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Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith ) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100 megapascals (1,000 bar ) or more, causing profound physical or chemical changes.
The metamorphic facies is not usually considered when classifying metamorphic rock based on protolith, mineral mode, or texture. However, a few metamorphic facies produce rock of such distinctive character that the facies name is used for the rock when more precise classification is not possible.
Metamorphic banded gneiss. Metamorphic rocks are formed by subjecting any rock type—sedimentary rock, igneous rock or another older metamorphic rock—to different temperature and pressure conditions than those in which the original rock was formed.
Phyllite – Type of foliated metamorphic rock – A low grade metamorphic rock composed mostly of micaceous minerals Pseudotachylite – Glassy, or very fine-grained, rock type Pages displaying short descriptions of redirect targets – A glass formed by melting within a fault via friction
A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion. Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. [1]
These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine-grained rock would be known as basalt. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks. [2]
Broadly, the Franciscan can be divided into two groups of rocks. Coherent terranes are internally consistent in metamorphic grade and include folded and faulted clastic sediments, cherts and basalts, ranging from sub-metamorphic to prehnite-pumpellyite or low-temperature blueschist (jadeite-bearing) grades of metamorphism.
Andalusite crystals in Dalradian metamorphic rock at Boyndie Bay in the Buchan metamorphic zone of north-east Scotland Often only part of the series can be found. Another metamorphic facies series is the Buchan series , that sees a fast increase in temperature but a relatively small increase in pressure.