Here's why:
* Metamorphism: This process involves changes in the mineralogy and texture of rocks due to heat, pressure, and chemically active fluids.
* Grain size: The size of individual mineral crystals within a rock.
* Increasing Metamorphism: As the intensity of metamorphism increases, the temperature and pressure rise. This leads to:
* Recrystallization: Existing minerals are dissolved and reprecipitated, forming larger crystals.
* Neocrystallization: New minerals form from the original minerals.
Therefore, with increasing metamorphism, the recrystallization and neocrystallization processes result in larger mineral grains, making the rock coarser-grained.
Example:
* Shale: A fine-grained sedimentary rock.
* Slate: A low-grade metamorphic rock derived from shale, with very small grains.
* Schist: A higher-grade metamorphic rock, still derived from shale, but with larger, visible mineral grains.
* Gneiss: A very high-grade metamorphic rock, with large, well-defined mineral grains.
