Tuff rock forms from of volcanic activity as the result violent eruptions. Volcanic ash, frothy magma, dust and fine rock particles from the eruption settle and build up on the land surface. The ejected particles form a rock with a soft, porous texture. The particles may eject from the main volcanic vent or may escape through cracks in the walls of the volcano, called fissures. These fissures are often smaller than the central vent but extend from the magma chambers toward the land surface.
The magma and rock particles that settle from the volcanic eruptions can form tuff rocks in several ways. Some tuff rocks form when the particles become buried under layers of other rock and harden into stone through compaction. Alternately, the particles can form a rock through cementation, typically when calcite or quartz in solutions precipitates and glues the particles together into a rock. Sometimes, the temperature of the rock particles is very high as they erupt from the volcano, and they fuse together, forming a welded tuff.
Unlike most rock, tuff can contain a variety of minerals and particle sizes from sample to sample. Individual grains within the tuff rock can be ashy, glassy or crystalline. Tuffs may include particles of minerals such as augite, biotite, plagioclase and leucite, among others. Because of the variety of minerals in the rock, samples can have different colors and appearances. Tuffs may be brown, gray, pink, green or brown. The presence of dark minerals, such as augite and biotite, can provide a speckled appearance.
Large or well-documented tuff deposits are present in Italy from the eruption of Vesuvius, in England and Wales from ancient volcanic eruptions, and in other locations in Germany, Greece, Japan, New Zealand and Peru, among other volcanic areas around the world. In the United States, Yellowstone Park in Wyoming has well-known deposits of tuff rock. Tuffs range in age from Precambrian to present time, because they form very frequently during volcanic eruptions.