Teacher sheet · Ages 15–16 · about 45–50 minutes
Magma, gas and plate tectonics
Lesson page: echoesof.science/school/volcanoes/ages-15-16/ · Pupil worksheet: echoesof.science/school/volcanoes/ages-15-16/print/
Learning aims
- Relate magma composition (silica, volatiles) to viscosity and eruption style.
- Explain volcanism at subduction zones, spreading ridges and hotspots.
- Carry out calculations with plate speeds and investigation data, including uncertainty.
- Describe volcanic hazards and how volcanoes are monitored.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: compare a Hawaiian lava flow with an explosive ash column. What controls the difference? |
| 15 min | Read the text; worked example: spreading over 1 million years. |
| 15 min | Bubble-rise investigation and calculations. |
| 5 min | Play Which setting? |
| 5 min | Quiz and discussion of the model's limits. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Hotter magma is always more explosive. | Explosiveness depends mainly on viscosity and gas content; runny mafic magma is usually effusive. |
| Volcanoes form only where plates collide. | They also form at divergent boundaries and over hotspots such as Hawaii. |
| Lava flows are the deadliest volcanic hazard. | Lava usually moves slowly enough to escape; pyroclastic density currents and lahars are far more dangerous. |
Worksheet answers
- Oceanic plate descending beneath another plate along a trench; magma rising from the region above the descending plate; a steep composite volcano built of lava and ash layers on the overriding plate.
- Speed = 10 cm ÷ mean time; uncertainty from ± half the range; syrup (or glycerol) is the slowest and best models viscous felsic magma.
- Felsic magma has more silica, so it is more viscous, and it holds more volatiles (4–7%); the gas cannot escape, pressure builds and it erupts explosively. Mafic magma has less silica and fewer volatiles (1–3%), so gas escapes and lava flows out.
- About 800–1,000 °C; colour depends on judgement, lighting and the surface cooling, so it gives only a range, not an exact value.
Quiz answers
- The Mid-Atlantic Ridge spreads at 2.5 cm per year. How far does it widen in 1 million years? 25 km. 2.5 cm × 1,000,000 = 2,500,000 cm = 25 km.[5]
- Why are felsic magmas more viscous than mafic magmas? They contain more silica. More silica means more viscous magma.[9]
- What is the most abundant volatile in magma? Water. Water, then carbon dioxide and sulphur dioxide.[9]
- Where are most volcanoes found? At convergent boundaries (subduction zones). Most volcanoes are associated with subduction zones.[10]
- Why are Kauai's rocks older than those on the Island of Hawaii? The Pacific Plate carried Kauai away from the hotspot. Plate motion carries islands away from the stationary hotspot.[6]
- Which hazard can travel at hundreds of km/h? A pyroclastic density current. Pyroclastic density currents are very fast; lava flows are slow.[10]
Going further
Use a world map of plate boundaries to classify ten volcanoes (for example Etna, Kīlauea, Mount St. Helens, Hekla in Iceland and Mount Fuji) as subduction, ridge or hotspot volcanoes, and test whether their usual eruption style matches the magma type you would expect.
Sources used in this lesson
- [1] How do volcanoes erupt? (US Geological Survey (USGS)): reliability High
- [2] About Volcanoes (USGS Volcano Hazards Program): reliability High
- [5] This Dynamic Earth: Understanding plate motions (US Geological Survey (USGS)): reliability High
- [6] This Dynamic Earth: Hotspots (US Geological Survey (USGS)): reliability High
- [8] Volcanoes (British Geological Survey (BGS)): reliability High
- [9] Magma Composition and Eruption Style (Physical Geology, section 4.2) (Physical Geology, open university textbook (eCampusOntario Pressbooks)): reliability Medium-high
- [10] Chapter 4 Volcanism: summary (Physical Geology) (Physical Geology, open university textbook (BCcampus OpenText)): reliability Medium-high
Full list with links and a note on each source's reliability: echoesof.science/school/volcanoes/sources/
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