The Chuska Mountains, in northwestern New Mexico and adjacent Arizona, consist of flat-lying Miocene (?) sedimentary rocks locally overlain by pyroclastic and flow rocks of the Pliocene Navajo-Hopi volcanic province. The volcanic rocks were extruded into deep valleys cut into the top and flanks of the mountains.
Five volcanic centers within the Chuska Mountains were examined in detail. (1) The depression of Washington Pass is floored by three lava flows underlain by fluvial deposits of pyroclastic origin and overlain by a rubble dome intruded by plugs and dikes of minette. Minor caldera subsidence is indicated by faults in the Chuska sandstone underlying the volcanic rocks, by folds in the lava and tuff, and by steep inward dips of the volcanic rocks in the rim of the circular depression. (2) The Palisades represent an erosional remnant of a thick valley fill of coalesced domes of lava and agglomerate from several closely spaced vents. The flow rocks overlie fluvial beds of sandy tuff eroded from the slopes and deposited in the valley bottoms. The lava and tuff rest on slopes as high as 60°. (3) East Sonsela Butte is capped by three lava flows deposited in valleys cut into the Pliocene Chuska Mountains, and West Sonsela Butte is a remnant of a low, flat dome of lava and rubble capping a crater filled with coarse explosion breccia. (4) Tsailee Butte is a plug of columnar lava intruding a cone of coarse tuff-breccia, and South Tsailee Butte is a remnant of a fluvial crater filling of interbedded sand and tuff cut by a thick cone sheet of minette. (5) Roof Butte is formed from two funnel-shaped explosion pipes, one of which is capped by a low lava dome.
The vents of the Chuska Mountains have similar eruptive and structural histories. The volcanism was explosive in its early stages, and large quantities of pyroclastic material were ejected in numerous phreatic explosions. Between eruptions the craters of the low, flat cones were quickly filled with fluvial deposits of sand and tuff stripped from bordering valley walls. Collapse and inward sliding of the soft fluvial layers in the crater walls are shown by faults, crumpled and sheared layers, and folds. Volcanic activity ended with quiet extrusion of viscous lava and intrusion of dikes and plugs. The volcanic rocks consist of alkalic sanidine trachybasalt and minette.