<P> The term ` closed system' degassing refers to the case where gas and its parent magma ascend together and in equilibrium with each other . The composition of the emitted gas is in equilibrium with the composition of the magma at the pressure, temperature where the gas leaves the system . In ` open system' degassing, the gas leaves its parent magma and rises up through the overlying magma without remaining in equilibrium with that magma . The gas released at the surface has a composition that is a mass - flow average of the magma exsolved at various depths and is not representative of the magma conditions at any one depth . </P> <P> Molten rock (either magma or lava) near the atmosphere releases high - temperature volcanic gas (> 400 ° C). In explosive volcanic eruptions, the sudden release of gases from magma may cause rapid movements of the molten rock . When the magma encounters water, seawater, lake water or groundwater, it can be rapidly fragmented . The rapid expansion of gases is the driving mechanism of most explosive volcanic eruptions . However, a significant portion of volcanic gas release occurs during quasi-continuous quiescent phases of active volcanism . </P> <P> As magmatic gas travelling upward encounters meteoric water in an aquifer, steam is produced . Latent magmatic heat can also cause meteoric waters to ascend as a vapour phase . Extended fluid - rock interaction of this hot mixture can leach constituents out of the cooling magmatic rock and also the country rock, causing volume changes and phase transitions, reactions and thus an increase in ionic strength of the upward percolating fluid . This process also decreases the fluid's pH . Cooling can cause phase separation and mineral deposition, accompanied by a shift toward more reducing conditions . At the surface expression of such hydrothermal systems, low - temperature volcanic gases (<400 ° C) are either emanating as steam - gas mixtures or in dissolved form in hot springs . At the ocean floor, such hot supersaturated hydrothermal fluids form gigantic chimney structures called black smokers, at the point of emission into the cold seawater . </P> <P> The gas release can occur by advection through fractures, or via diffuse degassing through large areas of permeable ground as diffuse degassing structures (DDS). At sites of advective gas loss, precipitation of sulfur and rare minerals forms sulfur deposits and small sulfur chimneys, called fumaroles . Very low - temperature (below 100 ° C) fumarolic structures are also known as solfataras . Sites of cold degassing of predominantly carbon dioxide are called mofettes . Hot springs on volcanoes often show a measurable amount of magmatic gas in dissolved form . </P>

What do we call a gas vent associated with volcanic activity