... nonwelded ignimbrites. First, development of a fault gouge, with a decrease in grain size caused by grain comminution, decreases the permeability of this element with respect to the host rock. Second, long open fractures paralleling the fault have an increased permeability with respect to the host rock...

.... Spatially variable postdepositional crystallization exerts a secondary control on mode of failure, inferred from the observation that nonwelded, crystallized ignimbrites exhibit both shear fractures and deformation bands. Widespread alteration and cementation of deformation bands provide evidence...

... km to the south and southwest it is 120 m thick and includes probable nonwelded ignimbrite. The Upper Chalmers sediment is overlain by a dacitic tuff which has a 5-m-thick, blocky to columnar-jointed interval near the base termed the Chalmers welded tuff. The tuff contains strongly spherulitic pumice...

... ). The first collapse phase, probably trap-door style, erupted the ≥900-m-thick ignimbrite of Hannegan Peak at 3.722 ± 0.020 Ma. This single cooling unit, generally welded, has an uppermost facies of nonwelded ignimbrite and fine ash. A short period of localized sedimentation followed. Eruption...

..., debris-flow deposits, and reworked pyroclastic deposits; and (4) the distal facies assemblage, including nonwelded ignimbrites and fluvial deposits. These facies assemblages are characteristic of stratovolcano complexes; in the Mount Wrightson Formation, however, the paucity of debris-flow deposits...

..., and crystal shattering) may lead to high porosity and permeability, the best example of which is a nonwelded ignimbrite with well-developed gas-pipe zones. Secondary processes (different types of alteration), however, tend to decrease primary porosity. However, certain secondary processes, such as dissolution...