What this lesson teaches
I can explain the hazards, impacts and benefits of earthquakes and volcanoes for nature and people.
Syllabus K335, Topic 4.2, Key Question 4. How might tectonic hazards affect the natural and human systems?
Make a guess
A volcano's eruption column collapses and a cloud of hot gas, ash and rock races down the slope. Is this lava?
- Yes. Volcanic flows down a slope are lava.
- No. It is a lahar.
- No. It is a pyroclastic flow.
- Yes, lava that has turned into gas.
Show the answer
No. It is a pyroclastic flow.
A pyroclastic flow is a fast, ground-hugging cloud of hot gas, ash and rock. Lava is molten rock that moves over the ground.
Lava, pyroclastic flows and collapsing slopes move differently, so name the material and how it moves before explaining the damage.
A lava flow is molten rock moving over the ground. Its speed depends on how runny it is, how much there is and the slope. Lava burns or buries plants, buildings, roads and farms. Even a slow flow can cut off roads.
A pyroclastic flow is a fast, ground-hugging cloud of hot gas, ash and rock. It forms when an eruption column or lava dome collapses. Its heat and speed kill living things and flatten settlements. It is not liquid lava.
A volcanic landslide happens when part of a volcano collapses. Steep, weak slopes, earthquakes and eruptions can cause it. It can bury land or block rivers, and if the debris mixes with enough water it can start a lahar. A slope can fail even when the volcano is not erupting.
Valleys channel many flows, but large, fast currents can spread beyond them. Each hazard needs its own map. A simple circle around the summit cannot show every route or prove that a place is safe.
- Lava flow
- Molten rock moving over the ground.
- Pyroclastic flow
- Fast, hot cloud of gas, ash and rock.
- Volcanic landslide
- Part of the volcano collapses.
Step by step
Lava
Molten rock burns or buries land.
Pyroclastic flow
Hot gas and rock rush along the ground.
Slope collapse
A weak flank falls and buries land.
Real examples to use
Mount Merapi, Indonesia, 2010
Pyroclastic flows from Merapi killed more than 300 people and forced hundreds of thousands to evacuate.
Mount St Helens, USA, 1980
The volcano's north side collapsed in the largest landslide ever recorded, releasing a sideways blast that flattened forests.
Worked example: Read each hazard map separately
A town on a valley floor may be outside the lava-flow zone. But it could still be hit by a lahar coming down the valley. One hazard map does not prove other hazards are absent.
Watch out for this
Pyroclastic flows and lava flows are both rivers of molten rock.
Pyroclastic flows are hot gas and rock fragments. Lava flows are molten rock.
Check your understanding
An eruption column collapses and sends hot ash, gas and rock racing along the ground. What is this?
- A lahar from the summit.
- A pyroclastic flow.
- A fast lava flow.
Show the answer
A pyroclastic flow.
Yes. It is a fast, hot cloud, not liquid lava.