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Earth and Environmental Science 11–12 · Year 12

Magma viscosity: flow rate of model lavas and what it means for eruption style

Module 6: Hazards

PracticalLow risk

The idea

Silica-rich magma is far more viscous than basaltic magma, traps gas and erupts explosively, while runny basalt degasses and flows.

What you need

  • model lavas: water, cooking oil, golden syrup, and syrup thickened with flour (in order of rising viscosity); each at two temperatures
  • a tilted glass sheet or board at 20 degrees with a start and finish line 30 cm apart, stopwatch
  • a bottle with a balloon for the gas-escape comparison, effervescent tablet
  • thermometer, water bath

How to do it

  1. Pour 20 mL of each liquid at the start line and time its front over 30 cm; repeat three times.
  2. Warm each liquid to 40 °C in the water bath and repeat.
  3. Rank the liquids by flow time at each temperature.
  4. Drop an effervescent tablet into water and into syrup and record how gas escapes from each (streams of bubbles against trapped, swelling foam).
  5. Match the ranking to basalt (Hawaii-type effusive) and rhyolite or dacite (Pinatubo-type explosive) and to the syllabus list of eruption types.

What you should see

Flow time rises steeply from water to thickened syrup; warming shortens the time of the oil, the syrup and the thickened syrup, so both composition and temperature set viscosity. Water crosses the 30 cm too fast for a stopwatch to separate its two temperatures, although its viscosity falls from 1.002 to 0.653 mPa·s between 20 and 40 °C (NIST Chemistry WebBook), so it serves as the low-viscosity reference. Gas escapes from water in a fine stream and is trapped in syrup as expanding foam, the model of why viscous magma erupts explosively.

What changes

What you change
liquid (composition) and temperature
What you measure
time for the front to travel 30 cm
What you keep the same
  • volume poured
  • slope angle
  • distance
  • surface material

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • All volcanoes explode.
  • Hotter lava is always more dangerous (hot basalt flows, cooler rhyolite explodes).

Safety card

Low riskLearners carry it out

Hazards

  • warm liquids
  • slips on spilt oil

Controls

  • water bath below 50 °C
  • work over trays
  • clean spills

Note

Heat or hot water is used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1, and record a RiskAssess risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
  • Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-11-12-curriculum-resources/earth-and-environmental-science-year-11-and-12
  2. www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
  3. www.earthlearningidea.com/PDF/38_Lava_viscosity.pdf
  4. www.earthlearningidea.com/PDF/126_Bubblemania.pdf
  5. webbook.nist.gov/chemistry/fluid

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