WELDED TUFFS OF CHIRICAHUA NATIONAL MONUMENT, ARIZONA
Chiricahua National Monument, Arizona, and much of the western part of the Chiricahua Mountains contain extensive bedded rhyolite deposits of Cenozoic age heretofore termed flows. Most of these volcanic rocks, however, are more correctly classified as welded rhyolite tuff or ignimbrite, the result of many eruptions of nuees ardentes. These deposits are characterized by vertical gradation from a coherent, nonporous base exhibiting prominent eutaxitic structure to a less coherent, porous top. By following vertical joints of tectonic origin and the eutaxitic structure developed by flattening and aligning of blocks of pumice in the deposit, weathering has developed spectacular columns and balanced rocks. Study of the petrography, structure, conditions of deposition, and origin of this unusually thick section of rock permits an evaluation of features which have been termed characteristic of welded tuffs, and facilitates identification of this important rock type.