Tephra is unconsolidated pyroclastic material produced by a volcanic eruption. The particles are formed by magma and fragments of rock; they vary in size and composition, and form layers of material when they land. On the ground, tephra can be transported by water and in time consolidates to form a soft rock known as tuff.
This picture shows a 6-foot-high (1.8 m) boulder of tephra photographed on the beach near Brown Bluff, a volcano on the Antarctic Peninsula that formed in the past million years when it erupted under a glacier. The larger, dark-coloured particles are fragments of alkali basalt, which are embedded in layers formed from volcanic ash.Photograph credit: Andrew Shiva