their origins and how materials are used in construction. Properties, such as mass ('weight'), volume, strength, hardness, flexibility, compressibility and solubility should be investigated and measured. Children should explore chemical change in a number of everyday materials, such as mixing Plaster of Paris, making concrete and firing clay. They should find out the common use of materials and relate the use to the properties which they have investigated, such as changes brought about by heating and cooling. They should learn about the dangers associated with the use of everyday materials, such as bleach and hot oil.

be able to group materials according to their characteristics

know that heating and cooling materials can cause them to melt or solidify or change permanently

know that some materials occur naturally while many are made from raw materials

be able to list the similarities and differences in a variety of everyday materials

be able to make comparisons between materials on the basis of simple properties, strength, hardness, flexibility and solubility

be able to relate knowledge of these properties to the everyday use of materials

know that solids and liquids have 'weight' which can be measured and, also, occupy a definite volume which can be measured

understand the sequence of changes of state that results from heating or cooling

be able to classify materials into solids, liquids and gases on the basis of their properties

know that gases have 'weight'

be able to classify aqueous solutions as acidic, alkaline or neutral, by using indicators

c be able to give an account of the various techniques for separating and purifying mixtures

ATTAINMENT TARGET 9: EARTH AND ATMOSPHERE Pupils should develop their knowledge and understanding of the structure and main features of the Earth, the atmosphere and their changes over time.

Key Programme of study stage

Level Statement of attainment

1 Children should collect, and find differences and similarities in, natural materials found in their locality, including rocks and soil. 1 They should compare samples with those represented or described at second hand. They should observe and record the changes in the weather and relate these to their 2 everyday activities.

en should investigate na 2 Childr materials (rocks, minerals, soils), should sort them according to simple criteria, and relate them to their uses and origins, using books and other sources. They should be aware of local distributions of some types of natural materials (sands, soils, rocks). They should observe, through urban or rural fieldwork, how weather affects natural materials (including plants) in their surroundings and how soil develops. They should also consider the major geological events which change the surface of the Earth. They should have the opportunity to make regular, quantitative observations and 4

keep records of the weather and the seasons of the year.

Pupils should

know that there is a variety of weather conditions

be able to describe changes in the weather

know that there are patterns in the weather which are related to seasonal changes

know that the weather has a powerful effect on people's lives

be able to record the weather over a period of time, in words, drawings and charts or other forms of communication

be able to sort natural materials into broad groups according to observable features

be able to describe from their observations some of the effects of weathering on buildings and on the landscape

know that air is all around us

understand how weathering of rocks leads to the formation of different types of soil

be able to give an account of an investigation of some natural material (rock or soil)

be able to understand and interpret common meteorological symbols as used in the media

be able to measure temperature, rainfall, wind speed and direction; be able to explain that wind is air in motion

know that climate determines the success of agriculture and understand the impact of occasional catastrophic events

know that landscapes are formed by a number of agents, including Earth movements, weathering, erosion and deposition, and that these act over different time-scales

be able to explain how earthquakes and volcanoes are associated with the formation of landforms

be able to explain the water cycle

STUDY COMMENTARY FOR UNIT 27

TABLE 1 Levels of the attainment targets covered in Unit 27

Level 2

L ATs 1 1

Level 3

L Level 5

2 13

Note: a, b, c, etc. refer to the statements of attainment. For the complete statements, please see pp. 3 and 4.

4 5 16 16 9 1 1 0 / 1 1 1 2 1 1 3 1 4 / 1 5

SCIENCE FOR PRIMARY TEACHERS

COMMENTARY GUIDE The main ideas introduced and developed in this Unit concern geological cycles and how rocks are formed and changed. You will learn about the different elements of the cycles and their contribution. The work builds on the ideas introduced and taught in Units 5 to 8, and provides some possible explanations for the origin and formation of the common rock types you have already met.

The topic of soil is not covered directly in Unit 27. At the end of this Study Commentary you will find a supplementary Section on soil that will enable you to address AT9. There are Study notes and Teaching notes associated with this material.

You should study all Sections of the Unit; the Study notes will guide you through the areas that are most relevant to your needs. There is much information in this Unit that will be useful for such topics as 'Rocks', 'Materials', 'Volcanoes', 'Water', 'The weather' and 'Change'.

1 INTRODUCTION Main attainment targets and levels addressed in Section 1: AT6: levels 2 and 4; AT9: level 5

STUDY NOTES When studying how rocks are formed, we need to be aware of the natural cycles at work within the Earth. This Unit focuses on one important geological cycle: the rock cycle. Looking at natural cycles helps us to understand how the Earth 'works'.

TEACHING NOTES Since this Unit deals with the concept of cycles, we shall take some time here to explore how you might go about introducing the concept into topic work.

The idea of a 'cycle' of events is not an easy one for young children to appreciate. They may have been introduced to the idea in terms of an 'energy chain' or 'food web', but geological cycles present the added difficulty of the enormous time-scale involved. At this stage the most important requirement is to foster their interest in Earth sciences, and to ensure that they gain the understanding and knowledge that is required to meet the necessary programmes of study; you should not try to teach them the full range of events that make up a geological cycle. If you wish to embark on work involving some cyclic phenomena, be aware that young children may not be able to make the necessary connections to enable them to have a full understanding of a 'cycle' of events; what will be within their understanding, however, is the consideration of elements of the cycle as discrete entities.

An obvious cycle to work with is the hydrological, or water, cycle, since this can bring together work on rocks, weather, life on Earth, change and so on, and many of the processes involved in the water cycle can be investigated by means of simple experiments, as the raw materials are readily obtainable. You will appreciate that when we describe phenomena in terms of cycles, we are simplifying what really happens; they are an example of the way we use models.

Before children can begin to appreciate the water cycle, they must have an opportunity to play with, and explore, the properties of water. Make sure that they get used to pouring water into containers and down piping; they should also be beginning to understand concepts such as water finding its own level and flowing downhill. Once children have taken part in activities such as these, they will derive more benefit from investigations that build on these basic ideas.


Discussions may be prompted by asking open-ended questions such as:

Where do we use water in our homes? Where is it used in school or in the local community?

What happens to water when it gets very hot? What does it look like? How do we use it?

What happens to water when it gets very cold? What does it look like? How do we use it?

What is steam? What is condensation? How can we make steam and condensation?

Where does water come from? Where does it go?

First-hand observations can help the children to approach these questions. You could boil a kettle of water in the classroom, and ask the children to watch and listen to what happens as the water is heated. (CAUTION-Never leave the children unsupervised in a room where water is boiling.) With careful supervision, older children could perhaps measure the temperature of the water at various stages during this heating-up process.

It may be possible for older children to perform simple cooking activities, such as heating soup or making hot drinks. Again, it is very important that children are never allowed to perform these activities unsupervised.

Similarly, children can freeze water to make ice-cubes, using a refrigerator in the school kitchen or staffroom. Ask them to time how long it takes for ice crystals to start forming, and then how long it takes for hard ice-cubes to form. Encourage the children to look closely at the ice-cubes, hold them in their hands and describe what they see when the ice-cubes melt. Again, simple culinary activities, such as making ice-lollies, could be useful.

The processes of evaporation and condensation are fundamental to an understanding of the water cycle, and are very difficult concepts for young children to grasp. When children have had an opportunity to explore water and some of its properties, you may wish to develop work on these elements of the water cycle. Some ideas are presented here that could be used to explore children's understanding of these terms, and suggestions are made about how to develop their comprehension.

You may consider having a small tank containing water in the classroom. The children could be invited to write down their ideas about, or observations on, the water, perhaps illustrating their notes with drawings. At this stage, do not intervene or attempt to teach them anything about the water. After about a week, when a variety of items has been collected, the children's .ideas can be explored. You can then use their own experiences and observations as starting-points for subsequent work.

If the children have had the opportunity to observe a water tank left in the classroom, they may have realized that water 'disappears'. This observation can be used as a starting-point; ask them structured questions, similar to those suggested below, to set them thinking about what might have happened to it.

Get the children to make handprints by pressing one hand on to a wet cloth and then on to a paper towel. Ask them to wave the paper towel in the air for a few moments, to dry it, and then ask them questions such as:

What has happened to the handprint?

Where do you think the water has gone?

Do you think you could make the water come back?

How can you make the handprint last longer?

How can you make the handprint disappear more quickly?


You could ask similar questions about other simple activities, such as washing their hands and drying them in air, or painting a picture and watching the paint dry.

The process of condensation, like that of evaporation, is a difficult one for young children to comprehend. Here are some simple activities to use as starting-points to help you to assess the children's understanding of it. Put some ice and water into a tin and then ask the children to look carefully at the outside of the tin. Again, using structured questions, try to establish the level of their understanding of the process. Some suggested questions are:

What do you think is on the outside of the tin?

Where do you think it has come from? Get them to make a list of ideas.

Do you think you can make it go away again? Encourage them to develop ways to try out their ideas.

Can you think of anywhere else you have seen this happen? The children could make a list of similar occurrences.

A second activity could involve the children breathing on to a mirror and then looking carefully at what happens. In the discussion that follows, ask them questions such as:

What happens when you breathe on to a mirror?

What do you think the substance is?

Where has it come from?

Can you make it go away again? Again, encourage them to try out their ideas.

Where do you think it goes?

The activities arising from these questions should allow you to further the children's understanding of these complex processes. When children have grasped the concept of water 'changing state'-that is, from solid to liquid, and liquid to gas-you can introduce and discuss such ideas as where rain comes from, what clouds are made of, where rain goes to, and so on. As always, it is important for children to base their ideas on observations and first-hand experiences rather than simply repeating facts about the water cycle that are beyond their comprehension. (Other work involving the water cycle is included in The Weather..)

Ideas about the rock cycle are best introduced to children after they have studied rocks in more detail (see Section 2 of this Study Commentary), and will only be appropriate for older junior children.

KEY POINTS The existence of natural cycles shows that natural processes do not act independently but are part of a continuing chain of events. The two cycles introduced here are the hydrological cycle and the rock cycle.

Geological cycles enable us to understand the way the Earth works. They represent types of models.


2 ROCK MATERIALS Main attainment target and levels addressed in Section 2: AT6: levels 1 to 4

STUDY NOTES This Section provides much information on rocks and minerals that you will find useful when you come to work on Earth sciences topics in the classroom. Work through the Section carefully, including the AV sequence and Experiment; it will stand you in good stead when you read the later Sections.

To work through the AV sequence 'Minerals' you will need to have samples of the following common rock-forming minerals: quartz, feldspar, mica, calcite and iron pyrites. Samples of these minerals are readily available from suppliers, and are included in a Fossil/Mineral Kit that is available from the address given in the Resources Section at the end of this Study Commentary. The Kit will be used again in the Study Commentary for Units 28-29, where it is needed for several activities; you will also find these common minerals useful for many classroom investigations.

The AV sequence builds on your knowledge and experience of the rocks introduced in Units 5-6 and helps you to identify the main physical and chemical properties of some common rock-forming and ore minerals. During the sequence you will be asked to do a series of tests on the given minerals. As you perform the tests, you will come to appreciate that each mineral has its own unique set of physical and chemical properties. Some of these identifying properties are colour, density and hardness. It is important to realize that no one test will necessarily be diagnostic for any one mineral, but the information accumulated from the different tests will enable you to come to some conclusions about the identification of a mineral and its characteristics.

Section 2.2 gives details about the silicate tetrahedron, which is the basic building unit of common minerals. As you consider the way that igneous rocks form, you will find Tables 3 and 4 (on p.9 of the Unit) essential reading; the information they contain enables us to make predictions about the possible composition and temperature of crystallization of common minerals and rocks considered here. Try to remember that quartz (SiO,) has the lowest temperature of crystallization and, of the common igneous rocks that you will meet, granite contains the highest percentage, by mass, of SiO,. Peridotite and basalt are both silica-poor rocks and contain much lower percentages (by mass) of SiO, than granite, and so have a much higher temperature of crystallization. The composition of the rock, and in particular the proportion of SiO, controlling the temperature range of crystallization of magma (Figure 4, on p. 10 of the Unit), is an important idea; ITQ 1 (on p. 11 of the Unit) will help to reinforce your understanding of this Section.

The Experiment to investigate the density of rocks and minerals is a useful one to do, and a way of adapting the method for class use is given in the following Teaching notes. The results you obtain from the Experiment will also help you

. to revise some of the earlier work on the nature of the Earth's crust (in Units 5 to 8) and density (in Unit 3). SAQs 1 and 2 (on pp. 16-17) should be completed before you move on to Section 3.

TEACHING NOTES There are many different kinds of rocks, and dividing them into three main groups according to how they were formed is a sensible way of coping with the great variety. You may already have done some work on rock properties and identification using the ideas introduced in the Study Commentary for Units 5-6.

When children begin to study rocks, an early observation to be made is that each rock is made up of different materials. In some rocks, such as sandstone, one


material predominates, whereas others, such as granite, contain many different materials. The AV sequence introduced criteria for observing and identifying different minerals, and you can use similar tests to identify rock types. These 'tests' can readily be adapted for use in the classroom and offer an ideal opportunity to develop skills of scientific enquiry, such as observation, testing and sorting.

Some of the tests can be used by the children to discover the properties of both rocks and minerals; others are more appropriate for either rocks or minerals. What you need to remember is that rocks are made up of minerals, so it is not appropriate to test rocks for hardness, for example, since the minerals they contain may well have different hardness values.

To investigate their properties, the children need to make a collection of rocks and minerals. These could be brought in from home, or collected from local environments or school trips. However, before the children embark on work using their own rock samples, it may be appropriate for you to introduce the tests using known samples; this will enable the children to become familiar with some of the common rock properties before they tackle more challenging tasks.

A good starting-point is just to get the children to look at the rock andfor mineral specimens, both with the naked eye and with the aid of a hand lens. Make sure the children know how to use the hand lenses correctly (see the AV sequence 'Rocks and rock textures' in Units 5 - 6 ) . Asking them to draw or paint enlarged versions of what they see will focus the children's attention on colour, texture and particle size. The rocks could also be sorted using colour: for example, rocks that are all one colour, rocks that are mainly one colour, rocks that are two or more colours. (See the Study Commentary for Units 5 - 6 . )

We present a variety of different tests here. You may prefer, however, to encourage the children to test and rank their collection of rocks and minerals according to their own criteria, and they may well devise other tests. However, do remember the message from the AV sequence 'Minerals'-no one test is necessarily diagnostic for identification purposes, and some tests will be more useful than others in identifying different minerals; e.g. hardness, rather than colour and appearance, is a diagnostic test for quartz, whereas colour and appearance is diagnostic for iron pyrites.

In planning your teaching sequence, you may well decide to introduce minerals before moving on to work on rocks. As children begin to appreciate that minerals are frequently seen in their crystalline form, they could be encouraged to examine everyday materials that have specific and individual crystal structures, such as sugar, salt and ice. An investigation to grow crystals is given in the Study Commentary for Units 5-6, and further work on crystals is included in the chemistry materials. Children can be encouraged to carefully observe crystal shapes and be aware that crystals are quite common and widespread. Ask them, for example, where they would expect to find crystals. If they have made crystals using a variety of chemicals, encourage them to look at their shapes under a microscope. A collection of different crystals could be made; encourage the children to record any observations they make-perhaps in a table or by building models of crystals. These have the added advantage of making attractive Christmas decorations!

ROCK AND MINERAL TESTING It is important for children to appreciate that even professional geologists cannot always name every rock or mineral at first sight. They have to perform tests on the rock or mineral in question before they can reach any conclusions about what it might be. A specimen that is particularly difficult to identify may need to be taken back to the laboratory for further, more sophisticated, tests.

Using different rocks or minerals, encourage the children to think about the sorts of 'tests' they could use. You may well have to help them with some of the tests or show them other tests they may not otherwise be aware of. Tests that could easily be used in the classroom include the following.


Colour Record the different colours in a rock, or the colour(s) of a mineral, using either paint or coloured pencils. Some children may be able to use the correct vocabulary to describe the appearance of a rock sample, e.g. dull, shiny, matt, speckled, etc. If you are able to break the specimens, the children will see whether the colours inside are the same as or different from those on the outside. (CAUTION-Take great care to avoid dangerous splinters of rock. Wear safety goggles and smash the rock under a cloth using a geological hammer.)

Reaction with acid Some minerals react with acid-that is, they fizz. Rocks that contain the mineral calcium carbonate (CaCO,) fizz and give off a gas when drops of acid come into contact with the rock. Thus for the mineral calcite (CaCO,), the 'acid' test is diagnostic. Strong vinegar can be used as the acid in this test. Calcium carbonate makes up part of rocks such as limestone, chalk and marble. Test these rocks to see whether they react. Get the children to test blackboard chalk. Does this contain calcium carbonate? Some animals use this material to build their shells. Try testing snail shells or sea shells with acid.

Hardness The children can test different minerals for hardness. They will need to be provided with a variety of items that can be used as 'scratchers', which they can then arrange into order of hardness, such as:

HARDEST SOFTEST

steel file + penknife blade + brass nail + 2p coin + fingernail

When the children have assembled their collection of scratchers, they could try using each one to make a scratch on their specimens. They need to know that making a scratch means that the scratcher actually cuts into a surface rather than just marking it. How will they record their results? When doing this activity the children should be thinking about two variables:

the order of hardness of the materials tested; and

the order of hardness of the scratchers.

Encourage the children to think about why it is important for us to know how hard different rocks are. Engineers and miners, for example, require this information if they are to construct tunnels or drill wells. Some rocks are easy to cut into, whereas others are so hard they need special drills with diamonds embedded in them. The overall hardness of a rock may also tell us something about its usefulness in the construction industry.

Density The density of a rock or mineral specimen can provide a good clue to its identification. Get the children to check, say, five or six specimens (rocks or minerals) that are about the same size. If rocks of the same size have different masses, then they must have different densities.

Ask the children how they could compare the density of these rocks. By simply picking up and handling each specimen they should be able to put the rocks in a rough order of density. They can then check their estimated order by weighing and recording the mass of each specimen. For some, it may be difficult to understand that although things can be the same size they can have different masses. Reinforce this concept by getting them to examine familiar objects around the home or classroom.

If we wish to compare the density of rocks with greater precision we need to know their volume (see the Experiment on p. 13 of the Unit), and so calculate their relative densities. The Experiment can be adapted for use in the classroom.

We can find out the volume of a rock sample by weighing it in water, using a measuring cylinder and a known volume of water. First, pour some water into the measuring cylinder and note the water level; then lower the rock specimen into the water (you will need to tie a piece of thread or thin wire around the rock). Note the new water level. The rise in the water level gives the volume of the rock. Repeat this with each of the specimens and record the results.


To compare the density of each specimen accurately we would have to have specimens of exactly the same size. Clearly this is not possible. However, we can work it out as follows.

We know the volume of each specimen (in cm3) and we know its mass (in g). Now we have to calculate the mass of 1 cm3 of the rock. Do this calculation and record the result.

Now, for each rock specimen, find out for each one how heavy 1 cm3 is, using the following equation:

mass of rock specimen Relative density = mass of an equal volume of water

given that 1 cm3 of water has a mass of 1 g.

This activity is obviously more suitable for older junior children, for whom it would provide plenty of opportunity to bring in mathematical skills and concepts.

Permeability Some rocks will allow water to pass through them-these rocks are said to be permeable. Ask the children to devise a test to investigate whether the rock specimens are permeable or impermeable.

Magnetism If a rock contains magnetic minerals then the rock will be magnetic-i.e. it will be attracted to or repelled by magnetic minerals, especially iron. Test the rock samples using a magnet to see whether they are magnetic. Record the results.

Cleavage or split Some rocks, such as slate, will cleave or split quite easily along particular lines. Carefully examine different rocks. Look for signs to see whether the specimen would split easily in a certain direction. Try to obtain samples of roofing slates to see how they have been cut.

When the children have been introduced to some or all of these tests they could perform them on their own collection of rocks and minerals, and keep records of the results for their own collections. Do ensure that they record where the specimen was found, when it was found, and who found it. They may also be able to suggest a possible identification. If all the specimens are logged in this way, the collection will be of value in that many different children will be able to use it; the procedure also encourages them to be systematic, and gives them an opportunity to try their hand at cataloguing.

INVESTIGATION 1 : MAKING A COLLECTION OF ROCKS To make a rock collection you will need: several egg cartons; some identification books on rocks; some sticky labels or paper and glue with which to make labels; and a collection of local rocks.

Collect as many different rock specimens as you can, either from your local environment, or from further afield; the children could bring them back from family outings, holidays and so on.

Egg cartons are useful to hold the specimens. Get the children to examine the rocks closely, and decide on how they will label each specimen. They may suggest giving them specimen numbers. The place where the specimen was found is also worth including on the label. Can you suggest why this information might be important?

When the collection of rock specimens has been labelled, the children could draw up an identification chart, such as Table 2, on which to record the characteristics of each rock.


TABLE 2 A suggested chart for recording data on rock characteristics

I I Specimen 1 / Specimen 2

crystals

hardness

1 magnetic properties I I

Specimen 3 1

Older children could use a rock identification book, and try to name the specimens based on the data they have recorded.

As further rock specimens are added to the collection, other distinguishing features, such as fossils, may be included on the chart.

Testing rocks and minerals is ideal for small-group work and gives children the opportunity to do a test, make a decision based on the results of that test, and classify items accordingly.

KEY POINTS Minerals are naturally occurring, inorganic materials and represent the most common solid materials on Earth.

Minerals and combinations of minerals have many different uses: e.g. silicon in electronics, diamond in jewellery, talc in blackboard chalk, graphite in lubricants.

Minerals have different properties that are diagnostic for their identificatioh.

Different minerals, like other substances, crystallize at different temperatures; if the composition of a rock is known, its temperature of formation can be estimated.


3 IGNEOUS PROCESSES Main attainment targets and levels addressed in Section 3: AT6: levels 1 to 4; AT9: level 5

STUDY NOTES This is an important Section, which deals with the origins and processes of formation of igneous rocks, and it is perhaps the most demanding Section in this Unit.

From Units 5-6 you will remember that igneous rocks form from a melt and the crystal size reflects the rate of cooling of that melt. Units 7-8 introduced the idea that the source of basaltic magma is the low-speed layer in the upper mantle, and that within the mantle are convection currents thought to be responsible for 'driving' the plates. Section 3.1 provides a possible explanation for the mechanism of this 'driving force' of these convection cells-that is, the radioactive decay of potassium, thorium and uranium.

Much of this Section is not directly relevant to key stages 1 and 2. However, the information provided will enable you to deal confidently with questions relating to the origins and characteristics of igneous rocks.

Allow yourself time to reflect on the concepts introduced in the Section, and then read through the notes below to reinforce your understanding.

VOLCANOES AND VOLCANIC ROCKS The way in which magma is erupted out of a volcano varies, depending on the composition of the magma and its dissolved gas content. Viscosity is the resistance of a liquid to flow, and lavas of different compositions display different viscosities. Basaltic lava (silica-poor) has a low viscosity and is usually free- flowing-often at very high speeds (up to 50 km h-'). This type of lava forms low-angled volcanic cones typical of those found in Hawaii. Lavas rich in silica (rhyolites) have a viscosity about 103 greater than basaltic lavas, and tend to form steep-sided cones.

Where there is a high dissolved gas content, the eruption is much more vigorous: basalt volcanoes display fire fountaining, whereas granitic magma can suffer violently explosive eruptions as the gas forces its way out of the highly viscous liquid.

CONSTRUCTIVE PLATE MARGINS AND THE ORIGIN OF BASALTS At constructive plate margins large quantities of basaltic lava are erupted. This magma has originated in the mantle. We know that the mantle is composed of peridotite and that it is likely that the origin of the magma erupted at constructive margins is the low-speed layer of the upper mantle (see Units 7-8). We now need to consider how basaltic magma, which forms new oceanic crust at constructive plate margins, is derived from mantle peridotite. It is generally thought that basaltic lava is produced by the partial melting of mantle peridotite. Partial melting is a process in which a mixture-in this case peridotite--does not 'melt' completely at one specified temperature. Pure substances (if you need

' to refresh your memory about what this means, you should refer back to the chemistry materials) melt at a fixed temperature. Rocks, however, are usually mixtures and so melt progressively over a range of temperatures. Peridotite contains mainly olivine, pyroxene and a little feldspar. Feldspar and pyroxene will start to melt before olivine (refer to Figure 4 on p. 10 of the Unit). So an early magma of peridotite will tend to have a pyroxene-feldspar composition- i.e. the composition of basalt.


PLUTONIC ROCKS Plutonic rocks are formed below the surface of the Earth as magma solidifies after being squeezed, i.e. intruded, into older rocks. You have already seen examples of plutonic rocks, for example granite and gabbro, and know that their texture is coarsely crystalline (i.e. they have large crystals) as a result of slow cooling.

Like the melting process, the crystallization process of rock is far from simple. The different minerals in a magma crystallize over different temperature ranges. As minerals are taken out of the magma as a result of crystallization, the magma gradually changes in chemical composition; the usual effect of this is that the magma becomes more silica-rich. This process is called fractional crystallization.

DESTRUCTIVE PLATE MARGINS AND THE ORIGIN OF ANDESITES At destructive plate margins, the ocean trench represents the place where two plates are converging, with one plate-which is always an oceanic plate--diving below the other and eventually becoming resorbed into the mantle. As the slab of oceanic lithosphere descends into the mantle, it becomes heated, partly by friction along the Wadati-Benioff zone, and partly by conduction from the surrounding hotter mantle. The overriding crustal rocks will also be heated. Partial melting of the subducting plate will occur. Magma will rise through the overlying plate, often producing plutonic cores to the fold mountains, or perhaps being erupted as lava.

This process at destructive plate margins involves the down-going plate moving continuously, but the overriding plate staying in the same position with respect to the subduction zone beneath it. This means that, with time, more magma will pass upwards through the same zones in the overriding plate. Remember that the process of partial melting always takes out the lowest temperature fraction-so SiO, becomes increasingly concentrated in the 'magma. Thus if peridotite is partially melted, there is an increase in the silica content of the resultant magma, thereby producing a basalt. When basalt is partially melted, the resulting magma will be andesitic and, as the rocks at the base of the crust melt, granitic magma will be formed. It is important to understand that both partial melting and fractional crystallization processes have the same overall effect-they produce an increasingly siliceous material-and that whereas basaltic material can be resorbed into the mantle, the production of andesitic and rhyolitic magma is a one-way process: magma of this composition cannot be resorbed back into the mantle, because its density is too low. It therefore forms a permanent addition to the crust.

Section 3.6 ends with an explanation of the origin of continents. The Summary forms a useful list of the main concepts introduced in this Section.

TEACHING NOTES We have given some suggestions for work on rocks and rock textures in the Study Commentary for Units 5-6 and in Section 2 of this Study Commentary. The material is appropriate also for this Section.

If you do any work on volcanoes and volcanic eruptions, this could lead to exploration of the children's own ideas about the interior of the Earth (see the Study Commentary for Units 5-6). The effect of insulation may be a useful idea to investigate here, so that children can appreciate how some things can stay hot for a very long time.

Children could set up investigations to answer the questions:

Can you find the best material to help keep a hot drink hot for a long time?

Can you make a container that will stop ice-cubes from melting?


Materials that the children will need to collect, or be provided with, to enable them to answer these questions include: a stop-watch or timer; a variety of containers of the same shape, size and material, such as empty cans, plastic pots, etc.; a collection of materials, such as cotton, cotton wool, nylon, wool, plastic 'bubble' material, thin foam, etc.; rubber bands and sticky tape; sticky labels; some ice; a kettle with which to make hot drinks; and a thermometer. This work relates well to that covered in the Study Commentary for Unit 9.

Volcanoes can be studied using secondary sources, such as atlases and other reference books. This will allow the children to find out where active volcanoes are found today, and note the differences/similarities in their appearance. Older children may be interested to discover why volcanoes have different shapes, and erupt different types of lava. They may well have been on holiday to a volcanic area and brought back samples of local rock derived from lava. Do not despair if no one is holidaying in a volcanic area-a local chemist should be able to supply you with some pumice, which is of volcanic origin.

Carefully supervised cooking activities, such as making fudge and toffee, may help the children to appreciate the property of viscosity and how this affects the movement of liquids. (Other practical work involving viscosity is covered in the chemistry materials.)

Crystalline rocks are frequently used as facing stones for buildings; a local churchyard will perhaps contain a large variety of rocks that have been made into headstones. A town or churchyard trail may be an appropriate and enjoyable activity to develop with the children. It will allow them to use their observational skills to look at what the local buildings or memorials are made of, and will also provide an opportunity to practise classifying rocks as either 'crystalline' or 'fragmental'.

KEY POINTS Magmas are complex chemical mixtures, containing many elements that become distributed between several minerals as the magmas crystallize. Each mineral in an igneous rock has crystallized at a different temperature. A magma crystallizes over a range of temperatures, and does not, in general, solidify all at once.

Partial melting helps to explain the wide diversity of igneous rocks that occur in the Earth's crust.

Fractional crystallization is the process by which the minerals crystallizing out from a melt leave a residual liquid of a different composition.

4 SEDIMENTS AND SEDIMENTARY ROCKS Main attainment targets and levels addressed in Section 4: AT6: levels 1 to 4; AT9: levels 2, 3 and 5

STUDY NOTES This Section is concerned with the formation of sedimentary rocks, and contains much information that you should find useful in your teaching. The TV programme 'From Snowdon to the sea' clearly shows the different processes involved at various stages, as a river flows from source to mouth, and the different types of sediment that are deposited at each stage.

Sections 4.2 and 4.3 deal with the breakdown or weathering of rocks and the products of this breakdown. The soil-a product of weathering-is dealt with in detail in a supplementary Section (see pp. 21-32). The concept of relative


resistance of minerals to weathering is important, because it is this relative resistance that results in the landscape as we see it.

Sections 4.4 to 4.7 deal with the transportation and subsequent deposition of weathered material. The nature of the sedimentary rock that is formed as a result of the accumulation of the products of weathering is, to a large extent, determined by the method of transportation of these products, together with the distance over which they have been transported. So by careful examination of a sedimentary rock, and using data from present-day environments, we can make deductions about the environmental conditions existing when the sediments were deposited.

TEACHING NOTES Many of the ideas and activities relating to the watei cycle that were introduced in the Teaching notes to Section 1 are also relevant here, and provide starting- points for thinking about the role of moving water in sediment transportation.

Practical work involving moving water is difficult to set up in the classroom, although many children will be aware of its action and effects if they have played at damming part of a river or stream. A visit to a local beach or river would be invaluable, so that the children could see and feel particles, stones and rocks being moved by the water. They may also be able to observe the way in which materials have been laid down in layers, or 'ripples', by the moving water. Some of the practical activities in exhibitions such as the Launch Pad at the Science Museum, London, or other interactive science centres, allow children to experiment with moving water.

Young children may find it hard to appreciate that water is capable of 'wearing something away'. You could get them to investigate this phenomenon using a slowly dripping tap and a bar of soap.

INVESTIGATION 2: ERODING SOAP For this investigation you will need a new bar of soap and a tap that can be made to drip slowly.

Before the children set up this investigation they need to think about how they will be able to determine whether any soap has been 'washed away', or eroded.

Get the children to position a new bar of soap in a sink so that a slow, steady drip of water splashes on to it from a tap above. The investigation should be set up before the children go home in the afternoon. Ensure that they make a careful note of the time they started the investigation, and that no one touches the tap or soap after it has been positioned. The water should be allowed to drip all night.

In the morning, the children should record the time the tap is turned off, and work out the length of time the tap has dripped. They may even be able to calculate the average number of drips per minute.

The children now need to determine whether any of the soap has been eroded. One way to do this is to weigh the soap before and after the investigation to determine how much of its mass has been washed away.

Some suggestions for further investigations in this area are given below.

A shower-head-type spray can be fixed at different heights. The children could investigate whether changing the velocity of the water drips (i.e. changing the height of the spray) has an effect on how much soap is eroded. The results could be compared with those obtained from the dripping tap in Investigation 2.

Several different soaps could be used, to see whether some soaps are more resistant than others to the effect of dripping water. How do these harder


soaps compare in price? The children may be able to make suggestions as to which brands therefore represent better value.

Large rain drops, which occur in cooler weather, cause more erosion. Craters formed by different-sized drops could be measured in fine powder or flour.

Another powerful eroding and transporting force is the wind. Ask the children to describe what a slight breeze feels like; what does it feel like when the wind is very strong? Have they seen objects being lifted up by the wind? What sorts of things can the wind pick up and move about?

The effects of abrasion are likely to be visible at various places around the school or in the children's local environment. Stone steps often have indentations in them due to the abrasive effect of foot traffic over many years. If you have a set of worn steps like this in your school, the children could attempt to measure the amount of material that has been worn away. They could also look at the effects of erosion on local buildings; do parts of a building that face in different directions suffer different amounts of erosion? If you can visit a churchyard containing old gravestones, they will be able to see the effects of erosion on several different types of rock. They may be able to make comparisons of the degree of resistance to erosion of the different types of rocks.

As children come into contact with various types of rock, they will learn to recognize the 'feel' or texture of the different types. Sandstones, for instance, generally feel 'sandy' or 'gritty', whereas marble feels very smooth. They may like to set up an investigation to see whether their friends can recognize rocks from their texture.

INVESTIGATION 3: CAN YOU FEEL THE DIFFERENCE? To do this investigation you will need: some specimens of rock that have different textures, such as sandstone, mudrock and granite (these should all be of approximately the same size-about 5 cm3); a small cardboard box placed on its side-a shoe box without a lid works well; and a piece of thick cloth

! to cover the box.

Cover the open side of the box with the cloth. (It may be best to use sticky tape to secure it.) An opening will need to be cut in the piece of cloth so that someone can put a hand through it and reach into the box. Before securing the cloth, place all three rock specimens in the box. The children can then ask their friends (or teachers!) to reach into the box and arrange the rocks into a sequence by texture-for instance by placing the smoothest to the left and the most coarse to the right. The children can then check the results of each person's test and record the data. Encourage them to test at least 10 people.

What other rock specimens could they include in the box?

KEY POINTS The environment of deposition helps to determine the character and properties of a sedimentary rock.

The importance of the principle of uniformitarianism is emphasized when considering sedimentary rocks.


5 TECTONIC AND METAMORPHIC PROCESSES Main attainment targets and levels addressed in Section 5: AT6: levels 1 to 4: AT9: level 5

STUDY NOTES Sections 5.1 to 5.3 deal with the deformation of rocks; Section 5.4 is concerned with metamorphic processes. Faults and joints are common features and are the result of a rock being subjected to external forces of tension or compression at relatively low temperatures. Structures produced under tension and compression differ: normal faults develop under tension and reverse faults under compression. At higher temperatures, such as the conditions under which metamorphic rocks form, different structures tend to occur as a result of plastic deformation. Metamorphic rocks represent the group of rocks that have been changed since their formation. This 'changing' is due to heat or pressure or, more typically, a combination of both.

The AV sequence 'Metamorphic rocks' introduces some common metamorphic rocks, together with their characteristics and conditions of formation, and is well worth working through. Metamorphism is not an easy concept to understand; perhaps one way in which it can be appreciated is by thinking about metamorphic processes with which we are familiar. If your house is built of brick and roofed with baked clay tiles, then, like most of us, you owe your home to metamorphism! Both these building materials are produced by working wet clay into the appropriate shape and firing it in a kiln. This transformation from soft clay to hard bricks and tiles can be regarded as a type of metamorphism brought about by heat-reactions have taken place that have resulted in permanent physical and chemical changes. If the reactions were readily reversible, the products would be of no value as building materials. In fact, they weather almost imperceptibly, mainly by mechanical processes, and may take centuries before they are broken down into separate particles.

In hotter, sunnier climates than ours, bricks are frequently made by allowing the clay blocks to dry in the Sun. The resulting bricks are quite hard, but need protection from tropical storms-e.g. by using wide overhanging roofs. Thus it would seem that a certain minimum temperature must be achieved for 'metamorphic' changes to take place.

So when rocks are heated, under certain conditions, they are changed by metamorphic processes, but the overall chemical composition of the metamorphosed rocks is not affected. Apart from some loss of water, there has been no change in the chemistry of the brick during its firing-new elements have not been added to the brick, and none have been lost from it.

This everyday analogy illustrates three fundamental aspects of the metamorphism of-rocks. First, all the changes occur in the solid state, with minimal change of shape and volume. In metamorphism there is usually no melting of the rocks. Second, there is usually no significant change in the overall chemical composition of the rocks-just as nothing is added to or taken from the articles being fired in the kiln, other than the addition of heat and the loss of water, along with a little CO, and other less important gases. Third, the original minerals form new minerals, and this produces a new rock. The chemical elements that make up the original minerals (clay minerals in our analogy) are redistributed into new crystal structures.

There are two main kinds of metamorphic process.

Contact metamorphism In this process, surrounding rocks are heated during intrusion of igneous rocks of younger age. The resulting rocks show changes in texture and crystallization. The rocks have been 'baked', but there is usually no


strong directed pressure involved. The manufacture of pottery and bricks is thus analogous to contact metamorphism.

Regional metamorphism This type of metamorphism affects vast areas, and the metamorphism of huge volumes of rocks is usually linked with large-scale tectonic movements. Strong directed pressures are involved with regional metamorphism.

Temperature and pressure represent important variables in the metamorphic process: different temperatures and pressures result in different rock types. Figure 1 shows how a mudrock can be changed into different rocks as a result of increasing temperature and pressure. The degree of metamorphism is referred to as the metamorphic grade.

mudrock

site

J phyllite

J schist

-1 gneiss

low-grade metamorphism

increasing l

temperature I and

pressure 1 high-grade

metamorphism

FIGURE 1 Sequence of changes affecting a mudrock: an example of metamorphic grade.

From this Figure we can see that slate is a low-grade metamorphic rock, whereas gneiss is a high-grade metamorphic rock resulting from much higher temperatures and pressures.

TEACH ING NOTES Children may have observed folding or faulting either when looking at rocks along the coast or in photographs. To investigate the processes involved in producing these structures, Plasticine could be used to make models upon which lateral pressure can be exerted. Sandwiches using brown and white bread and a variety of fillings are also fun to use. They have the advantage that buckled 'rock formations' can be eaten at the end of the lesson! Investigating the properties of foods such as toffee can help children to understand how the sudden application of pressure can cause rocks to crack or break, whereas the application of pressure over a long period can cause them to bend.

Some rocks crack into a characteristic pattern--e.g. the joints in limestone rocks, which can be studied from photographs or field visits. Substances such as mud, of which the children will have had experience, can also exhibit this feature when they dry out.

Two common activities, the firing of clay and cooking, can be of considerable use in explaining how heat can alter materials. The children could examine clay or cake ingredients before and after they have been subjected to heat, to see in what ways the material has changed.

KEY POINT Metamorphic rocks are rocks that have been changed by heat andlor pressure, but that have not changed their chemical composition. The original minerals produce new minerals under these conditions, thus forming a new rock. Everyday 'metamorphic processes' include baking a cake, and making bricks and pottery.

). b . .. S -


SUPPLEMENT: SOlL Main attainment target and levels addressed in this Section: AT9: levels 2 and 3

STUDY NOTES This Section considers factors affecting soil formation and the main goil types found in Britain.

Soil is essential for many of our activities. It is also a basic part of the natural environment. Soil, together with the plant life it suppoPts, the rock on which it lies, and the climate it experiences, forms a finely balanced natwal system.

Depending upon the context, the word 'soil' may have very different meanings. A simple definition of soil is the material that plants grow in, and which provides them with physical support and nutrients. There are other more particular views of soil-engineers, geologists, hydrologists, farmers and gardeners are all concerned with different aspects or properties of soil. The geologist, for example, calls this layer the regolith, and regards it as the weathered material of the underlying bedrock.

When fresh rock is first exposed at the surface of the Earth there is no soil. Soils are produced by physical and biological agents acting on a parent material. This may be solid rock or fragmented materials such as glacial deposits. Initially, the parent material is acted upon (weathered) by climatic factors, such as rainfall and temperature. Later, when colonized by plants, it is also acted upon by these and other biological agents. The build-up of organic matter in the soil, from the breakdown of plant litter, is a particularly important process.

THE SOlL PROFILE If you dig through the soil down to the upper part of the underlying rock, a cross-section of the soil is exposed. This is called the soil profile (Figure 2). A soil profile often has a number of distinct layers, or horizons, characterized by differences in colour, composition and texture. It is possible to make many subdivisions and classifications of soil by using these horizons, but we can start with a simple threefold division-which can be designated by certain letters.

horizon A (topsoil)

horizon B (subsoil)

horizon C (weathering parent material)

FIGURE 2 A so profile is a set of soil horizons.


Horizon A-sometimes called topsoil

horizon B-sometimes called subsoil

horizon C-parent material.

The parent material underlies the soil. The term 'parent material' includes soft substances, such as glacial and river deposits, as well as hard rocks, such as sandstones, limestones and granites.

HORIZON CATEGORIES

Horizon A This comprises the material at the top of the soil profile. It is usually dark in colour because of the decay of roots in situ and the incorporation of plant litter from above. It is this horizon that is most affected by the activities of living things. Most roots are found in this layer, together with plant and animal life-from the larger animals such as moles and earthworms, to the microscopic bacteria and fungi.

Horizon B This horizon consists chiefly of altered rock fragments. It contains very little plant material, although live roots and some plant and animal life occur. Within this horizon, mineral materials are actively broken down and altered, plant nutrients released, and the size of the soil particles made smaller. This horizon also receives material washed down from above, for example, fine clay particles or iron oxides.

Horizon C This horizon is the parent material from which the soil has developed. The parent material is often weathered but otherwise little altered. The depth at which the parent material occurs depends on the nature of the underlying rock as well as the length of time during which soil formation has been taking place.

THE ROLE OF WATER IN THE SOIL PROFILE There is an enormous range of very different soil profiles produced by different soil processes operating at varying rates and in different combinations. Despite the diversity of processes involved, the movement of water (i.e. percolation) through the soil profile represents one of the main factors in its development. The amount of water retained by the soil depends on the relative rates of input of water to and loss from it. Factors affecting these rates include quantity and rate of rainfall, temperature, and soil composition and depth. Water is mainly held within the soil profile in holes and cracks of varying sizes: some, of very small diameter, are full of water under most normal conditions; other, larger, holes and cracks become filled as water percolates through the soil after rainfall. Around the soil particle a layer of water is held tightly by surface tension and never released. Capillary water around the particles moves by attraction to other water molecules.

Water is a major factor in bringing about the rearrangement of weathered material (see Figure 3), and so producing the distinctive soil profiles. Rearrangement takes place chiefly via the transfer of material by water through the soil profile. Most transfers are vertical-upwards and downwards. In the UK, where precipitation almost always exceeds evaporation, upward movement is mainly biological, and results from the uptake of water and nutrients by plants. The downward movement of water, carrying minerals in suspension or solution, produces the characteristic soil profiles found in Britain. The amount and speed of water flow through the soil determines both the rate at which the products of weathering are removed and, in part, the nature of the resulting soil.

, . STUDY COMMENTARY FOR UNIT 27 - .

d d d d d d d d d

d d d d

d d d d d d d d d

d d d d d d d d d

d d d d

FIGURE 3 The role of water is important at every stage of the soil-forming process: (a) when rock is first exposed to the process of weathering; (b) when plant lie begins to grow on and in the weathered rock; (c) in the rearrangement of material to produce the soil profile.

In a soil that allows rapid percolation, a high annual rainfall produces a substantial throughput of water, and hence a substantial potential for moving material from one part of the soil to another. Much of this water drains freely out of the soil and through the material below, taking with it dissolved or suspended material. This process is termed leaching.

Some soils provide clear evidence for the transfer of soil constituents. The podzol is one such soil commonly found in Britain. In their natural environment, podzols often occur under heather or coniferous woodland, on sandy, freely draining, parent materials. They have a very distinctive profile, which can be divided into four broad horizons from the surface downwards (see Figure 4).

raw acid humus in and above the A layer marked bleached layer

marked black or red-brown iron-rich horizon

weathering parent material, e.g. sandstones

FIGURE 4 Podzol soil profile.

Notice that this profile includes a horizon E. This denotes a layer that is usually depleted of iron oxides; in the podzol profile, horizons E and B should be considered together, for it is here that the effects of the downward transfer of soil material by water are clearly seen. Horizon &the eluviated, or leached, layer- consists chiefly of bleached sand grains (that is, sand grains from which the surface coatings have been removed). In contrast, horizon B often appears to consist of coated sand grains cemented together by black organic materials andlor red-brown iron-rich materials. Horizon C is the little-altered parent material from which the soil has developed.


The soil-forming process responsible for this type of profile involves the transfer, by percolating water, of iron, aluminium and organic matter from horizon E to its deposition in horizon B. This process produces one of the most visually distinctive soil profiles to be seen in Britain.

SOIL-FORMING FACTORS Soil is only one part of the natural environment; it interacts with other components, and so forms an ecosystem. The wide variety of possible interactions is responsible for the large number of soil types found. In considering which components of the environment exert a major influence on the nature and distribution of soils, it is possible to isolate four major factors:

1 parent material

2 climate

3 terrain

4 plants and animals.

In addition to these factors, it is important to take account of how long the interactions have been taking place. We should perhaps consider time as a fifth factor.

Parent material

Parent material strongly influences the soil and its properties, particularly during the early stages of soil development. For example, the weathering of a coarse sandstone parent material produces a well-drained, coarse sandy soil. In contrast, a mudrock generally weathers to give a fine-textured soil, which may allow water to flow through it only very slowly.

Limestone and chalk (predominantly made up of CaCO,) exert a distinctive influence as they are easily soluble. When they are weathered, much of the original rock goes into solution and is thus removed, so what remains has often only a very small mass; consequently soils developed on limestone or chalk are often shallow, with an organic A .horizon directly overlying weathered parent material.

Climate

Climate is another important factor to consider in relation to soil formation. On a world scale, there are broad climatic regions, and it has become traditional to distinguish complementary soil and vegetation regions which often extend latitudinally across the globe. On a smaller scale, climate remains of fundamental importance, and in Britain soil variations occur because of differences in temperature and rainfall. At this level, the major climatic influence on soil development is probably rainfall. Rainfall is the major source of soil water and, as we have seen, the presence of water and its percolation through soil is an essential process in the development of a soil profile. Climate also has a marked influence on soils developed from materials of low permeability. When rainfall is heavy, such soils are likely to become waterlogged, forming gleys or peats.

Terrain

Terrain influences soil development in a rather complex way, reflecting both the varying conditions of drainage and water flow within the landscape, and the patterns of erosion and deposition. If we take an idealized landform developing on one uniform parent material, the sequence of soils across that landscape reflects the varying conditions of soil moisture and drainage at each site (see Figure 5).

On the flat crest, the soil is well above the water table-the underground surface below which the rocks are saturated with water-so water percolates rapidly to


give a soil that is excessively freely drained. Moving downslope from the crest, over the upper slope convexity, the depth of soil changes, which results in the water table appearing to move closer to the surface, but drainage is enhanced by lateral flow, either over or through the soil; these soils are typically freely drained. This convex part of the slope may also be subject to soil erosion, and consequently the soil may be shallow. Further downhill, on the middle and lower slopes, the soil is again influenced by both vertical and lateral movements of water; but this is an area where material eroded from upslope accumulates, so the soils are deeper and freely drained. At the slope foot, the water table is likely to be close to the surface, and there is little gradient to promote lateral flow; vertical percolation becomes restricted and the soil may be waterlogged for part or all of its depth. Valley soils are often deep because of the accumulation of eroded material, and they can often be waterlogged. In some circumstances the flat valley floor soil may be sufficiently wet to lead to the formation of peat.

V>rn>>>T> - zone of transfer - net accumulation

e x >> S > deep. )) imraz;~l~>> waterlogged ) freel dramed freely dramed freely dramed

FIGURE 5 The effect of terrain on soil development.

Plants and animals

Weathering and soil development are accelerated by the appearance and growth of plants on the surface of the bare rock, or on weathered debris. These plants, which include lichens and mosses, photosynthesize, or 'fix', atmospheric nitrogen and incorporate it as plant protein. When these plants die, they return a variety of organic materials to the surface of the weathered rock-this is the first soil material. By returning organic material to the surface, these first colonizers of the rock provide both nourishment and a 'foothold' for a succession of plants and organisms-from lichens and mosses, through grasses and shrubs, and eventually to trees. This is the beginning of the biogenic cycle, which gives soil many of its distinctive characteristics. Its importance is not just that it adds plant material to the soil; in addition, -a large population of soil organisms (e.g. earthworms, fungi and bacteria) helps break down materials for incorporation with the weathered minerals. The biological activity of plants and animals-in particular the action of plant roots, which penetrate the underlying rock and force it apart-ontributes greatly to the process of rock disintegration.


Time

Time is difficult to establish as an independent influence on soil development. Nevertheless, the time during which the foregoing factors have been interacting to produce soils will be reflected in the nature of the soils developed. It is often suggested that the influence of parent material is greatest during the earliest stages of soil development, but that with time climate may become dominant. Since most soils in Britain have developed since the end of the last ice age (about 10 000 years ago), it is important to ask how well developed they are, and how they might change in the future.

HOW DO SOILS DIFFER? Given the complex interactions between the soil-forming factors, it is not surprising that there is an enormous range of soils in Britain. Climatic and, as we have seen, biological factors strongly influence the development of soils and have led to the formation of four major soil types in Britain: podzol; brown earth; organic (rendzina is an example of an organic soil); and gley (see Figure 6). The balance between water input and drainage is of major importance in determining soil type (see Figure 7j.

A raw acid humus in and above the A layer

E marked bleached layer

marked black or red-brown ~ron-rich horizon

podzol brown earth

--L'---

rendzina

A organic matter well incorporated in soil


B brown, with good crumb structure


C hard limestone

B grey or mottled profile

FIGURE 6 The four main soil types in Britain.

Well-drained soils

Rainfall arriving at the surface of a well-drained soil percolates rapidly through it, and often carries soluble soil constituents, which may be transferred from one part of the soil to another, or completely removed. If the process of removal has been operating for long enough, the soil is described as leached. The rate at which a leached condition is attained will depend on the nature of the parent material; in sandy soils it may occur rapidly.

The podzol discussed earlier is a well-drained soil, developing on a sandy parent material, often under coniferous woodland. Within the upper part of the profile a distinctive grey 'bleached' horizon (E) is clearly visible. The black or red-brown layer (B) is evidence of the rapid percolation of water through the profile, and consequent leaching. Organic matter and iron is removed from the bleached layer and deposited in the layer beneath.


good

9 C .- E -0

poor

A

low input

LEACHED

good drainage

C

low input

GLEY

poor drainage

high input

PODZOL

good drainage

D

high input

PEAT

poor drainage

FIGURE 7 Soil water balance and the formation of soil types.

A brown earth develops under a deciduous forest or grassland area. There is often no distinct boundary between the A and B, and B and C horizons, and the soil has a good crumb structure. Horizon A is usually dark brown in colour with abundant soil organisms. This grades into horizon B, which is lighter in colour and passes into the weathering parent material. Both acid and calcareous brown earths occur, depending on the nature of the parent material.

Rendzinas and rankers are generally thin soils with the A horizon developed over the bedrock. Rendzinas are thin, alkaline, organic soils; rankers are thin, acid, organic soils.

Waterlogged soils

Many soils are affected by waterlogging periodically and for varying lengths of time. The cause of waterlogging may be a high groundwater table, slow percolation of rainfall through the soil, or both. The characteristic features of waterlogged soils are predominantly grey and bluish-grey colours in the zone of permanent waterlogging, and a patchwork of grey and bluish-grey colours together with orange and yellow colours (this patterning is known as mottling) in the zone where waterlogging occurs for part of the year, or for short periods throughout the year.

The most extensive waterlogged or seasonally waterlogged soils are gley soils. Gleys are often waterlogged and anaerobic, with iron present mainly in the grey- coloured reduced state-iron(I1); mottling occurs in horizons that dry out and where some oxygen penetrates and oxidizes iron locally to the red-coloured state-iron(II1). Peats accumulate under wet conditions.

SOIL TEXTURE

Soil is made up of a range of mineral particles, in some cases intimately mixed with organic particles. The relative proportions of the different-sized particles present in a soil determine its texture. Broad categories of soil, such as sand, silt and clay, are fairly easy to distinguish (Table 3), although a number of intermediate soil classifications exist. Mixtures of different-sized particles are known as loams; we can thus refer to, say, sandy loams or clay loams.


TABLE 3 Size distribution '(in mm) of the main soil particles

sand 2.0-0.02

silt 0.02-0.002

clay less than 0.002

Soil texture can be quickly assessed in the field by rubbing a moist sample of soil between the fingers. The feel of the soil depends to a great extent on the relative proportions of sand, silt and clay, and is generally described as soil texture.

Sandy soils feel gritty; the coarse particles can be clearly seen and look like sandpaper. A moistened silty soil feels smooth and rather soapy. A clayey soil has a large proportion of fine particles and feels sticky. These three categories- sandy, silty and clayey-are just the broad textural classes. Finer subdivisions are made to take account of soils with different mixtures of sand, silt and clay. For example, a soil described as having a sandy clay loam texture will have approximately 57% sand, 25% clay and 18% silt.

Soil texture refers only to how soils feel to the touch. Soils having the same texture may look very different. Alternatively soils may look very similar but have quite different textures.

SOIL STRUCTURE The components of soil do not usually occur as separate sand, silt or clay particles, but are normally 'stuck together' and organized into aggregates; the soil is then described as having structure. Where there is no aggregation, the soil is' described as structureless.

There are many types of soil structure. Some are produced by the actions of farmers or gardeners when they artificially break up the soil. These structures are very irregular in shape and called clods or fragments. Naturally occurring soil structures have different shapes and sizes, ranging from near ball-like structures of a few millimetres in diameter (granular or crumby structure) through to large, vertical, pillar-like structures (columnar or prismatic structure), which may be more than 20cm long. Structure helps to determine how successful a soil will be in supporting good plant growth and allowing water to percolate.

Blocky structures result when the soil particles are aggregated into irregular cube- like shapes. This arrangement, with blocky structures of different sizes, results in a loosely packed soil with large connected holes down through the subsoil. Soil water percolates freely and plant roots grow easily into the soil to take up water and plant nutrients (see Figure 8a). Under these conditions crop growth should be good.

In contrast, platy structures result when the soil particles are aggregated into plate-like forms, arranged horizontally. These structures are often very compact, with few vertical holes or cracks. They restrict the free percolation of water from topsoil to subsoil and, under moist conditions, may produce waterlogging within the topsoil and restrict plant growth. Plant growth is further restricted when roots are unable to penetrate platy structures and reach the soil below; the plant cannot gain access to water and nutrients within the subsoil, and crop growth is limited (see Figure 8b).

In normal British summers, the water in the topsoil may be insufficient to satisfy the needs of plants, which may wilt and die. Platy structures occur naturally within the soil, but they are also produced by the farmer ploughing, or the gardener digging the soil, when it is wet.


(a) (b) FIGURE 8 The effect of soil structure on plant growth. (a) Blocky structure. (b) Platy structure.

TEACHING NOTES Soil is an ideal medium for children to work with since it is readily available almost everywhere. A variety of investigations and topic work can be developed using soil. Your school may have gardens or grounds, which will be useful resources, and some children will have access to other gardens; it may also be possible to involve parents who are keen gardeners.

In work on soil you should encourage the children to think of it as a precious resource-and one that should not be contaminated or wasted. Research into past disasters, such as the creation of the dust bowl in the USA, can provide cross- curricular links and reveal the problems that societies have had to face when soil has been abused. We should not be complacent today, ,since numerous contemporary examples can be cited of soil contamination spoiling vast areas of land that previously had been productive. Thus, the study of soil can provide an opportunity for the discussion of environmental issues.

For younger children, work on soil can involve gathering information about what soil is and finding out that not all soils look the same. Key stage 2 work can progress to children finding out, by testing, what soil is made up of, and how different soils vary.

An early question to ask the children is: what is soil? Some children may know that soil is made up of different-sized particles and contains decaying plant and animal material, called humus.

To begin investigative work, children could collect a sample of soil from their garden or local environment. As with the study of rocks and minerals, beforexhe children perform any other activity, allow them time to look carefully at their soil sample, using a hand lens. If you have a variety of soils, encourage them to describe what they can see; ask whether they can identify any differences and similarities-and perhaps talk to them about what they think soil is made of and what we use it for. Their sample may contain clues as to some of the things it is made of--e.g. small pieces of leaf or twig. Handling the soil will encourage them to begin thinking about soil texture; they may be able to use the appropriate vocabulary to describe what their sample feels like, such as smooth, sticky, damp, gritty, and so on. They may also discover that the smaller the size of the soil particles, the smoother it feels. Young children could record their observations as illustrations, using coloured crayons or paints.

If you have access to a bank of sieves-i.e. a series of sieves having different- sized meshes-further analysis can be done to separate the different-sized particles; comparisons can be made between a variety of soils from different localities. Older children could weigh each sieve fraction and compile histograms


for the different soils. When comparing soils, the children will need to think about how to ensure that their tests are fair, and make careful records of the results. (Note: Ensure that soils are dry before you carry out any investigations on them.)

If you are able to obtain soil samples from different localities, older children may begin to appreciate that there is a relationship between underlying rock type and soil characteristics. As with rock samples, you could build up display and - - resource materials of different soil types.

When children become more aware that soils differ, they could devise tests to investigate these differences. Investigation 5 suggests a method of discovering whether or not all soils hold the same amount of water.

INVESTIGATION 4: ARE ALL SOILS THE SAME? Get the children to collect samples of soil from different places in the local environment, e.g. gardens, parks, fields, woods, waste ground. First, ask them to look carefully at the soils and say whether they think they are all the same. Can they devise a test to discover whether their hypothesis is correct?

One method of investigating whether or not soils differ is to put a small sample of soil into a clean, empty container with a lid, such as a jam jar, and add water to almost fill the jar. Put the lid on tightly and shake the jar carefully. Leave the jar undisturbed overnight-r longer if you can.

After a suitable time has elapsed, ask the children to look carefully at the jar. Can they see different layers in it? How can they record their observations so that the different types of soil can be compared? Can they find out what makes up each layer? Can they identify what is floating on top of the water? If they examine some of the material that is floating, they should be able to identify pieces of dead plant material. This makes up the humus in a soil.

INVESTIGATION 5: DO ALL SOILS HOLD THE SAME AMOUNT OF WATER? For this investigation you will need to collect together three known masses of different soil samples--e.g. sand, clay and loam (a mixture of sand and clay) or peat; some water; three filters-the top part of a squash bottle with a piece of muslin over the neck makes a good filter; three containers for the filters to sit in; a jug; and a stop-watch.

The investigation should be set up as in Figure 9, using known volumes of water. The children will need to think about whether they should weigh each soil sample before they begin, and whether they should test an equal quantity of soil. The same volume of water is poured into each filter; the children can then time how long it takes for the water to pass through each soil sample.

bottle with , bottom I

removed , n n n muslin over neck peat

FIGURE 9 Investigating how much water a soil can hold.

STUDY COMMENTARY FOR UNIT 2 7

What does this investigation suggest about the amount of water the different soil types can hold? Discussions arising from such an investigation could focus on the suitability of different types of soil for different agricultural activities.

A similar investigation could be done to find out whether different soils contain different amounts of air. Try using large stones or pebbles, and gravel to demonstrate that air spaces exist between particles. You can then pour known volumes of water on to dry sand, clay and loam samples, to discover which soil sample takes up the most water to fill its air spaces.

At key stage 2 more advanced investigations could include measuring the pH of a soil and assessing the effect on plant growth of adding different amounts of fertilizer to the soil.

Children may well have encountered the term 'pH' in previous work, e.g. when doing investigations on rainwater. The pH value of a soil expresses its degree of acidity or alkalinity, and is a measure of the hydrogen ion concentration in a solution (you may wish to refer to the chemistry materials to refresh your memory about this). Children need to be aware that the scale normally runs from pH 0 to pH 14, with pH 7 as neutral.

acid soil neutral soil alkaline soil

Gardeners and farmers need to know the properties of their soil so that they can decide which plants will grow best. Working.on a variety of soils, children could design an investigation to test the pH of a soil sample. (Note: Chemicals in tap water may affect the results, so it is best to use distilled water to form the soil solution.) Adding a few drops of Universal indicator to a soil solution in a test- tube should enable the children to estimate the pH. To illustrate different soil pH values it is best to compare, say, a podzol and a rendzina. Small differences in pH may not be detectable using this method.

If your local soil is acid, the children can find out what methods can be used to improve the soil by making it less acid-e.g. adding lime to it.

The use of fertilizer gives another opportunity for older juniors to use their process skills in designing a series of investigations to determine the optimum level of fertilizer, andtor the 'best' fertilizer, for plant growth. Rapid-cycling brassicas (see the Resources Section at the end of this Study Commentary) are ideal plants to use for these investigations. Activities such as these would also be useful for a class project; small groups of children could work on the different variables and then combine their results.

Work on the environment may well instigate a discussion about soil erosion or pollution; the children could carry out experiments on rapid-cycling brassicas using alum to investigate the effects of chemical pollution, or a heavy lubricating oil to simulate the effects of an oil spillage.

Throughout this Section, we have stressed the fine balance that exists between the soil and its environment. In Britain there is considerable evidence of this balance: we have a wide range of soils, reflecting the often intricate interactions of environmental factors. In a land as densely populated as Britain it is not surprising that our activities) have to date been a major influence on soil development; it appears that this influence will continue and perhaps increase in degree. In your own locality it may be possible to examine the changes in soil type that occur across the landscape as one of the natural soil-forming factors changes. Look at the different soils as you go down a hillslope or as you pass from grassland to woodland. These soil differences often reflect soil-environment interactions that have taken place, with only minor alterations, for hundreds or thousands of years. It is almost impossible, however, not to see the impact of our activities on the landscape, in agriculture, forestry, urban development or recreation.


QUESTIONS These questions are designed to test your knowledge and understanding of the Unit material. You should do them after studying the Unit and then check your answers with your tutor.

Q1 From the list below, select one pair of rocks in which the second rock type could not have been derived directly from the first rock type at some stage in the rock cycle.

A Granite + mudrock

B Gabbro + mudrock

C Granite + andesite

D Gneiss + granite

E Peridotite + gabbro

F Mudrock + schist

G Andesite + rhyolite

H Granite + sandstone

Q2 to Q4 The list below contains several statements about a rock, or rocks. Each statement describes either a characteristic feature of the rock or a stage in its formation. Note that you may need to select the same statement(s) for more than one question.

A The rock formed at depth beneath the Earth's surface

B Clay minerals are important constituents of this rock

C The rock was hot at the time of its formation (i.e. more than 600 "C)

D The rock contains undecomposed olivine as an important constituent

E The rock has a crystalline texture

F The rock contains undecomposed amphibole as an important constituent

G The rock is coarse grained

H The rock contains garnets as an important constituent

Q2 Select one statement that could describe only an igneous rock.

Q3 Select one statement that could describe only a sedimentary rock.

Q4 Select the only two statements that could be used to describe a rhyolite.

Q5 and Q6 Figure 10 shows the variation of temperature with depth in three continental areas of the USA: the Basin and Range Province, central USA and the Sierra Nevada. The first two of these are shown also in Figure 25, on p. 5 1 of Unit 27.

Q5 What is the thermal gradient beneath the Sierra Nevada? From the list below select the option that is closest to your own.

A 5 "Ckm-l

B 8"Ckm-l

C 12"Ckm-'

D lS°Ckm-l


depthkm

FIGURE 10 For use with Q5 and Q6.

Q6 Imagine that thick sequences of mudrocks have been buried to a depth of 40 km below each of the areas shown on Figure 10. Below is given a list of possible rocks or processes that might occur at 40 km depth in these areas today. Which two statements are correct?

Phyllites beneath the Basin and Range Province

Gneisses beneath central USA

Schists beneath the Sierra Nevada

Phyllites beneath the Sierra Nevada

Schists beneath the Basin and Range Province

Melting of the crustal rocks to produce granitic magmas beneath the Basin and Range Province

Gneisses beneath the Sierra Nevada

Melting of the crustal rocks to produce granitic magmas beneath central USA

NOTES

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