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Magma is molten rock below the surface, together with crystals and dissolved gas; once erupted it is lava. Magma rises because it is less dense than the surrounding rock and collects in magma chambers.

Magma composition controls viscosity. Felsic magmas contain more silica and are more viscous than mafic magmas. They also carry more volatiles: about 4–7% in felsic magma against 1–3% in mafic magma. The most abundant volatile is water, followed by carbon dioxide and sulphur dioxide.

In viscous magma the gas cannot escape easily, so pressure builds as gas comes out of solution, leading to an explosive eruption. Runny mafic magma lets gas escape, giving effusive eruptions of lava flows and domes.

Experiment: Bubble-rise test: how viscosity traps gas

1. Fill each tube with one liquid, leaving the same air gap (about 1 cm) at the top, and close it tightly.
2. Mark two lines 10 cm apart on each tube.
3. Turn a tube upside down and time how long the air bubble takes to rise from one line to the other.
4. Repeat three times for each liquid; calculate the mean time and the range.
5. Calculate the mean bubble speed in cm/s for each liquid, and give an uncertainty of ± half the range.
6. Relate your results to gas escaping from mafic and felsic magma, and list the limitations of the model.

Liquid	Time 1 (s)	Time 2 (s)	Time 3 (s)	Mean (s)	Speed (cm/s)
Water
Cooking oil
Golden syrup / glycerol

Questions

1. Draw a labelled cross-section of a subduction zone with a composite volcano above it.

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2. Use your bubble-test results to calculate the mean bubble speed in each liquid with an uncertainty. Which liquid best models felsic magma?

3. Explain, using silica, viscosity and volatiles, why felsic magma tends to erupt explosively and mafic magma effusively.

4. Lava glows orange. Use the USGS colour scale to estimate its temperature, and say why this is only an estimate.

Sources for every fact on this sheet: echoesof.science/school/volcanoes/sources/

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