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. 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 minerals forms sulfur deposits and small sulfur chimneys, called fumaroles. Very low-temperature (below 100 °C) fumarolic structures are also known as solfataras. Sites of cold degassing of predominantly carbon dioxide are called mofettes. Hot springs on volcanoes often show a measurable amount of magmatic gas in dissolved form. Current emissions of volcanic gases to the atmosphere Present day global emissions of volcanic gases to the atmosphere can be classified as eruptive or non-eruptive. Although all volcanic gas species are emitted to the atmosphere, the emissions of CO2 (a greenhouse gas) and SO2 have received the most study. It has long been recognized that eruptions contribute much lower total SO2 emissions than passive degassing does. Fischer et al (2019) estimated that, from 2005 to 2015, SO2 emissions during eruptions were 2.6 teragrams (Tg or 1012g) per year and during non-eruptive periods of passive degassing were 23.2 ± 2Tg per year. During the same time interval, CO2 emissions from volcanoes during eruptions were estimated to be 1.8 ± 0.9 Tg per year and during non-eruptive activity were 51.3 ± 5.7 Tg per year. Therefore, CO2 emissions during