It is apparent that pressure and temperature are environmental variables that can behave very differently over time. Two models have evolved for regional metamorphism. The normal or clockwise model is characteristic of most orogenic belts. Begins with the Burial phase - pressure builds up faster with temperature lagging. As the basin deepens and stabilizes temperature continues to rise while pressure remains constant.
Additional heat is provided from the mantle.
This is the Heating stage. The Uplift phase begins as an isostatic response to the lower density of the sediments and to heating which further decreases the density. During this stage, at least initially, the temperature continues to rise even as the pressure on the system drops. The Unroofing phase sees a rapid drop in both temperature and pressure as weathering removes the overlying rock cover. While most orogenic belts seem to be characterized by this clockwise trend, older granulite terrains seem to have evolved in a different manner, the reverse or counterclockwise trend.