The expansion of gases brings the magma closer to the surface and drives eruptions. The interaction between the viscosity and temperature of the magma and the gas content determines if an eruption will be effusive or explosive. On a global scale, volcanic gases produced our atmosphere and our oceans. Without the atmosphere and oceans, life would not have evolved on Earth. Gases emitted by volcanoes continue to influence the atmosphere but not to the extent of man-made sources.
Gases also pose a hazard at many volcanoes.
At other volcanoes, the gradual release of gas acts as an irritant and may pose a long-term health hazard. The following pages provide an overview of many aspects of volcanic gases.