The ignimbrite erupted during the formation of Crater Lake caldera 6,800 years ago (Slide 8--Part I) shows magnificent compositional zonation. The lowermost (pale) component exposed here is rhyodacitic in composition (70-72% SiO2); the uppermost (dark) component is andesitic (56-62% SiO2). Tapping of a compositionally zoned magma chamber with more evolved dacitic magma at the top yielded zoned deposits, showing mirror-image relationships: the first-erupted rhyodacites are at the bottom. There are no obvious breaks in the sequence: the mechanisms of transport and emplacement of ignimbrites that can yield these relationships remain poorly understood. Note the splendid pillars, evidence of rapid erosion. (Fig. 10.11).

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