The rock resulting from the accumulation of pyroclastic fragments is also known as tuff. It is a volcanic analogue of sandstone. It is consolidated and lithified volcanic ash.
Although of volcanic origin, it has many of the characteristics of sedimentary rocks because the fragments composing tuff may settle out from suspension in the air and commonly are stratified like sedimentary rocks. Pyroclastic-fall tuff is composed of volcanic ash that fell more or less vertically out of the atmosphere. These layers mantle the hills and valleys. In contrast, an ash-flow tuff, or ignimbrite, forms from particles that move laterally across the surface in a gas-charged flow in which movement resembles that of a lava flow but is much more rapid. In some ash-flow tuffs, the ash may be fused or welded together in a tight, coherent mass, and the glass fragments may be flattened and bent out of shape. This unique texture indicates that at the time of deposition, the ash fragments were hot enough to deform and fuse from the weight of the overlying ash.
It is a product of explosive volcanic eruptions. Sometimes the term is used even for consolidated material that has undergone limited posteruption reworking1. This is not only that the general public is using the term too loosely, but geologists too use terms like “lapilliTuff” and “tuffBreccia” that contain lots of larger pyroclastic material.
H.E. Andrews;J.R. Besancon;sandatlas