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Foliation: Definition & Types

Foliation: Definition & Types
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  • 0:04 What is Foliation?
  • 0:51 How a Foliated Rock is Formed
  • 2:26 Slate
  • 3:32 Schist
  • 4:27 Gneiss
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Deep below the earth's surface resides a type of rock called metamorphic rock. In geology, repeating layers of this rock are called foliation. Through this lesson, you will learn about the process of foliation and its different types.

What is Foliation?

Throughout nature we can find a variety of rocks. They come in different colors, sizes, and even shapes. Since a rock is made from one or several minerals, scientists can classify it by the way it's formed. In particular, one type of rock is formed when older rocks are exposed to intense heat and pressure. We call this type of rock a metamorphic rock.

Formation of a metamorphic rock
formation

When a rock undergoes this type of stress, the minerals in the rock begin to reorient themselves. As the minerals continue to move around, the rock will change or undergo metamorphosis. You might have seen a metamorphic rock before. Marble is an example of a metamorphic rock. As the minerals in rocks reorient during metamorphosis, a preferred orientation might be favored. The ability to describe this preferred orientation is called foliation.

How a Foliated Rock is Formed

If we open up a Latin book, we'll discover that the word ''foliation'' comes from the Latin term ''folium'', which means ''leaf.'' How can a leaf describe a foliated rock? The relationship between a leaf and a foliated rock can be seen in the structure of thin layers in a foliated rock.

A simple diagram of what a foliated rock looks like
foliation

Known to be very thin, like a sheet of paper, these layers have an evenly distributed planar structure. Think of the term ''planar'' as meaning ''flat'' or ''thin''. As shown in this diagram, several thin sheets of minerals will align themselves in a parallel orientation. Recall that a metamorphic rock is formed from minerals that reorient themselves due to external stress.

When these minerals line themselves up into a thin, flat structure, a foliated metamorphic rock is formed. To be more specific, it takes three things to cause foliation: growth of a mineral, bending, and rotation into an orientation that's parallel. As an interesting fact, the formation of these types of rocks can take thousands to millions of years! If there was an award for the mineral grain that most often creates foliation, first place would go to mica minerals. As we'll see shortly, micas are commonly found in different types of foliated metamorphic rock.

There are three common types of foliated metamorphic rock: slate, schist, and gneiss. Keep in mind that all three have the signature thin layer caused by stress or pressure, although each type is different based on mineral grain size and foliation characteristics. Let's take a look at each of these types in more depth.

Slate

Slate is formed when mudstone (or shale) changes form by metamorphosis at very low temperatures and pressures. It often has a beautiful gray color or different shades of gray, and you might see it being used for flooring or on someone's patio. How is foliation in slate formed? Well, there are two common types of minerals in slate rock: mica and clay. When these mineral grains line up in a straight parallel fashion, they cause foliation in slate. As shown in this example, one characteristic of slate foliation is the way in which these parallel lines appear in the rock. Because of the low temperatures and pressure, we can label this type of foliated metamorphic rock as low-grade. This simply means it is too hard to view the minerals and foliation with the naked eye.

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