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The Quaternary Campo de la Piedra Pomez ignimbrite (CPPI) is a superbly exposed, partially indurated, rhyolitic ignimbrite emplaced on the southern Puna of Argentina. It is characterized by a variety of facies that record in unprecedented detail the flow dynamics of the parent pyroclastic density currents (PDCs). Detailed facies analysis and internal architecture defined using a sequential stratigraphy approach reveal that CPPI was formed by sustained PDCs, generated from a low fountain eruptive style (boiling over). The PDCs had overall flow conditions characterized by high particle concentration and limited capability to surmount topographic obstacles. The mobility of the PDCs was largely controlled by high pore pressure and the continuous supply at the source (sustained forced convection-dominated PDCs). The successive forestepping-backstepping stacking patterns identified in the CPPI reflect a marked unsteadiness of its parent PDCs due to a pulsating discharge rate at the source. The lateral facies variations in the CPPI record the non-uniform character of its parent PDCs as they flowed outward from the base of the collapsing fountain. Proximal-medial areas characterized by steady aggradation rates pass through to distal braided thalwegs with highly variable aggradation rates, to frontal edges characterized by the step aggradations of secondary decoupled pumice-rich lobes. Collectively, the facies variations in the CPPI record the complex dynamics of sustained PDCs that include pulsating discharge rate at the source, progressive modification of the original topography, and lateral flow transformations. [ABSTRACT FROM AUTHOR]

Pyroclastic density currents (PDCs) are one of the most dangerous volcanic phenomena. The correct interpretation and mapping of PDC deposits in the volcano record is important to establish the eruptive style and play a fundamental role in hazard assessment and risk management. The Copahue volcano is an active intermediate volcano of the Southern Volcanic Zone of the Andes (Argentina-Chile) that presents fragmentary evidence of explosive activity during its evolution, with unusual minor PDC deposits. The recorded historic eruptions were mainly phreatomagmatic due to the presence of a crater lake. In this study, four key deposits previously interpreted as PDC products corresponding to different stages of the Copahue volcano evolution (Pleistocene, Holocene, and Historic times) are analyzed and this origin is discussed: (i) A Pleistocene reddish succession located in the northeast flank of the volcano formed by stretched bombs in a coherent lava is interpreted as clastogenic lavas; (ii) a series of proximal bedded volcaniclastics deposits of Pleistocene age are interpreted as redeposition of hyaloclastic fragments from syn-eruptive subglacial meltwater flows associated with subglacial eruptions; (iii) a distal Holocene deposit located ~12 km east of the active crater consist mainly in fine-sized clasts forming aggregates is reinterpreted as a sedimentary (lacustrine) deposit with volcaniclastic input; and, (iv) a historic whitish-grey clastic deposit located on the eastern flank is considered a product of a mixed avalanche generated during the 1992–1995 activity. Consequently, the PDC occurrence during the Copahue volcano evolution is less than previously thought. Large PDCs are unlikely in the future and their influence area would be reduced near the active crater as observed in recent eruptions. Flows triggered by the melting of snow/ice during volcanic activity and sudden drainage of the crater lake appear to be a more likely potential hazard that should be considered during risk assessment. • Reinterpretation of four deposits previously interpreted as PDCs-products. • Clastogenic lavas, subglacial flows, lacustrine deposits, and mixed avalanches. • Overestimation of the PDCs occurrence in the Copahue volcano. • Melting of snow and sudden drainage of the crater lake are potential hazard. [ABSTRACT FROM AUTHOR]

The morphological evolution of a volcano records the main events of its formation, and its complexity is a function of overlapping constructive and destructive phases. Typical morphological evolutionary trends have been established: simple cones grow into larger cones or they widen into sub-cones or massifs. However, there are many exceptions to these general sequences. One exception is represented by Chimpa Volcano, an andesitic stratovolcano located in the Central Puna of the Andes. In order to evaluate the morphological evolution of Chimpa Volcano, its morphology was characterized, analyzed and quantified using a Digital Elevation Model (DEM). Chimpa Volcano is morphologically classified as a sub-cone, and three constructive stages of evolution and one destructive event have been recognized. The current morphology is related to the formation of an elongated depression flanked by scarps, named Quebrada Casana generated by a destructive event. Morphology and stratigraphy were linked using a parameter named emplacement balance (E *) which is defined as the volumetric ratio between the volcanic products emplaced far away from the vent and those which remain near vent. Chimpa Volcano's morphological evolution is directly related to the type of volcanic products emitted. The evolution began with the emission of pyroclastic density currents (PDCs), where E * remains high, favoring the development of a plateau (Stage I). Then lava dome emplacement in a central position and its destruction reduced the value of E * and a massif was created (Stage II). Finally E * reached the lowest value when the volcano began to emit lava flows forming a sub-cone (Stage III). The presented methodology to evaluate volcanic products in relation to their morphology can be applied to other volcanic edifices in the central Andes and elsewhere. • Morphology was characterized, quantified and analyzed through DEM study. • Chimpa volcano does not follow the typical morphological evolutionary trends. • Three constructive stages of evolution and a destructive stage were established. • Emplacement Balance parameter shows the link between morphology and stratigraphy. • Morphological evolution shows a close link with the stratigraphy. [ABSTRACT FROM AUTHOR]