Metamorphic rocks form under intense heat and pressure deep within the Earth’s crust. These extreme conditions transform existing rocks into new minerals and structures.
The temperature range for metamorphic rock formation is typically between 200°C and 900°C (392°F to 1652°F). This wide span reflects the diverse environments where metamorphism occurs.
Low-grade metamorphism begins around 200°C, while high-grade metamorphism can reach temperatures up to 900°C. The specific temperature depends on factors like depth, tectonic setting, and the composition of the original rock. Understanding these temperature ranges helps geologists interpret the Earth’s history and processes.
Fundamentals of Metamorphic Rock Formation
Metamorphic rocks form through the transformation of existing rocks under specific temperature and pressure conditions. These processes alter the original rock’s mineralogy, texture, and chemical composition.
Mineral Transformation and Recrystallization
Metamorphism involves the transformation of minerals into new, stable forms. As temperatures and pressures increase, atoms within minerals reorganize into more compact structures. This process, called recrystallization, results in larger, interlocking crystal grains.
Some minerals, like quartz, remain stable across a wide range of conditions. Others, such as clay minerals, transform into new species like micas or feldspars. The presence of fluids can facilitate these changes by enhancing ion mobility.
Metamorphic reactions often produce distinctive mineral assemblages. For example, the reaction of quartz and calcite under high temperatures can form wollastonite, a calcium silicate mineral.
Metamorphic Facies and Protoliths
Metamorphic facies represent specific pressure-temperature conditions that produce characteristic mineral assemblages. Each facies is named after a key rock type or mineral.
Common facies include:
- Greenschist: Low to medium grade, characterized by chlorite and actinolite
- Amphibolite: Medium to high grade, marked by hornblende and plagioclase
- Granulite: Very high grade, featuring pyroxenes and feldspars
The original rock type, or protolith, strongly influences the resulting metamorphic rock. Shales typically metamorphose into slates, phyllites, and schists. Limestones often form marbles, while sandstones transform into quartzites.
Protolith composition and metamorphic conditions together determine the final rock type and mineral assemblage. This relationship allows geologists to infer past pressure-temperature conditions from observed metamorphic rocks.
Temperature Range in Metamorphism
Metamorphic rocks form across a wide spectrum of temperatures, from relatively cool conditions to extreme heat. The temperature range plays a crucial role in determining the type and intensity of metamorphic changes.
Low-Grade Metamorphism
Low-grade metamorphism occurs at temperatures between 200°C and 450°C. These conditions typically produce subtle changes in mineral structures and compositions.
Rocks like slate and phyllite form in this temperature range. Chlorite and muscovite are common minerals that develop during low-grade metamorphism.
At the lower end of this range, around 200-300°C, very mild alterations take place. Original sedimentary or igneous textures may still be visible in the metamorphic rock.
High-Grade Metamorphism
High-grade metamorphism takes place at temperatures above 450°C, often reaching up to 700-800°C. These intense conditions lead to significant recrystallization and mineral transformations.
Rocks such as gneiss and granulite form under high-grade conditions. Minerals like garnet, sillimanite, and pyroxene are indicators of high-temperature metamorphism.
At extreme temperatures exceeding 700°C, partial melting of rocks can occur. This process, known as migmatization, results in mixed igneous-metamorphic textures.
The upper limit for metamorphism is around 850-900°C. Beyond this point, complete melting of the rock typically occurs, transitioning into igneous processes.