Tuff is a porous rock formed from the widespread deposition and consolidation of volcanic ejecta. The material is quite variable, however, and can exhibit a wide range of characteristics, even in a single occurrence of the rock.

View a second image of Welded Tuff

Many large geologic formations, such as cliffs, hills, and mountains, are often composed of tuff. Within these natural structures, the rock differs significantly in constitution and hardness. In some place, for instance, the tuff may be relatively soft and crumbly, while in others it may be more compact and block-like. These differences primarily depend upon how much a tuff has been welded together. The degree of welding is determined by how hot the volcanic ash that forms a tuff was when it accumulated at the surface of the Earth; the warmer the material, the easier it is for the glass particles to weld together under the weight of overlying deposits.

Welded tuff is significantly harder than other types of tuff and can be found in various locations around the world. Also known by the term ignimbrite, the rock covers vast expanses of land in Guatemala, New Zealand, Peru, and parts of the United States. One of the most famous occurrences of welded tuff can be seen at Yellowstone National Park in Wyoming. Created by a cataclysmic eruption more than 600,000 years ago, the huge welded tuff formation at the park is known as the Lava Creek Tuff.