Final answer:
The changes in texture and composition of metamorphic rocks result from exposure to high temperature and pressure, not from compaction and cementation. Metamorphic rocks, such as slate and gneiss, form when the pre-existing rock, or protolith, undergoes a physical and mineral transformation, a process called metamorphism.
Step-by-step explanation:
The changes in texture and composition of metamorphic rocks result mainly from the exposure of pre-existing rocks, known as the protolith or parent rock, to conditions of high temperature and pressure within the Earth's crust. Unlike sedimentary rocks that form from the compaction and cementation of rock fragments, metamorphic rocks are formed when pre-existing igneous, sedimentary, or even other metamorphic rocks are subjected to environments that alter their physical and mineral structure. This process is known as metamorphism, meaning to change form, and involves the recrystallization of minerals, resulting in moderated changes to the rock's texture, such as the alignment of minerals to form foliation or the growth of larger crystals under higher grades of metamorphism.
The metamorphic rock's characteristics, such as the mineral content and texture, tell us about the conditions under which it formed. Foliation is a texture common in metamorphic rocks, associated with layers or bands that result from the reorientation of minerals. Examples include slate, which is formed from shale, and gneiss, which typically originates from sedimentary rocks like granite.
Therefore, the processes of compaction and cementation are not directly involved in metamorphic rock formation but are characteristic of sedimentary rock formation. Instead, the primary factors influencing metamorphic rock formation are temperature, pressure, and sometimes chemically active fluids, all of which contribute to the recrystallization and realignment of minerals.