Processes responsible for volcaniclastic sedimentation range from purely igneous on the one hand, to normal sedimentary processes on the other, with complex interaction of igneous and sedimentary processes in the middle. This is the scheme in general use for the description of grain size of volcaniclastic sediments and rocks. Note the main terms, and the difference between blocks and bombs . Use of the standard Wentworth scale for clastic sediments gives the most accurate grain-size description. The images in this table are back-scattered electron images of Quaternary tuffs.
Pumice consists of fragments of vesicular magma formed during explosions.
A greater degree of explosive fragmentation leads to fragmentation of the pumice, and the formation of glass shards, bounded by individual vesicle walls. Light coloured crystal fragments are common in this view. Lithic fragments are mostly of volcanic origin - often from older rocks eroded from the sides of the volcanic vent during eruptions. In some cases, shards and pumice are still hot and plastic when they are deposited, particularly in some pyroclastic flow and pyroclastic fall deposits. Thus, they are easily compacted as more material is deposited on top of them, and the hot, plastic fragments cohere, forming a welded tuff .