ACTIVITY 4: Compaction and cementation
of sediments
Purpose:
To explain that for most sediments to become rocks the grains need
to be squeezed together (compacted) and/or glued together (cemented).
The extent of the compaction and the strength of the cementation
affect the properties of sedimentary rocks.
Notes:
The apparatus and materials required are: plastic syringe (20cm3),
with the nozzle cut off at the end of the barrel; petroleum jelly;
tray; goggles; disposable cups and spoons; 250g of dry sand;
10g of powdered clay; 10g of plaster of Paris powder; sample
of sandstone (sedimentary); water; apparatus for testing strength
of the pellets, e.g. file, variety of masses, heavy ball-bearings,
dropper bottle of dilute hydrochloric acid (0.5M). Caution: corrosive.
Carry out a risk assessment and wear safety
goggles when using acids. Do not let pupils pour plaster of Paris
down the sink.
Development of knowledge and understanding:
The extent of compaction depends upon the type of sediment. Very
little compaction takes place when sand is converted to sandstone.
In muds and clays, water may initially comprise 80% by volume
of the sediment. A great deal of compaction takes place when
this is squeezed out. The compaction of sand and mud can be modelled
with pennies and matchsticks, respectively, which can be “compressed” between
two pieces of wood on an overhead projector.
Sands become sandstones when cement crystallises in the pore spaces.
The cement is formed from new minerals that crystallise from circulating
waters.
Pupils simulate part of the sedimentary part of the ‘rock
cycle’
by first making a 'rock', then testing the strength of their rock
and consequently breaking it up.
Demonstration 4: Sediments into Rocks
(This activity is based on ESTA’S “Second-hand Rocks”
in the “Science of the Earth 11-14” series.)
Learning objective:
To simulate the formation of sedimentary rocks and investigate
how the properties of the samples of 'sedimentary rocks' so formed
are dependent on the extent of their compaction and cementation.
Sandstone was once loose sand. The grains of sand have become
stuck together in sandstone. Is it something that has happened
to the sand, or has something been added to the sand? How has this
happened?
Try this exercise (overleaf): it may explain how sand becomes
sandstone. HINT - don’t overdo the water! Use a plastic cup
for all mixing, before inserting the mixture into the syringe.
- Put some damp sand in a syringe which has had the nozzle cut
off and has been smeared with petroleum jelly on the inside.
- Put the open end of the syringe on the palm of your hand and
press the plunger in hard.
- Carefully push the sand pellet out of the syringe onto a piece
of paper and leave on one side.
- Mix three parts of damp sand with one part clay.
- Repeat steps 1, 2 and 3 using the sand/clay mix instead of
the sand.
- Repeat the same three steps but this time use a mix of five
parts of damp sand and one part of plaster of Paris powder.
- Leave your “rocks” to dry.
- Which of the pellets is most like a rock? Plan an investigation
which will give you evidence to show which is most rock-like.
- Carry out your investigation.
- Do your results support your conclusion?
- How could you improve your investigation if you could do it
again?
- Do you think that sandstone can be formed from sand by pressure
alone?
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