We report on the observation of a coupling between the dewetting of a Si layer on SiO2 induced by surface/interface energy minimization and the etching between both materials due to the Si thorn SiO2 -> 2 SiOg up arrow reaction. In the limit of a thin SiO2 layer (<= 10 nm) sandwiched between a Si layer and a Si handle wafer, the front of Si dewetting and the front of SiO2 etching coexist in a narrow region. The interplay between both phenomena gives rise to specific morphologies. We show that extended Si fingers formed by dewetting are stabilized with respect to Rayleigh-Plateau-type instability over tenth of microns thanks to a localized etching of the SiO2 layer. The breakup of this structure occurs abruptly by an unzipping process combining dewetting and etching phenomena. We also put in evidence that Si rings are created with a thin SiO2 layer in the center. These processes are thermally activated with an activation energy of 2.4 +/- 0.5 eV and 4.0 +/- 0.5 eV, respectively, for dewetting and the etching reaction. All these results highlight the respective roles of wetting and etching in Si/SiO2/Si system dynamics and could be a stepping stone for the development of advanced processes based on Silicon-On-Insulator technology.