Topic summary
Welded tuff
Extracted from the Wikipedia article Tuff.
Transport and lithification
Particles of volcanic ash that are sufficiently hot will weld together after settling to the surface, producing a welded tuff. Welding requires temperatures in excess of 600 °C (1,100 °F). If the rock contains scattered, pea-sized fragments or fiamme in it, it is called a welded lapilli tuff. Welded tuffs (and welded lapilli tuffs) can be of fallout origin, or deposited from ash flows, as in the case of ignimbrites. During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact together, resulting in a eutaxitic fabric. Welded tuff is commonly rhyolitic in composition, but examples of all compositions are known.