Metamorphic Rocks and Metamorphism
by Owen Borville
January 30, 2021
Learning, Geology, Science
Metamorphism is the process of changing the chemical structure or composition of a rock. Metamorphism can also be described as recrystallization as a result of increased temperature or pressure or both on the rock. Heat enhances metamorphism by increasing the rate of chemical reactions. Pressure enhances metamorphism by recrystallization, or the rearrangement of grains inside minerals. These grains can be condensed if the pressure is uniform in all directions. If the pressure is not uniform in all directions, the grains can be rearranged in a variety of patterns. The presence of fluids also enhances metamorphism by increasing the rate of chemical reactions.
Annealing is the increase in the grain size of rocks or minerals as a result of heat or increase in temperature. Large mineral grains that grow during metamorphism are termed porphyroblasts and are surrounded by finer grained crystals. Porphyroclasts are larger elongated minerals grains which have undergone shear stress and are surrounded by finer grained crystals. Xenoblastic is a rock texture in metamorphic rocks consisting of anhedral crystals or grains. Granoblastic describes a metamorphic texture with mineral grains that are approximately the same size.
There are three types of metamorphism. Contact metamorphism is caused by heat from an igneous intrusion causing high temperature metamorphism on the surrounding rock, or country rock. Factors such as the size of the intrusion, the temperature of the intrusion, and the composition of the intrusion and the surrounding rock affect the extent of the metamorphism. High temperature fluids associated with the intrusion also enhance metamorphism by increasing the rate of chemical reactions and producing new minerals. An aureole is a section of rock that surrounds an intrusion of magma and shows the effects of contact metamorphism. Metallic ore deposits are formed from contact metamorphism as mineral saturated fluids enter the surrounding rock. These ore deposits include iron, copper, lead, tin, zinc, and tungsten.
Dynamic metamorphism is a type of metamorphism of rock masses caused primarily by stresses that yield relatively high strain (deformation) rates such as along faults. Dynamic metamorphism produces a rock called mylonite.
Regional metamorphism affects a large region and is associated with mountain building events. Regional metamorphism is caused by high temperatures and pressures inside the Earth and occurs most commonly along convergent plate margins. Regional metamorphism can also occur along divergent plate margins.
Metamorphic Rock Classification
Foliation forms in metamorphic rocks when pressure squeezes flat or elongated minerals so that they become aligned in a parallel pattern. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
Metamorphic rocks are classified as either foliated or non-foliated.
Foliated Metamorphic Rocks
Slate is a low grade or low temperature regional metamorphic rock derived from claystone or clay-rich rocks, producing a fine grained texture. Slaty cleavage is a parallel foliation or layering of fine-grained platy minerals in a direction perpendicular to the direction of maximum stress. The process produces the rocks slate and phyllite.
Phyllite is a fine-grained, foliated metamorphic rock generally derived from shale or fine-grained sandstone and is a higher metamorphic grade than slate. Phyllite has a gray to green color and the foliation is commonly crinkled or wavy. Phyllite differs from less recrystallized slate by its brightness, which is produced by barely visible flakes of muscovite mica. The mineral chlorite in phyllite produces the green color. Phyllite is also coarser grained than slate and can be of low to intermediate grade.
Schist is a metamorphic rock commonly derived from fine-grained sedimentary rock such as shale. Individual minerals in schist grow during metamorphism so that they are easily visible to the naked eye. Schists are named for their mineral constituents such as mica schist, which is rich in micas (biotite or muscovite). Schist can have a low to high grade of metamorphism. Foliation in coarse grained metamorphic rocks is called schistosity.
Gneiss (pronounced “nice”) is a high grade metamorphic rock which is coarse-grained, foliated, and commonly has alternating bands of light and dark-colored minerals. These minerals include quartz, feldspar, hornblende and mica minerals.
Migmatite is a “mixed” metamorphic rock of high grade that can form when metamorphic rock is heated enough to partially melt and the molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dikes. Migmatites are usually mixed light and dark colored rocks, generally formed of dark colored gneiss or schist and lighter igneous intrusive dikes and sills intruded during deformation. Migmatites appear to have been mushed around while still hot and plastic and display prominent folding.
Nonfoliated metamorphic rocks do not have platy or elongate minerals. These include:
Marble is a metamorphic rock made of calcite or dolomite and forms from limestone or dolomite by metamorphic recrystallization. Marble can form as a result of contact or regional metamorphism.
Quartzite is a metamorphosed quartz sandstone or chert. Quartzite is a hard, light colored, somewhat glassy-looking rock composed almost entirely of quartz and produced by contact or regional metamorphism.
Hornfels are a dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion.
Amphibolite is a metamorphic rock consisting primarily of amphibole minerals such as hornblende and plagioclase feldspar. Amphibolite commonly is a product of intermediate or high grade metamorphism. Amphibolite can be foliated or non-foliated.
Greenschist (greenstone) is a metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, epidote, actinolite, as well as quartz. The chlorite and epidote make greenstones green.
Skarn is a metamorphic rock formed from calcium-rich silicate rocks by metasomatism, or a significant change in chemical composition caused by metamorphism and hot fluids associated most commonly with granite intrusions. Skarn commonly contains garnet, pyroxene, actinolite, iron oxides, and epidote. Skarn can form less commonly by metasomatism from sedimentary rocks.
Metamorphic grade is the level or extent of metamorphism on a particular rock and is classified as low grade (greenshist facies), intermediate grade (amphibolite facies), or high grade (granulite facies). As rocks are exposed to higher temperatures and pressures, they are altered in a progressive manner. The rocks become harder and more coarsely crystalline and develop particular metamorphic textures.
Metamorphic zones of a particular metamorphic rock are labeled on a map according to the degree of metamorphism and are represented by a particular group of minerals called index minerals. Index minerals form at specific temperatures and pressures. Using these minerals, geologists can classify zones of metamorphic grade such as low, intermediate, and high grade. A well-known group of index minerals was introduced by geologist George Barrow. These minerals include chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. Chlorite indicates the lowest grade of metamorphism while sillimanite represents the highest grade of metamorphism. These minerals originate from clay-rich rocks from the same location or region.
Other metamorphic zones include blueschist, which is a metamorphic rock that forms at high pressures and low temperatures. Blueschist produces blue amphibole minerals such as glaucophane and aegerine. Blueschist also contains pyroxenes such as jadeite. Eclogite is a high pressure granular metamorphic rock of mafic igneous origin consisting of garnet and sodic pyroxene. Eclogite forms in the mantle or lower continental crust and at subduction zones.
Graphite and diamond are both metamorphic minerals composed of the single element carbon. Graphite is formed in metamorphic rocks and is dark colored with a shiny luster. Graphite has uses in industry and as pencil lead. Diamond is formed in kimberlite volcanic pipes under high pressure and temperature conditions. Diamonds can be transparent or opaque and are found as placer deposits. Diamonds have a hardness of 10 and are the hardest naturally occurring minerals. Diamond is a highly valuable gemstone and has industrial uses.
Meteorites are large pieces of material or rock from outside the Earth that reach the surface of the Earth without burning completely inside the Earth’s atmosphere. Meteorites contain silicate minerals common on Earth but also can contain metallic elements such as iron and nickel. Chondrites are stony meteorites that have not been modified and contain rounded grains or chondrules. An achondrite is a stony meteorite that does not contain chondrules or round grains. Tektites are silica-rich rocks formed from a meteorite impact. An impact crater is the result of a meteorite colliding with the Earth or a planet. Impact metamorphism or shock metamorphism occurs when an asteroid collides with the Earth’s crust or during an explosive event.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Metamorphism is the process of changing the chemical structure or composition of a rock. Metamorphism can also be described as recrystallization as a result of increased temperature or pressure or both on the rock. Heat enhances metamorphism by increasing the rate of chemical reactions. Pressure enhances metamorphism by recrystallization, or the rearrangement of grains inside minerals. These grains can be condensed if the pressure is uniform in all directions. If the pressure is not uniform in all directions, the grains can be rearranged in a variety of patterns. The presence of fluids also enhances metamorphism by increasing the rate of chemical reactions.
Annealing is the increase in the grain size of rocks or minerals as a result of heat or increase in temperature. Large mineral grains that grow during metamorphism are termed porphyroblasts and are surrounded by finer grained crystals. Porphyroclasts are larger elongated minerals grains which have undergone shear stress and are surrounded by finer grained crystals. Xenoblastic is a rock texture in metamorphic rocks consisting of anhedral crystals or grains. Granoblastic describes a metamorphic texture with mineral grains that are approximately the same size.
There are three types of metamorphism. Contact metamorphism is caused by heat from an igneous intrusion causing high temperature metamorphism on the surrounding rock, or country rock. Factors such as the size of the intrusion, the temperature of the intrusion, and the composition of the intrusion and the surrounding rock affect the extent of the metamorphism. High temperature fluids associated with the intrusion also enhance metamorphism by increasing the rate of chemical reactions and producing new minerals. An aureole is a section of rock that surrounds an intrusion of magma and shows the effects of contact metamorphism. Metallic ore deposits are formed from contact metamorphism as mineral saturated fluids enter the surrounding rock. These ore deposits include iron, copper, lead, tin, zinc, and tungsten.
Dynamic metamorphism is a type of metamorphism of rock masses caused primarily by stresses that yield relatively high strain (deformation) rates such as along faults. Dynamic metamorphism produces a rock called mylonite.
Regional metamorphism affects a large region and is associated with mountain building events. Regional metamorphism is caused by high temperatures and pressures inside the Earth and occurs most commonly along convergent plate margins. Regional metamorphism can also occur along divergent plate margins.
Metamorphic Rock Classification
Foliation forms in metamorphic rocks when pressure squeezes flat or elongated minerals so that they become aligned in a parallel pattern. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
Metamorphic rocks are classified as either foliated or non-foliated.
Foliated Metamorphic Rocks
Slate is a low grade or low temperature regional metamorphic rock derived from claystone or clay-rich rocks, producing a fine grained texture. Slaty cleavage is a parallel foliation or layering of fine-grained platy minerals in a direction perpendicular to the direction of maximum stress. The process produces the rocks slate and phyllite.
Phyllite is a fine-grained, foliated metamorphic rock generally derived from shale or fine-grained sandstone and is a higher metamorphic grade than slate. Phyllite has a gray to green color and the foliation is commonly crinkled or wavy. Phyllite differs from less recrystallized slate by its brightness, which is produced by barely visible flakes of muscovite mica. The mineral chlorite in phyllite produces the green color. Phyllite is also coarser grained than slate and can be of low to intermediate grade.
Schist is a metamorphic rock commonly derived from fine-grained sedimentary rock such as shale. Individual minerals in schist grow during metamorphism so that they are easily visible to the naked eye. Schists are named for their mineral constituents such as mica schist, which is rich in micas (biotite or muscovite). Schist can have a low to high grade of metamorphism. Foliation in coarse grained metamorphic rocks is called schistosity.
Gneiss (pronounced “nice”) is a high grade metamorphic rock which is coarse-grained, foliated, and commonly has alternating bands of light and dark-colored minerals. These minerals include quartz, feldspar, hornblende and mica minerals.
Migmatite is a “mixed” metamorphic rock of high grade that can form when metamorphic rock is heated enough to partially melt and the molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dikes. Migmatites are usually mixed light and dark colored rocks, generally formed of dark colored gneiss or schist and lighter igneous intrusive dikes and sills intruded during deformation. Migmatites appear to have been mushed around while still hot and plastic and display prominent folding.
Nonfoliated metamorphic rocks do not have platy or elongate minerals. These include:
Marble is a metamorphic rock made of calcite or dolomite and forms from limestone or dolomite by metamorphic recrystallization. Marble can form as a result of contact or regional metamorphism.
Quartzite is a metamorphosed quartz sandstone or chert. Quartzite is a hard, light colored, somewhat glassy-looking rock composed almost entirely of quartz and produced by contact or regional metamorphism.
Hornfels are a dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion.
Amphibolite is a metamorphic rock consisting primarily of amphibole minerals such as hornblende and plagioclase feldspar. Amphibolite commonly is a product of intermediate or high grade metamorphism. Amphibolite can be foliated or non-foliated.
Greenschist (greenstone) is a metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, epidote, actinolite, as well as quartz. The chlorite and epidote make greenstones green.
Skarn is a metamorphic rock formed from calcium-rich silicate rocks by metasomatism, or a significant change in chemical composition caused by metamorphism and hot fluids associated most commonly with granite intrusions. Skarn commonly contains garnet, pyroxene, actinolite, iron oxides, and epidote. Skarn can form less commonly by metasomatism from sedimentary rocks.
Metamorphic grade is the level or extent of metamorphism on a particular rock and is classified as low grade (greenshist facies), intermediate grade (amphibolite facies), or high grade (granulite facies). As rocks are exposed to higher temperatures and pressures, they are altered in a progressive manner. The rocks become harder and more coarsely crystalline and develop particular metamorphic textures.
Metamorphic zones of a particular metamorphic rock are labeled on a map according to the degree of metamorphism and are represented by a particular group of minerals called index minerals. Index minerals form at specific temperatures and pressures. Using these minerals, geologists can classify zones of metamorphic grade such as low, intermediate, and high grade. A well-known group of index minerals was introduced by geologist George Barrow. These minerals include chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. Chlorite indicates the lowest grade of metamorphism while sillimanite represents the highest grade of metamorphism. These minerals originate from clay-rich rocks from the same location or region.
Other metamorphic zones include blueschist, which is a metamorphic rock that forms at high pressures and low temperatures. Blueschist produces blue amphibole minerals such as glaucophane and aegerine. Blueschist also contains pyroxenes such as jadeite. Eclogite is a high pressure granular metamorphic rock of mafic igneous origin consisting of garnet and sodic pyroxene. Eclogite forms in the mantle or lower continental crust and at subduction zones.
Graphite and diamond are both metamorphic minerals composed of the single element carbon. Graphite is formed in metamorphic rocks and is dark colored with a shiny luster. Graphite has uses in industry and as pencil lead. Diamond is formed in kimberlite volcanic pipes under high pressure and temperature conditions. Diamonds can be transparent or opaque and are found as placer deposits. Diamonds have a hardness of 10 and are the hardest naturally occurring minerals. Diamond is a highly valuable gemstone and has industrial uses.
Meteorites are large pieces of material or rock from outside the Earth that reach the surface of the Earth without burning completely inside the Earth’s atmosphere. Meteorites contain silicate minerals common on Earth but also can contain metallic elements such as iron and nickel. Chondrites are stony meteorites that have not been modified and contain rounded grains or chondrules. An achondrite is a stony meteorite that does not contain chondrules or round grains. Tektites are silica-rich rocks formed from a meteorite impact. An impact crater is the result of a meteorite colliding with the Earth or a planet. Impact metamorphism or shock metamorphism occurs when an asteroid collides with the Earth’s crust or during an explosive event.
