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Geology PAG 7: Orogenic processes

Suggested Activity 1: Modelling rock deformation

Instructions 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 students 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 [email protected].

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).

OCR recommendations:

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 students. 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.

This document may have been modified – if in doubt check the master version on OCR Interchange.

Version 1.1 – February 2019 1 © OCR 2019

DISCLAIMERThis resource was designed using the most up to date information from the specification at the time it was published. Specifications are updated over time, which means there may be contradictions between the resource and the specification, therefore please use the information on the latest specification at all times. If you do notice a discrepancy please contact us on the following email address: [email protected]

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IntroductionLike the silicate minerals in rocks, carrier bags are made from polyethylene which is a crystalline material. Many silicates and polyethylene, behaves differently when stretched along the a-axis or across the a-axis. Students can model the anisotrophic properties of the rocks using carrier bags. 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 less competent rock exceeding its elastic limit, rapidly stretching under plastic deformation and fracture (like brittle rock). Students are expected to be familiar with the concepts of stress and strain.

Aims to record data appropriately and evaluate sources of error and modify procedures. to compare the behaviour of a competent and an incompetent material under stress to use these observations to describe the materials behaviour as an analogy to rock

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(1) follows written procedures(3) safely uses a range of practical equipment and materials(4) makes and records observations

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.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.

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

loading (see photographs below). Basic single use carrier bags or cheap bin bags are ideal. Each student/pair of students will need at least four sample strips (2×along & 2×across the bags).

duct tape and a hole punch – to make reinforced hangers at each end of sample strips 10×100 g slotted masses on a holder metre ruler retort stand with a boss and clamp 2 × small wire hooks 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: You will have to trial 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. This activity was trialled using 300 mm × 15 mm strips cut from 5p Co-op carrier bags (see Trial data, below).

A common error, particularly when using scissors, is to make slight imperfections (‘knicks) when cutting the strip. These will cause the strips to fail (snap) prematurely. When this happens use it as an opportunity for students to trouble shoot and improve their method.

This activity provides opportunities for students to practice measuring extension. The material will ‘creep’ when each mass is added and students will have to modifying their procedure as a single measurement will not give valid data. Using a smartphone to record extension as the yield strength is approached could be a way to improve the method.

One of the objects of this activity is to get students to trouble shoot, identify sources of error and improve their method to get a valid set of accurate results. There are many errors that can be introduced by poor techniques and each pair of students will need to carry out the experiment at least twice to get a valid set of results. Possible errors include:



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o Sample strip breaks too early – slight scissor cut or suddenly adding new mass.o Sample strip (across bag) never reaches elastic limit/fracture point – strip is too wide.o Cannot measure extension – common error when students try to measure between

marks on sample strip rather than measure the total length of the sample strip.o Plotted data shows saw-tooth/erratic changes – measurements taken too soon after

mass added with some measurements taken before/after creep has taken place.o Hanger breaks before sample strip – poorly made hanger or hole to close to edge

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.

Answers and Guidance to Extension Activities10. Under load one sample should show elastic behaviour and may not reach its yield

strength/elastic limit. Curves should be annotated to show elastic and plastic strain, the yield strength and failure of the material. The other sample will behave more like an incompetent rock and exceed its yield strength, show plastic behaviour (as the polymers get stretched out straight) before and a higher strain rate before it fractures like a brittle rock. You should make your own stress–strain curve from your trial data to compare.

11. 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).

12. Platy mineral which form sheets will show high anisotrophy (different behaviours in different directions) while framework minerals (feldspar and quartz), and those only composed of tetrahedra (diamond, garnet) will have least anisotrophic behaviour (behave the same regardless in which direction the stress is being imposed). 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, students: 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, tabulate the data collected from their individual readings in a clear and logical format,

recording all measurements taken to the number of decimal places (resolution) appropriate for the apparatus used. This should be recorded clearly in a table format, with with appropriate units used (see Appendix 3 in the Practical Skills Handbook);

record the modifications they made to the 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.



Document updatesv1.0 May 2017 Original version.v1.1 February 2019 Example trial data and additional teacher guidance.

Trial dataSample A, 20 mm wide strip of polythene cut in the direction of carrier bag loading

Length / cm Mass added /Kg

Extension / cm

Strain / % Stress / N

29.9 0.000 0.0 0.00 0.00030.1 0.100 0.2 0.67 0.98130.3 0.200 0.4 1.34 1.96230.6 0.300 0.7 2.34 2.94330.8 0.400 0.9 3.01 3.92431.2 0.500 1.3 4.35 4.90531.6 0.550 1.7 5.52 5.39631.9 0.600 2.0 6.69 5.88633.7 0.675 3.8 12.71 6.622

Sample B, 20 mm wide strip of polythene cut across the direction of carrier bag loading

Length / cm Mass added /Kg

Extension / cm

Strain / % Stress / N

29.7 0.000 0.0 0.00 0.00029.8 0.100 0.1 0.34 0.98130.0 0.200 0.3 1.01 1.96230.3 0.300 0.6 2.02 2.94330.7 0.400 1.0 3.37 3.92449.7 0.500 20.0 67.34 4.90564.5 0.550 34.8 117.17 5.396

109.0 0.600 79.3 267.00 5.886134.3 0.675 104.6 352.19 6.622



Co-op carrier bag, 2cm strip



OCR Resources: the small printThis formative assessment resource has been produced as part of our free A Level teaching and learning support package. All the A Level teaching and learning resources, including delivery guides, topic exploration packs, lesson elements and more are available on the qualification webpages.

If you are looking for examination practice materials, you can find Sample Assessment Materials (SAMs) on the qualification webpages: here

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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 to model 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 variations in plastic and ductile deformation due to tensional stress. The observations you make may allow you to make comparisons with the behaviour of competent and incompetent rocks during folding and mountain building.

You will be plotting a stress versus strain graph to show the effect of increasing load on the crystalline material that makes the carrier bag (a stress strain curve).

Aims To record data appropriately, evaluate sources of error and modify procedures.To compare the behaviour of a competent and an incompetent material under stressTo use your observations to describe the materials behaviour as a model for rock

Intended class time1 hour

Equipment2 × carrier bag sample strips cut in line with the normal load direction (Sample 1) 2 × carrier bag sample strips cut at ninety degrees to the normal load direction (Sample 2)10×100 g slotted masses on holdermetre rulerretort stand with a boss and clamp2 × small wire hookspad 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 and measurements you need to make. Ensure that you record all of your results in a suitable format.

1. Measure the original length (L0) of your first sample strip (e.g. Sample 1).2. Load the sample material with 100 g and measure the new length of the strip (L1).3. Continue to load the sample material 100 g at a time, noting its new length after the

addition of each mass.4. Calculate the extension (strain) for each load using ((L1

- L0) ÷ L0) %.5. Calculate the stress from each load using weight = mass × gravitational field strength.6. Review how the experiment went and note down how you can improve the accuracy of

your data.7. Using your improved method repeat the experiment from step 18. Repeat the experiment with a different sample strip (e.g Sample 2)9. Plot a graph of strain (% extension) against stress (gravitational force, N) for each sample.

Extension opportunities10. Annotate your stress–strain curves to show elastic strain, yield stress, plastic strain

(ductile deformation) and rupture (brittle deformation).11. When the properties (such as strength) 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?

12. Many minerals show anisotrophic properties, 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.

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

your data table, including observations, measurements and conclusions. The data should be recorded in a clear and logical format, with appropriate units and all measurements taken to the appropriate number of decimal places (resolution) for the apparatus used;

your notes on the modifications you made to the supplied procedures.

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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