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Metamorphic Rocks Video
Chemical Changes
Characteristics that cause chemical changes in rocks also add to the formation of metamorphic rocks. Very hot liquids and gases can, under extreme pressures, fill the pores of existing rocks. These liquids and gases cause chemical reactions to occur, and over time, change the chemical composition of the existing rock. Metamorphism can take place instantly as in rock shearing at plate boundaries or can take millions of years as in the slow cooling of deeply buried magma.
It is important to remember that the changes that go on in metamorphism are mostly in rock texture. The chemical composition of metamorphic rock is altered very little. The basic changes that do occur include the addition or loss of water and carbon dioxide. The biggest changes of metamorphic transformation, then, have to do with the way minerals are rearranged.
A chemical shift in the composition of metamorphic rock can also be changed by the addition or removal of different elements. This can happen as a result of the intrusion of magma bringing new minerals into contact with existing rock. Sometimes this can be seen through color changes in minerals of the same basic chemical composition.
When hot, mineral-rich waters rise through magma, they carry a variety of elements. Some of these elements include sulfur, copper, sodium, potassium, silica, and zinc ions to name a few. These minerals come from magma and intruded rock, during the time that water is filtering upward through the crust. On this journey, they interact with other minerals and chemicals replacing some of their own minerals with others. This type of chemical interaction and substitution is called metasomatism. Metal deposits like copper and lead are formed in this way.
Index Minerals
A Scottish geologist, George Barrow, noticed that rocks having the same overall mineral make up (like shale) could be seen to go through a series of transformations throughout specific zones in a metamorphic region. He found that minerals in individual zones had specific mineral configurations. As he studied minerals across a zone, he found that when new metamorphic mineral configurations were created, it was predictable.
The first appearance of index minerals marks the boundary of lowto high-grade metamorphic rock changes in a specific regional zone.
Barrow found that mineral (shale) configuration changes happened with regard to index minerals. These index minerals acted like milestones in the low- to high-grade metamorphic rock transformation process. Barrow found that the domino effect of metamorphism happened in the following series:
chlorite -> biotite -> garnet -> staruolite -> kyanite -> sillimanite ->
Low grade->High grade
When Barrow and his team studied the geological maps of the Scottish Highlands, they were able to plot where certain minerals started and stopped. They marked the locations of certain minerals and called these connected places isograds.
An isograd is a marker line on a map connecting different areas of certain minerals found in metamorphic rock.
Metamorphic Rock Textures
Metamorphic rocks are divided into two categories, foliated and nonfoliated. Foliate comes from the Latin work folium (meaning leaf ) and describes thin mineral sheets, like pages in a book. Metamorphic minerals that align and form bands, like granite gneiss and biotite schist, are strongly banded or foliated.
When metamorphic mineral grains align parallel in the same plane and give rock a striped appearance, it is called foliation or foliated rock.
Initially, the weight of sedimentary rock strata keeps the sheet-like formation of minerals parallel to the bedding planes. As the mineral layers are buried deeper or compressed by tectonic stresses, however, folding and deformation take place. The sedimentary strata are shoved sideways and are no longer parallel to the original bedding. In fact, metamorphism changes the texture enough that when broken, the metamorphic rock breaks in the direction of the foliation not the original mineral’s composition.
Foliates are made up of large concentrations of mica and chlorite. These minerals have very clear-cut cleavage. Foliated metamorphic rocks split along cleavage lines that are parallel to the alignment of the rock’s minerals. For example, mica can be separated into thin, flat nearly transparent sheets.
Mica is said to have good schistosity, from the Latin word schistos meaning easily cleaved.
Schistosity is the parallel arrangement of coarse grains of sheetstructure minerals formed during metamorphism and increasing pressure.
For fine-grained rocks with microscopic mineral grains, the breakage property is known as rock cleavage or slaty cleavage.
Slaty cleavage is found in an environment of low temperature and pressure. In these less-intense conditions, grain sizes increase and single grains are easily seen. Foliation is present with slaty cleavage, but not in a flat plane. Intermediate and high-grade metamorphic rock commonly breaks along rolling, or somewhat distorted surfaces similar to the orientation of the grain of quartz, feldspar, and other minerals.
Rock cleavage or slaty cleavage describes the way rock breaks into plate-like pieces along flat planes.
Large crystal textures can also be formed in a fine-grained, support rock during metamorphism. When this happens, crystals found in both contact and regional metamorphic rock are called porphyroblasts. They grow as the elements are rearranged by heat and temperature.
We learned that structural deformation goes on during metamorphism. When two rock surfaces deep in the Earth’s crust grind against each other, crushing and stretching into bands, myolites are formed. These rocks are deformed under very high pressure. This deformation can take place before, during, or after metamorphic changes have happened and is part of the ongoing recycling of rock.
For example, shale may be changed into schist during deep burial without any deformation. Then, much later, when tectonic action hauls the schist layer upward in mountain building, higher-grade metamorphism may cause foliation and deformation. Then, if the rock is living a really interesting life, it may be heated during contact metamorphism and change yet again.
Naming
When naming metamorphic rock, the rules are more flexible than that of igneous or sedimentary rock naming. Since metamorphic rock tends tochange in composition and texture as temperatures and pressures change, the naming changes.
For example, shale is a fine-grained, clastic sedimentary rock containing quartz, clays, calcite, and some feldspar. With the start of low-grade metamorphism, muscovite and chlorite begins to form. Transformed shale is called slate. If the slate meets with further metamorphism, the mineral grains grow and intermediate-grade metamorphism happens with foliation and mica forms. Continued metamorphism causes the formation of even larger, coarsegrained rock with high schistosity and is known as schist. Then at high-grade metamorphism, the minerals group into separate bands with layers of mica-like minerals such as quartz and feldspar. This type of high-grade metamorphic rock is called gneiss from an old German word, gneisto, meaning to sparkle. So naming then depends on what can be seen. Slate and phyllite describe textures, while gneiss is described by the large mineral grains (that are easily seen) being named first. So specific gneiss might be named, quartz–plagioclase–biotite–garnet gneiss. In this way, another geologist would have a pretty good idea of all that the rock contained. Nongeologists would probably just call it garnet!
The Internet has several sites that provide photos of metamorphic rock types. There are even photos that illustrate the complete metamorphic rock series.
