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Unlock This CourseA volcanic eruption is not one danger. It is a whole set of hazards, and each one behaves differently. A hazard is something that can cause harm. Some move faster than a car. Some creep along slower than you can walk. Some stay on the mountain, and some cross whole countries. To compare them, geographers use four ideas.
Key terms:
Remember from Types of Volcano and Their Features that explosive eruptions throw out tephra and effusive eruptions let lava pour out. Most of the hazards below come from one of those two.
A fast-moving cloud of very hot gas, ash and rock that races down the side of a volcano. It can move at more than 100 km per hour and be hundreds of degrees Celsius. Nobody can outrun it, and it burns, buries and suffocates. It usually stays within a few kilometres to tens of kilometres of the volcano, so it is local but extremely deadly. In AD 79, flows from Vesuvius buried the Roman towns of Pompeii and Herculaneum.
A fast-flowing mudflow made of volcanic ash and rock mixed with water. The water comes from heavy rain, a crater lake or melting snow and ice. A lahar moves like wet concrete and follows river valleys, so it can travel many kilometres from the volcano. It can happen during an eruption or years afterwards, when rain washes loose ash downhill. In 1985 a lahar from Nevado del Ruiz in Colombia buried the town of Armero and killed more than 20,000 people.
A stream of molten rock pouring from the volcano at roughly 700 to 1,200 degrees Celsius. Lava is usually slow, often slower than walking pace, so most people can get away. The danger is to property: it burns and buries buildings, roads and farmland and is very hard to stop. It stays local. In 2018 lava from Kilauea in Hawaii destroyed hundreds of homes.
Rock fragments thrown into the air that fall back down, such as small stones and cinders. Tephra is the name for all the fragments thrown out, from ash up to bombs, but here 'tephra fall' means the stones and cinders in between. They land close to the volcano, usually within a few kilometres of the vent. They fall quickly from the sky, bury land, break windows and can set roofs on fire.
The biggest pieces of tephra: lumps of rock more than 64 mm across, thrown out while still partly molten. Some are as big as a car. They land hard, so they can kill or injure anyone nearby and smash buildings. They are local, because they only land close to the vent.
Ash is tiny, sharp pieces of rock and glass, smaller than 2 mm. It is blasted high into the air and drifts with the wind, so it spreads far. It is made in almost every explosive eruption. Thick ash is heavy, especially when wet, and can make roofs collapse. It ruins crops, smothers grazing land and causes breathing problems. It can also stop jet engines. In 2010 ash from Eyjafjallajokull in Iceland closed airspace across much of Europe for days.
Volcanoes release gases such as sulfur dioxide, carbon dioxide and hydrogen sulfide. Some are hard to notice. Sulfur dioxide irritates the lungs and causes acid rain. Carbon dioxide is heavier than air, so it collects in hollows and valleys and can suffocate people and animals. Gases are released in nearly every eruption, and also between eruptions. They can drift a long way on the wind.
Different hazards win on different measures. This table compares them and ranks them by danger to life close to the volcano only, most dangerous first.
| Rank | Hazard | Speed | Size | Frequency | Spread | Danger |
|---|---|---|---|---|---|---|
| 1 | Pyroclastic flow | Very fast | Large flows | Only some explosive eruptions | Local | Kills very quickly |
| 2 | Lahar | Fast | Fills whole valleys | Often, during or after eruptions | Medium, along rivers | Buries towns |
| 3 | Volcanic bombs | Fast, falling | Small, single lumps | Some explosive eruptions | Local | Kills near the vent only |
| 4 | Tephra fall | Fast, falling | Medium area | Most explosive eruptions | Local | Injures, buries land |
| 5 | Ash fall | Slow, drifts | Very large area | Almost every explosive eruption | Very wide | Harms health, farming and air travel |
| 6 | Toxic gases | Drift with the wind | Can be large | Nearly all eruptions | Wide | Harms health, can suffocate |
| 7 | Lava flow | Slow | Narrow but can be long | Common at effusive volcanoes | Local | Rarely kills, destroys property |
The hazard that kills most people close to the volcano is not always the hazard that affects most people overall. Pyroclastic flows are fast and deadly but local. Ash is less deadly but spreads far and affects air travel and farming. Lava is slow, so people usually escape, but property is lost.
Fig. 1.1 shows made-up information about three hazards during eruptions of one volcano.
| Hazard | Speed | Furthest distance from vent | Eruptions with this hazard in the last 100 years |
|---|---|---|---|
| A | 120 km per hour | 8 km | 2 |
| B | Carried by wind | 300 km | 9 |
| C | 2 km per hour | 5 km | 6 |
Fig. 1.1
(a) Identify which hazard in Fig. 1.1 is most likely to be a pyroclastic flow. [1 mark]
(b) State one reason why lava is less likely to kill people than a pyroclastic flow. [1 mark]
(c) Describe two ways in which ash fall harms people. [2 marks]
(d) Assess which hazard in Fig. 1.1 is the most dangerous. [6 marks]
(a) Hazard A, because it is very fast (120 km per hour) and only reaches a short distance. (1)
(b) Lava moves slowly, so people have time to walk away. (1)
(c) Any two: it makes roofs collapse when it builds up (1); it ruins crops and grazing land (1); it causes breathing problems (1); it can stop aircraft flying (1).
(d) Top level answer (5 to 6 marks): Hazard A is the most dangerous to life. At 120 km per hour nobody can outrun it, so people in its path have almost no time to escape. However, it only reached 8 km and happened in just two eruptions, so it threatens fewer people overall. Hazard B spreads 300 km on the wind and happened nine times, so it affects a much bigger area and many more people, for example through damage to farming and air travel. It is carried by the wind and usually falls as thin ash, so it rarely kills directly. Hazard C is slow at 2 km per hour, so people can usually escape, but it is fairly common and destroys property in its path. Overall, A is the most dangerous because its speed makes it deadly, although B affects the most people and has the widest economic impact. (A clear judgement, with evidence from the figure and from at least two hazards.)
In an assess question, compare at least two hazards using speed, size, frequency and spread, and finish with a clear decision that says which is most dangerous and why.