Edible Igneous Rocks

Igneous rocks are formed when molten rock cools and solidifies. If it cools slowly then large crystals have time to form. Granite is formed when molten rock under the surface of the Earth (also called magma) cools.

If molten rock cools above the surface (when it is called lava) it cools quickly and the resulting rock has small crystals, like basalt. If the lava cools extremely quickly crystals have no time to form. Obsidian and pumice are formed in this way.

You can model the way these igneous rocks form in your own kitchen using sugar rather than molten rock which is far safer and tastier. However, remember molten sugar can still get very hot so be extra careful – do not taste it before it has had a chance to cool down!

You will need:

• 500 g of white granulated sugar
• Milk
• Bicarbonate of soda
• Some tap water
• A saucepan
• A heatproof bowl e.g. pyrex
• A wooden spoon
• A teaspoon
• Two trays covered with baking paper. Put them in the freezer about an hour beforehand so they are really cold.

Instructions

1. Add the sugar to the saucepan and add a small amount of water.
2. Heat until the mixture turns a light brown, but not black.
3. Put about a third of the mixture into the heatproof bowl and add a small amount of milk. Be careful this can splash and splutter – mind your eyes.
4. Leave it to cool at room temperature. This is fudge.
5. Pour another third of the molten sugar onto one of the cold trays. (You might have to reheat it briefly if it has started to solidify). This is toffee.
6. To the final third add a small spoonful of bicarbonate of soda and give it a good stir. It will bubble up.
7. Pour this mixture onto the other cold tray. This is honeycomb.
8. Study the structure of your sweets – how is the crystal size different in each one?

Now try to answer these:

How did you change the speed of cooling? How did this affect the size of the sugar crystals?

Slow cooling was obtained by adding milk and stirring. This resulted in large sugar crystals in fudge. The toffee cooled down very quickly because it was put onto a cold metal tray. No crystals were formed.

Obsidian is formed when lava falls into cold water. Pumice is formed when lava shoots out of a volcano and mixes with the air. How did you model these forming?

The toffee was like obsidian forming because the molten sugar was poured onto a cold tray. The bicarbonate of soda reacted in the heat of the molten sugar to form bubbles of carbon dioxide gas. The resulting honeycomb contained lots of bubbles and modelled pumice.

For more information on types of rocks and how they are formed take a look at Smart facts – Earth Structure and Composition.