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dehydration curve (vapour pressure curve) The equilibrium curve for dehydration reactions on graphs showing increasing total pressure (vertically) against increasing temperature (horizontally). The slope of the dehydration curve varies with vapour pressure and depends on the relationship with total pressure. This is important in prograde metamorphism when many reactions involve dehydration reactions as increasing temperature produces water vapour. Retrograde metamorphism is more limited, as most of the water has been driven off previously and the breakdown of existing grains, nucleation, and growth are much slower in the absence of water.