Tephra, the Greek word for ash, is used to describe any material that is ejected by a volcano into the atmosphere. Tephra includes dense blocks and bombs, and lighter materials such as scoria, pumice and ash. As one moves away from a volcano, the tephra deposits become finer grained (the particles are smaller) and thinner. This is because small particles stay aloft longer and stay within the eruption cloud for a greater distance from the volcano.
Layers of tephra from ancient eruptions near Crater Lake, Oregon. Selective erosion has carved out pillar-like structures. Photo: J. Alean