Not all ignimbrites possess easily recognizable pumice and lithic clasts, nor do they show compositional zonation. The 2.2 million year old Cerro Galan dacitic ignimbrite is an excellent example of a very large ignimbrite (at least 1,000 cubic kilometers) which is remarkably homogeneous throughout, and is also pumice and lithic poor. This slide shows a typical close-up view, in which the abundance of crystals is evident. Black biotite is conspicuous, but quartz and plagioclase are also evident. Fragmentation of fragile dacite pumice and crystal concentration probably took place during emplacement. Landsat image of Cerro Galan volcano is shown in Slide 9--Part I. (Chapter 10 and Section 14.2.2; Fig. 14.17).

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