Volcanoes
Where magma reaches the surface, and why some do it quietly.
A volcano is an opening where molten rock, ash and gas reach the surface. Most sit at plate boundaries — the Pacific Ring of Fire traces subduction zones — though hotspot volcanoes such as Hawaii occur mid-plate.
Composition decides everything
Eruption style is set by magma viscosity, and viscosity by silica content. Basaltic magma is low in silica, runny, and lets gas escape, so it erupts as lava flows. Rhyolitic magma is silica-rich and stiff, traps gas until pressure wins, and erupts explosively. The difference between a lava fountain and a pyroclastic flow is chemistry, not size.
| Type | Shape | Magma | Eruption |
|---|---|---|---|
| Shield | Broad, gentle | Basaltic | Effusive lava flows |
| Stratovolcano | Steep, layered | Andesitic | Explosive, alternating |
| Cinder cone | Small, steep | Basaltic | Short, fragmental |
| Caldera | Collapsed basin | Rhyolitic | Catastrophic |
Hazards
Lava flows are the most visible hazard and among the least deadly — they are usually slow enough to walk away from. Pyroclastic flows of hot gas and ash move at hundreds of km/h and are not survivable. Lahars, volcanic mudflows, can travel far down valleys long after an eruption ends.