Andesitic shield volcanoes: the case of Barva, Irazú and Turrialba, Costa Rica

GROSSE, P.; ALVARADO, G. E.

Pucón

Congreso; IAVCEI General Assembly 2004; 2004

Volcano morphology is conditioned by several factors such as eruption rate, magma composition, lava/tephra abundance, distribution of vents, stage of growth, structural setting and degree of erosion of the edifice, which results in very diverse sizes and shapes. This is especially true in subduction settings, where composite cones can have very variable shapes, including shield-like. Little work has been done to describe and understand these shield-like volcanoes. Barva, Irazú and Turrialba volcanoes form part of the southernmost segment of the Central American Volcanic Front (CAVF) and are characterized by high-K basaltic andesite to andesite lava flows and minor associated tephras; high-Mg basaltic lava flows and tephras with tholeiitic affinities are subordinate. A digital elevation model (DEM) of the volcanoes was constructed digitalizing topographic maps of 1:50,000 scale. GIS software was used to obtain various morphological parameters, which indicate that Barva has clearly a shield morphology while Irazú and Turrialba seem to have intermediate characteristics. Considering the compositional and structural characteristics of these volcanoes, we suggest that the main factors that have enabled their present morphology are: high temperature-low viscosity magmas, change of vent location through time and high effusion rates. Comparison with morphological data of other Central American volcanoes suggests that there is no clear division of two types of volcanoes but there is rather a whole range of possible shapes and sizes in between typical cones and shields. This further suggests that volcanoes may evolve from simple symmetrical cones to large multi-vent cones to still larger shields.