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Water vapor (H2O in gaseous form) is by far the most abundant gas emitted during volcanic eruptions, typically accounting for 60-90% of total volcanic gas emissions. This occurs because hot magma contains dissolved water from the original molten rock and from interactions with groundwater. Carbon dioxide (CO2) is the second most abundant (10-40%), while sulfur dioxide (SO2) and other gases are emitted in much smaller quantities. Helium is rare.