Magma is a “reducing” environment, meaning it lacks free oxygen. The gases released are those that were trapped in the Earth’s interior since its formation or recycled through subduction. While nitrogen, hydrogen and carbon-based gases are common, oxygen only exists in the volcano as part of chemical compounds like CO2 or SO2. This is an important distinction in planetary science, as it shows that the early Earth’s atmosphere (formed by volcanic outgassing) was “anoxic” (lacked oxygen) until the evolution of cyanobacteria.