ACTIVITY 6: Crystallisation (and melting)
Purpose:
To explain the effects of heating and cooling on rocks, particularly
the observable outcomes of different rates of cooling of molten
rocks (magma).
Notes:
The apparatus and materials required are: Salol (phenyl salicylate),
approximately 5g; boiling tube; 250cm3 beaker; 6 microscope slides;
teat pipette; goggles; hand lens; hot water; access to a freezer;
sheet of coloured paper; samples of igneous rocks of various
grain sizes (e.g. granite, gabbro, rhyolite, basalt).
Development of knowledge and understanding:
Many kilometres below the surface of the Earth the temperature
is high enough to cause partial melting. The magma produced rises
upwards because it is less dense than the overlying rocks. Crystallisation
may take place within intrusions at different levels of the crust
or at the surface, if extrusion takes place. Intrusions may subsequently
be exposed at the surface by weathering and erosion processes.
Igneous rocks form from melts which may exceed 1000 degrees Celsius.
This activity models the behaviour of a cooling magma, but at a
much lower temperature (around 40 °C). Pupils will discover
a link between cooling rates and the sizes of crystals that are
produced.
The largest crystals of Salol are formed from the slowest cooling
melt. Igneous rocks show a similar behaviour so pupils can use
the crystal size to predict the relative depths at which the rocks
crystallised. Coarse-grained rocks, like granite, are formed by
slow cooling at depth, whereas volcanic rocks, like basalt, are
fine-grained. Igneous rocks are classified according to their mineral
content as well as their crystal size.
The Salol crystals provide an insight into the size of crystals
as a function of cooling, but they misrepresent real rock textures.
Igneous rocks typically contain several minerals which crystallise
at different times and rates. Pupils could examine specimens of
igneous rock, preferably with polished surfaces, to observe the
size, shape and relationships between crystals.
Experimental modelling of the crystallisation of two chemicals
is described in the book Science of the Earth 11-14: Magma - Introducing
Igneous Processes, ESTA.
Demonstration 6: Fast or slow cooling: large or small crystals?
(This activity is based on ESTA’s “Magma” in “The
Science of the Earth 11-14” series.)
Learning objective:
To illustrate relationships between the sizes of crystals formed
in a melt when it is cooled at different rates.
We can find out why the crystals in igneous rocks have different
sizes by growing crystals from a melt in the laboratory.
- Take a piece of paper and label it as shown below. Put a microscope
slide on the 'room temperature' part of the paper.
- Ask for a pair of microscope slides chilled in a freezer and
put one on the paper. Take a pipette with some melted Salol from
a water bath and quickly put a few drops on the slide from the
freezer and on the one at room temperature. Place slides of the
same temperature on top. Watch what happens.
- Answer the following questions:
(a) On which slide did crystals form first?
(b) On which slide did crystals grow fastest?
(c) On which slide did the largest crystals form?
- If you had warm slides, would you expect the crystals:
(a) To form straight away or after some time?
(b) To grow quickly or slowly?
(c) To be large crystals or small crystals?
Write down your predictions.
- Test your ideas by asking for a pair of warm slides. Put one
on the paper; add liquid Salol as soon as you can; put the second
slide on top and watch carefully. Were you right?
- Now apply your ideas. Some igneous rocks are made of large
crystals, did they cool slowly or quickly? At what rate did fine-grained
igneous rocks cool?
- Some igneous rocks melt and crystallise deep within the crust,
whereas others come to the surface in volcanoes. Which of the
resulting igneous rocks will have the smallest crystals?
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