Cobalt thin films deposited on SiO₂ were processed by rapid thermal annealing. Agglomeration of Co takes place during the annealing, and the formed Co globules penetrate into the SiO₂, resulting in crater-like erosion pits. The presence of reaction is evidenced by the formation of a crystalline orthosilicate compound (Co₂SiO₄). The reaction mechanism is discussed in view of the presence of water at the oxide surface.