How do volcanoes erupt?

1. Far below our feet, the Earth is extremely hot. In some places, rock melts into a glowing liquid called magma.
2. Magma is lighter than the solid rock around it, so it slowly rises, like a blob in a lava lamp. It collects in a magma chamber under the volcano.
3. Magma has gas dissolved in it, like the fizz in a soda bottle. As the magma rises, the squeeze on it drops, and gas bubbles appear and grow.
4. The growing bubbles push on the rock above. The ground can swell a little and small earthquakes happen. Scientists measure these signs to warn people early.
5. When the pressure wins, magma erupts. Runny magma lets gas escape easily, so lava flows out gently. Sticky magma traps the gas until it bursts out explosively as clouds of ash.
6. Lava cools into new rock and ash settles in layers, so volcanoes grow with every eruption. Runny lava builds wide, gentle shield volcanoes; sticky magma builds steep cones.

Tap each part of the volcano. Some of them move when you find them!

• Magma chamber. A large pocket of magma deep under the volcano. It fills up slowly, sometimes for hundreds of years, before an eruption.
• Main vent (conduit). The main pipe that carries magma from the chamber up to the crater.
• Gas bubbles. Gas bubbles grow as the magma rises, like the fizz in a soda. They push the magma up and power the eruption.
• Crater. The bowl-shaped hole at the top where magma, gas and ash come out.
• Ash cloud. Volcanic ash is not soft like ash from a fire. It is tiny, sharp pieces of rock and glass blasted into the sky.
• Lava flow. When magma reaches the surface it is called lava. Runny lava can flow a long way before it cools and hardens.
• Layers of lava and ash. This volcano is built from layer upon layer of hardened lava and ash, added by many eruptions over thousands of years.
• Side vent. Some magma squeezes through cracks and comes out of a side vent on the volcano's slope.

Change how runny or sticky the magma is and how much gas it holds. Watch the pressure, the eruption and the shape of the volcano.

Experiment: Which kind of magma makes an explosive eruption?

1. Set the gas to medium.
2. Try very runny magma and watch an eruption.
3. Now try very sticky magma and compare.

What we found: Sticky magma traps the gas bubbles, so pressure builds up until it bursts out in an explosive eruption of ash. Runny magma lets bubbles slip out easily, so lava flows out gently. Gentle eruptions build wide, flat volcanoes; explosive ones build steep cones.

Turn on JavaScript to try it yourself.

Choose the right magma for each eruption so the volcano grows into the dotted shape.

Deep under the ground it is very hot. In some places, rock melts into magma. Magma is lighter than the rock around it, so it slowly rises and collects in a magma chamber under a volcano.

Magma has gas mixed into it, a bit like the fizz in a bottle of soda. While the bottle is closed, you can't see the bubbles. When you open it, the pressure drops and bubbles appear. Rising magma does the same thing: the closer it gets to the surface, the more the bubbles grow.

What happens next depends on how runny the magma is. Runny magma, like the lava in Hawaii, lets the bubbles escape easily. It flows out in glowing rivers and builds wide, gently sloping shield volcanoes. Sticky magma traps the bubbles. Pressure builds until the magma bursts apart into ash, which builds steep, cone-shaped volcanoes.

Scientists called volcanologists watch volcanoes closely. They measure tiny earthquakes, the ground swelling up and gases coming out. These signs help them warn people early. If a volcano near you becomes active, listen to the experts and follow their plan.

Key words

Magma

Melted rock underground, often with gas and crystals mixed in.

Lava

Magma that has come out onto the surface.

Viscosity

How runny or sticky a liquid is. Honey has a higher viscosity than water.

Shield volcano

A wide volcano with gentle slopes, built by many runny lava flows.

Stratovolcano

A tall, steep volcano built from layers of lava and ash.

Volcanic ash

Tiny pieces of rock and volcanic glass blasted out in an explosive eruption.

Check your thinking

1. What do we call magma once it comes out of a volcano?

   Show the answer

   Lava. Magma is called lava once it reaches the surface.

2. What happens to gas bubbles as magma rises?

   Show the answer

   They grow bigger as the pressure drops. Less squeeze lets the dissolved gas come out and expand, like opening a soda.

3. Which magma is more likely to explode?

   Show the answer

   Sticky magma with lots of gas. Sticky magma traps the gas, so pressure builds until it bursts.

4. How do scientists know a volcano might erupt soon?

   Show the answer

   They measure small earthquakes, ground swelling and gases. Moving magma shakes the ground, makes it swell and releases gas.

5. A wide volcano with gentle slopes was built mostly by...

   Show the answer

   Runny lava flows. Runny lava flows far before it hardens, building a wide shield.