It has recently been shown that amorphization and melting of ice (O. MISHIMA, Nature, 384 (1996) 546) were intimately linked. In this letter, we infer from molecular-dynamics simulations on the SiO2 system that the extension of the alpha-quartz melting line in the metastable pressure-temperature domain is the pressure-induced amorphization line. It seems therefore likely that melting is the physical phenomenon responsible for pressure-induced amorphization. Moreover, we show that the structure of a "pressure glass" is similar to that of a very rapidly (10(13) to 10(14) kelvins per second) quenched thermal glass.