Volcanic and geothermal areas

Volcanoes release gases even between eruptions, mainly water vapour, carbon dioxide, sulphur dioxide and hydrogen sulphide. Carbon dioxide is heavier than air. When there is no wind it can collect in hollows, ditches and caves to dangerous concentrations and push out the air you need to breathe. The US Geological Survey describes this danger in its Volcano Hazards Program.

In geothermal areas hot water circulates beneath the surface. It softens the ground from below and dissolves rock. This can create a thin crust over boiling water or mud that looks solid from above. Anyone who steps on it can break through and be badly scalded.

An eruption adds further dangers: lava flows, ejected rocks, ash and mudflows in valleys. Mudflows can also occur without an eruption when heavy rain mobilises loose ash on the slopes.

Many volcanoes are monitored around the clock by observatories. They issue alert levels, often as colours or numbers, and set up exclusion zones. What the levels mean differs from country to country. So before every visit, check the current alert level and the advice of the responsible observatory or authority, and on site follow warning apps and local media.

We strongly recommend: visit active volcanoes and craters only with licensed local guides where these are required or recommended. They know the current gas vents, the safe paths and the signs that mean it is time to turn back. Barriers are not excessive; they are based on measurements and experience.

The situation at a volcano can change quickly. A path that was open yesterday may be closed today. Follow instructions from authorities and guides immediately and without argument.

Use only marked paths and boardwalks, respect barriers and hold children by the hand. Never leave the boardwalks, not even for a photo. Do not rest in hollows, ditches or caves near gas vents, especially when there is no wind. Children and dogs are at greater risk because they breathe closer to the ground, where heavy gases collect.

Hydrogen sulphide smells of rotten eggs, but at higher concentrations the sense of smell can fail. Carbon dioxide has no smell at all. So the absence of smell does not mean safety. With a pungent smell, coughing, eye irritation, breathlessness, dizziness or headache, leave immediately uphill and upwind and call the local emergency number (112 in the EU). People with asthma or heart and lung conditions are more sensitive to volcanic gases.

If someone collapses in a hollow, do not run in after them. The cause may be gas, and a helper can lose consciousness too. Call emergency services and wait for rescuers with breathing protection. In rain and when an eruption warning is in place, avoid valley floors on volcanic slopes because of possible mudflows.

A dust mask can help against ash and dust but does not protect against gases. Anyone who confuses the two feels safe where they are not. Against gases, only distance, wind and higher ground help.

Sturdy shoes are important, because volcanic rock is often sharp and loose, and ash can be slippery. Water, sun protection and a warm layer belong in the pack too, because many volcanoes are high and exposed. What to do in ashfall in towns is covered in a separate entry on volcanic ash.

Hazard zones at volcanoes are often well monitored and marked, and authorities issue alert levels. The International Volcanic Health Hazard Network provides information on the health effects of ash and gases. If you follow this information and stay on the paths, you can experience volcanic and geothermal areas as impressive places.

This text deliberately gives no gas concentrations or limit values and makes no eruption forecasts. That is the job of the observatories. For you, their current advice counts for more than any general rule.