Pablo Grosse

Morphometry and evolution of arc volcanoes

Volcanoes change shape as they grow through eruption, intrusion, erosion, and deforma- tion. To study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Topography Mission (SRTM) digital elevation models, we compute and defi ne parameters for plan (ellipticity, irregularity) and profi le (height/width, summit/basal width, slope) shape, as well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, and recognize several evolutionary trends. Many cones grow to a critical height (~1200 m) and volume (~10 km 3 ), after which most widen into sub-cones or massifs, but some grow into large cones. Large cones undergo sector collapse and/or gravitational spreading, without sig- nifi cant morphometry change. Other smaller cones evolve by vent migration to elliptical sub- cones and massifs before reaching the critical height. The evolutionary trends can be related to magma fl ux, edifi ce strength, structure, and tectonics. In particular, trends may be controlled by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance versus edifi ce resistance. Morphometric analysis allows for the long-term state of individual or volcano groups to be assessed. Morphological trends can be integrated with geological, geophysical, and geochemical data to better defi ne volcano evolution models.

NETVOLC: An algorithm for automatic delimitation of volcano edifice boundaries using DEMs

Abstract Accurately delimiting boundaries is required for characterizing landforms through measurement of their geomorphometric parameters. Volcanism produces a wide range of landforms, from symmetric cones to very irregular massifs, that can gradually merge with the surroundings and contain other elements, thus complicating landform delimitation. Most morphometric studies of volcanoes delimit landforms manually, with the inconvenience of being time-consuming and subjective. Here we propose an algorithm, NETVOLC, for automatic volcano landform delimitation based on the premise that edifices are bounded by concave breaks in slope. NETVOLC applies minimum cost flow (MCF) networks for computing the best possible edifice outline using a DEM and its first- and second-order derivatives. The main cost function considers only profile convexity and aspect; three alternative functions (useful in complex cases) also consider slope, elevation and/or radial distance. NETVOLC performance is tested by processing the Mauna Kea pyroclastic cone field. Results using the main cost function compare favorably to manually delineated outlines in 2/3rds of cases, whereas for the remaining 1/3rd of cases an alternative cost function is needed, introducing some degree of subjectivity. Our algorithm provides a flexible, objective and time-saving tool for automatically delineating volcanic edifices. Furthermore, it could be used for delineating other landforms with concave breaks in slope boundaries. Finally, straightforward modifications can be implemented to extend the algorithm capabilities for delimiting landforms bounded by convex breaks in slope, such as summit craters and calderas.

Petrology and geochemistry of the orbicular granitoid of Sierra de Velasco (NW Argentina) and implications for the origin of orbicular rocks

The Velasco orbicular granitoid is a small (65 × 15 m), irregularly-shaped body that crops out within the Huaco granite, central Sierra de Velasco, NW Argentina. It consists of ellipsoid-shaped orbicules of 3 to 15 cm length immersed in an aplitic to pegmatitic matrix. The orbicules are formed by a core made up of a K-feldspar megacryst, partially to totally replaced by plagioclase, an inner shell of radial and equant plagioclase crystals, a layer of tangentially oriented biotite laths, and an outer shell of plumose plagioclase crystals, containing diffuse rings of tangentially oriented biotite. The orbicular granitoid formed in situ in a pocket of evolved and volatile-rich melt segregated from the surrounding partially crystallized Huaco granite, possibly via a filter pressing mechanism. The segregated melt entrained relatively few K-feldspar megacrysts into the pocket, leaving behind a concentration of megacrysts around the pocket. High water concentration caused effective superheating of the melt and destruction of nuclei, with only the large megacrysts surviving as solids. Sudden water-pressure loss and exsolution of the volatile phase, perhaps related to a volcanic eruption or fracturing of the surrounding granite, caused rapid undercooling of the melt. The orbicules grew in the undercooled melt by heterogeneous nucleation on the megacrysts, which acted as nucleation seeds, and crystallization of reversely zoned radial plagioclase and sporadic crystallization of tangential biotite rings according to fluctuations in its saturation. Orbicular growth gave way to crystallization of the equiaxial inter-orbicular matrix in two stages, when sufficient polymerization of the melt was attained. The time scale of formation of the orbicular granitoid was fast, possibly a matter of a few weeks or months.

Construction and degradation of a broad volcanic massif: The Vicuña Pampa volcanic complex, southern Central Andes, NW Argentina

The Vicuna Pampa volcanic complex, at the SE edge of the arid Puna Plateau of the Central Andes, records the interplay between volcanic construction and degradational processes. The low-sloping Vicuna Pampa volcanic complex, with a 1200-m-deep, southeastward-opening depression, was previously interpreted as a collapse caldera based on morphological considerations. However, characteristic features associated with collapse calderas do not exist, and close inspection instead suggests that the Vicuna Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of mainly basaltic to trachyandesitic composition. Construction of the Vicuna Pampa volcanic complex occurred during two distinct cycles separated by the development of the depression. The first and main cycle took place at ca. 12 Ma and was dominated by lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the depression with the emplacement of domes. We interpret the central depression as the result of a possible sector collapse and subsequent intense fluvial erosion during middle to late Miocene time, facilitated by faulting, steepened topography, and wetter climate conditions compared to today. We estimate that ∼65% of the initial edifice of ∼240 km 3 was degraded. The efficiency of degradation processes for removing mass from the Vicuna Pampa volcanic complex is surprising, considering that today the region is arid, and the stream channels within the complex are predominantly transport limited, forming a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuna Pampa volcanic complex records the effects of past degradation efficiency that differs substantially from that of today.