Tuff is a type of rock which is formed from compacted volcanic ash and fragments of material associated with volcanic eruptions. There are a number of different types of tuff, with the rock being classified on the basis of what it contains, how large the particles embedded in the rock are, and how it formed. Tuff has limited commercial uses, although it was once used extensively in building and sculpture in some regions because it was so readily available.

When volcanoes erupt, they spew massive amounts of ash, fragments of rock, and other materials into the air. As these materials settle and cool, they form into an assortment of rock varieties, including tuff. Tuff commonly forms when volcanic magma is very stiff, allowing air bubbles and pockets to form, and it tends to be extremely porous and very soft; depending on the prevailing conditions, tuff may have several layers of material, reflecting multiple eruptions.

In some cases, tuff actually welds together, because the components of the rock are so hot. In this case, tuff is classified as a pyroclastic rock, and it is called “welded tuff.” Welded tuff is often very easy to identify, because it typically has large chunks of material interspersed with smaller ones, all welded together by the heat of the ash and other components of the tuff.

Tuff may also be classified on the basis of the composition of its fragments. Basaltic tuff, ultramafic tuff, rhyolite tuff, and andesitic tuff are some examples of various types of tuff. Many of these forms of tuff have small crystalline fragments, which can sometimes cause the rock to sparkle or glitter. These forms of tuff are classified as sedimentary rocks, because they are formed by the deposition and compression of sediment.

In construction, tuff can be useful, especially for things like walls, and numerous examples of tuff walls can be seen in places like the Mediterranean, where tuff is abundant. Tuff has also historically been used in sculpture. In areas where tuff is common, people have to be careful when they are building homes and other structures, as tuff is not always capable of supporting a great deal of weight. A house built over a tuff field may collapse during an earthquake, and tuff cuttings in railway embankments and along roads are also subject to crumbling and damage.