There are two main types of metamorphic rocks. Foliated metamorphic rocks form under directed pressure (pressure that is not the same in all directions) or shear stress (forces acting parallel to each other, and having the result of “smearing” the rock). Non-foliated rocks form where pressure is uniform, or near the surface with very little pressure at all. Non-foliated metamorphic rocks also form when the parent rock consists of blocky minerals (e.g., quartz, calcite) which can’t be aligned because they aren’t longer in any one dimension.
How Foliation Develops
When a rock is squeezed under directed pressure during metamorphism, this can change the texture such that minerals are elongated in the direction perpendicular to the main stress (Figure 10.5).
When a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, the new minerals can be forced to grow with their long axes perpendicular to the direction of squeezing. This is illustrated in Figure 10.6. On the left, bedding is represented by the diagonal lines. After both heating and squeezing, new minerals have formed within the rock (right), generally parallel to each other. The original bedding is largely obliterated.
Figure 10.7 shows an example of this effect. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. The rock also has a strong foliation, which is horizontal in this view, 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.
Planes of weakness within a rock that are caused by foliation are referred to as rock cleavage, or just cleavage. This is distinct from cleavage in minerals because mineral cleavage happens between atoms, but rock cleavage happens between minerals.
Most foliation develops when new minerals are forced to grow perpendicular to the direction of greatest stress (Figure 10.6). 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.
Types of Foliated Metamorphic Rocks
The types of foliated metamorphic rocks, listed in order of metamorphic grade or intensity of metamorphism are slate, phyllite, schist, and gneiss (Figure 10.8). Each of these has a characteristic type of foliation
Slate forms from the low-grade metamorphism of shale. It has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to break into flat sheets, a property described as slaty cleavage.
Phyllite is similar to slate, but has typically been heated to a higher temperature. As a result, the micas have grown larger. They still are not visible as individual crystals, but the larger size leads to a satiny sheen on the surface. The cleavage of phyllite is slightly wavy compared to that of slate.
Schist forms at higher temperatures and pressures and has mica crystals which are large enough to see. Other minerals such as garnet might also be visible, but the rock consists predominantly of a single mineral. The cleavage of schist is wavier than that of phyllite.
Gneiss forms at the highest pressures and temperatures, has crystals large enough to see, and has minerals that have separated into bands of different colours. Sometimes the bands are very obvious and continuous, but sometimes they are more like lenses. In the example shown in Figure 10.8d, the dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable.
While slate and phyllite typically form only from mudrock, schist and especially gneiss can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.
Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or amphibolite (Figure 10.9).
Types of Non-foliated Metamorphic Rocks
Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. Metamorphism that happens because of proximity to magma is called contact metamorphism. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels.
Marble is metamorphosed limestone. When it forms, the calcite crystals recrystallize (re-form into larger blocky calcite crystals), and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone is pure calcite, then the marble will be white (as in Figure 10.10). On the other hand, if it has impurities such as clay, silica, or magnesium, the marble could be “marbled” in appearance.
Quartzite is metamorphosed sandstone (Figure 10.11). It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.
Even if formed under directed pressure, quartzite is not foliated because quartz crystals don’t align with the directional pressure. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure. An example of this is shown in Figure 10.12. The quartz crystals (white, grey, and black) show no alignment, but the micas (colourful needle-shaped crystals) are all aligned, indicating that there was directed pressure (in the direction of the white arrows) during metamorphism of this rock.
Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals (e.g., micas, pyroxene, amphibole, and others) depending on the exact conditions and the parent rock, yet because the pressure wasn’t substantially higher in any particular direction, these crystals remain randomly oriented. The hornfels in Figure 10.13 (left) appears to have gneiss-like bands, however these actually reflect the beds of alternating sandstone and shale that were the protolith for the hornfels. They are not related to alignment of crystals due to metamorphism. On the right of Figure 10.13 is a microscopic view of another sample of hornfels, also from a sedimentary protolith. The dark band at the top is from the original bedding. Here you can see that the brown mica crystals are not aligned.
What Happens When Different Rocks Undergo Metamorphism?
As we’ve discussed, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions (temperature, pressure, fluids). The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 10.1.
Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Sandstone and limestone don’t change much either because their metamorphic forms (quartzite and marble, respectively) have the same mineral composition, but re-formed crystals.
On the other hand, some rocks can change substantially. Mudrock (e.g., shale, mudstone) can start out as slate, then progress through phyllite, schist, and gneiss. Schist and gneiss can also form from sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.
Migmatite: Not Quite Metamorphic, Not Quite Igneous
If a metamorphic rock is heated enough, it can begin to undergo partial melting in the same way that igneous rocks do. The more felsic minerals (feldspar, quartz) will melt, while the darker minerals (biotite, hornblende) do not. When the melt crystallizes again, the result is light-coloured igneous rock interspersed with dark metamorphic rock. This mixed rock is called migmatite (Figure 10.14).
A fascinating characteristic of migmatites is ptygmatic folding. These are folds which look like they should be impossible because they are enveloped by rock which does not display the same complex deformation (Figure 10.15). How could those wiggly folds get in there without the rest of the rock being folded in the same way?
The answer to the ptygmatic fold mystery is that the folded layer is much stiffer than the surrounding layers. When squeezing forces act on the rock, the stiff layer buckles but the surrounding rock flows rather than buckling because it isn’t strong enough. A close-up version of what this might look like under a microscope can be found in Kurt Hollocher’s book A Pictoral Guide to Metamorphic Rocks in the Field on p. 163, Figure 16.2A. Figure 10.16 below is a sketch of Figure 16.2A, and the original figure can be viewed at http://bit.ly/1KSogyY. In Figure 10.16 the green mineral crystals go from being aligned parallel to the axis of each wiggle in the grey folded layer, to being horizontal further away.
Exercise 10.2 Naming Metamorphic Rocks
Which metamorphic rock is described in each of the following?
- A rock with visible minerals of mica and with small crystals of andalusite. The mica crystals are consistently parallel to one another.
- A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.
- A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them.
- A rock that is dominated by aligned crystals of amphibole.