Volcanic gas 3060924 221166932 2008-06-23T09:38:54Z MER-C 1749459 Reverted edits by [[Special:Contributions/122.106.22.71|122.106.22.71]] ([[User talk:122.106.22.71|talk]]) to last version by Plrk {{Refimprove|date=November 2007}} '''Volcanic gases''' include a variety of substances given off by active (or, at times, by dormant) [[volcano]]es. These include gases trapped in cavities ([[vesicles]]) in [[volcanic rock]]s, dissolved or dissociated [[gas]]es in [[magma]] and [[lava]], or gases emanating directly from lava or indirectly through [[hydrothermal|ground water 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. == Magmatic gases and high-temperature volcanic gases == Gases are released from [[magma]] through volatile constituents reaching such high concentrations in the base magma that they evaporate. (Technically, this would be described as the exsolution and accumulation of the gases upon reaching excess supersaturation of these constituents in the host solution (magmatic melt), and their subsequent loss from the host by [[diffusion]] and [[phase separation]] into bubbles). Molten rock (either [[magma]] or [[lava]]) near the [[Earth's atmosphere|atmosphere]] releases high-temperature volcanic gas (>400 °C). In explosive [[volcanic eruption]]s, 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. == Low-temperature volcanic gases and hydrothermal systems== If the magmatic gas traveling upward encounters [[meteoric]] water in an [[aquifer]], steam is produced. Latent magmatic heat can also cause meteoric waters to ascent 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 (geology)|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 smoker]]s, at the point of emission into the cold [[seawater]]. == Non-explosive volcanic gas release == 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 salts forms sulfur deposits and small sulfur chimneys, called [[fumarole]]s. Very low-temperature <100 °C) fumarolic structures are also known as [[solfatara]]s. Sites of cold degassing of predominantly [[carbon dioxide]] are called [[mofette]]s. [[Hot springs]] on volcanoes often show a measurable amount of magmatic gas in dissolved form. == Composition == The principal components of volcanic gases are [[water vapor]] (H<sub>2</sub>O), [[carbon dioxide]] (CO<sub>2</sub>), [[sulfur]] either as [[sulfur dioxide]] (SO<sub>2</sub>) (high-temperature volcanic gases) or [[hydrogen sulfide]] (H<sub>2</sub>S) (low-temperature volcanic gases), [[nitrogen]], [[argon]], [[helium]], [[neon]], [[methane]], [[carbon monoxide]] and [[hydrogen]]. Other [[compound (chemistry)|compounds]] detected in volcanic gases are [[oxygen]] (meteoric), [[hydrogen chloride]], [[hydrogen fluoride]], [[hydrogen bromide]], [[nitrogen oxide]] (NO<sub>x</sub>), [[sulfur hexafluoride]], [[carbonyl sulfide]], and [[organic compounds]]. Exotic trace compounds include [[methyl mercury]], [[halocarbons]] (including [[CFC]]s), and [[halogen]] [[oxide]] [[radical (chemistry)|radicals]]. The abundance of gases varies considerably from volcano to volcano. However, water vapor is consistently the most common volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.<ref name="Encyclovolc">H. Sigurdsson et al. (2000) ''Encyclopedia of Volcanoes'', San Diego, Academic Press</ref> Volcanoes located at [[convergent plate]] boundaries emit more water vapor and [[chlorine]] than volcanoes at [[hot spot]]s or [[divergent plate]] boundaries. This is caused by the addition of seawater into magmas formed at [[subduction zone]]s. Convergent plate boundary volcanoes also have higher H<sub>2</sub>O/H<sub>2</sub>, H<sub>2</sub>O/CO<sub>2</sub>, CO<sub>2</sub>/He and N<sub>2</sub>/He ratios than [[hot spot]] or divergent plate boundary volcanoes.<ref name="Encyclovolc"/> == Sensing, collection and measurement == Volcanic gases were collected and analysed as long ago as 1790 by [[Scipione Breislak]] in Italy.<ref>N. Morello (editor) (1998), ''Volcanoes and History'', Genoa, Brigati</ref> Volcanic gases can be sensed (measured in-situ) or sampled for further analysis. Volcanic gas sensing can be: * within the gas by means of electrochemical sensors and flow-through [[infrared spectroscopy|infrared-spectroscopic]] gas cells * outside the gas by ground-based or airborne remote [[spectroscopy]] (e.g., COSPEC, FLYSPEC, DOAS, FTIR) Volcanic gas sampling is often done by a method involving an evacuated flask with [[caustic]] solution, first used by [[Robert Bunsen|Robert W. Bunsen]] (1811-1899) and later refined by the German chemist [[Werner F. Giggenbach]] (1937-1997), dubbed ''Giggenbach-bottle''. Other methods include collection in evacuated empty containers, in flow-through glass tubes, in gas wash bottles (cryogenic scrubbers), on impregnated filter packs and on solid adsorbent tubes. Analytical techniques for gas samples comprise gas [[chromatography]] with [[thermal conductivity]] detection (TCD), [[flame ionization detector|flame ionization detection]] (FID) and [[mass spectrometry]] (GC-MS) for gases, and various wet chemical techniques for dissolved species (e.g., acidimetric [[titration]] for dissolved CO<sub>2</sub>, and [[ion chromatography]] for [[sulfate]], [[chloride]], [[fluoride]]). The trace metal, trace organic and [[isotope|isotopic]] composition is usually determined by different mass spectrometric methods. == Volcanic gases and volcano monitoring == Certain constituents of volcanic gases may show very early signs of changing conditions at depth, making them a powerful tool to predict imminent unrest. Used in conjunction with monitoring data on [[volcanic tremor|seismicity]] and [[deformation]], correlative monitoring gains great efficiency. Volcanic gas monitoring is a standard tool of any [[volcano observatory]]. Unfortunately, the most precise compositional data still require dangerous field sampling campaigns. However, [[remote sensing]] techniques have advanced tremendously through the 1990s. == Hazards == Volcanic gases were directly responsible for approximately 3% of all volcano-related deaths of humans between 1900 and 1986.<ref name="Encyclovolc"/> Some volcanic gases kill by acidic [[corrosion]]; others kill by [[asphyxiation]]. The [[greenhouse gas]], carbon dioxide, is emitted from volcanoes, although volcanic emissions account for less than 1% of the annual global total.<ref>[[Royal Society]] ''Climate Change Controversies'', London, June 2007</ref> Some volcanic gases including sulfur dioxide, hydrogen chloride, hydrogen sulfide and hydrogen fluoride react with other atmospheric particles to form [[aerosol]]s. <ref name="Encyclovolc"/> == References == {{Reflist}} == External links == * [http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html USGS Volcano Hazards Program: Volcanic Gases and Their Effects] [[Category:Volcanology]] [[ar:غازات بركانية]] [[de:Vulkanisches Gas]] [[et:Vulkaaniline gaas]] [[ja:火山ガス]]