7.2 classification of Metamorphic Rocks

There are two main species of metamorphic rocks: those that space foliated since they have created in an setting with one of two people directed press or shear stress, and those that are not foliated since they have created in an environment without directed pressure or reasonably near the surface ar with very tiny pressure in ~ all. Some species of metamorphic rocks, such as quartzite and marble, i beg your pardon also kind in directed-pressure situations, perform not necessarily exhibit foliation because their minerals (quartz and calcite respectively) perform not tend to show alignment (see number 7.12).

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When a rock is squeezed under directed pressure throughout metamorphism it is most likely to be deformed, and this can result in a textural readjust such that the minerals are elongated in the direction perpendicular come the key stress (Figure 7.5). This contributes come the formation of foliation.

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Figure 7.5 The textural effects of squeezing during metamorphism.

When a rock is both heated and squeezed throughout metamorphism, and also the temperature readjust is enough for new minerals to type from currently ones, there is a likelihood the the brand-new minerals will certainly be compelled to thrive with their long axes perpendicular come the direction the squeezing. This is illustrated in number 7.6, whereby the parental rock is shale, with bedding together shown. After both heating and squeezing, new minerals have formed within the rock, generally parallel to every other, and also the initial bedding has actually been largely obliterated.

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Figure 7.6 The textural results of squeezing and also aligned mineral growth during metamorphism. The left-hand diagram to represent shale v bedding in the direction shown. The right-hand diagram to represent schist (derived from the shale), through the mica crystals orientated perpendicular to the key stress direction and the initial bedding no longer easily visible.

Figure 7.7 shows an example of this effect. This big boulder has bedding still visible together dark and also light bands sloping steeply under to the right. The rock also has a strong slaty foliation, i beg your pardon is horizontal in this view, and also has developed because the rock was being squeezed throughout metamorphism. The rock has separation from bedrock along this foliation plane, and you have the right to see that other weaknesses are present in the exact same orientation.

Squeezing and heating alone (as presented in number 7.5) and also squeezing, heating, and also formation of brand-new minerals (as presented in number 7.6) can contribute to foliation, but most foliation establishes when brand-new minerals are compelled to grow perpendicular come the direction of greatest stress (Figure 7.6). This impact is especially solid if the new minerals are platy favor mica or elongated favor amphibole. The mineral crystals don’t need to be large to develop foliation. Slate, for example, is characterized by aligned flakes the mica that are too little to see.

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Figure 7.7 A slate boulder top top the next of Mt. Wapta in the Rockies near Field, BC. Bedding is visible together light and also dark bands sloping steeply to the right. Slaty cleavage is apparent from the way the absent has broken and likewise from lines of weakness that very same trend.

The various varieties of foliated metamorphic rocks, provided in bespeak of the grade or soot of metamorphism and also the type of foliation are slate, phyllite, schist, and gneiss (Figure 7.8). As currently noted, slate is developed from the low-grade metamorphism of shale, and also has microscope clay and mica crystals that have grown perpendicular come the stress. Slate often tends to rest into level sheets. Phyllite is comparable to slate, yet has typically been heated to a higher temperature; the micas have grown larger and are visible together a sheen top top the surface. Whereby slate is commonly planar, phyllite can type in wavy layers. In the formation of schist, the temperature has actually been hot enough so the individual mica crystals are visible, and other mineral crystals, such together quartz, feldspar, or garnet may additionally be visible. In gneiss, the minerals may have separated right into bands of various colours. In the example presented in number 7.8d, the dark bands are largely amphibole while the light-coloured bands are feldspar and also quartz. Many gneiss has little or no mica due to the fact that it creates at temperatures higher than those under which micas are stable. Uneven slate and also phyllite, which frequently only kind from mudrock, schist, and especially gneiss, can kind from a range of parental rocks, including mudrock, sandstone, conglomerate, and also a selection of both volcanic and also intrusive igneous rocks.

Schist and gneiss can be called on the communication of important minerals that space present. For example a schist acquired from basalt is generally rich in the mineral chlorite, therefore we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or simply a mica schist, or if there are garnets current it could be mica-garnet schist. Similarly, a gneiss that originated as basalt and is overcame by amphibole, is an amphibole gneiss or, more accurately, one amphibolite.

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Figure 7.8 examples of foliated metamorphic rocks

If a absent is buried to a good depth and encounters temperature that room close come its melt point, it will partially melt. The resulting rock, which includes both metamorphosed and also igneous material, is well-known as a migmatite (Figure 7.9).

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Figure 7.9 Migmatite indigenous Prague, Czech Republic

As already noted, the nature the the parental rock controls the types of metamorphic rocks that can kind from it under different metamorphic conditions. The kinds of rocks that deserve to be meant to type at various metamorphic qualities from various parent rocks are noted in Table 7.1. Some rocks, such together granite, execute not adjust much in ~ the reduced metamorphic grades due to the fact that their minerals space still secure up to several hundred degrees.

Table 7.1 A rough guide to the types of metamorphic rocks that type from different parent rocks at different grades of local metamorphismVery low GradeLow GradeMedium GradeHigh GradeApproximate Temperature RangesParent RockMudrockGraniteBasaltSandstoneLimestone
150-300°C300-450°C450-550°CAbove 550°C
slatephylliteschistgneiss
no changeno changeno changegranite gneiss
chlorite schistchlorite schistamphiboliteamphibolite
no changelittle changequartzitequartzite
little changemarblemarblemarble

Metamorphic rocks that type under one of two people low-pressure problems or simply confining pressure execute not come to be foliated. In most cases, this is since they are not buried deeply, and the heat for the metamorphism originates from a human body of magma that has moved right into the upper component of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels.

Marble is metamorphosed limestone. Once it forms, the calcite crystals have tendency to thrive larger, and any sedimentary textures and fossils that can have been current are destroyed. If the original limestone to be pure calcite, climate the marble will likely be white (as in number 7.10), but if it had actually various impurities, such together clay, silica, or magnesium, the marble can be “marbled” in appearance.

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Figure 7.10 Marble through visible calcite crystals (left) and an outcrop of banded marble (right)

Quartzite is metamorphosed sandstone (Figure 7.11). That is overcame by quartz, and in countless cases, the initial quartz grains of the sandstone room welded together with additional silica. Most sandstone includes some clay minerals and also may additionally include various other minerals such as feldspar or pieces of rock, so many quartzite has some impurities v the quartz.

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Figure 7.11 Quartzite from the Rocky Mountains, discovered in the Bow flow at Cochrane, Alberta

Even if formed during regional metamorphism, quartzite walk not have tendency to it is in foliated since quartz crystals don’t align through the directional pressure. Top top the other hand, any clay present in the initial sandstone is most likely to be convert to mica throughout metamorphism, and any such mica is most likely to align with the directional pressure. An example of this is shown in figure 7.12. The quartz crystals present no alignment, yet the micas space all aligned, indicating that there to be directional press during regional metamorphism of this rock.

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Figure 7.12 intensified thin ar 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 rock is foliated, also though it could not appear to be if examined without a microscope, and so it should have created under directed-pressure conditions.

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Hornfels is one more non-foliated metamorphic rock that normally creates during call metamorphism of fine-grained rocks choose mudstone or volcanic absent (Figure 7.13). In part cases, hornfels has actually visible crystals that minerals prefer biotite or andalusite. If the hornfels developed in a case without command pressure, climate these minerals would certainly be randomly orientated, not foliated together they would certainly be if developed with command pressure.

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Figure 7.13 Hornfels from the Novosibirsk an ar of Russia. The dark and light bands space bedding. The rock has actually been recrystallized during contact metamorphism and also does not screen foliation. (scale in cm)