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Magma, gas and plate tectonics
How magma composition controls viscosity, gas content and eruption style; volcanoes at subduction zones, spreading ridges and hotspots; hazards and monitoring. With a bubble-rise investigation and plate-speed calculations.
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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.[2][1]
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.[9]
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.[9][8]
Tectonic setting matters. Most volcanoes are at convergent boundaries, where one plate is subducted beneath another; composite volcanoes are typical there, as in the Andes and the Cascade Range. Magma also rises at divergent boundaries, where new crust forms; the Mid-Atlantic Ridge spreads at about 2.5 cm per year.[10][5]
Hotspots are long-lasting, very hot regions in the mantle. The Pacific Plate moves over the Hawaiian hotspot, so the islands get older away from it: rocks on Kauai are about 5.5 million years old, while the oldest exposed rocks on the Island of Hawaii are less than 0.7 million years old. Most shield volcanoes are linked to mantle plumes and erupt mafic lava.[6][10]
Hazards include pyroclastic density currents, which can travel at hundreds of km/h; lahars (volcanic mudflows) that can destroy towns; and ash carried far by wind. Lava flows destroy what they cover but usually move slowly enough for people to escape.[10][2]
Scientists watch volcanoes by recording earthquakes, detecting volcanic gases and measuring how the flanks of a volcano swell. Yet most of the world's volcanoes are not well monitored or not monitored at all.[10][8]
Try it: Bubble-rise test: how viscosity traps gas
You need
- 3 identical clear tubes or small bottles with tight lids
- Water, cooking oil and golden syrup (or glycerol)
- A ruler and marker
- Stopwatch
- Paper towels
Safety
Use plastic tubes or bottles if possible; check the lids are tight. Wipe up spills at once. Do not taste the liquids. Wash hands afterwards.
What to do
- Fill each tube with one liquid, leaving the same air gap (about 1 cm) at the top, and close it tightly.
- Mark two lines 10 cm apart on each tube.
- Turn a tube upside down and time how long the air bubble takes to rise from one line to the other.
- Repeat three times for each liquid; calculate the mean time and the range.
- Calculate the mean bubble speed in cm/s for each liquid, and give an uncertainty of ± half the range.
- Relate your results to gas escaping from mafic and felsic magma, and list the limitations of the model.
Play: Which setting?
Is each feature typical of a subduction zone, a spreading ridge, or a hotspot?
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Quiz
Other ages: Ages 6–7 · Ages 10–11