Alternating layers of Si (400–500 A thick) and Rh (200 A thick), up to 21 layers altogether, have been deposited by electron beam evaporation under high vacuum conditions on to thermally oxidized silicon substrates at room temperature. Many of the as-deposited samples exhibit varying degrees of surface roughness. RBS and AES profiles of these samples show that layer intermixing has occurred; the number of layers consumed in the reaction correlates with the degree of surface roughness. The compounds Rh4Si5 and Rh3Si4 have been identified by x-ray diffraction. Both compounds are known to form by nucleation-controlled kinetics at temperatures in excess of 800 °C. However, in this case it appears that reaction occurred via an explosive crystallization process initiated in situ by the arrival of hot Si microparticles. Propagation of the crystallization wave front around secondary Si microparticies results in the formation of a variety of striking geometric surface morphologies. These morphologies can be grouped into three distinct classes or types. Though individual morphologies arise from a random process, the emission of Si rnicroparticles during evaporation, each class of morphologies is itself phenomenologically repeatable and relates to the energy balance governing the propagation of the explosive crystallization wave front. In addition, multilayer films left unreacted after deposition have been explosively crystallized at room temperature by tapping with a stylus or by pulse laser heating, with similar results. © 1986, American Vacuum Society. All rights reserved.