Aligned volcanoes can be used to divide the Cascade volcanic chain into six segments averaging 175 km long. The petrology and eruptive style of volcanoes vary with their position on the segmented structure. Large volcanoes along the volcanic front and in the interiors of segments have been called “coherent,” relatively uncomplicated composite volcanoes that erupt uniform porphyritic andesite with only a small range of composition. Volcanoes near segment boundaries are “divergent” volcanoes that erupt porphyritic basalts, andesites, dacites, and rhyolites. Volcanoes near the segment boundaries but behind the volcanic front are distinguished by a predominance of basaltic and rhyolitic volcanism and phenocryst-poor lavas, as compared with those near the segment boundary but on the volcanic front.

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