Volcano in the lab: a wax volcano in action: teacher’s notes
This activity is designed for students aged 11-14, as a simple demonstration of igneous activity.
English National Curriculum reference 3.3.2f
ACCAC (Wales) reference 22.214.171.124
It can also be used with students aged 14-16 when discussing of the structure of the Earth and the physical properties of its layers.
English National Curriculum reference 4.3.2r
Welsh National Curriculum reference 126.96.36.199
AQA modular (3468) reference 15.4
AQA linear (3462) reference 12.12
OCR A (1983) reference 3.7.13
OCR B (1977) reference CD3
OCR C (19740) reference Sc188.8.131.52
This activity consists of a teacher-led demonstration for the whole class in which layers of sand and wax in a beaker of water are used to model how igneous rocks form both underground and at the surface. It may be that some teachers would wish to allow students to carry out the practical under very close supervision.
Volcanoes are exciting – hence all the volcano footage on TV. They can be used to fire students’ imaginations, and safe analogues of the behaviour of molten rocks can be demonstrated in the school laboratory.
Students will have seen TV coverage of volcanic eruptions, and may even have spent holidays in volcanic regions. They will also know that temperatures generally increase with depth in the Earth.
It is a common misconception that there is a universal layer of molten rock lying just below the Earth’s crust. This imaginary layer is often erroneously equated with the mantle, which is, in fact solid. 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. Students also find it difficult to visualise that some molten rock can set below the Earth’s surface to form intrusive igneous rocks.
The reason why temperature increases with depth in the Earth is mainly 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. During radioactive decay of an element, new elements and sub-atomic particles are formed. The total mass of these is very slightly less than the mass of the original element and the difference (Δm) is converted into the equivalent amount heat energy (E) in line with the equation E = Δmc2, where c is the speed of light.
The demonstration itself takes about 10 minutes, with discussion to follow.
Melt red candle wax into the base of the beaker to about 1 cm depth. Cover this with a layer of sand about 1 cm thick above the wax. Add water to fill the beaker about three quarters full.
Apply a strong source of heat to one part of the base of the beaker and let it cool to form a solid layer. Students need to concentrate because the ‘eruption’ often happens without much warning, other than an ominous crackling sound as the wax melts! The heat source is removed whilst there is still some wax left on the bottom of the beaker since this allows ‘lava tubes’ and intrusions to form more effectively.
Figure 1 shows an example of what might be observed.
Figure 1 A volcano in the lab
Points to bring out
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 melt and rise (they are also the least dense minerals). This can be shown by preparing a mixture of chopped wax and gravel in a metal container. When heated in front of students 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 in this experiment.
The original idea for the wax volcano model came from Mike Tuke and is described in M. Tuke Earth Science Activities and Demonstrations, London: John Murray, 1991.