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Ch. 3.4 METAMORPHIC ROCKS Metamorphic rock is formed when existing rock is changed by heat and pressure. Metamorphism – is the process of forming metamorphic rock. These rocks look different from the rocks they were formed from. Formation of Metamorphic Rocks Metamorphic changes take place at high temperatures and pressure. They happen below the Earth’s surface and into the upper mantle. There are 2 settings it takes place in: contact metamorphism and regional metamorphism. Contact Metamorphism This is when magma moves into a rock. This creates small changes in the rock. Example: marble – formed when magma intrudes in limestone Regional Metamorphism This is large-scale deformation and high-grade metamorphism. That means that there is a lot of change in the rocks and a large amount of rock changes. Agents of Metamorphism The agents (things that cause change) of metamorphism are heat, pressure, and hydrothermal solutions. Heat This can cause minerals to recrystallize or cause new minerals to form. There are 2 sources: magma and change in temperature due to depth beneath the Earth. Magma bakes rocks, while heat because of depth causes new minerals to form. Pressure Pressure increases with depth and is applied in all directions. Pressure closes the spaces between mineral grains, making the rock more compact and therefore, more dense. Temperature and pressure cause the rock to flow, not to fracture, so minerals will flatten and elongate (grow longer). Reactions in Solution Water solutions that have dissolved substances that change to gases are important in some metamorphism. This solution allows minerals to change and if it is at a higher temperature it happens faster. Hydrothermal solutions are when these water solutions escape from magma. Classification of Metamorphic Rock Classified by texture and composition. Texture is either foliated or nonfoliated. Foliated Metamorphic Rock Rocks will become more compact and dense as it goes through metamorphism. If it also has recrystallized minerals that are at the right angles then it gives the rock a layered or banded look – this is foliated metamorphic rock. Nonfoliated Metamorphic Rock If the metamorphic rock is not banded it is called nonfoliated metamorphic rock. They usually only have one mineral.
