- Volcanic gas
Volcanic gases include a variety of substances 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 magmaand lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.
The sources of volcanic gases on Earth include:
* primordial and recycled constituents from the
* assimilated constituents from the
groundwaterand 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
magmathrough 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 diffusionand phase separationinto bubbles). Molten rock (either magmaor lava) near the atmosphere releases high-temperature volcanic gas (>400 °C). In explosive volcanic eruptions, 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
meteoricwater 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, causing volume changes and phase transitions, reactions and thus an increase in ionic strengthof the upward percolating fluid. This process also decreases the fluid's pH. Cooling can cause phase separationand mineraldeposition, accompanied by a shift toward more reducing conditions. At the surface expression of such hydrothermalsystems, 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.
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
fumaroles. Very low-temperature <100 °C) fumarolic structures are also known as solfataras. Sites of cold degassing of predominantly carbon dioxideare called mofettes. Hot springson volcanoes often show a measurable amount of magmatic gas in dissolved form.
The principal components of volcanic gases are
water vapor(H2O), carbon dioxide(CO2), sulfureither as sulfur dioxide(SO2) (high-temperature volcanic gases) or hydrogen sulfide(H2S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxideand hydrogen. Other compounds detected in volcanic gases are oxygen(meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, nitrogen oxide(NOx), sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include methyl mercury, halocarbons(including CFCs), and halogen oxideradicals.
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.H. Sigurdsson et al. (2000) "Encyclopedia of Volcanoes", San Diego, Academic Press]
Volcanoes located at
convergent plateboundaries emit more water vapor and chlorinethan volcanoes at hot spots or divergent plateboundaries. 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 spotor divergent plate boundary volcanoes.
Sensing, collection and measurement
Volcanic gases were collected and analysed as long ago as 1790 by
Scipione Breislakin Italy. [N. Morello (editor) (1998), "Volcanoes and History", Genoa, Brigati]
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-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 causticsolution, first used by 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
chromatographywith thermal conductivitydetection (TCD), flame ionization detection (FID) and mass spectrometry(GC-MS) for gases, and various wet chemical techniques for dissolved species (e.g., acidimetric titrationfor dissolved CO2, and ion chromatographyfor sulfate, chloride, fluoride). The trace metal, trace organic and 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 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 sensingtechniques have advanced tremendously through the 1990s.
Volcanic gases were directly responsible for approximately 3% of all volcano-related deaths of humans between 1900 and 1986. 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. [ Royal Society"Climate Change Controversies", London, June 2007] Some volcanic gases including sulfur dioxide, hydrogen chloride, hydrogen sulfide and hydrogen fluoride react with other atmospheric particles to form aerosols.
* [http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html USGS Volcano Hazards Program: Volcanic Gases and Their Effects]
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