Name: _____________________________ Class: ____________ Date: ___________
Lab Partner(s): ___________________________________________________
Earth and Space Science - Laboratory Investigation
Igneous Rocks – Relating Cooling Rate and Crystal Size
Background Information:
All rocks are made up of one or more minerals. Some rocks are formed from molten rock.
When molten rock cools and hardens, it forms igneous rocks. Igneous rocks may contain
crystals. The size of the crystals depends upon how quickly the molten rock that forms
the igneous rocks cools. Some igneous rocks do not have crystals, while others may have
small or large crystals.
In this investigation you will form crystals from melted material. You will observe how
the cooling rate affects the size of the crystals that are formed.
Problem:
How does cooling rate affect the size of crystals?
Experimental Design:
Be sure to identify and state the dependent variable, the independent variable and the
controlled variables for this experiment.
Materials and Apparatus Available
per group:
3 watch glasses or small plates
3 small test tubes
small beaker to hold test tubes
blank paper and pencil
watch with second hand or timing device
hand lens
paper towel
10 mL paradichlorobenzene (PDB) flakes – caution – note the hazard warning – use in
a well ventilated area and do not smell fumes directly.
rock samples of rhyolite, granite and obsidian
per class:
4 - 1000 mL beaker with crushed ice
4 - 1000 mL beakers with boiling water
Note: Lab safety goggles must be worn for all parts of this lab when you are
handling PDB.
What to do: As a lab group, write a detailed bullet pointed list in your note books of a
procedure that will allow you to answer the experimental question. Include a list of
apparatus and materiasls. Be sure to create a table or another way of recording your
observations/data. Give a verbal overview of the procedure you intend to follow to the
teacher, get an approval signature in your notebook and then proceed. Record your
observations. Record any changes to the procedure if you adjust that which you wrote
before. Use a laptop to write up the lab with the usual lab report sections – problem,
introduction with background theory and summary of the procedure and variables,
materials and apparatus list, procedure, observations/data including drawings (you’ll need
to copy these from your notebook, analysis and discussion questions (cut and paste from
the lab on the class website), conclusion and references (which will include your text
book in order to answer the discussion and analysis questions.) You do not need an
application section for this lab. The procedure should be a step-by-step description of
how the experiment was carried out. Someone should be able to recreate the experiment
from your procedure. Be sure to write the procedure in past tense (Do not give directions,
but describe what you did instead.) and in a passive voice (i.e. “The yellow powder was
placed in the test-tube”, rather than “I placed the yellow powder in the test-tube.”) You
will have 20 minutes at the beginning of the next class to finish writing it up, and then
you will be expected to submit it (so make sure you delegate any work that needs to be
done such as background theory research or answers to the discussion questions at the
end of the lab work class). One copy of the lab report submitted per lab group.
Suggestions
Be sure to
 Note the size and shape of the PDB crystals at some point.
 Think about how you can cool something quickly and how you can cool something
slowly.
 Think about how you are going to manipulate the independent variable, how you are
going to measure the dependent variable, and how you are going to control other
important experimental variables.
 Record the important observations and data.
 Look at the crystals in the samples of rhyolite, granite and obsidian with the hand
lens. Draw the crystals in each sample in the circles provided.
Observations – Put all of your observations in your lab notebook. Don’t write anything
below. You will reproduce your observations, including drawings in your final lab writeup.
Examples of Observation Tables and Drawings:
*note: This is only a suggestion – your tables and drawings may be different if you wish
Data Table - Time for PDB to Solidify Given Different Cooling Rates
Crucible
Time for PDB to solidify
A (room temp)
B (ice cold)
C (hot)
Drawings of Crystals
Crucible A
Crucible B
Crucible C
Drawings of Crystals from Igneous Rock Samples
Rhyolite
Other Observations:
Granite
Obsidian
Analysis and Discussion Questions:
*note - you may have to do some research for these questions: (use your text book and any other
resources such as the internet (ex. http://www.canadianrockhound.com/junior/rocks_igneous.html
or geology reference necessary).
1. Compare the crystals in the crucibles to the samples of granite, rhyolite and obsidian.
Which PDB crystals are most similar to the crystals in the rock samples?
2. How does the rate of cooling affect the size of crystals?
3. Granite, rhyolite, and obsidian are essentially made of the same materials. Explain
why they look different.
4. Where would igneous rocks have a chance to cool slowly?
5. Where would igneous rocks cool rapidly?
6. How would viscosity of the lava affect the size of the crystals that forms in the rock as it
cools? Explain. (Hint: viscous fluids flow more slowly)
7. Compare obsidian to pumice. A) How do they look? B) Why does pumice have so many air
bubbles in it? C) Where did the gas come from?
8. Both granite and gabbro are coarse grained rocks which form kilometers below the surface.
How do they look, compared to each other and why do they look different?
9. Explain the difference between intrusive and extrusive igneous rocks and give two examples
of each.
10. Describe what a dike, sill and batholith are. Draw pictures below to identify what each looks
like in the sedimentary layers.
Crystal Lab Report due date:_______Next class after 20 min. of time to put your research
together and print it.