Fumarolic mounds and ridges are prominent surface features of the southern part of the Bishop Tuff and are characterized by their greater induration relative to the surrounding ash or tuff. They stand 0.5 to 15 m above the surrounding terrain and are of two main types: (1) domical mounds up to 60 m in diameter, and (2) straight or curved vertical joint ridges 1.5 to 5 m high and up to 600 m long. Fumaroles were formed in greatest numbers where crystallization within the sheet was intense and are absent from areas where the sheet was thick and densely welded but remained vitric.
Early conjugate joint sets, which followed welding but predated escape of fumarolic vapors, controlled the distribution of fumarolic mounds and ridges. Later random orthogonal joints, which postdate vapor-phase activity, resulted from release of thermal stress stored in the cooling sheet. Welding, conjugate fumarolic jointing, fumarolic activity, and random orthogonal jointing represent successive discrete stages in the cooling history of the Bishop Tuff.
The general mineralogy of the fumarolic areas and the vapor-phase zone are similar, but hydro-biotite and marialite are identified in the inner alteration zone. Likewise, the chemical composition of the fumarolic mounds is similar to that of the vapor-phase zone. The inner zone around some fumarolic vents shows significant changes through a decrease in SiO2 and an increase in A12O3, K2O, and H2O.