Lithified ash (or ash mixed with pyroclastic fragments) forms a volcaniclastic rock called a Tuff.
- A form of rock called tuff is created when volcanic ash is blasted from a vent during an eruption.
- The ash is transformed into a rock after ejection and deposition. Tuff is defined as rock with an ash content of more than 75%, whereas tuffaceous refers to rock with an ash content of 25% to 75%.
- The thickness of tuff often decreases with distance from the volcano and is usually greatest close to the volcanic vent. The typical shape of a tuff deposit is that of a "lens," not a "layer."
- Tuff may also be thickest on the vent's side that faces away from the wind or on the side facing the direction of the blast.
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