Science: Volcanic Rocks - making sweets

Volcanic rocks

Rocks are formed by magma from volcanoes are igneous rocks; all igneous rocks start off deep within the Earth as magma (molten rock).  The word igneous  means of ‘fire’.

When a volcano erupts, the magma rises towards the surface and turn into lava or it may cool and solidify inside the earth’s crust. 

Rocks that are formed in this way are a mass of interlocking crystals.  They are very strong and good to use for building.

The size of the crystals in an igneous rock depends on how quickly the magma is cooled. 

Lavas that cool quickly contain very small crystals – Balsalt is a common type of igneous rock that is formed in this way.

Granite is cooled more slowly because they solidify inside the earth’s crust.  They have a grainy texture because the crystals had time to grow.

Then there is Pumice which is created when extremely hot heated, highly pressurised rock is violently ejected from a volcano. The foamy look of pumice occurs because the lava rapidly cool and quickly reduces the pressure of air or gas (also known as depressurisation).  It is the depressurisation that creates bubbles by lowering the solubility of gases, like water and CO₂, that are dissolved in the lava. 

This then causes the gases to rapidly exsolve, meaning two minerals in a solid solutions separate from one another at a crucial point in temperature - in this case, with pumice rock, setting it in it’s ‘bubbly state’.  It’s the same process as when the bubbles of CO₂ that appear when a carbonated drink is opened.    Because both parts are cooled at the same time the bubbles in the pumice freeze inside the rock.

Here some good experiments to try at home that show us how igneous rocks are forms but not at 1000 degrees Celsius like molten magma but still allow us to understand the concept with the crystals.

Ready for some yummy science?

 

TOFFEE

 

STEP ONE

Put some sugar in a pan

 

STEP TWO

Add a spoon full of water and gently heat it over the hob until it turns very light brown (don’t let it burn)

 

STEP THREE

 

Pour it onto a well greased tray and you will have toffee. 

 

Toffee cools very quickly, too quickly to form crystal and will become smooth and glossy just like Obsidian. 

 

Obsidian is a volcanic rock that is formed when lava cools quickly.  It is often known as volcanic glass for this black glass like appearance.

FUDGE

Here is a recipe for fudge:  http://www.carnation.co.uk/Recipes/67/Ultimate-Fudge  

 

If you make fudge you will see it takes a lot longer for it to cool, the crystals are large as they grew slowly in the same way as magma cools slowly in the earth’s crust.  Fudge is very much like granite.

 

 

 

HONEYCOMB

If you want to make your honeycomb follow the steps in making toffee but just as the sugar begins to colour add a teaspoon of baking powder and allow it to cool. 

Pour it out on a piece of grease proof sheet on a baking tray and then allow it to cool thoroughly before touching. 
 The bubbles in honeycomb are like the ones in pumice is a very light rock that is full of holes.  It is made when lava containing a lot of gas pours out of underwater volcanoes.   As I said above when the gas and the rock are cooled very quickly at the same time the gas gets trapped causing these bubbles, it is the same when you make Honeycomb. 

For other experiments go to my Physics page, Chemistry page or Biology page.