Volcanic fumaroles erupt hot water vapor and gasses. They are usually found at the surface in regions of intense underground thermal activity. As rain water and ground water encounter hot rocks and magma below they flash to steam and erupt through fumaroles. Sulfur mixed into the vapor can cause yellow crystals to harden around the fumarolic vent opening. Fumaroles that contain sulfur are also called solfataras. Fumaroles that spew mostly carbon dioxide, methane or nitrogen are termed Mofettes.