Incredible moment Anak Krakatau erupts, Oct Download Google Earth For Free. Remote Sensing Downloader. Thunder Egg. Home plate tectonics Contact Metamorphism Vs. Regional Metamorphism. Share on Facebook. Contact Metamorphism Vs.
Regional Metamorphism Contact Metamorphism Vs. Recommended For You Mantle plumes crack continents. Sea levels influence eruptions on volcanic island. Burial Metamorphism When sedimentary rocks are buried to depths of several kilometers, temperatures greater than o C may develop in the absence of differential stress.
New minerals grow, but the rock does not appear to be metamorphosed. The main minerals produced are often the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.
Shock Metamorphism Impact Metamorphism When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock. These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO 2 polymorphs coesite and stishovite.
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. 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.
Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels. Marble is metamorphosed limestone.
When it forms, the calcite crystals tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white as in Figure 7. Quartzite is metamorphosed sandstone Figure 7.
It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz. 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.
An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock.
Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7. Metamorphism typically occurs between diagenesis max. There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all.
Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust.
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