5.7 — Volcano and Ashfall

The reassuring part first: volcanoes are among the best-monitored hazards there are, and eruptions almost always give warning — days to weeks, sometimes months.

Which means the people at risk are overwhelmingly those who ignored an evacuation order, and the correct response is mostly to leave when told.

And the second useful fact: lava is almost never the danger. It moves slowly and you can walk away from it.

The things that actually kill people are different, and worth knowing.

The real hazards

Pyroclastic flows

The most lethal.

A ground-hugging avalanche of superheated gas, ash and rock, moving at speeds that can exceed 100 kilometres per hour, at temperatures of several hundred degrees.

They are not survivable and they cannot be outrun.

The only defence is distance and evacuation in advance. This is the entire reason exclusion zones exist and the entire reason evacuation orders are not negotiable.

Lahars

Volcanic mudflows: ash and debris mixed with water — from rain, from melting ice, or from a crater lake.

They follow valleys and river channels, they travel far and fast, and they can occur years after an eruption whenever heavy rain mobilises deposited ash.

They have caused some of the worst volcanic disasters in history, including at distances far from the volcano.

The rule: get out of valleys and off low ground near drainages, and get to high ground on the side of a valley rather than in it.

Ashfall

The most widespread and the one that affects the most people, frequently hundreds of kilometres away.

And this is the hazard most readers are actually likely to meet.

Gases

Volcanic gases — carbon dioxide in particular — are heavier than air and collect in hollows, basements and valleys.

They are invisible and odourless in the case of carbon dioxide.

Rule: avoid low-lying areas near a volcano, and never enter a depression, cave or basement in an active area.

Ashfall: what to actually do

Because this is the practical chapter.

Volcanic ash is not soft. It is pulverised rock and glass: abrasive, sharp, heavy, and it conducts electricity when wet.

Immediately

Go indoors.

Close all windows, doors and vents. Put damp towels at gaps.

Turn off air conditioning and anything that draws outside air.

Bring pets and animals inside.

If you must go outside

Wear a well-fitting mask — a proper particulate respirator if available, and a damp cloth as a poor substitute.

Wear goggles, not contact lenses. Ash under a contact lens causes corneal damage.

Cover skin. Long sleeves and trousers.

The roof

Ash is heavy, and wet ash is much heavier.

Roof collapse from ash loading is a real and common cause of death and injury in ashfall, particularly on flat or lightly built roofs.

Clear the roof if it is accumulating and you can do it safely — and this is genuinely dangerous work: wet ash is extremely slippery.

Do not send somebody onto a roof in poor visibility or during continuing ashfall.

Vehicles

Do not drive unless necessary.

Ash reduces visibility to nothing, is slippery, and clogs air filters — engines fail.

And driving stirs up ash, making it worse for everybody.

Water and food

Cover water tanks and disconnect rainwater collection.

Cover food and food preparation surfaces.

Ash contamination of water supplies is common and it makes water unpalatable and sometimes unsafe.

Cleaning up

Wet the ash lightly before sweeping — dry ash becomes airborne and is what damages lungs.

Wear a mask and goggles.

Do not wash ash into drains. It sets like concrete and blocks the drainage system.

And bag it and dispose of it as directed.

Health

**Short-term: irritation of eyes, nose, throat and airways. **Coughing. Aggravation of asthma and other respiratory conditions.

People with existing lung conditions are the most affected and should be indoors and have their medication available.

Long-term exposure to certain ash compositions carries additional risk, which is why proper masks matter for anybody doing extended clean-up.

Before

Five things, if you live near a volcano.

Know the hazard zones and the evacuation routes. These are mapped and published in monitored areas.

Know the alert level system your country uses and what each level means for you.

Have masks and goggles in the kit (9.2) — ordinary cloth masks are not adequate for ash.

Have a plan for animals.

And take evacuation orders seriously. The historical record is unambiguous: the deaths are concentrated among people who stayed, and the people who stayed generally did so because it did not look dangerous yet.

The one to remember

Because the whole chapter reduces to it.

Lava you can walk away from.

Pyroclastic flows you cannot outrun, lahars follow the valleys, and ash arrives hundreds of kilometres away and collapses roofs.

Which means the two actions are: leave when told, and if you are far enough away for ash rather than flows, go indoors, seal up, and clear the roof.

What to do with this page

Evacuation orders near a volcano are not negotiable. The flow is not survivable and cannot be outrun.

Get out of valleys and off low ground — lahars follow drainages and gas collects in hollows.

Ash: indoors, sealed, mask and goggles, and watch the roof load.

Wet the ash before sweeping and never wash it into drains.

And ash is pulverised glass. Treating it as dust is the mistake.

Next: 5.8 — landslide and avalanche, which give warning signs almost nobody knows how to read.