Metamorphic rocks form by the physical, and sometimes chemical, alteration of a pre-existing rock. The pre-existing rock could be igneous or sedimentary, and even metamorphic rocks can be altered into a completely different metamorphic rock. The pre-existing rock is distinguished from the new metamorphic rock by referring to the original rock as the protolith or parent rock. A key aspect of metamorphism is that it occurs under subsolidus conditions, meaning temperatures that are too low for melting to occur.

The rocks that we classify as either igneous, metamorphic, or sedimentary, could have belonged to a different rock classification in the past, as rocks are recycled throughout geologic time, driven by the motion of the tectonic plates. It’s easy to see that increasing the temperature of a rock can produce magma, and that rocks on Earth’s surface can break up into sediment that can ultimately lithify into a sedimentary rock. But how can we alter a solid rock into a new rock, without first melting it or making it become sediment?

All rocks form at certain temperatures and pressures on, or more commonly, beneath, Earth’s surface, and these rocks are the most stable at the conditions the same as the conditions present when they formed. Therefore, changing the temperature and/or pressure conditions may lead to a different rock—a metamorphic rock—that changed to a more stable form under new external conditions.

The term metamorphism refers to a change in form. You may have heard a similar term—metamorphosis—that refers to the process of transforming a caterpillar into a butterfly. For a rock the change in form could be a change in the shape of existing crystals, or the transformation of one mineral into another. The newly formed metamorphic rock will have an altered appearance that is sometimes strikingly different from what it used to look like before metamorphism occurred.

 All metamorphic rocks have at least one protolith that has altered during metamorphism, and the metamorphic rock you end up with is strongly dependent on the rock you started with before the metamorphic event. Of secondary importance is the agent of metamorphic change: was it due to increased temperature, pressure, or both, or were chemically reactive fluids involved? We must also consider how high the temperatures and pressures were, as metamorphism can occur in a range of temperature and pressure conditions (although always at temperatures below the melting point of the rock).

In this chapter you will learn to identify metamorphic rocks based on their mineral content and texture; for metamorphic rocks, texture refers to the orientation of the minerals in the rock, although crystal size does convey important information regarding the temperature conditions during metamorphism.

Learning Outcomes

After completing this chapter, you should be able to:

• Correctly identify common metamorphic rocks and their distinguishing features
• Recognize foliation in metamorphic rocks, and the type of foliation
• Determine the grade of metamorphism based on foliation and possible index minerals