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.
It can also be used with students aged 14-16 when
discussing of the structure of the Earth and the physical properties of
The aim of this topic is to simulate ways in which
igneous rocks may form.
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.
At the end of the activity students should appreciate
magma can cool at and below the Earth’s surface, forming rocks which
The demonstration can follow the showing of selected
video clips of volcanic eruptions such as those available at
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.
- one 500 cm3 or 600 cm3
- Bunsen burner
- heat proof mat
- safety screen
- red candle wax
- washed sand (sand can be washed by putting some in
a bucket and using rubber tubing to run water run into the bucket and
allowing the water to overflow into a sink until it runs clear)
- Wear eye protection.
- The activity is safer than it sounds - the only
potential hazard is a cracked beaker, when some localised spillage of
hot wax can occur: the water remains cold throughout.
- It is the responsibility of the teacher to carry
out an appropriate risk assessment.
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
- Both the sand and the water represent the
crust of the Earth – the water does not represent the sea.
- The wax layer represents a layer in the Earth
crust (called the
- The mantle is solid. At certain points it becomes
hot enough to melt.
- When the wax melts, it rises because of its lower
density. It represents molten rock, known as
- 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
lavas may cover huge areas, arising from fissure eruptions, which
produce a greater total amount of lava than the better-known individual
- 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. The location of this tube could be linked to a
point above a ‘weak spot’ in the rock layer, ie the tube forms
above this weakness finding the easiest way to the surface. This happens
in Nature too. Students may be surprised that such a vast amount of
magma has passed so quickly through such a small tube.
- 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
This is analogous to the removal of layers of rocks by weathering and
- Reference can be made to the geological map of
Great Britain and Ireland, Figure 2. 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
Figure 2 Geological map of Great Britain and
Click here for a
full-sized version or else click here to
download the map in Encapsulated Postscript format.
- Students 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 sets 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.
- Students can be reminded that slow cooling leads
to large crystals.
- The model can be related to
plate tectonic theory.
- 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
- Studies of the chemistry of
igneous rocks show that partial melting of rocks produces three main
magma, depending upon the nature of the
tectonic plate boundary.
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.