The rock is of volcanic origin and consists of volcanic glass, feldspar, quartz, rock fragments and amphiboles. Because of the variation in grain size tuffs can sometimes also be classified as medium-grained rocks.
Tuff (from the Italian "tufo") is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff should not be confused with tufa, another type of rock.
The products of a volcanic eruption are volcanic gases, lava, steam, and tephra. Magma is blown apart when it interacts violently with volcanic gases and steam. Solid material produced and thrown into the air by such volcanic eruptions is called tephra, regardless of composition or fragment size. If the resulting pieces of ejecta are small enough, the material is called volcanic ash, defined as such particles less than 2 mm in diameter, sand-sized or smaller. These particles are small, slaggy pieces of magma and rock that have been tossed into the air by outbursts of steam and other gases; magma may have been torn apart as it became vesicular by the expansion of the gases within it.