Two things to understand about metamorphic rocks:

So, we have metamorphic rocks and we know how they form. Now how do we recognize them? Two general methods:


Metamorphic textures

Foliated rocks: Typically with clay and feldspar-rich protoliths.

Non-foliated rocks: Generally consist of minerals without platy form. May result from any form of metamorphism:

Metamorphic minerals

We've said that metamorphic grade is a continuum going from low to high grade. Although this is true, the presence of certain metamorphic minerals in a rock represent absolute milestones that we can recognize in this continuum. Which minerals we would expect to see depends on the protolith and the type of metamorphism the rock experienced.

The chart at right shows the relationship between grade and the appearance or disappearence of specific minerals for a clay protolith experiencing regional metamorphism.
Mineral Isograds:

By mapping the distribution of these minerals onto a map, we can see the physical distribution of grades of metamorphism. The lines separating zones of different minerals represent points with equal grades of metamorphism and so are called isograds. (Compare with contour lines - lines of equal elevation, or isotherms - lines of equal temperature.
Variations: But note these two extra considerations:

Metamorphic facies: Summing information about metamorphic mineral sequences, geologists have recognized characteristic groupings of rocks and minerals that correspond to different conditions of temperature and pressure in which metamorphism occurred even when there were several different protoliths present,. These are metamorphic facies. These are named after characteristic rocks found in them (E.g. granulite, blueschist, amphibolite) Don't be confused. The facies may contain many more than the single rock type.

They do correspond to different plate tectonic settings. Thus:


Key concepts and vocabulary: