Intense explosive volcanism was simulated by preheating samples of 600 ¿C and subjecting them to dynamic stress pulses of 0.9 and 1.3 GPa. The resultant ejecta was examined optically and by transmission electron microscopy (TEM). Deformation and failure was entirely by brittle fracture, and most fractures were irregular and intergranular. None of the features typical of K/T boundary sediments and of ejecta from meteorite impact craters (planar deformation features, amorphization, mosaicism) were observed; there were also no signs of intracrystalline plasticity. We conclude that explosive volcanism is incapable of producing the microstructure features seen at the K/T boundary, and that they must have been caused by impact of a large meteorite. ¿ American Geophysical Union 1992