The characteristic "rotten egg" smell of many active volcanoes is derived from

Hydrogen sulfide gas

Volcanic gases are gases from volcanoes. These are gases trapped in the spaces between volcanic rocks. The sources of this gases are the recycled constituents of the earth mantle, assimilated constituent of earth crust and groundwater, earth atmosphere. The principal component of volcanic gases are water vapor, carbon dioxide, sulfur dioxide , hydrogen sulfide. Gases varies depending on the volcano, water vapor is the most common among the gases.