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Unit 4, Lesson 5: Unit 4, Lesson 5 Metamorphic Rocks
Metamorphic Rocks: Metamorphic Rocks Rocks that have been changed from an existing rock into a new type of rock
When already existing rocks are buried deep within the Earth, tremendous heat, great pressure, and chemical reactions may cause them to “morph” into different rocks with different textures and structures
Formation: Formation The definition of metamorphism is to ‘change form.’ This is a reflection of the origin of metamorphic rocks since they can form from igneous, sedimentary, or even other metamorphic rocks.
Since the formation of metamorphic rocks is completely hidden from view, metamorphism is one of the most difficult processes for geologists to study. Nonetheless, geologists are aware that the agents responsible for metamorphosis are heat, pressure and chemically active fluids.
During metamorphosis, rocks are often subjected to all three agents simultaneously.
Heat is probably the most important agent. Heat provides the energy to drive chemical reactions that recrystallizes minerals.
Changes: Changes Metamorphism of rocks causes many changes in the existing rocks. The degree of metamorphism is reflected in the rock’s texture and mineralogy, which is the basis of classifying metamorphic rock. This includes increased density, growth of larger crystals, the transformation of low-temperature minerals into high-temperature minerals, and foliation (reorientation of the mineral grains into a layered appearance). An example of a foliated texture metamorphic rock is gneiss, which forms from igneous rock. An example of non-foliated textured metamorphic rock is marble, which forms from limestone.
Grade: Grade A useful concept in classifying metamorphic rock is metamorphic grade, which is the degree to which heat and pressure have changed a rock. An example of a low grade metamorphic rock is slate. Slate forms from the sedimentary rock shale, once it has been subjected to temperatures and pressures only slightly greater than those associated with the compaction that lithifies sediment. At higher grades, more and more of the original minerals react chemically with each other, re-crystallizing to form new minerals, releasing water, carbon dioxide and other volatile constituents.