The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815.
Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size. Tephra is typically rhyolitic in composition as most explosive volcanoes are the product of the more viscous felsic or high silica magmas.
Volcanologists also refer to airborne fragments as pyroclasts or sometimes just clasts. Once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff. The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest and therefore closest to the vent, while smaller fragments travel further - ash can often travel for thousands of miles as it can stay in the stratosphere for several weeks.
Tephra fragments are classified by size:
Ash - particles less than 2 mm in diameter
Lapilli or volcanic cinders - between 2 and 64 mm in diameter
Volcanic bombs or volcanic blocks - greater than 64 mm in diameter
The words "tephra" and "pyroclast" both derive from Greek. Tephra means "ash". Pyro means "fire" and klastos means "broken"; thus pyroclasts carry the connotation of "broken by fire".
The use of tephra layers as temporal marker horizons is known as tephrochronology.