The core of Tehama which is sulfur has weathered out as very fine particles. This bright yellow patch is the core of Mt Tehama which was 11,000 feet high. We can see the old arc of the volcano as represented by Brokeoff Mtn. The Sulfur Works is where there is still heat . Geologically this is a young volcano so the heat is subsurface - either hot rock or magma.
Water seeps down and gets heated and the fluids and vapors come up here.
The pH is around 2 or 3. There is sulfur gas and sulfur is associated with magma. When mixed with water it forms sulfuric acid. There is a dacite boulder which whas been stewed, boiled, etc and reacted with vapors. It is clay that has been produced in a hurry.