Dynamic Rock Cycle

ACTIVITY 7: Extrusive igneous rocks

Purpose:
To explain that the viscosity of lava influences the shape of volcanic landforms and that the viscosity depends on the temperature of the lava and how much water, gas and solids it contains.

Notes:
The apparatus and materials could include: treacle or syrup; Petri dishes or tiles with 'props' to allow tilting; spatulas or teaspoons and stirring rods; stopclock or watch; ruler; Bunsen burner; camping gas stove or hot water bath; tripod and gauze; boiling tubes with bungs; thermometer, stand, clamps and bosses: teat pipette; syringe; sand; hot soapy water; test tube brushes and cloths; goggles.

The least messy way of doing this investigation is if pupils retain the treacle in the boiling tube throughout, by inserting a bung before tipping the tube.

Development of knowledge and understanding:
Pupils will discover that the viscosity of a liquid can be changed in a variety of ways. The laboratory behaviour of treacle is used to model the characteristics of lava. The violence of volcanic eruptions and the resulting products are shown to be related to the viscosity of the lava. The viscosity of lava and treacle is modified by changes in: temperature; gas content; water content; concentration of solids, e.g. crystals in lava, sand in treacle.

Evidence of the gas content is preserved as 'bubbles' in some lavas like pumice; others preserve larger crystals in finer grained background material. Lava viscosity also depends upon the chemical composition of the melt, but this cannot be simulated easily using treacle.

The eruptive behaviour of molten rock gives rise to a variety of volcano types and extrusive igneous rocks. Differences in the nature of the original magma and subsequent changes as it rises towards the surface, influence the type of lava erupted. Lava viscosity has a controlling influence on the character of eruptions and the types of volcanoes produced. Low viscosity lavas flow freely from volcanoes to form flat sheets of lava which can cover large areas. As the lava degasses, fire fountains can be produced and you can go and watch or film these as the eruptions are fairly safe. These volcanoes have shallow slopes of 10° or less. High viscosity magmas may just be squeezed out of a vent, but often they solidify in the mouth of a volcano. Then the pressure steadily increases until a catastrophic eruption blasts large blocks of lava and huge volumes of ash into the sky, these fall back to Earth producing steep-sided cones with slopes of 30-40°. Stay well away from this type of eruption.

Demonstration 7A: Lava in the laboratory

(This activity is based on ESTA’s “Magma” in “The Science of the Earth 11-14” series.)

Learning objective:
To investigate the relative effects of the factors which influence the runniness of treacle.

Magma is a rock which has been heated up so much underground that it has melted and formed a liquid. When magma flows out of the ground from volcanoes it is called lava. Lavas can be very sticky and slow-flowing or they can be runny and fast-flowing.

We can find out why some lavas are runny and others are sticky by carrying out some experiments. We cannot bring lava into the laboratory because it is too hot (it may be hotter than 1000 degrees Celsius) and the nearest volcano is too far away.

Treacle is a liquid like lava and the runniness of both of these liquids can be changed in similar ways. So we can carry out laboratory experiments on treacle and then use these to understand how lava flows.

1. Plan a treacle investigation using the guidelines below.
2. Carry out your investigation by following your plan. Be sure to clean up afterwards.
3. Do not forget to record your results and to plot them on a graph.
4. Write down what your investigation showed. Were your first ideas right or wrong?
5. Could your investigation have been done better? How?
6. Be ready to tell the rest of the group about your investigation and your results.

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