Volcanic Gases: What Volcanoes Breathe Out

The main volcanic gases

Every volcano has its own gas "fingerprint", and the mix changes as an eruption develops. Still, the same handful of gases dominate almost everywhere. Water vapour usually makes up the large majority of the plume — the white clouds you see above many craters are mostly condensed steam.

Gas proportions vary strongly between volcanoes and over time. Overview based on the USGS Volcano Hazards Program.

Why magma releases gas

Deep underground, magma is under enormous pressure, and that pressure keeps water, CO₂ and sulfur dissolved in the melt. As magma rises, the pressure drops and the gases can no longer stay in solution. They form bubbles — exactly like opening a bottle of soda. While the cap is on, you see no bubbles; release the pressure and the CO₂ fizzes out.

Different gases come out at different depths. CO₂ is much less soluble than water, so it starts escaping when magma is still many kilometres down. Water and sulfur follow closer to the surface. This is why a rise in CO₂ at the surface can be an early hint that fresh magma is moving at depth.

What happens next decides the style of eruption:

• Runny (low-viscosity) basaltic magma lets bubbles escape easily. The result is usually gentle lava fountains and flows, as in Hawaiʻi or Iceland.
• Sticky (high-viscosity), silica-rich magma traps the bubbles. Pressure builds until the magma shatters into ash and the gas expands violently.

In short: high viscosity + high gas content = explosive eruption. Gas is the fuel; viscosity decides whether it fizzes out or blows up. See types of eruptions and Volcanology 101 for more.

Health and local hazards

Volcanic gas is one of the quieter volcanic hazards, but it affects people even when a volcano is not erupting.

• Carbon dioxide pooling: CO₂ is colourless, odourless and about 1.5 times denser than air, so it flows downhill and collects in hollows, cellars, pits and snow caves. At high concentrations it causes unconsciousness and death within minutes. At Mammoth Mountain in California, CO₂ seeping from the ground since around 1990 has killed large areas of trees near Horseshoe Lake, and in 2006 three ski-patrol members died after falling into a snow hollow filled with volcanic CO₂.
• Vog in Hawaiʻi: SO₂ from Kīlauea reacts in the air to form tiny sulfate particles — vog, or volcanic smog. It can irritate eyes and throats and worsen asthma, especially downwind on the Island of Hawaiʻi.
• Hydrogen sulfide: recognisable by its rotten-egg smell at low levels. At high levels it numbs the sense of smell, so the warning disappears just when the danger is greatest.
• Fluorine poisoning: during the Laki fissure eruption in Iceland (1783–84), fluorine settled on grass and poisoned grazing animals. A large share of Iceland's livestock died, and the famine that followed killed roughly a fifth of the population.
• Acid rain: SO₂, HCl and HF dissolve in rain and fog, damaging crops, corroding metal and contaminating rainwater tanks near active vents.
• Ash plus gas: ash particles carry acidic gas coatings to the ground, which is why fresh ash can contaminate water supplies and pasture far from the volcano.

Limnic eruptions: the Lake Nyos disaster

A limnic eruption is a rare event in which a deep lake suddenly releases a huge cloud of dissolved CO₂. It happens in calm, deep crater lakes where volcanic CO₂ seeps in from below. The cold, dense bottom water holds the gas under pressure, much like the soda bottle. If something disturbs the layers — a landslide, a storm or slow saturation — gas-rich water rises, bubbles form, and the lake erupts in a runaway fizz.

On 21 August 1986, Lake Nyos in Cameroon released a CO₂ cloud that flowed down the surrounding valleys and suffocated about 1,700 people and thousands of cattle, some of them many kilometres from the lake. Two years earlier, in August 1984, a smaller release at nearby Lake Monoun had killed 37 people — a warning that was only understood afterwards.

Can it happen again in the same lake? Yes. The gas supply from below does not stop, so the deep water slowly recharges with CO₂. To prevent a repeat, scientists installed degassing pipes: a pipe reaches the bottom water, and once water is pumped up, the falling pressure makes gas bubbles form, which lifts the water further on its own. The pipe then works like a self-powered fountain, releasing CO₂ gradually and harmlessly. Pipes have operated at Lake Nyos since 2001 and at Lake Monoun since 2003.

Lake Kivu, on the border of Rwanda and the DR Congo, is a much larger concern. Its deep water contains large amounts of dissolved CO₂ and methane, and around two million people live along its shores. Methane is now extracted there to generate electricity, which also helps reduce the gas load, and scientists monitor the lake closely.

Volcanic gases and climate

The biggest climate effect of volcanoes comes not from CO₂ but from SO₂. A powerful eruption can blast SO₂ into the stratosphere, more than about 10–15 km up, where it turns into tiny sulfate aerosol droplets. These droplets reflect sunlight back to space and can stay aloft for a year or more, cooling the surface.

• Pinatubo, 1991: the eruption in the Philippines injected roughly 15–20 million tonnes of SO₂ into the stratosphere and lowered average global temperatures by about 0.5 °C for over a year.
• Tambora, 1815: the largest eruption in recorded history led to 1816, the "year without a summer", with crop failures and food shortages in Europe and North America.
• Hunga Tonga–Hunga Haʻapai, 2022: this underwater eruption was unusual. It released relatively little SO₂ but injected a huge amount of water vapour into the stratosphere, raising its water content by roughly 10%. Because water vapour is a greenhouse gas, scientists are still studying its net effect.

What about CO₂? Volcanoes do emit it, but in small amounts compared with people. The USGS estimates that human activities release roughly 100 times or more CO₂ each year than all volcanoes on land and under the sea combined. Volcanoes cannot explain today's global warming. Learn more about historic events in famous eruptions.

How scientists measure volcanic gas

Gas measurements are one of the most important tools for tracking volcanic activity, alongside earthquakes and ground deformation.

• COSPEC and DOAS: the correlation spectrometer (COSPEC), used from the 1970s, measured SO₂ by how it absorbs ultraviolet sunlight. Smaller, cheaper DOAS spectrometers have largely replaced it and now run in automatic networks on many volcanoes. UV cameras add images of the SO₂ plume.
• MultiGAS: portable or fixed sensor packages placed in the plume that measure CO₂, SO₂, H₂S and water vapour at the same time.
• Satellites: instruments such as OMI and TROPOMI (on Sentinel-5P) map SO₂ from space every day, detecting eruptions in remote places and tracking clouds that threaten aviation.
• Gas ratios: because CO₂ escapes from deeper magma than SO₂, a rising CO₂/SO₂ ratio can signal that new magma is moving up. At volcanoes such as Etna, such changes have been observed before eruptions.

No single measurement predicts an eruption, but combined with seismic and deformation data, gas trends help observatories decide when to raise alert levels.

Safety tips for visitors

• Iceland: during eruptions on the Reykjanes Peninsula, check the Icelandic Met Office gas forecast and stay upwind of the plume. Never enter hollows near the lava.
• Hawaiʻi: obey closures in Hawaiʻi Volcanoes National Park. People with asthma, heart problems, young children and pregnant visitors should avoid areas with strong fumes.
• Etna and other summits: go with a licensed guide, keep away from steaming vents, and turn back if you feel short of breath, dizzy or get a headache.
• Everywhere: do not linger in low spots, craters or snow caves where CO₂ can collect, and remember that a dust mask does not protect against gas.

More practical advice is in our volcano safety guide.

Frequently asked questions