Final answer:
The order of rocks from low to high metamorphic grade is shale, slate, and gneiss, with gneiss showing high-grade features like distinct mineral banding due to intense recrystallization.
Step-by-step explanation:
The metamorphic grade describes the intensity or degree of metamorphism and is mostly determined by the temperature and recrystallization that the protolith is subjected to. Here is the order of the listed rocks from low-grade to high-grade metamorphic rocks:
- Shale (low-grade)
- Slate
- Gneiss (high-grade)
The changes observed in this suite of rocks as the grade is increased include the reorientation of minerals to form foliated textures, recrystallization where existing minerals change size and shape, and neocrystallization where new minerals form.
In particular, gneiss exhibits high-grade metamorphism with distinct alternating bands of light and dark minerals and a more intense recrystallization compared to slate and shale.
Slate is formed from shale as a result of increased pressure and slight increase in temperature, leading to the alignment of its clay minerals. Shale is the original low-grade protolith that undergoes these transformations.
Foliation becomes more pronounced as we move from slate to gneiss, reflecting the increased levels of heat and pressure during metamorphism.