Evaluation of morphometry-based dating of monogenetic volcanoes—a case study from Bandas del Sur, Tenerife (Canary Islands) (original) (raw)
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Slope Dependent Morphometric Analysis as a Tool Contributing to Reconstruction of Volcano Evolution
Earth and Environmental Sciences, 2011
Morphometric analysis was applied to two case study areas, two volcanic complexes of distinct geotectonic setting, age and volcanic evolution. Selected volcanic areas encompass a www.intechopen.com Earth and Environmental Sciences 220 number of features and rock types associated with volcanic activity. The first case study was carried out in the Conchagua Volcanic Complex, El Salvador (Central America), while the second one was performed in the Doupovské hory Volcanic Complex, Czech Republic (Central Europe). 2.1 Conchagua Volcano The Pacific coast of Central America is bordered by a chain of active subduction-related volcanoes. This chain is called the Central American Volcanic Arc (CAVA) and extends from Guatemala via El Salvador, southern Honduras, Nicaragua and Costa Rica to western Panama (e.g., Carr et al., 2003). The volcanic arc is associated with the subduction of the Cocos plate beneath the Caribbean plate and it is divided into several segments by traverse faults. Conchagua Volcano is located near one of these segment boundaries (Carr, 1984). Conchagua Volcano (Fig. 1), on which our research has been focused, is the easternmost volcano of the Salvadorian mainland. Conchagua volcano is located on the Conchagua Peninsula surrounded by the Pacific Ocean and the Gulf of Fonseca. The area of the Gulf of Fonseca including the Conchagua Peninsula is characterized by the presence and intersection of three important tectonic structures. The Median Trough (syn. Salvadorian Depression called the Nicaraguan Depression further to the SE) is parallel to the Middle America Trench. The Trough originated in response to extension related to the subduction roll-back of the Cocos Plate (Phipps-Morgan et al., 2008; Funk et al., 2009). The tension on oblique subduction is accommodated by dextral strike-slip movements on the El Salvador Fault Zone (the northern edge of the Salvadorian Depression-Corti et al., 2005). Extension related to eastward escape of the Chortis Block is thought to be the main reason for formation of the Comayagua Graben (Burkart & Self, 1985). The Guayape Fault running from the Gulf of Fonseca to the northeast (Finch & Ritchie, 1991) is interpreted as a Mesozoic terrane boundary, originally being part of the Guayape-Papalutla Fault Zone (Silva-Romo, 2008). Early studies assumed sinistral movement on the Guayape Fault (Burkart & Self, 1985), but sinistral displacement exceeding 50 km was documented by Finch & Ritchie (1991). The latter authors have also observed several dextral strike-slip basins providing evidence for a later dextral movement phase. Dextral movements on this fault may result from anticlockwise rotation of the Chortis Block (Gordon & Muehlberger, 1994). The eruptive history of the Conchagua Peninsula has been recently reconstructed by Rapprich et al. (2010). The oldest rocks cropping out in this area are Playitas welded rhyolitic ignimbrites of Miocene age. The next stage is represented by non-welded pyroclastic deposits of La Unión unit (mean K-Ar age: 13.3 ± 3.7 Ma). The presence of banded pumice, deposits containing both mafic scoria and felsic pumice fragments is interpreted as being a result of mingling between basaltic and dacitic magmas. Eruptions of this unit were most likely triggered by injection of basaltic magma into a dacitic magma chamber. Rocks of the subsequent Pozo unit are poorly exposed and strongly altered. Andesite lavas alternate with mafic pyroclastic flow deposits. As the non-welded pyroclastic and strongly altered effusive and pyroclastic rocks have similar surface features, the products of these two phases were combined in this study. Subsequent activity became much calmer and was predominated by effusions of basaltic andesite to andesite lavas. The lava sequences were subdivided into two formations in relation to their geochemical constraints (Rapprich et al., 2010). The earlier of the two formations,
2011
Modeling of volcanic morphometry provides reliable measurements of parameters that assist in the determination of volcanic landform degradation. Variations of the original morphology enable the understanding of patterns affecting erosion and their development, facilitating the assessment of associated hazards. A total of 24 volcanic Holocene eruptions were identified in the island of Gran Canaria (Canary Islands, Spain). 87% of these eruptions occurred in a wet environment while the rest happened in a dry environment. 45% of Holocene eruptions are located along short barrancos (S-type, less than 10 km in length), 20% along large barrancos (L-type, 10-17 km in length) and 35% along extra-large barrancos (XL-type, more than 17 km in length). The erosional history of Holocene volcanic edifices is in the first stage of degradation, with a geomorphic signature characterized by a fresh, young cone with a sharp profile and a pristine lava flow. After intensive field work, a careful palaeo-geomorphological reconstruction of the 24 Holocene eruptions of Gran Canaria was conducted in order to obtain the Digital Terrain Models (DTMs) of the pre-and posteruption terrains. From the difference between these DTMs, the degradation volume and the incision rate were obtained. The denudation of volcanic cones and lava flows is relatively independent both their geographical location and the climatic environment. However, local factors, such as pre-eruption topography and ravine type, have the greatest influence on the erosion of Holocene volcanic materials in Gran Canaria. Although age is a key factor to help understand the morphological evolution of monogenetic volcanic fields, the Gran Canaria Holocene volcanism presented in this paper demonstrates that local and regional factors may determine the lack of correlation between morphometric parameters and age. Consequently, the degree of transformation of the volcanic edifices evolves, in many cases, independently of their age.
2011
a Departamento de Física (GEOVOL), Campus de Tafira, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain b Institute of Earth Sciences Jaume Almera, CSIC, Sole i Sabaris s/n, 08028 Barcelona, Spain c Fac. Geology, Univ. Barcelona, Marti i Franques s/n, 08028 Barcelona, Spain d Estación Volcanológica de Canarias, CSIC, 38206 La Laguna (Tenerife), Canary Islands, Spain e LSCE/IPSL, Laboratoire CEA-CNRS-UVSQ, Bât. 12, Avenue de la Terrase, 91198 Gif sur Yvette, France f Laboratoire Magmas et Volcans UMR 6524 CNRS-UBP-IRD, 5 rue Kessler, 63038 Clermont-Ferrand, France
2009
By using new high-resolution (2 m) digital elevation model derived from the 2005 LiDAR survey of Mt. Etna volcano (Italy), our study measured the classical morphometrical parameters for scoria cones, i.e. W co (cone width), W cr (crater diameter), H (cone height) as well as volume, inclination of cone slope and substrate, and a number of other parameters for 135 scoria cones of Mt. Etna. Volume and age distribution of cones shows that there is no direct structural control on their emplacement in terms of Etna's rift zones. The cones are progressively smaller in size toward summit, which can be explained by the large volcano's feeding system and progressively frequent lava burial toward top. A careful analysis of H/W co ratio (determined as 0.18 for other volcanic fields worldwide) shows that this ratio strongly depends on (1) the calculation method of H and (2) lava burial of cone. For Etnean cones, applying an improved method for calculating H relative to the dipping substrate results in a significantly lowered standard H/W co ratio (0.137), which in turn questions the validity of the classical value of 0.18 in the case of large central volcanoes. The reduction of the ratio is not only due to methodology but also to the common lava burial. This can be expressed even better if H mean is used instead of H max (H mean /W co = 0.098). Using this measure, at Etna, well formed cones have higher ratios than structurally deformed (e. g. double or rifted) cones. Furthermore, although the sampled scoria cones at Etna have formed in a relatively narrow time interval (b 6500 yrs BP), there is a slight decrease in H/W co corresponding to erosional changes detected globally (H/ W co = 0.143, 0.135 and 0.115 for three age classes of Etna's scoria cones, corresponding to average slopes of 26.6, 23.9 and 23.7°). Because the morphometrical effect of position on a dipping substrate as well as lava burial exceeds the effect of erosion, we call attention to use caution in simply using the H/W co ratio of scoria cones for detecting age, especially on large active volcanoes.
Geomorphology, 2012
Volcanic cones are the most common volcanic constructs on Earth. Their shape can be quantified using two morphometric ratios: the crater/cone base ratio (W cr /W co ) and the cone height/width ratio (H co /W co ). The average values for these ratios obtained over entire cone fields have been explained by the repose angle of loose granular material (i.e. scoria) controlling cone slopes. The observed variability in these ratios between individual cones has been attributed to the effect of erosional processes or contrasting eruptive conditions on cone morphometry. Using a GIS-based approach, high spatial resolution Digital Elevation Models and airphotos, two new geomorphometry datasets for cone fields at Mauna Kea (Hawaii, USA) and Lanzarote (Canary Islands, Spain) are extracted and analyzed here. The key observation in these datasets is the great variability in morphometric ratios, even for simple-shape and well-preserved cones. Simple analog experiments are presented to analyze factors influencing the morphometric ratios. The formation of a crater is simulated within an analog cone (i.e. a sand pile) by opening a drainage conduit at the cone base. Results from experiments show that variability in the morphometric ratios can be attributed to variations in the width, height and horizontal offset of the drainage point relative to the cone symmetry axis, to the dip of the underlying slope or to the influence of a small proportion of fine cohesive material. GIS analysis and analog experiments, together with specific examples of cones documented in the field, suggest that the morphometric ratios for well-preserved volcanic cones are controlled by a combination of 1) the intrinsic cone material properties, 2) time-dependent eruption conditions, 3) the local setting, and 4) the method used to estimate the cone height. Implications for interpreting cone morphometry solely as either an age or as an eruption condition indicator are highlighted.
Systematic morphometric characterization of volcanic edifices using digital elevation models
Geomorphology, 2012
Quantitative characterization of the size and shape of volcanic edifices is an essential step towards the understanding of factors controlling volcano growth and morphology. The recent advent of digital elevation models (DEMs) with worldwide coverage offers the opportunity to systematically document the morphometry of all types of volcanoes using quantitative well-formalized methodologies. We present a methodology for the morphometric characterization of volcanic edifices. After reviewing previous studies on volcano morphometry and the various existing DEM sources, we describe an integrated procedure that uses a DEM and its derived products (slope, curvature) to extract a coherent set of morphometric parameters for a given volcanic edifice. Edifice boundaries are manually defined by searching for breaks in slope around the base. The parameters describe the overall size (basal and summit region area and widths, height, volume), planar shape (ellipticity and irregularity index of elevation contours), profile shape (height-width ratios) and slope of the edifice. Similar parameters for relatively large (depending on DEM spatial resolution) summit craters/calderas are also computed. Slope values and ellipticity and irregularity indexes are extracted for successive height intervals providing detailed information of volcano shape as a function of height. The number of secondary peaks is also estimated. The method is tested on thirteen composite volcanoes in Nicaragua using three DEM datasets (90 m SRTM, 30 m ASTER G-DEM and an 80 m topographic mapderived DEM) and the resulting parameters are evaluated in terms of boundary delineation and DEM source. Finally, the parameters obtained for the Nicaraguan volcanoes are discussed as an illustrative example of the type of data and information that can be extracted systematically for volcanoes worldwide.
Syn-eruptive morphometric variability of monogenetic scoria cones
According to Wood's model, morphometric parameters such as slope angle can provide valuable information about the age of conical volcanic edifices such as scoria cones assuming that their initial slopes range from 30°to 33°, measured manually on topographic maps, and assuming that their inner architectures are homogenous. This study examines the morphometric variability of nine young (a few thousand years old) small-volume scoria cones from Tenerife, Canary Islands, using high-resolution digital elevation models in order to assess their slope angle variability. Because of the young age and minimal development of gullies on the flanks, their morphometric variability can be interpreted as the result of syn-eruptive processes including: (1) pre-eruptive surface inclination, (2) vent migration and lava outflow with associated crater breaching and diversity of pyroclastic rocks accumulated in the flanks of these volcanic edifices. Results show that slope angles for flank sectors differ by up to 12°among the studied volcanoes, which formed over the same period of time; this range greatly exceeds the 2-3°indicated by Wood. The greater than expected original slope range suggests that use of morphometric data in terms of morphometry-based relative dating and detection of erosional processes and settings must be done with great care (or detailed knowledge about absolute ages and eruption history), especially in field-scale morphometric investigation.