Geological Setting of La Garrotxa Volcanic Field
Situated in the northeast of the Iberian Peninsula, La Garrotxa Volcanic Field is part of the Catalan Volcanic Zone and one of the provinces of the Neogene-Quaternary alkaline volcanism associated with the European Rift System. It covers about 600 km2 and
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Geological Setting of La Garrotxa Volcanic Field Joan Martí, Xavier de Bolós and Llorenç Planagumà
Situated in the northeast of the Iberian Peninsula, La Garrotxa Volcanic Field is part of the Catalan Volcanic Zone and one of the provinces of the Neogene-Quaternary alkaline volcanism associated with the European Rift System. It covers about 600 km2 and lies between the cities of Olot and Girona (Fig. 2.1). This basaltic volcanic field contains over 50 cones (including both cinder and scoria cones), lava flows, tuff rings and maars dating from the Middle Pleistocene to the early Holocene, which rest either on upper Palaeozoic granites and schists or on sedimentary Eocene and Quaternary substrata. Available petrological and geochemical data indicate that this region consists of a suite of intracontinental leucite, basanites, nepheline basanites and alkali olivine basalts, which in most cases represent primary or near-primary magmas, their geochemical characteristics being very similar to analogous petrologic types found in other European Cenozoic volcanic zones. La Garrotxa Volcanic Field embraces two geographically distinct zones, the larger area located in the north of the county of La Garrotxa, mostly corresponding to La Garrotxa Volcanic Zone Natural Park, and a southerly area that contains fewer but larger and more complex volcanic edifices (Fig. 2.1). Although both correspond to tectonically controlled depressions, the northern zone has substrata consisting of thick layers of Tertiary and Quaternary sediments, whereas the southern zone is underlain by unconsolidated Quaternary sediments in combination with the Palaeozoic basement.
J. Martí (&) X. de Bolós Institute of Earth Sciences Jaume Almera CSIC, Barcelona, Spain e-mail: [email protected] X. de Bolós e-mail: [email protected] L. Planagumà Tosca, Environmental Services, Olot, Spain e-mail: [email protected]
Volcanic activity in La Garrotxa Volcanic Field is characterised by numerous small cinder cones built during short-lived monogenetic eruptions occurring along tectonic-related volcanic fissures. The total volume of extruded magma in each eruption was between 0.01 and 0.2 km3 (DRE). Strombolian and phreatomagmatic episodes alternated in most of these eruptions and gave rise to complex stratigraphic sequences with a broad range of pyroclastic deposits. The eruption sequences differ from one cone to another and demonstrate that the eruptions did not follow a common pattern, particularly in cases of magma/water interaction. This complex eruptive behaviour is likely to be due to the differing stratigraphic, structural and hydrogeological characteristics of the substrata below each volcano rather than to any differences in the physicochemistry of the erupting magmas, which are generally fairly homogeneous throughout La Garrotxa Volcanic Field. The existence of this volcanism is linked to the complex geodynamic evolution of the area following the Alpine orogeny that involved great stretching and breakage of the continental lithosphere, thereby
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