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a small vent or fissure from which hot gases issue. The gases may include H2O, HCl, HF, SO2, CO2, CO, H2S, and H2. If the gases are emitted from magma, the vent is called a primary fumarole; if they are emitted from lava flows or pyroclastics that have not yet solidified, the vent is referred to as a secondary, or rootless, fumarole.
Fumaroles may be located in the crater, on the slopes, or at the foot of a volcano. The gases are emitted under pressure, frequently resulting in a loud hissing or roaring. As the temperature decreases, the water vapor undergoes a transformation to the liquid state. Depending on thermodynamic conditions, certain gases emitted along with the water vapor are dissolved in the liquid water, as are various gases and substances that are produced as a result of reactions with the wall rock and that are encountered on the way to the earth’s surface. In this way, hydrothermal solutions form hot springs in the vicinity of active volcanoes.
The deposition of sublimates of halides, sulfates, native sulfur, and other minerals is associated with fumaroles.
V. I. VLODAVETS