We demonstrate the vertical self-catalyzed molecular beam epitaxy (MBE) growth of GaAs nanowires on an amorphous SiO2 substrate by using a smooth [111] fiber-textured silicon thin film with very large grains, fabricated by aluminum-induced crystallization. This generic platform paves the way to the use of inexpensive substrates for the fabrication of dense ensembles of vertically standing nanowires (NWs) with promising perspectives for the integration of NWs in devices.