Petrologic and structural contrast of the quaternary volcanoes of Guatemala

Abstract

An imposing chain of volcanoes that forms a narrow belt parallel to the Pacific coast of Guatemala displays a variety of petrologic and eruptive features that appear to be related to differences in their structural environment. In western Guatemala most of the volcanoes are large composite cones of pyroxene andesite that bear only a few parasitic cones of basaltic cinders on their flanks. However, many of the volcanoes, during their later stages of growth, discharged immense volumes of dacite pumice from their summit vents, and some of them erupted domes of viscous andesite and more siliceous lavas far down their slopes. The huge cauldron of Lake Atitlan and the Krakatoan-type caldera of Lake Ayarza were formed by subsidence related to voluminous eruptions of lava and pumice.

In eastern Guatemala, however, most of the volcanoes consist mainly or wholly of basalt; minor basaltic cones are unsually abundant, both as parasites and as independent, short-lived forms alined along faults. The volcanoes, instead of being restricted to a narrow belt, are widely scattered along fissure systems, many of which trend north south. Although dacite pumice is relatively scarce, some of the largest flows of rhyolitic obsidian on the continent are found here where they are closely associated in time and place with olivine-rich basalts. This intimate basalt-rhyolite association, the only one of its kind known in Central America, appears to represent a late stage of extreme fractional crystallization of a large body of basaltic magma.