Hydrothermal Activity at Mount Lassen

As water from rain and melting snow seeps into the porous volcanic soils it becomes heated by contact with a mass of hot rocks that have been baked by this magma chamber. This water reaches temperatures of 465 degrees F. causing boiling with an increased pressure which forms a steam and gas reservoir over the hot water. This intense pressure causes the super-heated (above the boiling point) water to rise back towards the surface as steam, boiling water, super-heated fumaroles, acid-sulfate hot springs, and bubbling mudpots. These thermal pools can reach temperatures of 240 degrees F. at the surface. Bumpass Hell was named after Kendall Vanhook Bumpass who discovered it by stepping in just such a thermal pool, loosing his leg as a result of the burns he suffered.

You will catch the smell of rotten eggs in the air which is hydrogen sulfide, one of the volcanic gases released by the magma below. Sulfobus bacteria lives in these hot, murky pools and gets its energy by converting sulfur to sulfate. When this sulfate combines with hydrogen, sulfuric acid is produced which is highly acidic and corrodes the rocks surrounding this area. Cyanidium alga thrives in this highly acidic and hot environment and can be seen on the rocks in the steam or on the edges of the surrounding soil. Fumaroles are small vents that let out volcanic gases and are found throughout Lassen Park. A drowned fumarole is when steam and gases vent into standing bodies of water and are commonly seen in Bumpass Hell. Bright yellow and yellow-orange crystalline deposits of pure sulfur line the active fumarole vents.

The Sulphur Works and Little Hot Springs Valley are colorful areas filled with residue from hot water and acids that have attacked the lava flows of ancient Mt. Tehama for thousands of years. The color of these residual deposits is caused by the hydrothermal action on the minerals found in the rocks with a blue residue attributed to manganese, red to iron, and yellow to sulphur. Kaolin and Opals are common products here. The Sulphur Works area once offered hot mineral baths to visitors and was considered as an avenue for profit at one time. Today, the Sulphur Works is protected by the National Park Service and is enjoyed as one of the most interesting geothermal areas in the park.