6: Metamorphic Rocks
- Page ID
- 28243
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Metamorphic rocks, meta- meaning change and –morphos meaning form, is one of the three rock categories in the rock cycle. Metamorphic rock material has been changed by temperature, pressure, and/or fluids.
The rock cycle shows that both igneous and sedimentary rocks can become metamorphic rocks. And metamorphic rocks themselves can be re-metamorphosed. Because metamorphism is caused by plate motion, metamorphic rock provides geologists with a history book of how past tectonic processes shaped our planet [1].
Reference
1. Yardley BWD (1989) An introduction to metamorphic petrology. Prentice Hall
- 6.1: Metamorphic Processes
- Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting. Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Most metamorphic processes take place deep underground, inside the earth’s crust.
- 6.2: Metamorphic Textures
- Metamorphic rocks are either foliated or nonfoliated. Foliation is a term used that describes minerals lined up in planes. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf. Nonfoliated rocks have minerals that do not show a preferred orientation.
- 6.3: Metamorphic Grade
- Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions.
- 6.4: Metamorphic Environments
- As with igneous processes, metamorphic rocks form at different zones of pressure (depth) and temperature as shown on the pressure-temperature (P-T) diagram.
Thumbnail: Folded foliation in a metamorphic rock from near Geirangerfjord, Norway. (CC-SA-BY 3.0; Siim);