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ROCK DETECTIVES
Erosion & deposition
demonstration
Apparatus
 Anything from which a waterproof channel can be made, e.g. a gutter; a shallow plastic box; or
even a cardboard box covered with a plastic sheet
 washed sand, to half-fill the channel
 a small quantity of gravel or small pebbles
 water, either from a running source, or poured from a container
 if being carried out indoors, a container to catch the overflow and to prevent sand blocking a sink
The activity
Set up a sloping “stream table” using a piece of square section gutter or even a box covered in a
plastic sheet. Half fill it with washed sand and add some small pebbles or gravel. Make a straight
channel through the sand with your finger.
Ask the pupils:
 What do you think will happen if we pour water slowly on to the sand at the top end of the gutter?
 Which will move first – the sand or the pebbles? Explain your answer.
 As more water is poured onto the surface, will the channel become deeper, or will it get filled up?
 What will happen if we pour the water more quickly?
 Where do you think the sand and pebbles will settle down after they have been moved?
With the pupils watching carefully, pour water onto the sand, either from a container, or from a hose
pipe if running water is available. Ask the pupils to see if their predictions were correct. Also, ask if
they can spot how the sand grains and pebbles are moving – are they sliding, bouncing or rolling over
the bed of sand? Continue to watch as channels fill with sediment and switch over, forming a braided
pattern – and, if a pool forms at the bottom, how a micro-delta can build out.
Following up the activity
 Prop up the gutter at a steeper angle and then
investigate any differences from the first run.
 Investigate reports of flooding or loss of land by river
erosion. These may come from local knowledge or
from the media or web.
Wren’s Nest
National Nature Reserve
ROCK DETECTIVES
Make your own rock
practical
Apparatus
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sand
expendable small plastic cups or pots
water
a range of “cements”, e.g. salt, sugar, plaster of Paris. Note: Care should be taken in using plaster
of Paris, since it gives out heat as it sets; only tiny quantities are needed. Builders’ cement powder
is alkaline and should not be used.
 stirring sticks
 scissors, for cutting up the plastic cups
The activity
Ask a pupil to take a handful of sand and see if it is possible to make a ‘rock’ by squeezing it as hard
as possible. To become a hard rock, most sediments need to be cemented together. This idea can
be tested with a variety of ‘cements’. Before carrying out each part of the activity, ask pupils to predict
how strong they think each ‘rock’ will be when it has dried. Dampen some sand with water and pack
it tightly into the bottom of an old plastic cup or small pot. Cut away the plastic carefully and leave the
sand pellet to dry.Repeat this several times, but mix the sand beforehand with any suitable ‘cements’
that come to hand. Use a ratio of about one part of ‘cement’ to four parts of damp sand. The
‘cements’ could include salt, sugar, plaster of Paris, etc.
Note:
Pupils should be warned not to confuse manufactured builders’ cement powder with the materials
that they are using. An alternative name for these could be ‘rock glue’.
Ask the pupils:
to devise a fair way to test the strength of their rocks after they have hardened. When the ‘rocks’ have
become hard (which may take a day or so), pupils can then be invited to test them to destruction, to
see which were made with the strongest cements. If possible, give them a piece of real sandstone to
test as well. If they plan a series of tests, they should try the least destructive one first!
Following up the activity
Mixing gravel and pebbles with the ‘cement’ could lead into a conversation about conglomerates. A
mix of sand, pebbles and ‘cement’ would give a realistic looking rock with could be compared with
sandstone pebble inclusions.
Wren’s Nest
National Nature Reserve
ROCK DETECTIVES
Plate tectonics
demonstration (by teacher)
Apparatus
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large tin of Golden Syrup
baking tray
packet of thin biscuits
Bunsen burner, tripod and heatproof mat
access to a fridge
Safety note: take care with hot syrup
The activity
Pour the syrup into the baking tray and leave in the fridge for 1 hour. This is to 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 baking tray and contents on a tripod without a gauze in place to ensure localised heating.
Heat the baking tray using a low blue flame directed at the centre of the baking tray. 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.
Wren’s Nest
National Nature Reserve