In the year 79 CE the Roman town of Pompeii vanished under metres of ash in a single day when Mount Vesuvius erupted. Yet nearly two thousand years later, three million people live in its shadow — drawn by rich soils and warm land. A volcano is one of nature's fiercest and most tempting neighbours.
How a volcano works
A volcano is an opening in the crust through which molten rock, ash and gases escape. Deep down, rock in the mantle melts into magma. Because magma is less dense than the solid rock around it, it rises, collecting in a magma chamber. When pressure builds too high, it forces its way up a vent and bursts out of the crater. Once it reaches the surface, magma is renamed lava.
Two shapes of volcano
The kind of magma decides the shape. Runny magma flows far and builds wide, gentle volcanoes; sticky, gas-rich magma traps pressure and builds steep, explosive ones.
| Feature | Shield volcano | Composite volcano | |
|---|---|---|---|
| Lava | Runny, flows far | Sticky, thick | |
| Shape | Wide, gently sloping | Tall, steep-sided | |
| Eruption | Gentle, frequent | Violent, explosive | |
| Found at | Hotspots / constructive | Destructive boundaries | |
| Example | Mauna Loa (Hawaii) | Mount Fuji (Japan) |
Where volcanoes form
- Destructive boundaries — subducting plates melt, feeding violent composite volcanoes.
- Constructive boundaries — magma rises into the gap between plates, forming gentler volcanoes (as in Iceland).
- Hotspots — plumes of magma punch through the middle of a plate, far from any boundary. Hawaii sits over one.
Effects of an eruption
Geographers split effects into two groups. Primary effects happen directly and instantly: lava flows, ash falls, buildings destroyed, people injured. Secondary effects follow on afterwards: fires, mudflows, blocked roads, ruined crops, homelessness, and disrupted air travel from ash clouds.