What Determines Crystal Size in Igneous Rock?

What determines crystal size in igneous rock?

Setting the scene

Igneous rocks form when liquid rock, called magma or lava, freezes. Magma is found in the mantle, under the Earth’s crust. When a volcano erupts, magma escapes to the Earth’s crust. It cools and will eventually become solid rock.
Salol is a waxy substance that can be used to model liquid rock.

Aims

In this practical you will:

use information to write a hypothesis on how crystal size will change with temperature
test your hypothesis by observing how salol cools at different temperatures
decide how temperature affects crystal size in igneous rocks.

You will be working scientifically to:

use what you know about granite and basalt to formulate a hypothesis about how temperature affects crystal size in igneous rocks
record your observations
interpret your observations and decide if your hypothesis was correct
present explanations of your observations in relation to your hypothesis.

Safety

Salol is an irritant. Avoid getting it on your skin.
Handle glass slides with care.
Take care when using hot water. If hot water gets onto your skin, run the area under cold water and ask your teacher for assistance.

Equipment and materials

six glass slides
test tube containing salol
glass beaker containing warm water
glass beaker containing ice
empty glass beaker
plastic pipette
black card

ruler
thermometer
hand lens
paper towels

Hypothesis

Granite is an igneous rock that is made when magma cools down within the Earth’s crust. Granite contains very large crystals that are easy to see.

Basalt is an igneous rock that is formed when liquid rock cools down outside the Earth’s crust. Outside the crust, the temperatures are lower. Basalt contains fine crystals that are hard to see without a hand lens.

Use the information above to write a hypothesis about how you expect the crystal sizes to change as the temperature changes. Explain your answer.

Prediction

Predict how the crystal size of salol will be different when the salol has cooled quickly to when it has cooled slowly.

Method

1Collect your test tube of salol and place it into the beaker containing hot water
(water temperature should be around 60 °C).

2Collect two slides from the beaker containing ice and place one onto black card.

3Use a pipette to place one or two drops of salol onto the glass slide.

4Quickly place the second slide on top. Try to avoid getting the salol on your hands.

5Observe what happens to the salol, paying particular attention to the size of the
crystals formed. Use a hand lens to look at crystals if necessary. (If possible, use
a ruler to measure crystal sizes.)

6Repeat the process collecting slides from the empty beaker and then the slides
from the hot water.

Results

Design a results table for your observations at each of the three temperatures.
Remember: if you were successful in measuring crystal sizes using a ruler, these should also be included in your results table.

Questions

1State at which temperature the salol cooled most rapidly.

2State which temperature produced the largest crystal sizes.

3Describe what the three beakers (where the glass slides were kept) represent in the formation of igneous rock.

4Explain how the formation of metamorphic rock differs from the formation of igneous rock (that you modelled using salol).

5State whether your hypothesis was correct. Explain your answer using your observations.

Extension

1Explain why temperature affects the crystal size seen in igneous rocks.

2Suggest whether the experiment you have carried out is able to answer your
hypothesis fully. Explain your answer.

This resource sheet may have been changed from the original.