Final answer:
Metamorphic rocks are created from pre-existing rocks exposed to high temperatures and pressures. Contact metamorphism involves heat from nearby magma, while regional metamorphism occurs over larger areas due to tectonic forces. Grain shape is influenced by these processes, with foliation common in regional metamorphism and varying grain sizes indicating the metamorphic grade.
Step-by-step explanation:
Understanding Metamorphic Rocks: Contact vs Regional Metamorphism
Metamorphic rocks are formed from existing rocks that undergo changes due to high temperatures and high pressures. The process that leads to the formation of metamorphic rocks is known as metamorphism. There are two types of metamorphism: contact metamorphism and regional metamorphism. Contact metamorphism occurs when rocks are heated by nearby molten magma, leading to changes in the rock at the contact zone. On the other hand, regional metamorphism takes place when rocks are subjected to high pressures and temperatures over large areas, typically due to tectonic forces associated with mountain building.
The connection between these metamorphic processes and grain shape lies in the conditions under which the rock forms. During contact metamorphism, the grain size might increase as minerals grow in the heated conditions, but without strong directional pressure, foliation (layering) typically does not occur. Alternatively, regional metamorphism often results in foliation due to the directed pressure, and the grain size can vary significantly with larger, visible crystals forming in higher-grade metamorphic rocks, such as schist or gneiss, than in lower-grade rocks like slate or phyllite. The texture of metamorphic rocks, which includes the size, shape, and arrangement of mineral grains, is a key feature in their classification and reveals information about the metamorphic environment in which they formed.