Which Process Can Change Metamorphic Rock Into New Metamorphic Rock?

10.3 Classification of Metamorphic Rocks

Metamorphic rocks are broadly classified as foliated or non-foliated. Non-foliated metamorphic rocks do non have aligned mineral crystals. Not-foliated rocks form when pressure is uniform, or most the surface where pressure is very low. They tin can also form when the parent rock consists of blocky minerals such as quartz and calcite, in which individual crystals do not marshal considering they aren't longer in whatever one dimension. This distinction breaks down in zones of intense deformation, where fifty-fifty minerals like quartz can be squeezed into long stringers, much like squeezing toothpaste out of a tube (Figure ten.xiii).

**Figure 10.xiii** Rocks from the Western Carpathians mount range without deformation (left) and after deformation (right). Scale bar: 1 mm. Left- An undeformed granitic rock containing the mica mineral biotite (Bt), plagioclase feldspar (Pl), potassium feldspar (Kfs), and quartz (Qtz). Right- A metamorphic rock (mylonite) resulting from extreme deformation of granitic rocks. Quartz crystals have been flattened and deformed. The other minerals have been crushed and deformed into a fine-grained matrix (Mtx). *Source: Farkašovský et al. (2016) CC BY-NC-ND. Click the prototype to view the original figure captions and access the full text.*

Types of Foliated Metamorphic Rocks

Four common types of foliated metamorphic rocks, listed in order of metamorphic class or intensity of metamorphism are slate, phyllite, schist (pronounced "shist"), and gneiss (pronounced "nice"). Each of these has a characteristic type of foliation

Slate

Slate (Figure 10.14) forms from the low-grade metamorphism of shale. Slate has microscopic dirt and mica crystals that take grown perpendicular to the maximum stress direction. Slate tends to suspension into flat sheets or plates, a holding described as slaty cleavage.

Phyllite

Phyllite (Figure 10.15) is similar to slate, but has typically been heated to a higher temperature. Every bit a effect, the micas have grown larger. They still are not visible as individual crystals, merely the larger size leads to a satiny sheen on the surface. The cleavage of phyllite is slightly wavy compared to that of slate.

Schist

Schist (Effigy 10.16) forms at higher temperatures and pressures and exhibits mica crystals that are large plenty to encounter without magnification. Individual crystal faces may wink when the sample is turned in the light, making the stone appear to sparkle. Other minerals such every bit garnet might also exist visible, merely information technology is non unusual to find that schist consists predominantly of a unmarried mineral.

Gneiss

Gneiss (Figure 10.17) forms at the highest pressures and temperatures, and has crystals big enough to see with the unaided eye. Gneiss features minerals that accept separated into bands of unlike colours. The bands of colours are what define foliation within gneiss. Sometimes the bands are very obvious and continuous (Effigy 10.17, upper correct), but sometimes they are more similar lenses (upper left). Nighttime bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica considering it forms at temperatures higher than those under which micas are stable.

While slate and phyllite typically form only from mudrock protoliths, schist and particularly 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 ground of important minerals that are present: a schist derived from basalt is typically rich in the mineral chlorite, then nosotros call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or merely a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or amphibolite (Figure 10.18).

**Effigy 10.18** Amphibolite in thin department (2mm field of view), derived from metamorphism of a mafic igneous stone. Greenish crystals are the amphibole hornblende, and colourless crystals are plagioclase feldspar. Note horizontal crystal alignment. *Source: D.J. Waters, University of Oxford view source/ view context. Click the paradigm for original figure caption and terms of apply.*

Types of Non-foliated Metamorphic Rocks

Metamorphic rocks that grade under low-force per unit area atmospheric condition or under the effects confining pressure level, which is equal in all directions, practise not get foliated. In most cases, this is because they are non buried deeply plenty, and the heat for the metamorphism comes from a trunk 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

Marble (Figure 10.19) is metamorphosed limestone. When it forms, the calcite crystals recrystallize (re-grade into larger blocky calcite crystals), and whatsoever sedimentary textures and fossils that might take been nowadays are destroyed. If the original limestone is pure calcite, then the marble will exist white. On the other hand, if information technology has impurities such every bit clay, silica, or magnesium, the marble could exist "marbled" in advent (Figure 10.nineteen, bottom).

Quartzite

Quartzite (Figure 10.twenty) is metamorphosed sandstone. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with boosted silica. Sandstone oftentimes contains some dirt minerals, feldspar or lithic fragments, so quartzite tin too incorporate impurities.

Even if formed under directed force per unit area, quartzite is generally not foliated because quartz crystals do not normally 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.

Hornfels

Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks similar mudstone or volcanic rocks. Hornfels have dissimilar elongated or platy minerals (eastward.yard., micas, pyroxene, amphibole, and others) depending on the exact conditions and the parent rock, even so because the pressure wasn't substantially higher in whatsoever item direction, these crystals remain randomly oriented.

The hornfels in Figure ten.21 (left) appears to have gneiss-similar bands, but these actually reflect the beds of alternate sandstone and shale that were in the protolith. They are not related to alignment of crystals due to metamorphism. On the right of Figure 10.21 is a microscopic view of another sample of hornfels, also from a sedimentary protolith. The dark ring at the summit is from the original bedding. Here you can run into that the brown mica crystals (biotite) are not aligned.

**Effigy x.21** Hornfels, a non-foliated metamorphic rock formed from a fine-grained protolith. Left- Hornfels from the Novosibirsk region of Russia from a sedimentary protolith. Dark and low-cal bands preserve the bedding of the original sedimentary rock. The rock has been recrystallized during contact metamorphism and does not brandish foliation. (scale in cm). Correct- Hornfels in sparse section from a sedimentary protolith. Annotation that the brownish mica crystals are not aligned. The nighttime band at the top reflects the layering inside the sedimentary parent rock, similar to the mode those layers are preserved in the sample on the left. *Source: Left- Fedor (2006) Public Domain view source; Right- D.J. Waters, University of Oxford view source/ view context. Click the image for terms of utilize.*

What Happens When Different Rocks Undergo Metamorphism?

The nature of the parent stone controls the types of metamorphic rocks that can form from information technology nether differing metamorphic conditions (temperature, pressure, fluids). The kinds of rocks that tin exist expected to course at different metamorphic grades from diverse parent rocks are listed in Table 10.1.

*Source: Karla Panchuk (2018) CC Past 4.0, modified afterwards Steven Earle (2015) CC Past 4.0 view source. Click the table for a text version.*

Some rocks, such as granite, practice non modify much at the lower metamorphic grades because their minerals are notwithstanding stable up to several hundred degrees. Sandstone and limestone don't change much either because their metamorphic forms (quartzite and marble, respectively) take the same mineral composition, but re-formed larger crystals.

On the other mitt, some rocks can change essentially. Mudrock (e.thousand., shale, mudstone) can kickoff out as slate, then progress through phyllite, schist, and gneiss, with a variety of different minerals forming along the fashion. Schist and gneiss can likewise course from sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Migmatite: Both Metamorphic and Igneous

If a metamorphic rock is heated plenty, it can begin to undergo partial melting in the same fashion that igneous rocks exercise. The more felsic minerals (feldspar, quartz) will melt, while the more than mafic minerals (biotite, hornblende) practice not. When the melt crystallizes once again, the result is lite-coloured igneous rock interspersed with dark-coloured metamorphic rock. This mixed stone is chosen migmatite (Effigy x.22). Annotation that the foliation present in the metamorphic rock is no longer present in the igneous rock. Liquids cannot support a differential stress, and then when the melt crystallizes, the foliation is gone.

**Effigy 10.22** Migmatite photographed nigh Geirangerfjord in Norway. *Source: Siim Sepp (2006) CC Past-SA three.0 view source*

A fascinating feature of migmatites is ptygmatic (pronounced "tigmatic") folding. These are folds expect like they should be impossible considering they are enveloped by rock which does not brandish the same complex deformation (Figure 10.23). How could those wiggly folds get in there without the residual of the stone being folded in the aforementioned way?

**Effigy 10.23** Ptygmatic folding from Cleaved Hill, New Due south Wales, Australia. Ptygmatic folding happens when a potent layer within a rock is surrounded by weaker layers. Folding causes the stiff layer to crinkle while the weaker layers deform around information technology. *Source: Roberto Weinberg (http://users.monash.edu.au/~weinberg) view source. Click the image for terms of use.*

The respond to the ptygmatic fold mystery is that the folded layer is much stiffer than the surrounding layers. When squeezing forces human action on the rock, the potent layer buckles but the surrounding rock flows rather than buckling, because it isn't potent enough to buckle.

Which metamorphic stone 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 1 another.
A very hard stone with a granular appearance and a burnished 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.

References

Farkašovský, R., Bónová, K., & Košuth, M. (2016). Microstructural, modal and geochemical changes equally a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia). Geologos 22(three), 171-190. doi: 10.1515/logos-2016-0019 View full text

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