Volcanic gas
Water vapor dominates the air released by active volcanoes, making up more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of these releases. Other compounds include sulfur dioxide, hydrogen sulfide, nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen. Exotic trace compounds like mercury and halocarbons appear in smaller amounts. The abundance of gases varies considerably from volcano to volcano with tectonic setting. Volcanoes located at convergent plate boundaries emit more water vapor and chlorine than those at hot spots or divergent plate boundaries. This difference is caused by the addition of seawater into magmas formed at subduction zones. Primordial and recycled constituents from Earth's mantle provide the initial source. Assimilated constituents from Earth's crust add another layer to the mix. Groundwater and the Earth's atmosphere also contribute to the final composition.
As magma ascends towards the surface, ambient pressure decreases. This decrease reduces the solubility of dissolved volatiles within the rock. Once solubility drops below volatile concentration, gases come out of solution to form a separate gas phase. The gas initially distributes throughout the magma as small bubbles that cannot rise quickly. As magma continues its ascent, bubbles grow through expansion and further decreases in solubility. Depending on viscosity, bubbles may coalesce or remain fixed until they connect into a network. In volcanoes with an open path to the surface like Stromboli in Italy, bubbles reach the top and pop to create small explosions. If gas cannot escape fast enough, it fragments the magma into ash particles. This fluidized ash accelerates the mixture and drives explosive volcanism. Whether gas escapes gently or causes an explosion depends on total volatile content and magma viscosity.
When magmatic gas encounters meteoric water in an aquifer, steam is produced. Latent magmatic heat can cause meteoric waters to ascend as a vapor phase. Extended fluid-rock interaction leaches constituents from cooling magmatic rock and country rock. This process increases ionic strength and decreases fluid pH. Cooling causes phase separation and mineral deposition accompanied by a shift toward more reducing conditions. At the ocean floor, hot supersaturated hydrothermal fluids form gigantic chimney structures called black smokers. These chimneys emit into cold seawater at specific points of emission. Over geological time, this process generates economically valuable ore deposits through concentration and redeposition. Low-temperature volcanic gases below 400°C emanate as steam-gas mixtures or dissolve in hot springs. Sites of advective gas loss often feature precipitation of sulfur and rare minerals forming fumaroles.
Fischer et al estimated that SO2 emissions during eruptions were 2.6 teragrams per year from 2005 to 2015. Non-eruptive periods of passive degassing released 23.2 ± 2 Tg per year during the same interval. CO2 emissions from volcanoes during eruptions were 1.8 ± 0.9 Tg per year while non-eruptive activity released 51.3 ± 5.7 Tg per year. Therefore, CO2 emissions during volcanic eruptions are less than 10% of those released during non-eruptive activity. The 15th of June 1991 eruption of Mount Pinatubo in the Philippines released a total of 18 ± 4 Tg of SO2. Such large VEI 6 eruptions occur only once every 50 to 100 years. The 2010 eruptions of Eyjafjallajökull in Iceland emitted a total of 5.1 Tg CO2. VEI 4 eruptions happen about once per year. Recent estimates by Burton et al suggest 540 Tg CO2 per year when accounting for diffuse emissions.
Volcanic gases were collected and analyzed as long ago as 1790 by Scipione Breislak in Italy. Volcanic gas sensing occurs within the gas using electrochemical sensors and flow-through infrared-spectroscopic gas cells. Ground-based or airborne remote spectroscopy includes Correlation spectroscopy and Differential Optical Absorption Spectroscopy. Sulphur dioxide absorbs strongly in ultraviolet wavelengths making it ideal for monitoring. Satellite-based instruments allow global monitoring while ground-based instruments like DOAS estimate flux near well-monitored volcanoes. The Multi-Component Gas Analyzer System remotely measures CO2, SO2 and H2S. Direct sampling often involves an evacuated flask with caustic solution first used by Robert W. Bunsen. This method was later refined by German chemist Werner F. Giggenbach into what is now called the Giggenbach-bottle. Analytical techniques include gas chromatography with thermal conductivity detection and mass spectrometry.
Certain constituents of volcanic gases show very early signs of changing conditions at depth. These changes make them a powerful tool to predict imminent unrest. Used in conjunction with seismicity and deformation data, correlative monitoring gains great efficiency. Volcanic gas monitoring remains a standard tool of any volcano observatory. An increase in CO2 content at Stromboli has been ascribed to injection of fresh volatile-rich magma at depth. The most precise compositional data still require dangerous field sampling campaigns. Remote sensing techniques have advanced tremendously through the 1990s. The Deep Earth Carbon Degassing Project employs Multi-GAS remote sensing to monitor nine volcanoes continuously. Sudden changes in gas composition often presage a change in volcanic activity before other indicators appear.
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Common questions
What percentage of volcanic gas emissions is water vapor?
Water vapor dominates the air released by active volcanoes, making up more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of these releases.
How do tectonic settings affect the composition of volcanic gases?
Volcanoes located at convergent plate boundaries emit more water vapor and chlorine than those at hot spots or divergent plate boundaries. This difference is caused by the addition of seawater into magmas formed at subduction zones.
When did Scipione Breislak first collect and analyze volcanic gases?
Volcanic gases were collected and analyzed as long ago as 1790 by Scipione Breislak in Italy. Direct sampling often involves an evacuated flask with caustic solution first used by Robert W. Bunsen.
Why does carbon dioxide from non-eruptive activity exceed eruption emissions?
CO2 emissions during volcanic eruptions are less than 10% of those released during non-eruptive activity. Non-eruptive periods of passive degassing released 23.2 ± 2 Tg per year while CO2 emissions from eruptions were 1.8 ± 0.9 Tg per year between 2005 and 2015.
What happened during the Mount Pinatubo eruption on the 15th of June 1991?
The 15th of June 1991 eruption of Mount Pinatubo in the Philippines released a total of 18 ± 4 Tg of SO2. Such large VEI 6 eruptions occur only once every 50 to 100 years.