Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Tephra horizons in south-central Iceland: The thick and light-coloured layer at the centre of the photo is rhyolitic tephra from Hekla.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff. Tephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed. When a volcano explodes, it releases a variety of tephra including ash, cinders, and blocks. These layers settle on the land and, over time, sedimentation occurs incorporating these tephra layers into the fossil record. Often, when a volcano explodes, biological organisms are killed and their remains are buried within the tephra layer. These fossils are later dated by scientists to determine the age of the fossil and its place within the fossil record.
A 2007 eruptive plume at Mount Etna produced volcanic ash, pumice, and lava bombs.
Tephra is unconsolidated pyroclastic material produced by a volcanic eruption. It consists of a variety of materials, typically glassy particles formed by the cooling of droplets of magma, which may be vesicular, solid or flake-like, and a varying proportions of crystalline and mineral components originating from the mountain and the walls of the vent. As the particles fall to the ground, they are sorted to a certain extent by the wind and gravitational forces and form layers of unconsolidated material. The particles are further moved by ground surface or submarine water flow.
The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest, therefore closest to the vent, while smaller fragments travel further – ash can often travel for thousands of miles, even circumglobally, as it can stay in the stratosphere for days to weeks following an eruption. When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions (or from a multitude of smaller eruptions occurring simultaneously), they can reflect light and heat from the sun back through the atmosphere, in some cases causing the temperature to drop, resulting in a temporary “volcanic winter”. The effects of acidic rain and snow, the precipitation caused by tephra discharges into the atmosphere, can be seen for years after the eruptions have stopped. Tephra eruptions can affect ecosystems from miles to kilometers depending on the size of the eruption.