Dynamic Rock Cycle

A volcano in the laboratory

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Volcanoes are exciting – hence all the volcano footage on TV. They can be used to fire pupils’ imaginations, and safe analogues of the behaviour of molten rocks can be demonstrated in the school laboratory. This activity consists of a teacher-led demonstration for the whole class. It also demonstrates how ‘rocks’ may form below ‘ground’, as well as on the surface.

Prior knowledge

Pupils will have seen TV coverage of volcanic eruptions, and may even have spent holidays in volcanic regions. They will also know that temperatures generally rise with depth in the Earth. (This is because of radioactive decay of minerals within the Earth, and the fact that the hundreds of kilometres of overlying rock provide a very good insulator).

Starting points/misconceptions to avoid

The demonstration can follow the showing of selected video clips of volcanic eruptions. Many pupils (and writers of school textbooks!) believe that there is a universal layer of molten rock lying below the Earth’s crust. They often erroneously equate this imaginary layer with the mantle, which is, in fact solid. Pupils also find it difficult to visualise that some molten rock can set below the Earth’s surface, to form intrusive igneous rocks.

Level

Either KS3 (11-14 year olds), as a simple demonstration of igneous activity , or can be extended to KS4 (14-16 year olds), with discussion of the structure of the Earth and the physical properties of its layers. It can also be related to plate tectonics.

Time

The demonstration itself takes about 10 minutes, with discussion to follow.

Equipment and safety notes

• One 500ml or 600ml glass beaker
• Red candle wax, about 1cm deep, previously melted and poured into the base of the beaker
• Washed sand, in a layer about 1cm deep above the wax
• Very cold water to top up the beaker (about three-quarters full)
• Bunsen burner, heat proof mat, tripod and gauze, gas supply (or bottled gas burner etc).
• Safety screen

Risk assessment

Materials: Water, sand, candle wax – all non-toxic

Risks: The water remains cold throughout, so poses no risk. A thin layer of wax is melted in a beaker. It could crack the beaker and exude onto the bench, but this is unlikely. If spectators are watching close to the beaker, a safety screen is advisable; viewing from a few feet away poses no risk. The presenter should wear eye protection.

Activity

Pupils watch attentively from behind the safety screen, whilst the teacher applies a strong Bunsen flame to the base of the beaker, on the tripod and gauze. They must not lose concentration, because the “eruption” often happens without much warning, other than an ominous crackling sound as the wax melts! The Bunsen is removed whilst there is still some wax left on the bottom of the beaker.

Points to bring out (depending on the level of the pupils)

• The sand and the water represent the crust of the Earth.
  The wax layer represents a layer in the Earth below the crust. (The mantle).
• The mantle is solid. It is being heated at a point source.
  When the wax melts, it rises, because of its lower density. It represents molten rock, known as magma.
• Some of the wax rises rapidly to the surface, imitating a volcanic eruption. It is very runny and spreads out evenly over the surface of the water (usually). This represents the way in which some lavas may cover huge areas, arising from fissure eruptions, which are quantitatively more important than the better known individual volcanoes.
• Some of the wax can be seen rising through “tubes” of wax, which insulate it from the surrounding cold water and enable it to reach the surface. This happens in Nature too.
• Some of the wax sets very quickly in the cold water, forming grotesque shapes. These represent intrusive igneous rocks. Once the wax has all set, the “lava layer” may be removed and the water poured off, in order to study the shapes of the “intrusions”. This is equivalent to the removal of layers of rocks by weathering and erosion.
• Reference can be made to the geological map of the UK. Widespread sheets of lava form the Antrim plateau in Northern Ireland: masses of intrusive igneous rocks are shown as big red blobs in Devon and Cornwall, Southern Uplands of Scotland etc.
• Pupils can be challenged to say which aspects of the model are not consistent with the natural world. The most important one is that the surface eruption set very slowly, whilst the “intrusions” set very quickly. In reality, the reverse would be true, because of the higher ambient temperatures at depth and the insulating properties of several kilometres of rock. Lavas may become solid within days, months or years, whereas a deep-seated intrusion of several tens of cubic kilometres may take millions of years to cool to the ambient temperature. Of course, the wax merely sets: it does not form crystals.

Possible extension

• At KS4, the model can be related to plate tectonic theory.
• In reality, complete melting of rocks below ground is seldom achieved. Rocks partially melt, and the minerals of lowest melting points are the ones which rise (they are also the least dense minerals). This can be shown by preparing a mixture of chopped wax and gravel in a container. When heated, in front of pupils, the wax melts, and rises, whilst the gravel does not. It is possible to do this at the same time as the volcano demonstration, but experience shows that glass beakers tend to be more susceptible to cracking!
• When students study the properties of seismic waves in the Earth, they will appreciate that the mantle is generally solid, with only about 5% of liquid, in between the crystals of the rocks. Localised heating, and/or reduction in pressure, lead to partial melting, but the magma chambers which form are only tens of kilometres across, not mantle wide.
• Studies of the chemistry of igneous rocks show that partial melting of rocks produces three main types of magma, depending upon the nature of the tectonic plate boundary.

Acknowledgements

The original idea for the wax volcano model came from Mike Tuke and is described in his Earth Science Activities and Demonstrations, published by John Murray.

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