Changes in a preexisting rock's mineralogy, texture, and/or composition that occur predominantly in the solid state when the rock is exposed to a physical or chemical environment that is significantly different from that in which it initially formed. Any parameter that can effect such a change is one that could cause metamorphism. These include temperature, pressure, the nature of the fluid phase, and the state of stress. Although the processes of weathering and diagenesis (changes in sediments) are essentially identical to those of metamorphism, and solid metamorphic rock masses can coexist with melts during partial melting, metamorphism is rather arbitrarily confined to conditions between those of diagenesis and large-scale melting. Given that weathering and diagenesis occur only in the thin uppermost veneer of sediments, and that large-scale melting is an exceptional process with respect to normal conditions at depth, metamorphism is considered the dominant process taking place throughout most of the Earth's crust and mantle. Significant uplift and erosion typically is required to expose metamorphic rocks at the surface of the earth.