The primary gas emitted by volcanoes during an eruption is water vapor, which constitutes a significant portion of volcanic gases. Other gases released include carbon dioxide, sulfur dioxide, and various other trace gases. Water vapor can accumulate in the atmosphere as the magma in the Earth's crust rises to the surface, causing it to rapidly decompress and generate steam. This steam pressure, along with the other gases, contributes to explosive volcanic eruptions. Understanding these emissions is important for volcanology and environmental health assessments.