Rocks tell a story.
In particular, in order to understand the processes of mountain building and collisional orogenesis, we go to the crystalline metamorphic rocks found in the cores of these mountain belts. At least in part, the story of the collision of continents is contained in the chemistry and textures of metamorphic minerals. For example, subduction and continental collision cause lithosphere plates to be deeply buried. All the while, the rock is reacting, forming new minerals, the chemistry of which is strongly controlled by the pressure and temperature. Some minerals, such as garnet and plagioclase, quite faithfully retain their composition from the time at which they were produced.