The particulate emission of a volcano consists of glass particles (up to 2mm in size) formed by the bursting of rising gas bubbles in the magma, and is referred to as volcanic ash. Volcanic ash should not be confused with the ash resulting from combustion processes.
Volcanic ash can be carried over large distances following the eruption of a volcano. It has very high density and can cause the roofs of buildings to collapse under its weight. For example, when Pinatubo erupted in June 1991, many persons were killed under collapsing roofs.