Volcanic gas

• 1. Introduction

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  Volcanic gas has been shaping Earth's atmosphere, soils, and living things long before any human set foot near a crater. In 1790, the Italian naturalist Scipione Breislak became one of the first scientists to collect and analyze the fumes rising from an active volcano, setting in motion a field of study that now sits at the center of hazard prediction, climate science, and even ore prospecting. What exactly pours out of the Earth's openings? Where does it come from? And what does it mean that much of it never arrives during a dramatic eruption at all, but seeps invisibly through the ground for years on end? The answers reach from the deepest recycled materials of the mantle all the way to the air you are breathing right now.

• 2. What Volcanoes Exhale

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  Water vapor dominates what volcanoes exhale, consistently making up more than 60 percent of total volcanic gas emissions. Carbon dioxide follows, typically accounting for 10 to 40 percent of the mixture. The rest of the roster includes sulfur dioxide at high temperatures, hydrogen sulfide at lower temperatures, nitrogen, argon, helium, neon, methane, carbon monoxide, and hydrogen. More exotic compounds turn up in smaller quantities: hydrogen chloride, hydrogen fluoride, hydrogen bromide, sulfur hexafluoride, carbonyl sulfide, and a range of organic compounds. Trace amounts of mercury, halocarbons including CFCs, and halogen oxide radicals have also been detected.

  Not every volcano exhales in the same proportions. Volcanoes sitting at convergent plate boundaries, where one tectonic plate dives beneath another, emit markedly more water vapor and chlorine than those at hot spots or divergent plate boundaries. The reason lies in the addition of seawater into magmas formed at subduction zones. These convergent volcanoes also show higher ratios of water to hydrogen, water to carbon dioxide, carbon dioxide to helium, and nitrogen to helium, all traceable to that oceanic input. The sources feeding any particular eruption draw on primordial and recycled materials from the mantle, assimilated material from the crust, groundwater, and even the atmosphere itself.

• 3. The Physics of Bubble and Blast

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  Stromboli, the small island volcano off the coast of Italy, offers one of the most visible demonstrations of how volcanic gas behaves inside a magma column. There, bubbles that form as rising magma depressurizes can make it all the way to the surface, popping in small explosions that give Stromboli its reputation as the lighthouse of the Mediterranean. The physics behind this is straightforward: as magma ascends, ambient pressure drops, solubility of dissolved gases falls, and gas exsolves to form a separate bubble phase. Those bubbles grow and, depending on the viscosity of the surrounding magma, either rise and coalesce or stay embedded until they link into a permeable network.

  At Santiaguito, part of the Santa Maria volcano in Guatemala, and at Soufriere Hills Volcano on Montserrat, gas flows rapidly through just such a permeable network toward the surface. When neither escape route is available and gas cannot get out fast enough, the consequences are far more violent. The magma fragments into fine ash. That fluidized ash has much lower resistance to motion than viscous magma, so it accelerates, driving further gas expansion and rapid movement of the mixture. This sequence is the primary mechanism of most explosive volcanic eruptions. Whether an eruption is gentle or catastrophic depends on the magma's total volatile content and on its viscosity, which itself is controlled by chemical composition.

• 4. Hidden Seeps and Hydrothermal Plumbing

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  Carbon dioxide, colorless and odorless, collects in low-lying ground called mofettes at sites where cold degassing seeps from predominantly CO2 sources. These spots can reach hazardous concentrations for people, animals, and vegetation without any visible warning. Fumaroles, which are vents of hotter volcanic gas, deposit sulfur and rare minerals around their openings and, at temperatures below 100 degrees Celsius, are also called solfataras. Both represent the non-explosive end of volcanic gas release, moving through fractures by advection or diffusing broadly across permeable ground through structures known as diffuse degassing structures.

  Deeper in the system, magmatic gas rising through an aquifer encounters meteoric water and produces steam. The resulting hot mixture leaches constituents from cooling magmatic rock and the surrounding country rock, increasing ionic strength and decreasing pH as it percolates upward. Cooling then causes phase separation and mineral deposition. On the ocean floor, this same hydrothermal process produces black smokers: gigantic chimney structures that form where hot, supersaturated hydrothermal fluids meet cold seawater at the point of emission. Over geological time, the same leaching, alteration, and redeposition cycle concentrates certain minerals into economically valuable ore deposits.

• 5. Passive Degassing Versus Eruption

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  Fischer et al., publishing in 2019, estimated that between 2005 and 2015, SO2 emissions during volcanic eruptions averaged 2.6 teragrams per year. The figure for non-eruptive passive degassing over the same decade was 23.2 plus or minus 2 teragrams per year. For carbon dioxide, the contrast was sharper still: eruptions contributed 1.8 plus or minus 0.9 teragrams per year, while non-eruptive activity released 51.3 plus or minus 5.7 teragrams per year. Eruptive CO2 thus accounts for less than 10 percent of total annual volcanic CO2 output.

  Large eruptions can, however, still deliver enormous single-event pulses. The eruption of Mount Pinatubo in the Philippines on the 15th of June 1991, rated at Volcanic Explosivity Index 6, released approximately 18 plus or minus 4 teragrams of SO2. Events of that scale occur only once every 50 to 100 years. The 2010 eruptions of Eyjafjallajokull in Iceland, a smaller VEI 4 event of the kind that occurs roughly once per year, emitted a total of 5.1 teragrams of CO2. Meanwhile, Le Quere and colleagues estimated that human burning of fossil fuels and cement production released 9.3 gigatons of carbon per year from 2006 through 2015, equivalent to 34.1 gigatons of CO2 annually. Some more recent estimates of total volcanic CO2 are higher than Fischer's figures; Burton et al. in 2013 put the figure at 540 teragrams per year, and Werner et al. in 2019 estimated 220 to 300 teragrams per year, both incorporating diffuse soil degassing from volcanic regions.

• 6. Measuring the Invisible

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  Sulfur dioxide absorbs strongly in ultraviolet wavelengths and occurs at low background concentrations in the atmosphere, making it the preferred target gas for remote volcanic monitoring. Differential Optical Absorption Spectroscopy, known as DOAS, can detect SO2 from ground-based arrays placed near well-monitored volcanoes to estimate emission flux. Satellite-based instruments extend that reach to global monitoring. The Multi-Component Gas Analyzer System, or Multi-GAS, remotely measures CO2, SO2, and hydrogen sulfide simultaneously. Emissions of other gases are typically estimated by measuring their ratios against SO2 in the volcanic plume, using Fourier Transform Infrared Spectroscopy, electrochemical sensors at the crater rim, or direct sampling, then multiplying by the known SO2 flux.

  When direct sampling is necessary, scientists still use a method tracing back to the German chemist Werner F. Giggenbach, who lived from 1937 to 1997 and refined the evacuated flask containing a caustic solution first introduced by Robert W. Bunsen, who lived from 1811 to 1899. The resulting container is called a Giggenbach bottle. Laboratory analysis of collected gas samples draws on gas chromatography with thermal conductivity and flame ionization detection, mass spectrometry, acidimetric titration for dissolved CO2, and ion chromatography for sulfate, chloride, and fluoride. An increase in the CO2 content of gases at Stromboli, for instance, has been linked to injection of fresh volatile-rich magma at depth, demonstrating how chemical signals at the surface reflect conditions far below. The Deep Earth Carbon Degassing Project uses Multi-GAS remote sensing to monitor nine volcanoes on a continuous basis.

• 7. Hazard and Early Warning

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  Volcanic gases were directly responsible for approximately 3 percent of all volcano-related human deaths between 1900 and 1986. Some gases kill through acidic corrosion; others cause asphyxiation. Sulfur dioxide, hydrogen chloride, hydrogen sulfide, and hydrogen fluoride all react with atmospheric particles to form aerosols. The hazard is not confined to eruption events. Passive degassing can push CO2 and other gases through large areas of porous ground far from a volcanic vent, where the danger is neither visible nor immediately obvious. Changes in soil and water chemistry from even small gas emissions affect plants, animals, and ecosystems in the surrounding area.

  Because certain gas constituents show early signs of changing conditions at depth, monitoring them alongside seismic and ground-deformation data gives volcano observatories a powerful early-warning capability. Gas composition shifts often presage changes in volcanic activity. Remote sensing techniques advanced significantly through the 1990s, reducing the need for the dangerous field sampling campaigns that still produce the most precise compositional data. Passive degassing, which can persist for years even when a volcano appears quiet, means that gas monitoring must be continuous rather than reserved for periods of obvious unrest.