The most densely welded ignimbrites are glassy rocks hard to distinguish from lavas. More commonly, it is only the pumice clasts that are conspicuously glassy: they are squashed into black, glassy flattened pancakes. These, seen in cross section, resemble candle flames, and are known by their Italian name of fiamme. Similar textures can be seen on a microscopic scale, with tiny glass shards flattened and molded over one another. Extreme degrees of flattening can be produced by squashing of shards at the base of thick ignimbrites, whereas higher up, where pressure is less, the flattening is proportionally less. Before ignimbrites were properly understood, eutaxitic texture was the main diagnostic criterion used to identify them. Most ignimbrites, however, look more like Slide 26 than 25. Eutaxitic texture is still a valuable criterion for identifying ancient ignimbrites. (Fig. 10.32).

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