Here's why:
* Metamorphism: This process involves the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) due to intense heat and pressure. These conditions cause changes in the mineral composition and texture of the rock.
* Crystallinity: This refers to the size, shape, and arrangement of crystals within a rock. As rocks are metamorphosed, the heat and pressure cause:
* Recrystallization: Existing minerals are broken down and new, larger, and more stable minerals form.
* Grain growth: The individual crystals within the rock grow larger, leading to a more coarse-grained texture.
* Alignment: Pressure can cause the minerals to align themselves in a specific direction, resulting in a more foliated (layered) appearance.
Therefore, as rocks undergo metamorphism, their overall crystallinity increases, leading to more well-defined crystals and a more distinct texture.
