The geothermal areas in Lassen Volcanic National Park include several groups of hot springs and fumaroles, as remnants of former volcanic activity, exist in Lassen Volcanic National Park. Most of these lie in or are closely adjacent to Mount Tehama's caldera. Bumpass Hell is the most spectacular of these, but others of importance are Sulphur Works, Little Hot Springs Valley, Boiling Springs Lake and Devil's Kitchen. In each thermal area, the highest temperature of water generally is close to the boiling temperature at the altitude of the particular spring or fumarole—198F at Bumpass Hell and 191F on the northwest flanks of Lassen Peak. Temperatures as high as 230F have been recorded in the park.
Spring activity varies with water supply. Abundant water results in clear springs during early summer, but as the season progresses and the water supply decreases, springs change successively to turbid, warm pools, spattering mudpots, and finally steaming fumaroles. There are no true geysers within Lassen Volcanic National Park.
Gases from hot springs are composed mostly of steam and carbon dioxide, with minor amounts of other gases. These react with the rocks around the springs to ultimately form opal if temperature and acidity are high, and kaolin if they are low. Deposits of sulfur, pyrite, quartz and other substances are also found around the springs and in their runoff channels.
Solfataric alteration within the caldera of Mount Tehama covers about five square miles, much more extensive than the present hot springs basins—indicative of its former extent, and suggestive of its waning activity. It is the altered materials in the caldera which yielded most readily to the forces of erosion. Diamond Peak is a body of unaltered rock which still remains because it is more resistant.