Rock Making

CSF Science - Natural & Processed Materials, Level 3
Recognise and describe the conditions involved in producing new materials. Distinguish between reversible and irreversible changes

CSF Science - Earth & Beyond, Level 5
Explain the methods of formation and uses of common rocks.

Rock Making .....
Turning sediments into rocks

This simple activity can be used during discussions about the formation of rocks or the "rock cycle". It is suitable for both upper primary and junior secondary students and is a great springboard for many related concepts.

You need:

• sand (or other sediment)
• saturated sugar solution
• eye dropper
• old dessert spoon or foil patty pan and peg
• bunsen burner or short candle in holder
• heat proof mat
• hand lens or magnifying glass (optional)

What to do:

1. Fill a level dessert spoon or half fill the patty pan with sand.
2. Saturate the sand with the sugar solution using the dropper.
3. Warm the spoon over the bunsen burner for about a minute or heat the patty pan over the candle for about two minutes holding it with the peg.
4. Place the spoon or patty pan on the heat proof mat and allow to cool completely.
5. Examine your 'rock' with a hand lens or magnifying glass.

Why it works!

Sand grains are "cemented" together to form rocks. As the solution cools, the chemicals (in this case sugar) are precipitated .

Questions/Discussion:

1. Where do sediments come from in the first place?
2. Sugar solutions do not occur naturally in the earth. What naturally occurring, water soluble chemical compound(s) could cement sand grains together?
3. Find out the names of rocks that may form in a similar way.
4. Relate this model to natural occurrences in the earth's crust.

Extensions:

1. Soluble salts such as bi-carb soda, washing soda and Epsom salts can be used to try and make rocks. What are the chemical names for these everyday products? (Secondary students could try other soluble chemicals of their choice).
2. What are conglomerate rocks? How would you make some? Try it!

More Rock Making Tasks

These methods take longer but could be set up as class demonstrations for students to monitor.

Method 1: (works best in areas of hard water)

1. Take a large icecream container and nearly fill it with builders' sand. Bury an object, such as a shell, to show the rock is 'artificial' and how natural objects can be trapped in 'real' rocks resulting in fossils.
2. Saturate the sand with water and leave for about a week.

This method is a useful demonstration because it uses normal water and produces large lumps of rock. It works very well in areas of hard water owing to the higher concentration of dissolved salts. It will not work at all if 'pure' water is used.

Method 2:

1. Fill a margarine container with sand.
2. Saturate with a concentrated (common) salt solution.
3. Leave for a week or so to evaporate.

The top will become quite hard as the water evaporates. The formation of this 'salt rock' can be likened to the formation of sandstone - water has been driven off.

Question
Solutions of common salt can be found in underground rocks.
Why is common salt never found as a cement in surface rocks?

For more information contact MEV