THE NATURE OF GEOTHERMAL GASES Geothermal gases occur in areas of anomalously hot rock and in areas of normal geothermal gradients where groundwater circulates through excessively deep fractures. These areas are usually associated with young volcanism or tectonism. Geothermal gases originate mainly from magma degassing, and from thermal metamorphism of rocks conveying hot waters. Gases consist predominantly of carbon dioxide but may include hydrogen sulfide and methane. Dynamic geother­ mal systems feature groundwater convection cells that convey gases to shallow depths. This chapter describes the basic elements of geothermal systems capable of placing hazardous concentrations of gas in the near-surface underground. The geology of geothermal areas is described in more detail in Chapter 8.