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Spot that Rock:rock identification

Demystifying rock identification

Earth Science for KS3

Earth Science Education Unit


© The Earth Science Education UnitCopyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them.

Spot that Rock: rock identification– Earth Science for KS3

Spot that Rock leads you step by step through a series of investigative practical activities that will allow you to teach pupils to investigate and sort most rocks from first principles (and distinguish them yourself). The techniques are then applied in describing and classifying a series of unknown rocks.


Workshop outcomes The workshop and its activities provide the following outcomes:• an introduction to a structured scheme of rock description and

identification based upon the reliable characteristics of rocks, involving:

• identifying key terms in describing rocks;• identifying key terms in describing the grains in rocks, using

a magnifier/ hand lens;• testing for permeability;• modelling rock structures;• classifying and naming rock groups;

• practical activities giving opportunity for investigation and discussion;

• a focus on the main types of rocks likely to be used in teaching the National Curriculum for Science (2007) at KS3.

The workshop uses the following sequence of activities:


Activity 5Which are the strongest rocks?

Activity 6Classifying three main groups

Activity 7Grouping rocks

Activity 8Naming the rock groups

Finding how easy it is to remove grains

How to sort the three rock groups

Applying the principles to other rocks

Sedimentary, igneous, metamorphic names

Activity 1What is a rock?

Activity 2Difference between grains

Activity 3Predicting properties

Activity 4Modelling rock differences

Recognise rock properties

Recognise grain properties

Testing porosity/ permeability

Making 2D and 3D rock models

Activity 9 Processes KS3/4

Leading into the activity

1. What is a Rock?2. Are There Differences Between the Grains of Rocks?3. Rocky Predictions and Explanations4. Rocky Modelling: a tessellation exercise5. Weak or Strong: which are the strongest rocks?6. Classifying Rocks: the Earth Scientist’s way7. Grouping Rocks (classification)8. Naming Rock Groups9. Processes KS3/4 – controls on characteristics and investigating them


Leading into the activity

Show the pupils an image showing a rock face of different rock types as seen in the world around us. You can find pictures of a rock face from ‘Steps towards the rock face’ (an ESTA publication) – as below.Also show the pupils a collection of different rock types that (you say) were found in the beach below the rock face.

Have a brief discussion on what can be seen on the images, without assuming any prior geological knowledge. Imagine that some of the rocks from the images are included in the collection.

You might like to ask:• Why should we want to know?• What extra can the rock samples tell us?• How can we group them?


Figure 1: A rock face and pebble beach at Portishead, Somerset

© Peter Kennett, ESEU

1. What is a Rock?Divide the class into groups of three and give each group a sample of granite and a sample of sandstone. It is better not to name them at this stage, just call them the speckled rock and the red/brown rock. Ask one child to pick up the speckled rock and describe it (what it looks like, feels like, etc.) to a second child. The third child notes down key words that are used. When this is done, the pupils swap over, one describes the red/brown rock and another makes notes.

Ask the pupils‘Which key words were used in both descriptions?’


Possible answerthe words bits, colour, heavy and rough/smooth are often used.

Explain that we will now use a scientific word for the ‘bits’ in rocks - grains.

Conclude – not all rocks are the same, there can be clear differences.

2. Are There Differences Between the Grains of Rocks?

Ask the pupils To use a magnifier or hand lens to ‘describe the grains of the two rocks’ to one another whilst the third makes notes.

Note: The correct way to use a magnifier or hand lens, is to hold the magnifier/hand lens up to your eye, and then move the object you’re viewing closer and closer until it comes into focus.

Figure 2: Looking closely at a rock using a magnifier

© ESEU


Possible answerThe words, shape, size, colour and shininess are often used.

We are going to investigate the importance of the shape of the grains further now.

3. Rocky Predictions and Explanations

Ask the pupils to weigh the red/brown rock (sandstone) and the speckled rock (granite) and write down the results so they can be used later.

Ask the pupils‘Predict what will happen to the masses of the rocks when they are placed in water, and explain your predictions.’

Give three options - the rocks could: stay the same weight; get heavier; or become lighter. Ask the pupils to write their predictions down, then ask the class so that you can get a feel of what they are predicting.


Possible answer Pupils often predict that the red/brown rock will get heavier but the speckled rock will stay the same. When asked why they have made these predictions, they may answer that water will get into the red/brown rock and make it heavier but it won’t get into the speckled rock.


•Ask them to put the samples, at the same time, into a clear plastic container of water and watch carefully to see what happens. They should take them out again after about 30 seconds.

•Ask them to carefully dry the rocks on towels (paper or cloth), re-weigh them and check their results against the previous masses.

They will find the sandstone has increased markedly in mass but that the granite hasn’t (unless very sensitive scales are used, when the remaining wetness of the granite causes a small increase in mass). So, the pupils probably predicted correctly.



If they have observed the samples carefully, they will have seen a few bubbles on the surface of the granite. But many more bubbles come from the sandstone, and continue to bubble as the air is driven out.

Figure 3: Sandstone ‘bubbling’ in a plastic beaker


Figure 4: Granite not ‘bubbling’ in a plastic beaker




It is useful to have a discussion on how the air is driven out. Do most of the bubbles come from the bottom or the top? Does most of the water go in at the bottom or the top? Are the pores (gaps between the grains) likely to be interconnected?

The air rises from the top, as it has a lower density than water, this allows atmospheric pressure to push water into the bottom to replace it, showing that the pore spaces must be interconnected and the rock is permeable (permeability is the flow of fluid through a material). So, the bubbles come from the top as water is pushed into the bottom.



Note: The rate of flow of a fluid (liquid or gas) through a material is its permeability – the property mentioned in the National Curriculum for Science. This depends on the porosity – which is the percentage of pore space in the material (and is not a term used in the National Curriculum). Rocks with a high percentage of porosity (lots of pore space), where the pores are interconnected and not too small for fluid to flow through, also have high permeability. Low porosity gives low permeability.


4. Rocky Modelling: a tessellation exercise

Figure 5: Tessellation exercise© Peter Kennett, ESEU

Ask the pupils to model the grains in the sandstone and granite in a tessellation exercise using sticky shapes stuck onto paper, magnetic shapes on a magnetic board or shapes on an interactive whiteboard.

They should use circular and square or rectangular shapes that don’t overlap to cover up as much space as possible. It quickly becomes clear that the circular shapes leave gaps, whilst the rectangular shapes do not.


Develop this 2D pattern into a 3D concept using an interlocking wooden model (or a Lego model) and marbles in a glass or jar.

Ask the pupils‘How much space do you think there is in the interlocking model and in the container of marbles?’



Possible answerThe interlocking model will have no space; The container of marbles may have about a quarter space.

Figure 6: Container of marbles © ESEU

Figure 7: Interlocking wooden model © ESEU

Ask the pupils‘How much space do you think there is in the interlocking model and in the container of marbles?’



The pupils will often be amazed as you pour more and more water into the container of marbles – which is almost half space. This is a measure of porosity – the porosity here is nearly 50%. Rocks with high porosity are usually very permeable, allowing fluids to flow through (except when pore spaces are very small and water can’t flow through, as in clay). The interlocking model has no pore-space.

The marbles in the container are like the sandstone – with lots of gaps between the spherical grains. The interlocking model is like the granite, made of interlocking grains with no spaces.

Ask the pupils‘When the pot is tipped the marbles fall out – but you can’t tip out the grains from a sandstone. Why not?’



Possible answerThe grains in the sandstone are stuck together – by some sort of ‘glue’

This natural ‘glue’ is called ‘cement’ by Earth scientists.

Note: Grains are naturally glued together to form rocks when cement is deposited between the grains. As water flows through the pore spaces minerals crystallise out, sticking the grains together. Minerals are naturally formed chemical compounds and common mineral cements are quartz (silicon dioxide) and calcite (calcium carbonate).

5. Weak or Strong: which are the strongest rocks?

This is an investigation into which makes a rock stronger, having interlocking grains or cemented grains. Ask the pupils to compare the two rocks by scraping them with a spoon on to a piece of paper.


The difference is usually very clear – rocks crumble if the cement between the grains is not very strong (e.g. the pupil’s sample of sandstone), whilst having interlocking crystals makes rocks strong (e.g. granite).

Teaspoon © S Sepp

6. Classifying Rocks: the Earth Scientist’s way

This now allows us to sort rocks into two great groups:• rocks that have grains stuck together (by glue or cement)• rocks that have interlocking grains

The clues are in how the grains are linked to each other – the grain relationships.


• rocks that have grains stuck together

• rocks that have interlocking grains

http://www.earthscienceeducation.com/virtual_rock_kit/conglomerate_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/limestoneA_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/shale_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/granite_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/gabbro_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/gneiss_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/slate_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/schist_HS.htm

http://www.earthscienceeducation.com/virtual_rock_kit/andesite_HS.htm

6. Classifying Rocks: the Earth Scientist’s way

The rocks with interlocking grains can be subdivided into two major types, based on their grain arrangements:


• interlocking grains scattered through the rock

• the interlocking grains might be scattered through the rock (random orientation) as in the granite,

• the interlocking grains may be lined up (parallel orientation) as in rocks like slate and schist.

• Interlocking grains aligned








7. Grouping Rocks (classification)

Give the pupils a selection of rocks and ask them to group them, using the criteria they have been using (scrape test, careful study of the grains and possibly the water test). Pupils are likely to have difficulty with fine-grained specimens so will need to go back to testing them for permeability or crumbliness.

You may wish to give the pupils rock group description cards to help them with this section.











• crystalline rocks

8. Naming Rock Groups

We can now give names to the rock groups.

The first level of classification is into those with grains stuck together (called sedimentary) and those with interlocking grains, or crystals (called crystalline).

• sedimentary rocks











Note: Sedimentary rocks are so called because they are formed of sediment. Sediment grains are pieces of material which have been transported before being deposited. Crystalline rocks are made of crystals which have grown during formation of the rock and so are interlocking. The crystals are minerals. Minerals are naturally formed chemical compounds and sometimes, chemical elements.


• crystalline rocks• sedimentary rocks











• sedimentary rocks • metamorphic rocks


The second level of classification of the crystalline rocks is into those with crystals scattered through the rock (randomly orientated crystals, called igneous rocks) and those with aligned crystals (called metamorphic rocks).

Rock group definition cards may be useful here.

• igneous rocks











A summary of sedimentary, igneous and metamorphic rocks:

Features • rounded grains• non-interlocking

(+ cement)• grain orientation

usually random

• angular grains• interlocking

grains• random grain

orientation

• angular grains• interlocking grains• banded or layered

grain orientation

Rock group

Sedimentary Igneous Metamorphic

Further sub-division

• grain size• colour and

mineral content• structure

• grain size• colour and

mineral content (chemistry)

• grain size• mineral content• structure



Note: As we are dealing with ‘the real world’ there are some important exceptions to this classification method that participants should be aware of:

• some sedimentary rocks are well cemented with a tough cement and so are not porous or weak – but are nevertheless formed of grains stuck together ;

• some metamorphic rocks are not formed under pressure (but mainly by heat) and so have random grain orientation;

• some metamorphic rocks contain only one mineral, and so banding or layering cannot be seen;

• some igneous rocks can be weakened by gas bubbles or weathering and so can be fairly weak.


9. Processes KS3/4 - controls on characteristics and investigating them


Rock Characteristics Controls Lab investigation?

Sedimentary

grain size

colour and mineral content

structure

Igneous

grain size

colour and mineral content (chemistry)

Metamorphic

grain size

mineral content

structure

Pupils are asked to consider what other factors might control the key features of rocks, then to plan ways in which they might be investigated in the lab.

9. Processes KS3/4 - controls on characteristics and investigating them - some answers


Rock Characteristics

Controls Lab investigation?

Sedimentary

grain size distance/energy of transport shake rocks in a plastic container

colour and mineral content

the characteristics of the original rocksexamine a variety of beach pebbles of sedimentary rocks

structure eg. beds, ripple marksmake beds by dropping spoonfuls of different coloured sands into a beaker of water

Igneous

grain size rate of crystallisationcrystallise materials (eg. Salol or CuSO4) quickly and slowly

colour and mineral content (chemistry)

composition of the original meltexamine a variety of beach pebbles of igneous rocks

Metamorphic

grain sizeamount of metamorphism; greater = larger crystals

none

mineral content composition of the original rockconsider the changes: mudstone → slate → schist → gneiss compared with limestone → marble and sandstone → metaquartzite

structurewhether metamorphism is by heat/pressure or just heat

none


ESEU’s Virtual Rock KitSee http://www.earthscienceeducation.com/virtual_rock_kit/index.htm to see a ‘kit’ of common rocks in hand specimen, enlarged, under the microscope, in exposure and in use.

Copyright


Granite © ESEU

Fossiliferous limestone © ESEU

Sandstone © ESEU

Figure 1 © Peter Kennett, ESEU

Figure 2 © ESEU




Figure 6 © ESEU

Figure 7 © ESEU

Teaspoon © S Sepp

Rock samples – ESEU rock virtual rock kit


Workshop outcomes The workshop and its activities provide the following outcomes:• an introduction to a structured scheme of rock description and

identification based upon the reliable characteristics of rocks, involving:

• identifying key terms in describing rocks;• identifying key terms in describing the grains in rocks, using

a magnifier/ hand lens;• testing for permeability;• modelling rock structures;• classifying and naming rock groups;

• practical activities giving opportunity for investigation and discussion;

• a focus on the main types of rocks likely to be used in teaching the National Curriculum for Science (2007) at KS3.


Spot that Rock:rock identification

Demystifying rock identification

Earth Science for KS3

Earth Science Education Unit


© The Earth Science Education UnitCopyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them.

www.earthscienceeducation.com spot that rock: rock identification demystifying rock identification...

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