Chapter 7 Metamorphism and Metamorphic Rocks
7.2 Classification of Metamorphic Rocks
There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all. Some types of metamorphic rocks, such as quartzite and marble, which can form whether there is directed-pressure or not, do not typically exhibit foliation because their minerals (quartz and calcite respectively) do not tend to show alignment (see Figure 7.2.8).
When a rock is squeezed under directed pressure during metamorphism it is likely to be deformed, and this can result in a textural change such that the minerals appear elongated in the direction perpendicular to the main stress (Figure 7.2.1). This contributes to the formation of foliation.
When a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, there is a strong tendency for new minerals to grow with their long axes perpendicular to the direction of squeezing. This is illustrated in Figure 7.2.2, where the parent rock is shale, with bedding as shown. After both heating and squeezing, new minerals have formed within the rock, generally parallel to each other, and the original bedding has been largely obliterated.
Figure 7.2.3 shows an example of this effect. This large boulder has bedding visible as dark and light bands sloping steeply down to the right. The rock also has a strong slaty foliation, which is horizontal in this view (parallel to the surface that the person is sitting on), and has developed because the rock was being squeezed during metamorphism. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation.
Squeezing and heating alone (as shown in Figure 7.2.1) can contribute to foliation, but most foliation develops when new minerals are formed and are forced to grow perpendicular to the direction of greatest stress (Figure 7.2.2). This effect is especially strong if the new minerals are platy like mica or elongated like amphibole. The mineral crystals don’t have to be large to produce foliation. Slate, for example, is characterized by aligned flakes of mica that are too small to see.
The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are: slate, phyllite, schist, and gneiss (Figure 7.2.4). As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the