Geothermal areas in Lassen Volcanic National Park

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.

These react with the rocks around the springs to ultimately form opal if temperature and acidity are high, or kaolin if they are low.

Solfataric alteration within the caldera of Mount Tehama covers about five square miles (13 km2),[1] much more extensive than the present hot springs basins.

As in many hydrothermally active areas, the rocks at Sulphur Works and Little Hot Springs Valley in Lassen Volcanic National Park have been chemically altered into bright-colored clays.

Sulphur Works is said to be the volcanic center of the ancestral Mount Tehama.Near Little Hot Springs Valley is Bumpass Hell, a hydrothermally altered geothermal area that spans 16 acres (6.5 ha) and has hot springs, fumaroles, and boiling mudpots.

As part of Mount Tehama's main vent, Bumpass Hell is the result of fissures that tap the volcanic heat, thought to be a cooling mass of andesite, perhaps three miles (5 km) below the surface.

It is named after Kendall Vanhook Bumpass, a cowboy and early settler who worked in the Lassen Peak area in the 1860s.