7.2 classification of Metamorphic Rocks

There space two main types of metamorphic rocks: those that are foliated because they have developed in an environment with either directed pressure or shear stress, and also those that space not foliated because they have formed in an setting without directed pressure or reasonably near the surface with very little pressure in ~ all. Some varieties of metamorphic rocks, such as quartzite and marble, i m sorry can type whether there is directed-pressure or not, execute not typically exhibit foliation since their minerals (quartz and calcite respectively) perform not have tendency to display alignment (see number 7.2.8).

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When a rock is squeezed under command pressure during metamorphism it is most likely to it is in deformed, and this can result in a textural readjust such that the minerals appear elongated in the direction perpendicular come the main stress (Figure 7.2.1). This contributes to the formation of foliation.

Figure 7.2.1 The textural impacts of squeezing during metamorphism. In the original rock (left) there is no alignment the minerals. In the squeezed absent (right) the minerals have been elongated in the direction perpendicular to the squeezing.

When a absent is both heated and also squeezed during metamorphism, and also the temperature adjust is enough for brand-new minerals to form from currently ones, over there is a solid tendency for new minerals to flourish with their long axes perpendicular to the direction of squeezing. This is shown in figure 7.2.2, wherein the parent rock is shale, v bedding together shown. After ~ both heating and also squeezing, brand-new minerals have developed within the rock, typically parallel to every other, and also the original bedding has been mostly obliterated.

Figure 7.2.2 The textural impacts of squeezing and mineral growth during regional metamorphism. The left diagram is shale v bedding slanting down to the right. The best diagram to represent schist (derived from the shale), with mica crystals orientated perpendicular to the main stress direction and the initial bedding no longer quickly visible.

Figure 7.2.3 shows an instance of this effect. This huge boulder has actually bedding visible together dark and also light bands sloping steeply down to the right. The rock additionally has a solid slaty foliation, i beg your pardon is horizontal in this see (parallel come the surface ar that the person is sitting on), and also has developed because the rock to be being squeezed throughout metamorphism. The absent has split from bedrock follow me this foliation plane, and also you deserve to see that various other weaknesses are present in the same orientation.

Squeezing and heating alone (as shown in number 7.2.1) can add to foliation, yet most foliation creates when new minerals room formed and also are compelled to prosper perpendicular to the direction of best stress (Figure 7.2.2). This impact is especially solid if the new minerals are platy choose mica or elongated choose amphibole. The mineral crystals don’t have to be huge to produce foliation. Slate, for example, is characterized by aligned flakes of mica that room too tiny to see.

Figure 7.2.3 A slate boulder on the side of Mt. Wapta in the Rockies close to Field, BC. Bedding is visible together light and dark bands sloping steeply come the best (yellow arrows). Slaty cleavage is evident from the way the rock has broken (along the level surface that the person is sit on) and also from present of weak that room parallel to that very same trend (red arrows).

The various types of foliated metamorphic rocks, noted in bespeak of the grade or soot of metamorphism and the kind of foliation are: slate, phyllite, schist, and also gneiss (Figure 7.2.4). As currently noted, slate is created from the low-grade metamorphism the shale, and also has microscopic clay and also mica crystals that have actually grown perpendicular come the stress. Slate often tends to rest into level sheets. Phyllite is comparable to slate, however has frequently been heated come a greater temperature; the micas have grown larger and also are visible as a sheen top top the surface. Where slate is generally planar, phyllite can type in wavy layers. In the formation of schist, the temperature has been hot enough so the individual mica crystals are big enough to be visible, and also other mineral crystals, such as quartz, feldspar, or garnet may likewise be visible. In gneiss, the mineral may have actually separated into bands of various colours. In the example displayed in figure 7.2.4d, the dark bands are largely amphibole when the light-coloured bands are feldspar and also quartz. Many gneiss has small or no mica due to the fact that it forms at temperatures higher than those under i beg your pardon micas room stable. Unequal slate and phyllite, which commonly only kind from mudrock, schist, and also especially gneiss, can form from a selection of parental rocks, including mudrock, sandstone, conglomerate, and also a selection of both volcanic and intrusive igneous rocks.

Schist and gneiss have the right to be named on the basis of crucial minerals that space present. For example a schist acquired from basalt is commonly rich in the mineral chlorite, for this reason we speak to it chlorite schist. One acquired from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets current it can be mica-garnet schist. Similarly, a gneiss the originated as basalt and also is dominated by amphibole, is an amphibole gneiss or, much more accurately, an amphibolite.

Figure 7.2.4 examples of foliated metamorphic rocks: (A) Slate, (B) Phyllite, (C) Schist, (D) Gneiss.

If a absent is hidden to a an excellent depth and encounters temperature that are close to its melting point, it may partially melt. The resulting rock, which consists of both metamorphosed and also igneous material, is recognized as migmatite (Figure 7.2.5).

Figure 7.2.5 Migmatite native Prague, Czech Republic

As already noted, the nature that the parent rock controls the species of metamorphic rocks the can form from it under different metamorphic conditions. The type of rocks that have the right to be expected to form at various metamorphic grades from assorted parent rocks are provided in Table 7.1. Part rocks, such together granite, perform not readjust much in ~ the reduced metamorphic grades because their minerals are still stable up to number of hundred degrees.

Table 7.1 A rough guide to the types of metamorphic rocks that type from various parent rocks at various grades of regional metamorphism.Parent RockVery low Grade (150-300°C)Low grade (300-450°C)Medium grade (450-550°C)High class (Above 550°C)
Graniteno changeno changealmost no changegranite gneiss
Basaltchlorite schistchlorite schistamphiboliteamphibolite
Sandstoneno changelittle changequartzitequartzite
Limestonelittle changemarblemarblemarble

Metamorphic rocks that type under either low-pressure conditions or simply confining pressure execute not become foliated. In most cases, this is since they are not buried deeply, and also the warmth for the metamorphism originates from a human body of magma that has moved right into the upper part of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks room marble, quartzite, and also hornfels.

Marble is metamorphosed limestone. As soon as it forms, the calcite crystals tend to prosper larger, and also any sedimentary textures and also fossils that can have been present are destroyed. If the initial limestone to be pure calcite, climate the marble will likely be white (as in figure 7.2.6), however if it had various impurities, such together clay, silica, or magnesium, the marble might be “marbled” in appearance. Marble that develops during regional metamorphism—and in fact that consists of most marble—may or might not build a foliated texture, yet foliation is generally not basic to watch in marble.

Figure 7.2.6 Marble through visible calcite crystals (left) and also an outcrop the banded marble (right).

Quartzite is metamorphosed sandstone (Figure 7.2.7). It is conquered by quartz, and in numerous cases, the initial quartz seed of the sandstone space welded together with additional silica. Most sandstone includes some clay minerals and also may likewise include other minerals such as feldspar or pieces of rock, so most quartzite has actually some impurities with the quartz.

Figure 7.2.7 Quartzite native the Rocky Mountains, uncovered in the Bow flow at Cochrane, Alberta.

Even if formed throughout regional metamorphism, quartzite (like marble) does not tend to look at foliated due to the fact that quartz crystals nothing align through the directional pressure. On the other hand, any type of clay present in the original sandstone is likely to be convert to mica throughout metamorphism, and any such mica is most likely to align v the directional pressure. An instance of this is presented in figure 7.2.8. The quartz crystals display no alignment, however the micas space all aligned, indicating that there to be directional press during regional metamorphism that this rock. Since these micas are an extremely small, this rock would not show up to be foliated to the naked eye.

Figure 7.2.8 amplified thin section of quartzite in polarized light. The irregular-shaped white, grey, and black crystals are all quartz. The small, thin, brightly coloured crystals space mica. This absent is foliated, even though it might not appear to be if examined there is no a microscope, and also so it need to have developed under directed-pressure conditions.

Hornfels is an additional non-foliated metamorphic rock the normally forms during call metamorphism that fine-grained rocks prefer mudstone or volcanic rock (Figure 7.2.9). In some cases, hornfels has actually visible crystals that minerals choose biotite or andalusite. If the hornfels formed in a situation without directed pressure, climate these minerals would be randomly orientated, not aligned with one-another, together they would be if developed with command pressure.

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Figure 7.2.9 Hornfels native the Novosibirsk region of Russia. The dark and light bands room bedding. The rock has actually been recrystallized during contact metamorphism and does not display screen foliation. (scale in centimetres).