Geology of Crater Elegante, Sonora, Mexico

Crater Elegante is a circular, flat-floored, volcanic depression 1.6 km in diameter located in the Pinacate volcanic field, northwestern Sonora, Mexico. Exposed in its steep walls is an eruptive sequence of late Pleistocene age that is made up of numerous flows and pyroclastic units of porphyritic alkali basalt and hawaiite. Rapid alternation of effusive and pyroclastic activity is evident. The forms of intravolcanic sills and dikes are displayed in vertical cross section in the crater walls, as are structural relationships between flows and pyroclastic layers deposited on them while the flows were still mobile.

Eruptive activity at Crater Elegante culminated catastrophically with ejection and base-surge dispersal of large quantities of pyroclastic materials. These form a blanket of tuff breccia and contain vesicular juvenile ash as their dominant constituent at the crater rim. Fine-grained accidental ejecta in the tuff breccia deposits increase in relative abundance with distance from the rim. The volume of accessory debris in the ejecta blanket is much smaller than the volume of the crater. Although the diameter of the original vent is not known, it must have been much smaller than the diameter of Crater Elegante. The crater formed chiefly by wholesale collapse of the volcanic edifice following eruption of ultravesiculating magma and evisceration of a large chamber located within fine-grained, water-bearing sediments beneath the volcanic pile. Thus, Crater Elegante in some respects resembles a maar but is in fact a small collapse caldera.