Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from lava, from volcanic craters or vents. Volcanic gases can also be emitted through groundwater heated by volcanic action. The sources of volcanic gases on Earth include:
primordial and recycled constituents from the Earth's mantle, assimilated constituents from the Earth's crust, groundwater and the Earth's atmosphere. Substances that may become gaseous or give off gases when heated are termed volatile substances. Volcanic gases are not only released during an eruption; they can also seep out slowly when a volcano is inactive through a continuous process known as passive degassing, which can persist for years. Even small emissions can affect the surrounding environment by changing soil and water chemistry and impacting plants and animals. By studying changes in gas composition and emission rates, scientists can monitor volcanic activity and assess potential hazards before an eruption occurs.
Composition
The principal components of volcanic gases are water vapor (H2O), carbon dioxide (CO2), sulfur either as sulfur dioxide (SO2) (high-temperature volcanic gases) or hydrogen sulfide (H2S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen. Other compounds detected in volcanic gases are oxygen (meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include mercury, halocarbons (including CFCs), and halogen oxide radicals. The abundance of gases varies considerably from volcano to volcano, with volcanic activity and with tectonic setting. Water vapour is consistently the most abundant volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions. Volcanoes located at convergent plate boundaries emit more water vapor and chlorine than volcanoes at hot spots or divergent plate boundaries. This is caused by the addition of seawater into magmas formed at subduction zones. Convergent plate boundary volcanoes also have higher H2O/H2, H2O/CO2, CO2/He and N2/He ratios than hot spot or divergent plate boundary volcanoes.
Magmatic gases and high-temperature volcanic gases Magma contains dissolved volatile components, as described above. The solubilities of these different volatiles depend on pressure, temperature and magma composition. As magma ascends, ambient pressure decreases, decreasing solubility of the dissolved volatiles. When solubility falls below the volatile concentration, gases exsolve, forming a separate gas phase, and the magma becomes supersaturated in volatiles. The gas will initially be distributed throughout the magma as small bubbles that cannot rise quickly. As the magma ascends, the bubbles grow due to decompression and further exsolution caused by decreasing volatile solubility. Depending on magma viscosity, bubbles may either rise and coalesce or remain relatively fixed in place until forming a continuous network. In the first case, the bubbles can accumulate at vertical surfaces, such as the roof of a magma chamber. In volcanoes with an open path to the surface, such as Stromboli in Italy, bubbles may reach the surface producing small explosions as they pop. In the second case, gas can flow rapidly through the permeable network towards the surface, a mechanism observed at Santiaguito, Santa Maria volcano, Guatemala and Soufrière Hills Volcano, Montserrat. If gas cannot escape fast enough, magma fragments into fine ash. The fluidized ash has much lower resistance to motion than viscous magma, so accelerates, causing further gas expansion and rapid motion of the mixture. This sequence drives explosive volcanism. Whether gas escapes gently (passive eruptions) or violently (explosive eruptions) depends on magma's total volatile content and its viscosity, which is controlled by composition. The term "closed system" degassing refers to a process in which gas and its parent magma ascend together while remaining in equilibrium. The composition of the emitted gas reflects equilibrium with magma at the pressure and temperature where the gas leaves the system. In "open system" degassing, gas separates from its parent magma and rises through the overlying magma without remaining in equilibrium. As a result, the gas released at the surface represents a mass-flow average of the magma exsolved at multiple depths and does not correspond to magma conditions at any single depth. Molten rock (magma or lava) near the atmosphere releases high-temperature volcanic gas (>400 °C). In explosive eruptions, the sudden release of gas from magma can cause rapid movements of the molten rock. When the magma encounters water, such as seawater, lake water or groundwater, it may be rapidly fragmented. Rapid gas expansion is the primary driving mechanism of most explosive volcanic eruptions. However, a significant proportion of volcanic gas is also released during quasi-continuous, quiescent phases of active volcanism.
Low-temperature volcanic gases and hydrothermal systems 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. Over geological time, this process of hydrothermal leaching, alteration, and/or redeposition of minerals in the country rock is an effective process of concentration that generates certain types of economically valuable ore deposits.
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