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Why do some volcanoes pour out rivers of lava, and others explode?
Volcanic Eruption lab · Last updated: 16 October 2026
You erupt different volcanoes and compare them: some pour out gentle rivers of glowing lava, while others blow apart in huge explosions of ash. They find out that the difference lies in the magma itself, how runny or sticky it is and how much gas is trapped inside.
What you'll discover
- Magma contains dissolved gases, like the fizz in a sealed bottle of soda.
- Runny magma lets gas bubble out easily, so the lava flows out gently.
- Thick, sticky magma traps the gas until pressure builds up and the volcano explodes.
The science in plain words
A volcano is an opening in Earth's crust where molten rock, called magma, reaches the surface. Magma forms deep underground where rock melts, often where tectonic plates pull apart or where one plate sinks beneath another. Because it is less dense than the solid rock around it, magma rises and collects in chambers beneath the volcano.
Magma contains dissolved gases, mostly water vapour and carbon dioxide. Under high pressure deep down, the gas stays dissolved, like the fizz in a sealed soda bottle. As magma rises, the pressure drops and the gas forms bubbles. What happens next depends on how runny the magma is, a property called viscosity. Magma with less silica, such as basalt, is runny, so bubbles escape easily. The lava pours out in flowing rivers and fountains, as in Hawaii.
Magma rich in silica is thick and sticky, a little like cold honey. Gas bubbles cannot escape easily and pressure builds up. When it finally breaks through, the gas expands violently and shatters the magma into ash and fragments, producing an explosive eruption such as Mount St Helens in 1980. Explosive volcanoes often build steep cones, while runny lava builds broad, gently sloping shield volcanoes. India's Barren Island in the Andaman Sea is an active volcano, and the Deccan Traps were made by enormous ancient lava flows.
Where it fits
- India, NCERT: Volcanoes are not a chapter of the NCERT science syllabus for classes 6 to 10; they are taught in NCERT geography.
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-ESS2-2, MS-ESS3-2
Common questions
Why do some volcanoes explode?
Volcanoes explode when their magma is thick and sticky, usually because it is rich in silica. Gas cannot bubble out easily, so pressure builds until the magma bursts apart into ash and rock fragments. Runny magma lets gas escape gently, so it flows out as lava instead.
What is the difference between magma and lava?
Magma and lava are both molten rock. It is called magma while it is underground and lava once it reaches Earth's surface. Lava also loses much of its dissolved gas as it erupts, so its behaviour can be different from the magma it came from.
Are there volcanoes in India?
Yes. Barren Island in the Andaman Sea is India's only active volcano and has erupted several times in recent decades. The Deccan Plateau is built from the Deccan Traps, thick layers of ancient lava that poured out around 66 million years ago.
Related labs
- Plate Tectonics: The ground moves as fast as your fingernails grow. What does that do in a million years?
- Rock Cycle: Follow a grain of rock: worn away, buried, cooked, melted and born again.
- Earthquake Waves: Two kinds of wave race through the Earth. How do they tell us where a quake was, and what is inside?
See all earth and space labs or every Taz science lab.
Try it yourself
The Volcanic Eruption lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.