The transformations of different crystalline phases in SiO2, the main component of the earth's crust, were examined. Using improved metadynamics algorithm, the complex structural phase transitions with several intermediate states were simulated. Most of the simulations were conducted using the force field by van Beest, Kramer and van Santen(BKS) to qualitatively reproduce the phase boundaries between quartz, coesite and stishovite. In quartz II, Si atoms were arranged in alternating layers of tetrahedral and octahedral coordination. Three different abrupt transitions occurred after high barriers were overcome. In the first transition fourfold coordinated silicon atoms disappeared and a distorted octahedral structure was created. The surprising transition was to an anatase structure which has not been reported for silica and it represented an unknown artifact of the BKS potential.