In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock. Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified.
Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. Those that are bounded by their own crystal faces are termed idioblastic. Those that show none of their own crystal faces are termed xenoblastic. From examination of metamorphic rocks, it has been found that metamorphic minerals can be listed in a generalized sequence, known as the crystalloblastic series , listing minerals in order of their tendency to be idioblastic.
In the series, each mineral tends to develop idioblastic surfaces against any mineral that occurs lower in the series. This series is listed below:. This series can, in a rather general way, enable us to determine the origin of a given rock. For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase.
Another aspect of the crystalloblastic series is that minerals high on the list tend to form porphyroblasts the metamorphic equivalent of phenocrysts , although K-feldspar a mineral that occurs lower in the list may also form porphyroblasts.
Porphyroblasts are often riddled with inclusions of other minerals that were enveloped during growth of the porphyroblast. These are said to have a poikioblastic texture. Most metamorphic textures involve foliation. Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate , if it has a phyllitic foliation, it is termed a phyllite , if it has a shistose foliation, it is termed a schist.
A rock that shows a banded texture without a distinct foliation is termed a gneiss. All of these could be porphyroblastic i. A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite , if the grain size is large and individual minerals can be easily distinguished with a hand lens.
Protolith Protolith refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith. As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith.
Bulk Chemical Composition The mineral assemblage that develops in a metamorphic rock is dependent on The pressure and temperature reached during metamorphism The composition of any fluid phase present during metamorphism, and The bulk chemical composition of the rock.
Just like in igneous rocks, minerals can only form if the necessary chemical constituents are present in the rock i. Based on the mineral assemblage present in the rock one can often estimate the approximate bulk chemical composition of the rock. Some terms that describe this general bulk chemical composition are as follows:. These are as follows:. In general, metamorphic rocks do not drastically change chemical composition during metamorphism, except in the special case where metasomatism is involved such as in the production of skarns, as discussed above.
The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism. Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.
This pressure and temperature environment is referred to as Metamorphic Facies. This is similar to the concept of sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition.
The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism. A high geothermal gradient such as the one labeled "A" , might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies. Under a normal to high geothermal gradient, such as "B", rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism or depth of burial increased.
Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels. Marble Figure When it forms, the calcite crystals recrystallize re-form into larger blocky calcite crystals , and any sedimentary textures and fossils that might have been present are destroyed.
If the original limestone is pure calcite, then the marble will be white. Quartzite Figure It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.
Even if formed under directed pressure, 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 is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals e.
The hornfels in Figure They are not related to alignment of crystals due to metamorphism. On the right of Figure The dark band at the top is from the original bedding. Here you can see that the brown mica crystals biotite are not aligned. The nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions temperature, pressure, fluids.
The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees.
On the other hand, some rocks can change substantially. Mudrock e. Schist and gneiss can also form from sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks. If a metamorphic rock is heated enough, it can begin to undergo partial melting in the same way that igneous rocks do. The more felsic minerals feldspar, quartz will melt, while the more mafic minerals biotite, hornblende do not.
When the melt crystallizes again, the result is light-coloured igneous rock interspersed with dark-coloured metamorphic rock. This mixed rock is called migmatite Figure Note that the foliation present in the metamorphic rock is no longer present in the igneous rock. Liquids cannot support a differential stress, so when the melt crystallizes, the foliation is gone.
These are folds look like they should be impossible because they are enveloped by rock which does not display the same complex deformation Figure Technical Announcements. Employees in the News. Emergency Management. Survey Manual. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors.
Conditions like these are found deep within the Earth or where tectonic plates meet. Process of Metamorphism: The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks.
Pressure or temperature can even change previously metamorphosed rocks into new types. Metamorphic rocks are often squished, smeared out, and folded. Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would become igneous rocks!
Common Metamorphic Rocks: Common metamorphic rocks include phyllite, schist, gneiss, quartzite and marble. Foliated Metamorphic Rocks: Some kinds of metamorphic rocks -- granite gneiss and biotite schist are two examples -- are strongly banded or foliated.
Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance. Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied. Non-Foliated Metamorphic Rocks: Non-foliated metamorphic rocks do not have a platy or sheet-like structure.
There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate. No matter how much pressure you apply, the grains will not align! Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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