Metamorphic Facies

Metamorphic facies show how metamorphic minerals form.

It is a series of metamorphic mineral assemblages that form in a specified range of pressures and temperatures.

In the early 1900s, Scandinavian scientists realised that metamorphic minerals don't form randomly.

They realised that metamorphic rocks contain distinct mineral assemblages formed under certain pressures and temperatures.

 eskolas facies
 Eclogite facies. By GOC53 via

Those assemblages depended not only on pressures and temperatures, but also the chemical composition of the original rock.

In other words, the same metamorphic rock contains different minerals depending on at which temperatures and pressures it was formed.

And two different rocks don't contain the same minerals just because they formed under the same pressures and temperatures, because the composition of the original rock was different.

But - two separate masses of the same rock, located at different locations, will have the same minerals if they were formed under the same pressures and temperatures.

This observation lead to Eskola's facies:

Zeolite - low pressures and temperatures

Hornfels - low pressure, high to medium temperatures.

Greenschist - low to medium pressures and temperatures

Amphibolite - high to medium pressures and temperatures

Granulite - highest temperatures, medium pressures

Blueschist - high pressures, low to medium temperatures

Eclogite - highest pressure, high to medium temperatures.

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