A volcano is an opening where magma reaches the surface. How a volcano erupts, and what shape it builds, depends mostly on how runny its magma is and how much gas it holds. Most volcanoes sit along plate boundaries.
| Type | Magma | Eruptions and shape | Example |
|---|---|---|---|
| Shield volcano | Runny, low in silica (basalt) | Gentle lava flows build a broad, gently sloping mountain | Mauna Loa, Hawaii |
| Cinder cone | Gassy blobs of lava | Small, steep cone of loose cinders | Parícutin, Mexico |
| Composite volcano (stratovolcano) | Sticky, rich in silica, gas-rich | Explosive eruptions; tall, steep cone of lava and ash layers | Mount St. Helens, Mount Fuji |
Key idea
Runny, low-silica lava lets gas escape gently and builds broad shield volcanoes. Sticky, high-silica magma traps gas until it bursts out explosively, building steep composite volcanoes.
Why it works
Silica (SiO₂) makes molecules link into chains, which makes magma thick and sticky, like cold honey. Gas bubbles can't rise easily through sticky magma, so pressure builds until it blasts the magma apart into ash. In runny basalt, bubbles slip out easily, like bubbles in boiling soup, so the lava just pours out. That is why Hawaiian eruptions are mostly gentle while Mount St. Helens exploded in 1980.
Worked example: Predict the eruption
A volcano's magma is high in silica and full of dissolved gas. What kind of eruption should you expect?
- High silica: sticky magma.
- Gas trapped in sticky magma builds pressure.
- Expect explosive eruptions of ash, typical of a composite volcano.
Answer: Explosive
Worked example: Read the shape
A volcano is about 100 km wide but has slopes of only a few degrees. What kind is it, and what is its lava like?
- Broad and gentle means lava flowed far before cooling.
- That takes runny, low-silica basalt: a shield volcano.
Answer: A shield volcano with runny basalt lava
Common mistake
Thinking lava flows are the main danger. Most lava flows move slowly enough for people to walk away. Fast clouds of hot ash and gas (pyroclastic flows) and volcanic mudflows (lahars) have killed far more people. In 1985 a lahar from Nevado del Ruiz in Colombia killed about 23,000 people.