4.16: Metamorphic Rock Classification

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What's the difference in these two rocks?

The rock on the left was the sedimentary rock shale. Now it's the metamorphic rock slate. Slate looks very similar to shale, but it is harder and more platy. The rock on the right is gneiss. The light and dark minerals have separated into bands. Gneiss is produced by higher temperature metamorphism. The original rock was different in both cases.

Common Metamorphic Rocks

Some of the most common metamorphic rocks are listed below (Table below). Their parent rock and the type of metamorphism are also mentioned.

The table shows some common metamorphic rocks and their original parent rock.

Rock Name	Type of Metamorphic Rock	Comments
Slate	Foliated	Metamorphism of shale
Phyllite	Foliated	Metamorphism of slate, but under greater heat and pressure than slate
Schist	Foliated	Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite
Gneiss	Foliated	Metamorphism of various different rocks, under extreme conditions of heat and pressure
Hornfels	Non-foliated	Contact metamorphism of various different rock types
Quartzite	Non-foliated	Metamorphism of quartz sandstone
Marble	Non-foliated	Metamorphism of limestone
Metaconglomerate	Non-foliated	Metamorphism of conglomerate

Summary

Foliated metamorphic rocks are platy.
Non-foliated metamorphic rocks are massive.
The more extreme the amount of metamorphism, the more difficult it is to tell what the original rock was.
Marble is metamorphosed limestone.