Volcanoes erupt, they spew ash, their scarred flanks sometimes run with both lava and landslides. But only occasionally. A less dramatic but important process is continuous gas emissions from volcanoes; in other words, as they exhale. A number of volcanoes around the world continuously exhale water vapor laced with heavy metals, carbon dioxide, hydrogen sulfide and sulfur dioxide, among many other gases. Of these, sulfur dioxide is the easiest to detect from space.