Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism. [source]

Volcanologists also refer to airborne fragments as pyroclasts. However, once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff. [source]

This particular block of tephra was photographed at Brown Bluff, Tabarin Peninsula, Antarctic Peninsula. To give an idea of scale, the height at the center of the object is at least 6 ft (1.8 m).