Inside the cave is a fumarole that releases carbon dioxide of volcanic origin. It was first recognized in volcanic fumarole deposits. Discharge zones can be breccia diatremes, or simple fumarole conduits. At the summit, there are four large craters which contain active fumarole fields. A fumarole field is associated with the Salton Buttes. Associated minerals include sodium alum, native sulfur and other fumarole minerals. The most powerful fumarole observed at Craters of the Moon occurred December 1967. Carbon dioxide is the most important fumarole gas, followed by hydrogen sulfide. The fumarole deposits of the volcano are noted as sources for several rare minerals. The sulfur itself formed at temperatures of in a now extinct fumarole. The volcano has two summits, with a fumarole field between them. It occurs as fumarole deposits in the Vesuvius area. At Vesuvius it occurs as alterations from gypsum within leucite tephrite and as fumarole deposits. The fumarole on Deception Island also supports moss species found nowhere else in Antarctica. Fumarole activity was observed in 1962.