7.2 category of Metamorphic Rocks

There are two main varieties of metamorphic rocks: those that room foliated since they have developed in an environment with either directed press or shear stress, and also those that room not foliated due to the fact that they have created in an environment without directed pressure or fairly near the surface ar with very little pressure at all. Some types of metamorphic rocks, such as quartzite and also marble, i beg your pardon also kind in directed-pressure situations, perform not necessarily exhibit foliation due to the fact that their minerals (quartz and also calcite respectively) carry out not often tend to display alignment (see number 7.12).

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When a absent is squeezed under command pressure during metamorphism the is likely to be deformed, and this can result in a textural adjust such the the minerals are elongated in the direction perpendicular to the main stress (Figure 7.5). This contributes to the development of foliation.

Figure 7.5 The textural impacts of squeezing throughout metamorphism.

When a rock is both heated and also squeezed during metamorphism, and also the temperature adjust is sufficient for brand-new minerals to form from currently ones, over there is a likelihood the the brand-new minerals will be forced to thrive with their long axes perpendicular come the direction of squeezing. This is depicted in number 7.6, wherein the parental rock is shale, with bedding as shown. ~ both heating and also squeezing, brand-new minerals have created within the rock, normally parallel to each other, and also the initial bedding has actually been greatly obliterated.

Figure 7.6 The textural impacts of squeezing and aligned mineral growth during metamorphism. The left-hand diagram represents shale with bedding in the direction shown. The right-hand diagram to represent schist (derived from that shale), through the mica crystals orientated perpendicular come the main stress direction and the initial bedding no longer quickly visible.

Figure 7.7 shows an instance of this effect. This huge boulder has bedding quiet visible as dark and light bands sloping steeply down to the right. The rock additionally has a solid slaty foliation, i m sorry is horizontal in this view, and also has occurred because the rock was being squeezed throughout metamorphism. The absent has break-up from bedrock along this foliation plane, and you have the right to see that other weaknesses are current in the same orientation.

Squeezing and also heating alone (as displayed in figure 7.5) and also squeezing, heating, and also formation of new minerals (as presented in number 7.6) can contribute to foliation, but most foliation develops when new minerals are forced to flourish perpendicular to the direction of best stress (Figure 7.6). This impact is especially strong if the brand-new minerals space platy favor mica or elongated favor amphibole. The mineral crystals don’t need to be huge to create foliation. Slate, because that example, is defined by aligned flakes the mica that are too little to see.

Figure 7.7 A slate boulder top top the side of Mt. Wapta in the Rockies close to Field, BC. Bedding is visible together light and dark bands sloping steeply come the right. Slaty cleavage is noticeable from the way the absent has broken and likewise from currently of weak that very same trend.

The various varieties of foliated metamorphic rocks, provided in bespeak of the grade or strongness of metamorphism and also the form of foliation room slate, phyllite, schist, and gneiss (Figure 7.8). As already noted, slate is developed from the low-grade metamorphism that shale, and has microscope clay and also mica crystals that have grown perpendicular to the stress. Slate often tends to rest into flat sheets. Phyllite is similar to slate, but has frequently been heated come a greater temperature; the micas have actually grown larger and also are visible together a sheen top top the surface. Whereby slate is commonly planar, phyllite can form in wavy layers. In the formation of schist, the temperature has actually been hot sufficient so the individual mica crystals space visible, and also other mineral crystals, such as quartz, feldspar, or garnet may additionally be visible. In gneiss, the mineral may have actually separated right into bands of different colours. In the example displayed in figure 7.8d, the dark bands are mainly amphibole if the light-coloured bands space feldspar and also quartz. Most gneiss has tiny or no mica since it develops at temperatures higher than those under which micas space stable. Unequal slate and phyllite, which generally only form from mudrock, schist, and especially gneiss, can form from a range of parent rocks, including mudrock, sandstone, conglomerate, and a selection of both volcanic and also intrusive igneous rocks.

Schist and also gneiss can be called on the basis of crucial minerals that are present. For instance a schist acquired from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One acquired from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it could be mica-garnet schist. Similarly, a gneiss the originated as basalt and also is dominated by amphibole, is an amphibole gneiss or, an ext accurately, an amphibolite.

Figure 7.8 examples of foliated metamorphic rocks

If a rock is hidden to a great depth and also encounters temperature that space close to its melting point, that will partly melt. The resulting rock, which consists of both metamorphosed and also igneous material, is well-known as a migmatite (Figure 7.9).

Figure 7.9 Migmatite indigenous Prague, Czech Republic

As currently noted, the nature of the parent rock controls the types of metamorphic rocks the can form from the under differing metamorphic conditions. The kinds of rocks that have the right to be intended to type at different metamorphic qualities from miscellaneous parent rocks are detailed in Table 7.1. Some rocks, such as granite, do not readjust much at the reduced metamorphic grades due to the fact that their minerals space still stable up to number of hundred degrees.

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

Metamorphic rocks that kind under either low-pressure problems or just confining pressure perform not come to be foliated. In many cases, this is since they space not buried deeply, and the warm for the metamorphism comes from a body of magma that has actually moved into the upper part of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks room marble, quartzite, and hornfels.

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

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

Quartzite is metamorphosed sandstone (Figure 7.11). That is dominated by quartz, and also in numerous cases, the initial quartz seed of the sandstone are welded together with extr silica. Many sandstone consists of some clay minerals and may likewise include other minerals such together feldspar or pieces of rock, so many quartzite has some impurities with the quartz.

Figure 7.11 Quartzite native the Rocky Mountains, found in the Bow flow at Cochrane, Alberta

Even if formed throughout regional metamorphism, quartzite walk not tend to it is in foliated due to the fact that quartz crystals don’t align through the directional pressure. On the other hand, any type of clay present in the original sandstone is most likely to be convert to mica throughout metamorphism, and also any together mica is likely to align with the directional pressure. An example of this is presented in number 7.12. The quartz crystals show no alignment, however the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock.

Figure 7.12 amplified thin ar of quartzite in polarized light. The irregular-shaped white, grey, and also black crystals are all quartz. The small, thin, brightly coloured crystals room mica. This absent is foliated, even though it could not appear to it is in 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 another non-foliated metamorphic rock that normally forms during call metamorphism the fine-grained rocks favor mudstone or volcanic absent (Figure 7.13). In some cases, hornfels has actually visible crystals that minerals like biotite or andalusite. If the hornfels created in a case without command pressure, then these minerals would certainly be randomly orientated, no foliated together they would certainly be if developed with command pressure.

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