SNA/KS4/Prep

From Earth Science On-Site

Snableazes & Cullernose Point, North'berland

KEY STAGE 4 PREPARATION AND FOLLOW-UP IDEAS
© GeoconservationUK ESO-S Project, 2013

It is anticipated that the ideas and materials presented here will be adapted by schools, and others, to be more appropriate for their own purposes and programmes of study.

In such circumstances please acknowledge the source as the Earth Science On-Site project.

Many ideas involved in this Earth-Science On-Site excursion will revise ideas from Key Stage 3 work. See document SNA6 KS3 prep.

At Key Stage 4, in addition to the knowledge and understanding of geological processes gained in Key Stage 3 Physics, the pupils’ knowledge of the response of materials to deforming forces, needs to be revised and slightly extended. See parts 1 and 2 below.

Introductory Work

In addition to the Key stage 3 concepts the following themes should form the basis of the preparatory lesson in school within a week prior to the field visit. The three themes are focused on understanding faulting, folding and igneous intrusion.

Part 1: The Response Of Materials To Bending Forces

Time: about 15 minutes

In KS3, pupils are likely to have investigated the behaviour of springs and rubber bands when they are stretched. Under lower stresses, both show a linear relationship (known as Hooke’s law) between force (load) and extension. This is called elastic deformation. However as the stress increases, the behaviour of the two materials begins to differ; neither obeys Hooke‘s law any more, but the spring becomes permanently deformed, while the elastic band becomes much more difficult to stretch further, and eventually snaps, demonstrating brittle failure.

However, it is unlikely that pupils will have investigated behaviour of materials under bending forces. For the purpose of this preparatory lesson, a few quick qualitative demonstrations should be enough to achieve the following learning objectives:

• know that under low bending forces, a strip of material will exhibit elastic deformation;
• know that under higher bending forces, a strip of material will exhibit plastic deformation, becoming permanently bent;
• know that under very high bending forces, a strip of material may snap, suffering brittle fracture;
• know that some materials deform in these ways more readily than others.

For quick demonstrations the teacher will need to ‘sacrifice’ e.g. a few (old) wooden rulers (or wooden skewers), a few (old) plastic rulers (or similar plastic strips which do eventually show brittle fracture) and a few metal (steel) rulers (or similar metal strips which can be bent by hand). If a variety of metals in strip form such as copper, zinc, aluminium, are available for comparative purposes, so much the better. A steel wire coat hanger could be used to show brittle fracture after ‘working’ in the plastic stage.

Part 2: That Folds Are Formed Gradually, Under Compressive Stresses

Time about 20 minutes

The beds on the Boulmer foreshore are simply tilted to the SE, but the beds at Cullernose Point have been folded. The north –south trends of the folds indicate an east-west compressive force, perhaps created in association with the stresses which also caused nearby faulting.

The activity below is taken from the Earth Science Education Unit (ESEU) workshop “The Dynamic Rock Cycle”. Visit the Earth Science Education Unit website^[1] for free materials relating to the teaching ideas of The Dynamic Rock Cycle. Contact eseu@keele.ac.uk for details of their facilitator scheme for free In-Service Training for science departments, funded by UK Gas and Oil.

Part 3. Make Your own Folds

Learning Objectives

1. Folds are caused by compression of rocks;
2. Folds are three dimensional, and form with their axes at right angles to the major stress;
3. Folds are evidence of ancient stress pattern in the Earth’s crust.

Equipment: a box with transparent sides (a chocolate box, or component drawer.) a spatula or desert spoon, a tray (to catch spilt sand) a cardboard paddle to fit snugly across the box, 500g of dry fine sand, 25g of flour, a photograph of folded rocks, digital camera (optional).

Teachers may want to do this as a demonstration, or, with multiple kits available teachers may want pupils to complete the exercise in small groups and discuss it afterwards to draw out the learning points.

Procedure: Place the cardboard paddle vertically at one end of the transparent box. Then build up several layers of sand and flour, but DO NOT fill the box more than half full. (It is useful to place the flour layer ONLY against the front face of the box, thus using less flour, and making the sand re-useable a second and third time.) (See Figure 1)

Very carefully, push the vertical paddle across the box, so that it begins to compress the layers. When you notice the layers beginning to bend, stop pushing. Hold the paddle upright and take a digital photograph, or draw a scaled diagram of the result.

Continue pushing the layers with the paddle until the sand is about to overflow the box. Hold the board upright and again photograph or draw a scaled diagram of the result. It should have features looking something like Figure 2. Photographs or sketches of the intermediate stages are also instructive.

The Discussion: Describe the folded nature of the layers, bringing out the following points;

• The layers have been compressed into about 40% of their original length.
• In order to do this they have deformed, or “folded” into upfolds and downfolds.
• That this bending or “folding” happened over a period of time.
• That the view is only of the end (or profile) of the fold, which actually runs all the way across the box, and formed at right angles to the main direction of compression.
• Real folds in real rocks are therefore evidence of ancient compression directions in the Earth’s crust.
• Then add arrows to your diagram (or printed digital photograph) to show the directions of the forces which were acting whilst you compressed the layers with the paddle.

Part 4. Igneous Processes

The central feature of this Earth Science On-Site visit is observation of igneous (dolerite) intrusions. The large dyke at Boulmer, (cutting across the bedding) and the sills at Snableazes and Cullernose Point (roughly parallel with the bedding) require some preliminary understanding of the geometry and origins of such features and their relationship to the bedding of the country rock they intrude.

Activities

Activity 1: Although videos and three-dimensional models are useful for establishing the main ideas and definitions, the ESEU workshop demonstration “A volcano in the laboratory”^[2] and the “Lava in the laboratory” pupil activity are extremely useful for demonstrating the processes involved, using red wax as a proxy for intrusive magma, and syrup as a proxy for extrusive lava.

Activity 2: Pupils should examine and describe crystalline igneous rocks and relate the crystal size to rate of cooling, and the overall colour to acid or basic magmas. . E.g. Granite, and rhyolite (both acid rocks), and basalt and dolerite (both basic rocks).

A summary of the central ideas and definitions is given below.

1. Magma is liquid rock underground. (In this case it is iron and magnesium rich, or basic, magma which crystallised underground as the dark coloured rock dolerite).
2. Basic magma derives from the partial melting of an otherwise solid upper mantle.
3. Igneous rocks, therefore, are characterised by interlocking crystals and joints which form as the solid rock continues to cool and contract. In sheet shaped intrusions these joints are perpendicular to the cooling surfaces. As a crude rule of thumb, vertical joints in sills, and horizontal joints in dykes. In ideal circumstances the stresses form hexagonal columns.
4. Hot magma moves upwards through cold “country rock” by virtue of being less dense. It follows lines of previous fractures such as faults and joints causing an extension of the crust equivalent to the width of the intrusion. Intrusions which cut across bedding in this way are called dykes. (At Boulmer the dyke is slightly over 30 metres wide.)
5. Lava is the molten rock erupted on the surface, and cools quickly to form Basalt. Basalt is the fine grained extrusive equivalent of dolerite, which crystallises more slowly underground and therefore has slightly bigger interlocking crystals than basalt.
6. Where the hydraulic pressure is insufficient to drive the intrusion further upwards through the country rock, the magma may spread out sideways, often along bedding surfaces, to form a sill. Here the ground’s surface is displaced upwards by the thickness of the intrusion. (At Snableazes and Cullernose Point this is around 20 metres.). In some places the sill may “step across” bedding planes and continue at a different level in the rocks. This is then called a transgressive sill, and is definitive evidence of an intrusive origin..
7. Both sills and lavas can be parallel to the bedding, but the heat from sills, being intrusions, metamorphose the country rock both above and below. Lavas cannot do this.
8. Dykes and sills effectively “cut across” the country rock and so are younger than them, (even if they occur above them in the quarry.) Principle of Cross-Cutting Relationships.

References

1. ↑ Earth Science Education Unit www.earthscienceeducation.com
2. ↑ A volcano in the laboratory www.earthscienceeducation.com/workshops/rockcycle/volcano.htm

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