Final answer:
The pre-existing rock that a metamorphic rock forms from is called the protolith, also known as the parent rock. It is fundamentally altered through changes in temperature, pressure, or chemical interactions to become a metamorphic rock with unique properties and textures.
Step-by-step explanation:
Protolith in Metamorphic Rocks
The pre-existing rock that is altered to form a metamorphic rock is called the protolith. This original, unmetamorphosed rock, sometimes referred to as the parent rock, undergoes changes due to variations in temperature and pressure, or interactions with chemically reactive fluids. As a result, a new metamorphic rock emerges with distinctive properties and textures such as slate, phyllite, schist, gneiss, or even migmatite when there's partial melting involved.
Comprehending the concept of a protolith is fundamental in understanding metamorphic rocks. This knowledge is essential not only for identifying the resultant metamorphic rock but also for understanding the geological processes behind the rock cycle as it allows you to trace the journey from the original rock form to the final metamorphic state.
Metamorphic grade refers to the changes in metamorphic temperature and pressure that a rock undergoes. Texture in these rocks can be foliated or non-foliated and is indicative of the conditions the protolith was subjected to during metamorphism. Each metamorphic rock is unique and is a direct result of the characteristics of its protolith and the specific conditions under which metamorphism occurred.