We describe unusual "plate tephra" particles that provide key information about rarely observed processes that occur during volcanic eruptions. The tephra formed during the 2008-2009 eruption of Llaima volcano, Chile, and dispersed as far as 9 km from the vent. The plates are angular clasts of vesicular basaltic andesite ranging in size from 1 to 14 cm and in thickness from 2 to 5 mm. External features such as ridges, varying degrees of curvature, and adhered material are present. Internal textures include strong crystal alignment and deformed enclaves. We propose that the plates are wall fragments formed during the rupture of large gas slugs associated with unsteady fire fountaining during the violent Strombolian phase of the eruption. The presence of plate tephra may be a diagnostic feature of highly unsteady activity where slug rupture is concurrent with the formation of a sustained eruption column.