teacher’s notes
Mantle convection moving plates: the golden
syrup / hobnob teacher demonstration
How convection currents in the mantle may be responsible for plate movement -the
Golden Syrup and biscuit demonstration.
Learning objectives:
currents of solid but ductile mantle move beneath the lithosphere carrying the
plates
where plates collide one is taken down into the mantle and ‘destroyed’; where
they move apart, new lithosphere is created.
Timing: 15 minutes from the start of heating
Preparation time: the syrup needs to be in the freezer for at least 1 hour in advance
Health and safety: the syrup gets very hot
Apparatus:
large tin of Golden Syrup
large beaker, 5 dm
3
packet of thin biscuits
Bunsen burner, tripod and heatproof mat
access to a freezer
Pour the syrup into the beaker and leave in the freezer for 1 hour. Do not allow it to
freeze, simply reduce the temperature to gain maximum viscosity.
Break a biscuit in half carefully and place the two halves, touching, on the surface of
the syrup. The biscuit halves need room to move apart, so ensure that the two halves
are not too big (nibble pieces off the edges if necessary).
Place the beaker and contents on a tripod without a gauze in place to ensure
localised heating.
Heat the beaker using a low blue flame directed at the centre of the beaker’s base.
Slowly the central cone section of the syrup will change in colour as its temperature
rises and its viscosity falls.
Close observation will clearly show convection taking place in this region. After
around 10 minutes (depending on the speed of heating and quantity of syrup used)
the biscuit halves will have been driven apart by the convection currents.
Convection currents in the mantle were thought, for many years, to be solely
responsible for plate tectonic movements, with the movement taking rocks down at
destructive margins and new rocks forming when plates spread. It is now thought
likely that there are three possible driving mechanisms for plate tectonics:
Movement of mantle convection currents as above
The mass of the subducted plate (the sinking slab) at the subduction zone
dragging the surface part of the plate across the surface
At constructive margins, the new plate material sliding off the higher oceanic
ridges.