Utilizing atomic-force microscopy, we have performed and characterized nanoindentations on a MgO(100) surface with depths varying between one monatomic layer and tens of nm. Our results show that plastic deformation is indicated by discrete events in the indentation curve which are associated with the number of atomic layers being expelled by the tip penetrating the surface. An estimation of the energy required to expel MgO ions from a monatomic deep cavity correlates well with our data. The critical shear strength was evaluated and lies in the same order of magnitude as the theoretical value.