Crystal Growth Rate

Here’s a quick investigation you can do at home, using equipment from your kitchen, to determine how the rate of crystal growth affects the size of the crystals. This is a great Earth Science experiment which relates to igneous rocks.
Igneous rocks
Igneous rocks form when molten rock cools and crystallises (to become molten rock must be heated to at least 6000C ).
Molten rock inside the Earth is called magma. Sometimes magma does not reach the surface before cooling. This is common at mountain building areas, where we have two continental plates crashing into one another, like at the Himalayas. When the magma crystallises, it forms intrusive igneous rocks. (Memory hack: INtrusive = INside Earth)
Continental collision with common responses/activities labelled (image courtesy of USGS)
When molten rock flows onto the surface of the Earth it is called lava. This is more common at divergent plate boundaries, where two tectonic plates are pulling apart from each other, such as at the Mid-Atlantic Ridge. Subduction zones, where one plate slides beneath another one, can also lead to volcanism (although volcanoes are usually more explosive, and more likely to spew out lots of ash than lava). Lava may also spill onto the Earth’s surface at hot spots, like the one that has formed the Hawaiian Islands. When lava cools it forms extrusive igneous rocks. (Memory hack:  EXtrusive = EXit the Earth)
Divergence with common responses/activities labelled (image courtesy of USGS)

You can imagine magma being like a person wrapped up inside a blanket and kept nice and snug. This means that magma takes a long time to cool down.  This causes the crystals to grow slowly, which allows them to become larger, so they can be seen clearly with the naked eye. Granite is a common intrusive igneous rock. Pegmatites are igneous rocks with crystals larger than 2.5 cm and can be spectacular.
Lava on the other hand is exposed quickly to the elements – like when your mum rips your blanket off you to get you out of bed in the morning. This means that it cools quickly and allows little time for crystals to grow, forming small crystals. Basalt is a common extrusive igneous rock, but probably most people’s favourite extrusive igneous rock is obsidian. Obsidian cools so quickly that it doesn’t even crystallise, and instead forms a glass (this is because of the introduction of water, speeding up the cooling process further).

Extrusive and intrusive igneous rock formation around a volcano

Investigating growth rate
To complete your own simple investigation, you will need:
  • Hot water (about 100 ml) *Younger students should complete this experiment with an adult
  • Salt (about 5- 10 tbs)
  • Food colouring
  • A glass
  • Two small dishes/eggshells
  • Spoon

Equipment for crystal growth rate investigation
  1. Create a saturated solution, by adding the solute (salt) to the solvent (hot water) and stirring until it can dissolve no more
  2. Add a few drops of food colouring
  3. Pour the solution into your dishes/eggshells
  4. Leave one in a warm place and the other in a cool place for a few days until all the water has evaporated

Which one crystallised the quickest?  Which one had the smallest crystals?
You can watch a video of how to do this experiment and my results by clicking here.
In my investigation, the egg which was placed in a cool location crystallised slowly and grew into one large crystal which covered the whole of the inside of the eggshell. This represents an intrusive igneous rock. The eggshell placed in a warm area crystallised quicker and formed lots of small crystals. This represents an extrusive igneous rock.
This model is a useful way to see the difference between the rate of crystal growth and the size of crystals. However, the weakness of this model is that crystallisation in this case is caused by evaporation (of water) not by cooling.  Which is not how magma crystallises.
Further investigations:
  • Does the temperature of the water affect how much salt can be dissolved?
  • Does the colour of the food colouring make any difference to a) the rate of evaporation b) the size of the crystals?
  • Does the surface area of the dish make any difference to a) the rate of evaporation b) the size of the crystals?

For more activities and resources relating to this topic check out:
Here are some more hands-on activities you can do on this topic at home (note: please don't attempt the potassium permanganate one at home!). With some feedback, once you have had a go, here.
For more information on the rock cycle check out our animation on YouTube.
·   Or for a more interactive experience download the WASP app, for iPad.