Welded tuff is a type of pyroclastic rock. A pyroclastic deposit consists of fragmented material that is formed from explosive volcanic activity or from aerial expulsion from a volcanic event.
These pyroclasts may be deposited in the form of pyroclastic flows [a dense, ground-hugging cloud of ashy material], forming ignimbrites (Greek for “fire cloud material”). Tuff is then the rock name for a sample taken from such a deposit, and if the deposits are emplaced at a high temperature, the lower portions will become welded by the internal head, forming welded tuff.
Source: Winter, Introduction to Igneous and Metamorphic Petrology
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Emily Zawacki, Ph.D. 