SES4U
Grade 12 Earth and Space – University
Unit: Earth Materials
Activity: Crystal Formation
Igneous rocks are formed when magma cools and crystallizes. The aim of this experiment
is to investigate why intrusive igneous rocks like granite contain large crystals, whilst
extrusive (volcanic) rocks like basalt are made up of tiny crystals. Have a look at a sample
(or photograph) of each rock type and discuss your ideas before you start. This
experiment will use molten Lauric Acid to model the behaviour of magma as it cools.
Pre-lab:
Examine the two igneous rocks provided (one black basaltic rock & one grey coloured
granite rock). Igneous rocks form when lava/magma cools. Make a prediction about which
of the rocks formed in hot temperatures and why and which formed in cool temperatures
and why.
Black basaltic rock: ____________________________________________________
Grey granite rock: _____________________________________________________
Materials:
lauric acid
- 1 test tube
- thermometer
- 1 beaker for warm water bath
- tweezers
- medicine dropper/pipette
- 6 microscope slides
- hand lens
- hot plate
Procedure:
1. Put some microscope slides in the freezer.
2. Heat water to ~42°C. Do not overheat or it will not work. Do not allow all crystals of
Salol or Lauric Acid to dissolve. You may need some of the undissolved crystals to ‘seed’
the crystallization later on.
3. Melt 2-3g of Lauric Acid in a warm water bath (warm water in beaker and then
place lauric acid in test tubes in the bath). Place two microscope slides on side of hot
plate and allow warming.
4. Take two microscope slides from the freezer.
5. Using a stirring rod or pipette, quickly put a few drops of the melted lauric acid on
to one of the cold slides and cover it with the other cold slide.
6. Repeat the process with two slides at room temperature.
7. Repeat the process with two slides which are warm
8. Record your observations, use hand lens as needed
SES4U
Grade 12 Earth and Space – University
Observations:
Cold Slide
Crystal
Size
Unit: Earth Materials
Room Temp. Slide
Warm Slide
Crystal
Shape
Relative
time to
crystalize
Sketch
Analysis:
1) What is the relationship between the rate of cooling and the size of crystals
formed?
2) How can crystal size be used to predict the depth at which rocks crystalized? (hint
temperature increases with depth)?
3) What is the relationship between rate of cooling and crystal shape?
4) Go back and re-observe the two rocks that you looked at in the pre-lab activity.
Identify which crystalized deep within the crust of the earth and which crystalized
near or at the surface. For each justify why you made this decision.