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(or welded tuff), fragmental volcanic rocks with a tuff body consisting of lava and ash, with comparatively large inclusions of dark glass embedded in the body. In the marginal zones the glass and tuff usually merge so that the rocks appear to be welded. Against the gray background of the rock the glass inclusions appear either as irregular flakes and sparks or as lenses arranged in a subparallel manner which by their shape resemble tongues of flame. For this reason they are frequently called fiamme from the Italian word for flame. The formation of ignim-brite is explained by deposition from glowing ash clouds that occur during Katmai-type eruptions. Ignimbrite is most often found among persilicic volcanic rocks (rhyolites, trachytes, da-cites, and more rarely andesites) and occurs in the form of horizontal beds in volcanogenic masses. Ignimbrites are characteristic for many of the world’s volcanic regions.