What Causes Steam From The Ground

Geothermal activity produces plumes of steam from geysers.

Like all matter, water has multiple phases. Its liquid form fills lakes, rivers and seas. As a solid, it covers a pond in winter or floats in a glass of iced tea. Water vapor is its invisible gaseous state. When water vapor condenses, white steam appears. A sudden temperature or pressure change can cause this condensation. Steam erupts from the ground when hot water vapor meets cooler air.


This Yellowstone National Park geyser erupts so predictably that it bears the name Old Faithful.

Geysers occur when ground water seeps down far enough to reach a hot bubble of magma. When the water hits this superheated fluid, it boils away and becomes gaseous vapor. As the plume of water vapor hits the cooler air, it condenses into visible steam. True geysers also require a certain shape; otherwise, the water simply steams away. These reservoirs of water in geothermally active regions erupt periodically, giving rise to famous geysers such as Old Faithful in Yellowstone National Park in Wyoming.


Volcanoes and associated volcanic activity boil the groundwater above them, turning it to steam. While relatively rare geysers require specific “plumbing” and a water reservoir, volcanoes regularly vent live steam from the ground before and during eruptions. Underground springs, lakes and rivers turn to steam as a volcano’s magma pushes its way to the surface. As the superheated steam hits the air, it condenses. Volcanic eruptions themselves may consist partially or mainly of steam.

Subterranean Fires

Underground coal fires mimic the boiling action of geothermal and volcanic activity, heating subterranean water until it boils into water vapor and condenses into steam as it hits the surface. Coal fires can burn for decades or centuries. One of the largest burns under Centralia, Pennsylvania; this coal fire has raged since May 1962. The ground near Centralia still steams and smokes as the fire in the mines beneath the town heat the water that seeps into them.

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Solar Evaporation

Under certain conditions, the invisible water vapor that solar evaporation draws up condenses as steam. Water vapor becomes visible when a significant temperature differential happens; on very cold days or very hot days, the ground may seem to steam. On cold days, the surrounding air condenses water vapor into mist readily, while on hot days, dark surfaces may steam after a rainfall because the water heats sufficiently to produce a larger amount of vapor.

Artificial Heat Sources

When grates on city streets steam, the cause is almost certainly human agency. New York City’s Consolidated Edison Company (ConEd) produces and supplies steam power for heating portions of lower Manhattan. Some steam escapes the steam mains, while other steam comes from water that comes into contact with the heated pipes. Other human activity from laundry to cooking takes place in basements; the heat that these activities produce finds its way to the surface as steam. On cold nights, the warm water from bathtubs and sinks produces steam as it moves through pipes and into the sewer system.