Geology PAG 7: Orogenic processes

Suggested Activity 1: Modelling rock deformationInstructions for teachers & techniciansThis practical activity is composed of two parts; a teacher/technician section and the learner activity which can be found on page 7. This practical activity supports OCR AS/A Level Geology.

When distributing the activity section to the learners either as a printed copy or as a Word file you will need to remove the teacher instructions section.

This is a suggested practical activity that can be used as part of teaching the AS and A Level Geology specifications, and helps to fulfil the requirements of the Practical Endorsement.

These are not required activities, nor are they coursework tasks.

You may modify these activities to suit your learners and centre. Alternative activities are available from, for example, ESTA, Earth Learning Idea, CLEAPSS and publishing companies.

Support for mapping activities to the requirements of the Practical Endorsement is available from OCR – see www.ocr.org.uk/positiveaboutpractical or email us at PASS@ocr.org.uk.

Students can collaborate during the activities but each student must individually demonstrate competence in each of the practical skills being assessed (see Practical Skills below).

It is possible for a student to achieve some but not all of the practical skills involved in an activity (and this can be recorded as individual skills in the OCR PAG Tracker).

Further details are available in the specifications (Practical Skills Topics).

Before carrying out any experiment or demonstration based on this guidance, it is the responsibility of teachers to ensure that they have undertaken a risk assessment in accordance with their employer’s requirements, making use of up-to-date information and taking account of their own particular circumstances. Any local rules or restrictions issued by the employer must always be followed.

CLEAPSS resources are useful for carrying out risk-assessments: (http://science.cleapss.org.uk).

Centres should trial experiments in advance of giving them to learners. Centres may choose to make adaptations to this practical activity, but should be aware that this may affect the Apparatus and Techniques covered by the learner.

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IntroductionLearners will investigate the anisotrophic properties of the material used to make a carrier bag. Carrier bags are designed so that the polymer molecules run parallel to the direction of loading. Under load the polymers get stretched out straight when the strain rate drops mimicking the behaviour of ductile rock. When loaded perpendicular to the design direction they the bag will behave more like a brittle rock. Learners are expected to be familiar with the concepts of stress and strain.

Aims to determine the relationship between stress and strain (force and extension) to use these observations to describe the materials behaviour as an analogy to rock to record data appropriately and evaluate sources of error and modify procedures.

Intended class time 1 hour

Practical Skills – competence assessed by the teacher1.2.1 (b) safely and correctly use a range of practical equipment and materials1.2.1 (c) follow written instructions1.2.1 (d) make and record observations/measurements 1.2.1 (e) keep appropriate records of experimental activities1.2.2 (j) use of appropriate apparatus to record a range of quantitative measurements1.2.2 (l) use of methods to increase accuracy of measurements.

CPAC – competence assessed by the teacher(1) follows written procedures(3) safely uses a range of practical equipment and materials(4) makes and records observations(5) researches, references and reports – see extension oppertunities.

Links to Specifications2.1.1 (b) rock-forming silicate minerals as crystalline materials built up from silicon–oxygen tetrahedra to form frameworks, sheets or chains and which may have a range of compositions2.1.1 (c)(iv) the techniques and procedures used to measure mass, length and volume3.3.1 (b)(ii) the use of stress and strain diagrams5.4.1 (c) how the composition of the parent rock and conditions (strain rate, temperature and pressure) at the time of rock deformation determine the nature of that rock deformation6.2.1(a)(ii) the measurement of rock strength under compression and under shear – to include a qualitative understanding of peak strength and residual strength

Mathematical Skills – learning opportunity within activity Mathematical skills must be applied in the recording of the data and calculations, and in

analysing the data. These steps require the appropriate application of the following mathematical skills:o M1.1 Recognise and make use of appropriate units in calculations.o M1.2 Recognise and use expressions in decimal and standard form.

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o M1.3 Use an appropriate number of significant figures.o M1.4 Use ratios, fractions and percentages.o M2.9 Plot two variables from experimental or other linear data.o M3.7 Translate information between graphical, numerical and algebraic forms.

ChemicalsLabel Identity Hazard information

none used — —

EquipmentEach learner or group will require: samples of plastic bag cut both in line with the normal load direction and at ninety degrees

to that (see photograph below). Basic single use carrier bags are ideal. 100 g masses on holder calipers or vernier measurement system (possibly a micrometer for an extension) metre rule stand boss and clamp material clamps, for example SciChem catalogue number XPS130010 two short strips of wood (approximately 100×20×5 mm or lolly sticks) pad of soft protective material (e.g. towel or bubble wrap or newspaper)

Health and Safety Health and safety should always be considered by a centre before undertaking any

practical work. A full risk assessment of any activity should be undertaken including checking the CLEAPSS website (http://www.cleapss.org.uk).

Care should be taken such that when the material finally breaks the masses land safely without damaging equipment or endangering people. Make sure the masses are less than 10 cm above the desk, and that there is a soft protective material for them to land on after the sample material strip breaks.

NotesCentres are advised to trial this activity before using it with students. In particular: the centre should try the experiment in advance to determine the appropriate width of

plastic strip which demonstrates the initial elongation, the increase in stiffness and ultimate breaking point when using 10×100 g masses,

note that any imperfection in cutting the strip can cause it to break prematurely, other benefits of doing the activity in this way is that it allows more practice at measuring

extension than taking a single measurement would, as well as modifying the procedure; using a smartphone to record extension as the yield strength is approached. Skills of graph plotting and interpretation may also be examined at both AS and A Level. The plotting of the graph may be done using software (in which case competence in 1.2.1(g) and CPAC5 may be demonstrated), though learners should draw stress–strain curves by hand. Learners may require support with the graphical analysis.

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Answers and Guidance to Extension Activities1. Under load sample 1 should show elastic strain below the yield strength then plastic strain

before failing (the polymers get stretched out straight when the strain rate drops). Sample 2 will behave more like a brittle rock with a lower gradient showing only elastic strain before failing.

2. Carrier bags are manufactured by extruding the plastic material through a dye which aligns the polymers; this allows the bag to support the design load but makes it prone to splitting. If the plastic was harder it would stretch less but could fracture with no warning (like marble). If the plastic were softer it would stretch more and carry less load (like clay).

3. Platey mineral which form sheets will show high anisotrophy while framework minerals (feldspar and quartz), and those only composed of tetrahedra (diamond, garnet) will have least anisotrophic behaviour.

4. Learners may need support using the calipers (particularly if they have a vernier scale). The main issues will be with using appropriate units and expressing values appropriately. The shape of the curves drawn will be similar to the load against extension graph.

5. Curves should be annotated to show elastic and plastic strain, the yield strength and failure of the material. Ductile behaviour of rock is plastic deformation while break/rupture is brittle behaviour. Incompetent rocks display rapid elastic strain and then break, while competent rocks display lower elastic strain rates and rupture at higher stress (with or without plastic deformation).

RecordsAs evidence for the Practical Endorsement, learners: should not need to re-draft their work, but rather keep all of their notes as a continuing

record of their practical work, dating their work clearly, should have evidence of the data collected from their individual readings in a clear and

logical format, should record any measurements taken to the number of decimal places (resolution)

appropriate for the apparatus used. This should be recorded clearly in a table format, or clearly identified with appropriate units,

should record any modifications to supplied procedures, including their own risk assessments and methods where appropriate.

Extension questions help learners develop their understanding of the underlying geological theory and are a preparation for the written examinations. They also help learners to develop the practical science skills assessed indirectly in the written examinations and they should be encouraged to record their data appropriately, for example showing full workings in calculations, and stating final answers to the appropriate number of significant figures.

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Sample 1 Sample 2

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Geology PAG 7: Orogenic Processes

Suggested Activity 1:Modelling rock deformation

Learner activityIntroductionIn this experiment you will be investigating the properties of the material used to make a carrier bag as an analogue for the behaviour of rock under tensional stress. Like the minerals that compose rocks plastic bags are composed of crystalline polymers (such as polyethylene) which deform differently depending on the direction of the imposed stress.

You are expected to be familiar with the concept of stress and strain and the further concepts of the application of a force leading to the extension of an elastic material in tension.

You will be plotting a graph to show the effect of increasing load (a stress strain curve).

The observations you make may allow you to make analogies with the behaviour of competent and incompetent rocks during folding and mountain building.

Aims to determine the relationship between stress and strain (force and extension)to use these observations to describe the materials behaviour as an analogy to rockto record data appropriately and evaluate sources of error and modify procedures

Intended class time1 hour

Chemicals Label Identity Hazard information

none used — —

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Equipmentsamples strips cut from a plastic carrier bag, some cut in line with the normal load direction and some cut at ninety degrees to the normal load direction100 g masses on holdercalipers or vernier measurement systemmetre rulestandboss and clampmaterial clampstwo short strips of woodpad of soft protective material

Health and Safety Care should be taken such that when the material finally breaks the masses land safely

without damaging equipment or endangering people Make sure the masses are less than 10 cm above the desk, and that there is a soft

protective material for them to land on after the material breaks.

ProcedureBefore starting your practical work, read the information below.

Decide how you will organise your practical work, and which observations you need to make and/or which measurements you need to take. Ensure that you record all of your results in a suitable format.

1. Measure the original length, L, of the sample material (using either sample).2. Load the sample material with 100 g and determine the extension x.3. Add an additional 100 g mass and determine the new extension from L.4. Continue to load the sample material 100 g at a time, noting its behaviour after the

addition of each mass.5. Review how the experiment went and how you could improve the accuracy of your

observations.6. Implement your improvements and repeat the experiment with a sample of the same

orientation as step 17. Repeat the experiment with sample 28. Plot a graph of load (a force) against extension. 9. Explain your observations in terms of the structure of the material.

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Extension opportunities1. Explain the behaviour of the plastic bag samples using appropriate technical terminology.2. When the properties of a material change depending on orientation they are called

anisotrophic. How does the manufacturing of carrier bags take advantage of the anisotrophic properties of the sample material?

3. Many minerals show anisotrophy, for example mica and clays are anisotrophic, while the properties of others show change little with direction (quartz and olivine for example). Suggest how the crystalline structure of a mineral can predict how it will behave in response stress.

4. Stress is measured in Pascals (Nm−2) and strain as the ration of the change in length to the original length. Measure the thickness of the plastic bag using callipers and replot your load against extension graph as a graph of stress against strain.

5. Annotate your stress–strain curves to show elastic strain, yield stress, plastic strain (ductile deformation) and rupture (brittle deformation).

RecordsAs evidence for the Practical Endorsement, you need records of:

the data collected from their individual readings in a clear and logical format, with all measurements taken to the number of decimal places (resolution) appropriate for the apparatus used. These should be recorded clearly in a table format, or clearly identified with appropriate units,

any modifications to supplied procedures, including your own risk assessments and methods where appropriate.

All work should be clearly dated.

In addition you should have considered the above questions as the answers to these questions will aid you in preparation for your written examinations.

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