Amorphous silicon dioxide layers were implanted with 100 keV Ar ions to a relatively high fluence in a tentative to generate cavities in the oxide. Different oxide layers were used, obtained either by thermally growth or by chemical vapor deposition (CVD) on Si substrate. In all SiO2 layers, cavities are not formed in the as-implanted state. However, in the transmission electron microscope, under electron beam, the combined effect of irradiation induced defects and implanted rare gas leads to the formation of cavity bands giving the unique opportunity to observed in-situ cavity growth. The cavity morphology and their distribution are found to be dependent on the silicon dioxide growth process. For thermally grown SiO2 layer, a homogeneous cavity band is formed, centered at the mean ion path, with an average cavity size of 20 nm. For CVD SiO2 layer, slightly smaller cavities are formed in two distinct bands. The formation of cavities is discussed in light of gas and defects interaction and field-induced migration whereas the cavity distribution is discussed in terms of self-organization.