7.2 category of Metamorphic Rocks

There space two main types of metamorphic rocks: those that are foliated due to the fact that they have created in an setting with one of two people directed push or shear stress, and also those that are not foliated since they have formed in an setting without directed push or relatively near the surface ar with very small pressure in ~ all. Some varieties of metamorphic rocks, such as quartzite and marble, i beg your pardon also kind in directed-pressure situations, carry out not have to exhibit foliation since their mineral (quartz and calcite respectively) perform not have tendency 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 also this can result in a textural change 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 impacts of squeezing during metamorphism.

When a rock is both heated and also squeezed during metamorphism, and also the temperature adjust is sufficient for new minerals to type from present ones, there is a likelihood the the brand-new minerals will certainly be forced to thrive with their lengthy axes perpendicular come the direction that squeezing. This is illustrated in number 7.6, wherein the parent rock is shale, through bedding as shown. After both heating and also squeezing, brand-new minerals have created within the rock, usually parallel to every other, and also the original bedding has actually been greatly obliterated.

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Figure 7.6 The textural effects of squeezing and aligned mineral growth during metamorphism. The left-hand diagram represents shale through bedding in the direction shown. The right-hand diagram represents schist (derived from the shale), through the mica crystals orientated perpendicular come the key stress direction and also the original bedding no longer quickly visible.

Figure 7.7 shows an example of this effect. This big boulder has actually bedding quiet visible as dark and also light bands sloping steeply down to the right. The rock also has a strong slaty foliation, which is horizontal in this view, and also has occurred because the rock to be being squeezed during metamorphism. The rock has separation from bedrock follow me this foliation plane, and you have the right to see that other weaknesses are current in the exact same orientation.

Squeezing and heating alone (as presented in number 7.5) and also squeezing, heating, and formation of new minerals (as shown in number 7.6) can contribute to foliation, but most foliation establishes when new minerals are forced to grow perpendicular come the direction of biggest stress (Figure 7.6). This result is especially strong if the new minerals room platy prefer mica or elongated like amphibole. The mineral crystals don’t need to be big to develop foliation. Slate, for example, is identified by to adjust flakes the mica that are too small to see.

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Figure 7.7 A slate boulder on the next of Mt. Wapta in the Rockies close to Field, BC. Bedding is visible as light and also dark bands sloping steeply to the right. Slaty cleavage is evident from the means the absent has broken and also from currently of weak that exact same trend.

The various varieties of foliated metamorphic rocks, noted in stimulate of the grade or intensity of metamorphism and the form of foliation are slate, phyllite, schist, and gneiss (Figure 7.8). As already noted, slate is created from the low-grade metamorphism that shale, and also has microscope clay and mica crystals that have grown perpendicular come the stress. Slate tends to break into level sheets. Phyllite is similar to slate, but has commonly been heated come a greater temperature; the micas have grown larger and are visible as a sheen on the surface. Wherein slate is frequently planar, phyllite can kind in wavy layers. In the development of schist, the temperature has been hot enough so the individual mica crystals room visible, and other mineral crystals, such as quartz, feldspar, or garnet may additionally be visible. In gneiss, the mineral may have separated into bands of various colours. In the example presented in figure 7.8d, the dark bands are greatly amphibole when the light-coloured bands are feldspar and quartz. Many gneiss has tiny or no mica due to the fact that it forms at temperatures higher than those under i beg your pardon micas are stable. Unlike slate and also phyllite, which typically only type from mudrock, schist, and especially gneiss, can kind from a range of parent rocks, consisting of mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Schist and also gneiss deserve to be called on the communication of essential minerals that room present. For example a schist obtained from basalt is typically rich in the mineral chlorite, so we contact 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 might be mica-garnet schist. Similarly, a gneiss that originated as basalt and also is overcame by amphibole, is an amphibole gneiss or, much more accurately, one amphibolite.

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Figure 7.8 examples of foliated metamorphic rocks

If a absent is buried to a great depth and also encounters temperature that are close to its melt point, that will partially melt. The result rock, which contains both metamorphosed and igneous material, is well-known as a migmatite (Figure 7.9).

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Figure 7.9 Migmatite from Prague, Czech Republic

As currently noted, the nature that the parent rock controls the varieties of metamorphic rocks the can form from it under differing metamorphic conditions. The type of rocks that have the right to be supposed to form at different metamorphic grades from various parent rocks are detailed in Table 7.1. Some rocks, such as granite, carry out not change much at the reduced metamorphic grades due to the fact that their minerals space still stable up to several hundred degrees.

Table 7.1 A rough overview to the types of metamorphic rocks that type from different parent rocks at various 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 form under either low-pressure problems or just confining pressure execute not come to be foliated. In many cases, this is because they are not hidden deeply, and the warm for the metamorphism comes from a human body of magma that has moved into the upper component of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks space marble, quartzite, and also hornfels.

Marble is metamorphosed limestone. When it forms, the calcite crystals have tendency to flourish larger, and any sedimentary textures and fossils that can have been current are destroyed. If the initial limestone was pure calcite, then the marble will most likely be white (as in figure 7.10), yet if it had actually various impurities, such together clay, silica, or magnesium, the marble could be “marbled” in appearance.

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

Quartzite is metamorphosed sandstone (Figure 7.11). That is conquered by quartz, and in many cases, the original quartz seed of the sandstone are welded with each other with additional silica. Most sandstone includes some clay minerals and may likewise include other minerals such as feldspar or pieces of rock, so most quartzite has actually some impurities through the quartz.

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Figure 7.11 Quartzite native the Rocky Mountains, discovered in the Bow river at Cochrane, Alberta

Even if formed during regional metamorphism, quartzite walk not tend to be foliated since quartz crystals nothing align v the directional pressure. ~ above the other hand, any clay current in the original sandstone is most likely to be converted to mica throughout metamorphism, and any such mica is likely to align with the directional pressure. An instance of this is presented in figure 7.12. The quartz crystals show no alignment, however the micas room all aligned, indicating the there to be directional press during local metamorphism that this rock.

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Figure 7.12 magnified thin section of quartzite in polarized light. The irregular-shaped white, grey, and black crystals room all quartz. The small, thin, brightly coloured crystals room mica. This absent is foliated, also though it could not show up to be if examined there is no a microscope, and also so it have to have created under directed-pressure conditions.

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Hornfels is an additional non-foliated metamorphic rock that normally forms during contact metamorphism that fine-grained rocks like mudstone or volcanic rock (Figure 7.13). In part cases, hornfels has visible crystals of minerals like biotite or andalusite. If the hornfels created in a situation without command pressure, then these minerals would certainly be randomly orientated, not foliated together they would be if developed with command pressure.

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