Depositional facies and migration of the eruptive loci for Atexcac axalapazco (central Mexico): implications for the morphology of the crater

Abstract

Phreatomagmatic explosions produced an alternated stratified sequence of surge and fallout deposits that formed the Atexcac axalapazco in the eastern Mexican Volcanic Belt. We defined six different depositional facies based on variations in grain size, composition and depositional structures. The depositional facies defined in this study indicate that the Atexcac axalapazco was formed through phreatomagmatic, phreatic and magmatic explosions. The fluctuations on the availability of groundwater of a local aquifer allowed intermittent interaction with periodic injections of ascending magma bodies and resulted in explosions with different grades of efficiency in fragmentation. In some cases availability of groundwater was almost null, producing only magmatic explosions. The deposition of material of those defined facies was accompanied by ballistic blocks of different compositions (basaltic, limestone, andesitic, microdioritic, juvenile and altered rocks). Velocities of those blocks were modeled, resulting at infrasonic speed (~ 0.3 Mach). The compositions and directions of sampled ballistic blocks allowed us define three different explosive stages with various location zones inside the crater. The evolution   of these likely explosive zones suggests a migration and alternation, and sometimes simultaneity of the explosive locus that originated the elongated and irregular morphology of actual crater.