School › Volcanoes › Ages 10–11
Runny lava and explosive ash
Why does magma rise, why do some volcanoes explode while others pour out lava, and where on Earth are volcanoes found? With a fair-test 'lava race'.
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Deep underground some rock melts into magma. Magma is lighter than the solid rock around it, so it rises and collects in magma chambers. Some of it pushes up through cracks and vents; once it erupts it is called lava.[1]
Magma contains dissolved gas. In thin, runny magma the gas escapes easily and lava flows out of the volcano, as at Hawaii's volcanoes. In thick, sticky magma the gas cannot escape, pressure builds up and the gas bursts out violently, as at Mount St. Helens in the USA.[1]
Kīlauea's lava in Hawaii erupts at about 1,170 °C. You can estimate lava's temperature from its glow: yellow is about 1,000–1,200 °C, orange 800–1,000 °C and red 600–800 °C.[7]
There are different kinds of volcano. Shield volcanoes are wide with gentle slopes, built from runny lava flows. Composite volcanoes (stratovolcanoes) are tall and steep, built from layers of lava, ash and rocks. Cinder cones are small and made of pieces of lava.[2]
Volcanoes are not spread out randomly. Many lie where plates of the Earth's crust meet, such as the Ring of Fire around the Pacific Ocean. Others, like Hawaii's, sit over a hot spot in the mantle.[3][4][6]
There are about 1,350 potentially active volcanoes in the world, and roughly 50 to 70 eruptions happen each year. In Europe, Etna and Stromboli in Italy erupt almost all the time.[3][8]
Eruptions can be dangerous: ash can be carried far by the wind, and when hot volcanic material mixes with water or melted snow it can form fast mudflows.[2][1]
Try it: Lava race: runny or sticky?
You need
- A tray or board, propped up at one end on 2 books
- A ruler and masking tape
- 4 liquids: water, cooking oil, golden syrup or honey, ketchup
- 4 teaspoons
- A stopwatch
- Paper towels
Safety
Wipe up spills at once so nobody slips. Do not taste the liquids. Wash hands afterwards. Keep the slope low so the tray cannot slide off the table.
What to do
- Put a tape start line near the top of the tray and a finish line 20 cm lower.
- Plan a fair test: what will you keep the same? (Same amount of liquid, same slope, same start line.)
- Predict which 'lava' will be fastest and which slowest.
- Pour one teaspoon of the first liquid on the start line and time how long it takes to reach the finish line. Stop at 3 minutes if it has not arrived.
- Repeat for each liquid, then do the whole race a second time.
- Order the liquids from runniest to stickiest. Which would be like Hawaii's lava, and which like the magma of an explosive volcano?
What happens, and why?
Runny liquids flow quickly and sticky ones slowly. Magma is similar: runny magma lets gas escape and flows out as lava, while thick, sticky magma traps the gas until it bursts out in an explosion.[1]
Play: Flowing or exploding?
Is it more typical of a runny-lava eruption or an explosive eruption?
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Quiz
Other ages: Ages 6–7 · Ages 15–16