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Science project idea

Why volcanoes have different shapes: a lava thickness test

Runny lava spreads far and builds wide, low shield volcanoes; thick, sticky lava piles up near the vent and builds steep cones. Pouring liquids of different viscosity through a hole in a tray shows the two shapes forming.

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What it shows

Runny lava spreads far and builds wide, low shield volcanoes; thick, sticky lava piles up near the vent and builds steep cones. Pouring liquids of different viscosity through a hole in a tray shows the two shapes forming.

The science

Viscosity is a liquid's resistance to flowing. Lava rich in silica is viscous, like honey, and lava poor in silica is runny, like oil. A viscous liquid loses height slowly as it spreads, so it stops close to the vent in a steep mound, while a runny liquid flattens into a wide thin sheet. The same silica also traps gas, which is why thick-lava volcanoes erupt explosively.

Materials

Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A flat tray or a sheet of plastic on a table
  • A paper cup with a small hole in the bottom
  • Water with a drop of colour
  • Cooking oil
  • Honey or thick sugar syrup
  • A ruler and a stopwatch

Safety

For this project: Wipe spills at once; oil and syrup on the floor are slippery.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Hold the cup a few centimetres above the tray with a finger over the hole.

  2. Pour in 50 millilitres of coloured water, remove your finger and let it drain onto the tray.

  3. Measure how wide the puddle spread and how high its edge stands, and time how long it took to stop.

  4. Wipe the tray and repeat with 50 millilitres of oil.

  5. Repeat with 50 millilitres of honey, pouring slowly.

  6. Make a table of liquid, spread width, mound height and time to stop.

  7. Sketch the side view of each spread and name the volcano type it resembles.

  8. Write a conclusion linking viscosity to volcano shape and to the violence of eruptions.

What you should see

Water spreads widest and lowest, honey forms the narrowest and tallest mound, oil sits between. The table shows a clear link between viscosity and the height-to-width ratio.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is viscosity?

A liquid's resistance to flow; thick liquids have high viscosity.

2Which lava builds a shield volcano?

Runny, low-silica basaltic lava.

3Why are thick-lava eruptions more explosive?

Thick lava traps gas until the pressure bursts out.

4Give an example of a shield volcano.

Mauna Loa in Hawaii.

Free ebook: Viva Questions for Science Projects

Take it further

Warm the honey in a bowl of hot water and repeat to show how temperature changes viscosity and flow.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

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