“Tuff” is a name that is used for a broad range of materials. The only requirement is that the materials are ejecta produced by a volcanic eruption. Tuff can contain fragments of dust-size particles to boulder-size particles and be composed of many different types of material. Mount St. Helens tephra: Photograph of an outcrop of stratified…

Sometimes the ejecta is hot enough when it lands that the particles are soft and sticky. These materials “weld” together upon impact or upon compaction. The rock formed from this hot ejecta is known as a “welded tuff” – because the ejected particles are welded together. Some deposits might contain welded tuff near the vent…

A “tuff ring” is a small volcanic cone of low relief that surrounds a shallow crater. These craters, known as maars, are formed by explosions caused by hot magma coming in contact with cold groundwater. The explosion blasts fragments of bedrock, tephra, and ash from the crater. The tuff ring forms as these ejected materials fall back to…

Tuff is an igneous rock that forms from the products of an explosive volcanic eruption. In these eruptions, the volcano blasts rock, ash, magma and other materials from its vent. This ejecta travels through the air and falls back to Earth in the area surrounding the volcano. If the ejected material is compacted and cemented into a rock, that rock will be…