Understanding the high-pressure behaviour of SiO2, a prototypical network-forming material, is important for resolving many problems in the Earth sciences, For pressures of 1-3 GPa (similar to 1-3 x 10(4) atm), it has been shown that increases in pressure result in higher rate constants for atomic transport processes such as diffusion, viscous flow and crystal growth in SiO2 as well as in some silicate melts(1-5). Structural transitions and coordination changes observed beyond 10 GPa (refs 5-9) may also be related to this pressure-induced increase in atomic mobility. There must be limits, however, on the extent to which pressure can enhance mobility, as a migration barrier decreasing linearly with pressure should vanish at a critical pressure, beyond which a sudden change in behaviour should be observed(10,11), Here we report measurements of the pressure dependence of the growth rate of quartz from amorphous SiO2 for pressures up to 6 GPa. We observe a sharp peak in growth rate-implying a minimum in viscosity-at 3 GPa, which we interpret as evidence that the critical pressure is being traversed, The corresponding depth below the Earth’s surface at which this peak occurs (similar to 100 km) suggests that this critical pressure may be related to the ubiquitous cut-off in subduction-related volcanism observed when oceanic plates reach roughly this depth.