Tuff

Tuff, also known as tuff, is a volcanic rock that is considered a solidified igneous rock in petrography. It consists of more than 75% pyroclasts of all grain sizes and is therefore a pyroclastic rock. Tuffs found throughout the world vary in color, ranging from gray to yellowish, brownish, reddish, and vivid red. Due to trapped volcanic gases, tuff is often very porous. In the building industry, tuffs used to be popular as masonry blocks, tuff bricks and for panels on facades. Nowadays, they are mainly found in restoration work and in stone sculpture.

The IUGS (International Union of Geological Sciences) recommends that a rock be called tuff or ash tuff if it consists of at least 75% volcanic ash with a grain size of less than 0.2 cm. The remainder of the rock may consist of finer or coarser components, and overall the percentage of pyroclasts should be greater than 75%. However, the term tuff was formerly used much more broadly to refer generally to consolidated pyroclastic deposits of varying grain size. Today, the term tuff is often used to describe rocks that consist not only of solidified volcanic ash, but generally of solidified pyroclastic deposits of various grain sizes.
Depending on the proportion of pyroclasts in the rock, the term tuffite may be used when the proportion is between 25% and 75%. 

Tuffs are formed by volcanic activity. When a volcano erupts, the rising magma is cooled during the eruption and crushed into ash. This volcanic ash is deposited both near the volcano and windblown farther away. Older rocks that have plugged the volcanic vent or are at the ash deposition site may become trapped during the eruption. Deposition of additional layers of tuff eventually consolidates the loose material. Consolidation depends, among other things, on the surcharge and any cementing substances that may be present.

Tuffs are found all over the world. In Germany, for example, tuff is found around the Kaiserstuhl, in the Odenwald, in the Thuringian Forest, in the Eifel, Rhön, Vogelsberg, Hunsrück, Vogtland and around Chemnitz.