Room-temperature microplastic phenomena in oxide glasses are attributed to the combined effects of densification and shear flow, the origin of which, however, remains speculative. Using atomic force microscopy imaging to analyze the imprint topology created by microindentation in mixed alkali alumino-phosphate glasses, we show that the amount of matter displaced at the surface is correlated with the alkali mobility. These results lead to the conclusion that mobile alkali assists shear flow processes produced by indentation at room temperature and that these processes correspond to plastic rather than viscous flow.