When the rocks R1, R2 and R3 sink into a mountain range, their Temperature increases slowly with pressure.When the convergence which produces the thickening, ceases, the Pressure exerted on each of these rocks decreases while the Temperature continues to increase at the beginning of the exhumation. This in turn decreases as the rocks approach the surface, when the thickened crust returns to its normal thickness.
During this P-T-t evolution, the mineralogical association of the rocks changes, but in general it preserves the memory of the peak T conditions.
A few million years later, the geologist evaluates the P and T conditions of the parageneses of these rocks and draws the metamorphic gradient. metamorphic Gradient. If he looks carefully, he can find clues that will allow him to retrace part of the. P-T-t's path !
