Downloaded from geology.gsapubs.org on March 6, 2011 Morphometry and evolution of arc volcanoes Pablo Grosse1, Benjamin van Wyk de Vries2, Iván A. Petrinovic3, Pablo A. Euillades4, and Guillermo E. Alvarado5 1CONICET (Consejo Nacional de Investigaciones Científi cas y Técnicas) and Fundación Miguel Lillo, Miguel Lillo 205, (4000) San Miguel de Tucumán, Argentina 2Laboratoire Magmas et Volcans, Centre National de la Recherche Scientifi que-Unité Mixte de Recherche (CNRS-UMR6524), Université Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France 3CONICET–IBIGEO (Consejo Nacional de Investigaciones Científi cas y Técnicas–Instituto de Bio y Geociencias), Universidad Nacional de Salta, Mendoza 2, (4400) Salta, Argentina 4Instituto CEDIAC (Capacitación Especial y Desarrollo de la Ingeniería Asistida por Computadora), Universidad Nacional de Cuyo, Ciudad Universitaria, (5500) Mendoza, Argentina 5Área de Amenazas y Auscultación Sísmica y Volcánica, Instituto Costarricense de Electricidad, Apartado 10032-1000, Costa Rica ABSTRACT lava/tephra ratio, and deformation, and ulti- Volcanoes change shape as they grow through eruption, intrusion, erosion, and deforma- mately on underlying factors such as magma tion. To study volcano shape evolution we apply a comprehensive morphometric analysis to fl ux and tectonic setting. two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Since Cotton (1944) there have been rela- Topography Mission (SRTM) digital elevation models, we compute and defi ne parameters for tively few studies of volcano morphology, plan (ellipticity, irregularity) and profi le (height/width, summit/basal width, slope) shape, as although Francis (1993) and Thouret (1999) well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, gave broad overviews. The morphometry of and recognize several evolutionary trends. Many cones grow to a critical height (~1200 m) some specifi c volcano types has been studied in and volume (~10 km3), after which most widen into sub-cones or massifs, but some grow into detail, such as cinder cones (e.g., Wood, 1980; large cones. Large cones undergo sector collapse and/or gravitational spreading, without sig- Riedel et al., 2003), oceanic shields (e.g., Cul- nifi cant morphometry change. Other smaller cones evolve by vent migration to elliptical sub- len et al., 1987; Michon and Saint-Ange, 2008), cones and massifs before reaching the critical height. The evolutionary trends can be related to seamounts (e.g., Smith, 1996), and extraterres- magma fl ux, edifi ce strength, structure, and tectonics. In particular, trends may be controlled trial volcanoes (e.g., Plescia, 2004). Systematic by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance morphometric studies of polygenetic arc volca- versus edifi ce resistance. Morphometric analysis allows for the long-term state of individual noes are scarce at both individual and regional or volcano groups to be assessed. Morphological trends can be integrated with geological, scale (e.g., Wood, 1978; Lacey et al., 1981; geophysical, and geochemical data to better defi ne volcano evolution models. Carr, 1984; van Wyk de Vries et al., 2007), lead- ing to varying morphological classifi cations that lack consensus, with different and overlapping INTRODUCTION evolves depending on the prevailing processes. terms such as simple, composite, compound, Volcano edifi ce shape and size result from Thus, volcano morphology potentially contains complex, cluster, multiple, twin, shield-like, and the interplay between constructive and destruc- information on the balance of such factors as collapse scarred (e.g., compare classifi cations tive (erosional and deformational) processes age, growth stage, composition, eruption rate, given in Macdonald, 1972; Pike and Clow, (Fig. 1A). During a volcano’s life, its shape vent position and migration, degree of erosion, 1981; Francis, 1993; Simkin and Siebert, 1994; Davison and De Silva, 2000). Clearly, detailed morphometric studies are needed for a more Figure 1. A: Three-dimen- A rigorous quantitative classifi cation and a better sional (3-D) images de- rived from Shuttle Radar understanding of volcano shape evolution. Hone Topography Mission digital et al. (2007) went in this direction by means of elevation models show- cladistic analysis. 1 2 ing different shapes of arc We present a morphometric analysis of poly- volcanoes. 1—Concep- genetic volcano edifi ces from two continental ción (11.538°N, 85.623°W), simple symmetrical cone; subduction arcs, the Central American Volcanic 2—Ollagüe (21.308°S, Front) and the southern Central Andes Volcanic 68.180°W), more complex Zone. We quantify, characterize, and classify cone; 3— Aucanquilcha volcanic edifi ce morphology, and then detect (21.225°S, 68.469°W), com- 3 4 posite volcano with a sub- shape evolution trends that we relate to evolu- conical shape; 4—Rincón tionary processes. Here we specifi cally look for de La Vieja (10.809°N, and interpret general trends; complementary 85.319°W), com plex mas- detailed analyses of individual volcanoes should sif. B: 3-D image of Ara- Summit area Elevation B contours Shape descriptors 6.0 Summit area be a subsequent step. car (24.297°S, 67.783°W) Height - Ellipticity index (ei) showing acquired mor- - Irregularity index (ii) )mk ( 5.5 phometric parameters and Slopes ei (x10) MORPHOMETRIC PARAMETERS Volume noi tave corresponding diagram of 5.0 We have used 90 m spatial resolution digi- ii (x10) elevation versus slope, el- Slope tal elevation models (DEM) from the Shuttle lipticity index, and irregu- lE 4.5 Radar Topography Mission (SRTM). This is larity index. See the Data Edifice outline Lowest closed Best-fit contour the best high-resolution global DEM data set Repository (see footnote 1) Area, width surface 4.0 for all locations and data. 10 15 20 25 30 (e.g., Rabus et al., 2003), and it is adequate for © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2009; 2009 v. 37; no. 7; p. 651–654; doi: 10.1130/G25734A.1; 5 fi gures; Data Repository item 2009151. 651 Downloaded from geology.gsapubs.org on March 6, 2011 morphometric studies of stratovolcanoes (e.g., noes have a wide variety of shapes and sizes. 0.25), and circular (low ei) and regular (low ii) Wright et al., 2006; Kervyn et al., 2008). We They are contrasting examples of continental plan shapes (Fig. 3). Average fl ank slopes are have analyzed 59 Central American Volcanic margin arcs: the Central American Volcanic 21º–34° and maximum interval slopes are 27º– Front and 56 southern Central Andes Volcanic Front is developed on thin to thick crust, contains 37° (Figs. 3 and 4). There is a ~300 m height Zone edifi ces (see the GSA Data Repository1 for many young and historically active volcanoes, interval at 1140–1430 m (corresponding to vol- table, map, and additional material). Selected and has a humid, erosive climate; the southern umes of 9–13 km3) where there is a clear lack volcanoes have shown Holocene activity Central Andes Volcanic Zone is on thick crust, of cones (only one volcano, Azufre, is present) (Smithsonian Institution database, Siebert and most volcanoes are dormant or extinct, and it (Fig. 2). The cones above this “cone gap” have Simkin, 2002) or are morphologically fresh. has a very arid, low-erosion climate. slightly lower H/WB, generally higher WS/WB, The seamless SRTM DEMs from the CGIAR- Figures 2–4 graphically display the morpho- and are more irregular and elliptical (Figs. 3 and CSI (Consultative Group on International Agri- metric features (also see Table DR1 in the Data 4). Within this large cone subgroup is a set of cultural Research–Consortium for Spatial Infor- Repository). Edifi ces of both arcs are grouped paired or twin cones (Atitlán-Tolimán, Fuego- mation) were used (Jarvis et al., 2008). into four main shape classes: cone, sub-cone, Acatenango, and San Pedro–San Pablo), which A basic morphometric uncertainty is the massif, and shield. This classifi cation is not are characterized by higher WS/WB ratios and ei selection of volcano extent, as the aprons can absolute, as there is gradation and overlap in the values (Fig. 3). merge with the surrounding landscape. We data; it is based on a fi rst-order grouping using thus restrict our analysis strictly to the edi- the H/WB ratio, then refi ned using the WS/WB Sub-Cones fi ces, as they are generally clear landforms. ratio, the ei and ii, and the average fl ank slopes. Sub-cones have intermediate H/WB of 0.10– Consequently, size data are an estimation of Field knowledge and qualitative evaluation of 0.16; their WS/WB, plan shapes and slope val- edifi ce size only and not total erupted volume. DEMs, satellite images, and geological maps ues are very variable, but are also intermediate The outline of each edifi ce was user-estimated were used to sort out quantitatively uncertain (Figs. 3 and 4). The larger sub-cones (volumes (details in the Data Repository). cases. The morphometric differences between > 13 km3) tend to be more irregular than the Morphometric parameters were acquired cones and massifs are clearly evident, while smaller ones (Fig. 4). With the exception of using an expressly written IDL (interactive sub-cones are transitional. Within each type, unusually large Pular-Pajonales, the sub-cones data language) code (MORVOLC; see the differences between Central American Volca- have heights of 400–1400 m and volumes Data Repository for detailed descriptions of nic Front and southern Central Andes Volcanic between 1 and 46 km3. The lack of larger sub- the parameters used). Basal area and width are Zone edifi ces can be found, but are small com- cones with sizes equivalent to the larger cones obtained from the outline. The outline is also pared to differences between types.