Crust
The solid rock the volcano stands on. Magma rises through it along cracks and weaknesses.
Continental crust: About 30–50 km thick
Education example
Read-only showcaseA volcano, in cross-section
A volcano cut in half: from the magma chamber to the ash cloud, every feature explained.
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Layers of ash and lava
A stratovolcano is built of alternating layers: ash and rock from explosive eruptions, then lava from quieter ones. That is why it is also called a composite volcano.
- Type
- Stratovolcano (composite)
- Built from
- Alternating ash and lava layers
- Examples
- Fuji, Vesuvius, Mount St Helens
Magma chamber
A reservoir of molten rock a few kilometres down. When pressure builds - from new magma or from gas coming out of it - the volcano erupts.
- Typical depth
- 1–10 km
- Temperature
- About 700–1,300 °C
Main vent
The pipe magma rises through, from the chamber to the crater.
Also called: The conduit
Sill
A sheet of magma that pushed sideways between layers of rock and set there, parallel to them.
Runs: Along the layers
Dyke
A sheet of magma that cut across the layers of rock on its way up, then cooled where it stood.
Runs: Across the layers
Secondary vent
A branch off the main vent that breaks out on the volcano's side, building a small cone of its own.
Also called: A parasitic cone
Crater
The bowl at the summit where the main vent opens. It is shaped by eruptions blasting material out.
Where: At the top of the main vent
Lava flow
Molten rock that has reached the surface - here from the secondary vent. It runs downhill, slowing as it cools and crusts over.
- Temperature
- About 700–1,200 °C
- Usually moves
- Slower than walking pace
Pyroclastic flow
A ground-hugging avalanche of hot gas, ash and rock that pours down the side of the volcano - the most deadly thing it does.
- Speed
- Can exceed 100 km/h
- Temperature
- Hundreds of degrees
Volcanic bombs
Blobs of lava thrown into the air that cool into rock before they land.
Size: Over 64 mm across, by definition
Ash cloud
Fragments of rock and glass carried up by hot gas. A large eruption can lift ash into the stratosphere, where winds carry it around the world.
- Made of
- Tiny rock and glass fragments
- Can reach
- The stratosphere, above 10 km
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