Lava, pyroclastic flows and collapsing slopes behave differently. Identify the material and movement before explaining the damage.
A lava flow is molten rock moving over the surface. Its speed and reach depend on viscosity, supply and slope. Lava can burn or bury vegetation, buildings, roads and farmland. Even a slow flow can cause severe damage or cut off access routes.
A pyroclastic flow is a fast-moving, ground-hugging current of hot gas, ash and rock fragments. It can form when an eruption column or lava dome collapses. Its heat and impact can kill organisms and devastate vegetation and settlements. It behaves differently from liquid lava.
Volcanic landslides occur when part of a volcano becomes unstable and collapses. Steep slopes, weakened rock, earthquakes and eruptions can contribute. Collapse may bury land or block drainage. Mixing debris with enough water can also start a lahar. A slope can fail even when the volcano is not visibly erupting.
Valleys can channel flows, but large, energetic currents may spread more widely. Each process needs its own hazard map. A simple circle drawn around the summit cannot show every flow route or establish that a place is safe.
Step by step
Lava
Molten rock flows over the surface and can burn or bury land.
Pyroclastic flow
Hot gas and fragments move rapidly along the ground.
Slope collapse
An unstable volcano flank falls, burying land and sometimes blocking drainage.
Worked example: Hazard-specific map reading
A town on a valley floor may lie outside the expected lava-flow area yet still be exposed to a lahar from farther upstream. Do not use a map for one volcanic hazard as proof that all the others are absent.
Watch out for this
Pyroclastic flows and lava flows are two names for molten rivers.
Pyroclastic flows contain hot gas and fragmented material; lava flows consist of molten rock.
Check your understanding
An eruption column collapses and sends a hot mixture of ash, gas and rocks rapidly along the ground. What is described?
- A sea breeze
- Only a river of molten lava
- A pyroclastic flow