"Not all volcanoes erupt and not all eruptions are explosive. Volcanologists label volcanoes active, dormant and extinct, depending on the likelihood of an eruption occurring. Volcanoes that will never erupt again are considered extinct. Active volcanoes come in two classes -- volcanoes which are active, either erupting now or having recently erupted and dormant volcanoes, volcanoes which are currently quiet but are expected to erupt in the future. About sixty volcanoes are actively erupting each year." -- https://www.ngdc.noaa.gov/hazard/stratoguide/intro.html
For additional information on volcanic gases, see the Volcanic Lakes box on the Seamounts, Volcanic Lakes, Extra-Terrestrial Volcanoes page.