CERRO PIZARRO VOLCANO, PUEBLA, MEXICO: EVOLUTION OF A COMPLEX ISOLATED DOME

Cerro Pizarro (CP) is a ~220ka, isolated dome in the eastern Trans-Mexican Volcanic Belt. A five-part evolution of the complex comprises: 1) Initial dome-growth. A rising body of magma intruded into an older terrain of Cretaceous limestone overlain by Tertiary and Quaternary volcanic rocks. Early deposits contain abundant material derived from older, buried units and are interpreted as vent-clearing deposits. 2) Explosive phase. As the early dome grew, explosive eruptions deposited thin fall and surge units; dome growth was also accompanied by minor Merapi-type collapse events. A dense lava interior was surrounded by a vitrophyric layer. 3) Cryptodome stage. Continued expansion of the dome caused deformation of the earlier overlying units to orientations as steep as vertical or slightly overturned. This phase of dome growth was entirely endogenous and demonstrates how cryptodomal dome growth can affect dome facies as well as flat-lying country rock. 4) Sector collapse. The fourth phase of dome evolution involved Mount St. Helens-like sector collapse, a phenomenon rarely seen in domes. Rocks representative of all previous phases of dome evolution are preserved in breccias of the debris-avalanche deposit. 5) Final dome growth and eruption. The present shape of the volcano is conical, mimicking the cryptodome. Minor pyroclastic material from the final phase drapes the slopes of the volcano and coalesced into flows that lapped up against surrounding topography.

Chemistry of material representing various stages of this succession is generally homogeneous, and together with soft-sediment deformation seen in pre-CP units that were affected by cryptodomal deformation, suggests that the timing of the first four stages of volcano evolution was relatively short. Deposits from stage 5, however, overlie the Zaragoza Ignimbrite derived from Los Humeros caldera to the north, suggesting that the last eruptive episode occurred somewhat later.

Dome growth and destruction is generally considered in the context of larger systems in which domes often occur. The history of CP indicates that processes like sector collapse common to larger stratovolcanoes occur in smaller, more isolated systems as well, and that even in the absence of a larger magmatic system, dome growth may be a highly complex process.