Why Is Steam Coming Out of the Ground? Something Down There Is Still Hot

The vent releases gas rather than lava, and the gas has two possible origins that frequently mix. Some is groundwater heated by rock at depth until it boils and finds its way to the surface, which is mostly steam. Some comes from magma itself, which holds dissolved gases under pressure that separate out as the magma rises and the pressure falls, and that component carries carbon dioxide, sulphur dioxide, hydrogen sulphide, hydrogen chloride and hydrogen fluoride among others. Temperatures at the opening run from close to boiling to several hundred degrees, and the hottest are associated with magma nearest the surface.

Sampling the gas is one of the main tools of volcano monitoring:

• •Rising sulphur dioxide indicates magma approaching the surface
• •A shifting ratio of carbon dioxide to sulphur suggests new magma arriving
• •Rising temperature indicates a hotter source or a shorter path
• •Changes in chlorine and fluorine track how much water is involved
• •A sudden drop can mean the vent is sealing, which raises pressure
• •Measurements are taken by instrument, by drone and from satellites

The yellow crusts and crystals around these openings are a direct product of the chemistry. Sulphur compounds emerging as gas react and cool near the surface, depositing elemental sulphur as bright yellow crystals, and where the flow is strong the deposits build into chimneys and mounds. Those deposits have been mined, most famously at Kawah Ijen in Java, where workers still break sulphur from pipes installed at the vents and carry loads of eighty kilograms or more out of the crater on foot, breathing gas that damages lungs and teeth, which is among the more brutal industrial occupations remaining anywhere.

Several surface features arise from the same heat at depth and distinguishing them is useful. A hot spring is water emerging warm, where the supply is enough to keep a pool rather than escaping as steam. A geyser is a spring whose plumbing traps water until it flashes to steam and erupts periodically, which requires a specific underground geometry and is consequently rare. A mud pot forms where gas emerges through ground with little water, and the acid produced breaks the rock down into clay that bubbles. A vent emitting mostly carbon dioxide rather than steam is named separately again and is the most dangerous of all to approach.

The hazard is not obvious and kills people regularly, because the gases are heavier than air and invisible. Carbon dioxide is odourless and pools in hollows, in tents and in valley bottoms, displacing oxygen without any warning, and livestock and people have been found dead in such hollows with no sign of what happened. Hydrogen sulphide announces itself by smell at low concentrations and then deadens the sense of smell entirely at higher ones, so the warning disappears exactly when it matters. Ground near a vent can be thin crust over boiling mud. Visitors are killed at accessible sites every few years.

Gas venting without an eruption comes partly from heated groundwater and partly from magma releasing dissolved gases as it rises, and monitoring what emerges is a principal way of tracking what magma is doing. Sulphur deposited around the vents is mined by hand at some sites. Carbon dioxide is odourless and pools in hollows, and hydrogen sulphide destroys the sense of smell at exactly the concentrations that matter.