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Chapter 8 - Metamorphic Rocks and Processes |
1. Define metamorphism
2. List and describe the agents of metamorphism.
3. Describe the environments of metamorphism.
4. Describe the textures of metamorphic rocks.
5. Illustrate important metamorphic minerals.
6. List the most common metamorphic rocks and how they are classified.
7. Explain the relationship between metamorphism and plate tectonics.
8. Describe the role of metamorphism in the formation of oil, gas, and coal.
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| Keywords and Essential Concepts |
1. Define metamorphism
| Metamorphic rocks are exposed in mountain ranges and in old, stable portions of continents that were once of core of mountain ranges long ago. Metamorphic rocks consist of materials that have experienced long journeys through geologic time. Rocks exposed in the core mountain ranges may have once been buried tens of miles below the surface where heat and pressure changed them from what they had been previously—such as sediments deposited on the seafloor or material ejected from volcanic eruption. |
metamorphic—pertaining to the process of metamorphism or to its results. Metamorphism is the mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions imposed at depth below the surface and below surficial zones where processes of sedimentation, compaction, and cementation take place. Metamorphism may take place where a rock is subjected to conditions unlike those in which it formed. Examples of metamorphic rocks include slate, marble, quartzite, greenstone, gneiss, and schist.
metamorphic rocks—Rock that was once one form of rock but has changed to another under the influence of heat, pressure, or fluids without passing through a liquid phase (melting).
recrystallization—a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same. |
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| recrystallization |
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2. List and describe the agents of metamorphism.
| heat—contributes to the process in two ways. First, atoms may combine differently at different temperatures. This means that a mineral stable at one temperature might become unstable at a higher (or lower) temperature and be converted to a different mineral with a more stable atomic structure. Second, heat makes practically all chemical reactions go faster, meaning that mineral transformations are much easier at higher temperature. |
| chemically active fluids—fluids can serve to speed up metamorphic processes. Chemical reactions require water, and most proceed much faster as the amount of water goes up. Dissolved ions in the fluid also make those mineral transformations that require chemical changes in the minerals to occur, whether by supplying needed ions or flushing away excess ones. |
pressure—can control which minerals or forms of minerals are stable. Some minerals may be converted to minerals with similar composition but different atomic packing simply because pressure is increased.
Two types of pressure:
1. confining pressure—pressure on any point is equal in all directions, similar to a balloon held under water.
2.
directed pressure—squeezing of rocks, such as caused by converging tectonic plates.
Pressure reorients minerals with linear or platy structure or to create a preferred orientation of the new mineral grains as they form. Thus elongate minerals such as amphiboles, or platy minerals such as clays or micas tend to align themselves parallel to each other when under pressure. This only happens when there is directed pressure; confining pressure does not accomplish it. A texture of this sort in a metamorphic rock is called foliation.
mountain building and regional metamorphism—convergent plate boundaries, particularly between colliding continents, causes metamorphism to take place on a regional scale.
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| agents of metamorphism |
heat in metamorphism |
pressure in metamorphism |
forces of pressure |
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| confining & direct pressure |
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3. Describe the environments of metamorphism.
regional metamorphism—a type of metamorphism in which the mineralogy and texture of rocks are changed over a wide area by deep burial and heating associated with the large-scale forces of plate tectonics. In regional metamorphism, rocks that form closer to the margin of the tectonic plates, where the heat and pressure are greatest, often differ in their minerals and texture from those that form farther away. |
| contact metamorphism—metamorphism in which the mineralogy and texture of a body of rock are changed by exposure to the pressure and extreme temperature associated with a body of intruding magma. Contact metamorphism often results in the formation of valuable minerals, such as garnet and emery, through the interaction of the hot magma with adjacent rock. |
| dynamic metamorphism—metamorphism resulting exclusively or largely from rock deformation, principally faulting and folding. |
| hydrothermal alteration—metamorphism as a result of exposure to hot fluids passing through permeable rocks |
retrograde metamorphism—metamorphic changes that take place when rocks formed at great depths migrate to the surface via tectonic uplift and are exposed to lower pressure and more fluid-rich geologic settings, causing minerals to change to match surficial geologic environments.
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| regional metamorphism |
contact metamorphism |
hydrothermal metamorphism |
4. Explain the relationship between metamorphism and plate tectonics.
The movement of tectonic plates transports sediment and rocks into different geologic setting—these changes can result in metamorphism, particularly in zones where tectonic plates are converging, as in a subduction zone or where continental plates converge, pushing up high mountain ranges while material below the mountains are pushed down under increasing temperature and pressure condition. These are large scale regions that experience a wide range of conditions through time, and is called regional metamorphism. Areas that are subjected to definable conditions, in part based on the minerals formed, are called metamorphic facies. Examples include:
moderate pressure + low temperature: greenschist facies
high pressure + low temperature: blueschist facies
moderate pressure + high temperature: amphibolite facies
high temperature + high pressure: ecologite facies.
Even though the rock material may have the same chemical composition, their mineral compositions, texture, and appearance are different, and the rocks they comprise are classed into rocks of different metamorphic grades.
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| Metamorphic facies in Plate Tectonics |
metamorphic facies |
Temperature and pressure effects on metamorphism |
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5. Describe the textures of metamorphic rocks.
| foliation—any penetrative planar mineral-alignment fabric present in rocks, especially rocks affected by regional metamorphic compression typical of orogenic belts. |
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| How foliation forms |
rock cleavage—the capacity of a rock to split along certain parallel surfaces more easily than along others, such as bedding planes. Not all rocks have rock cleavage, but it is perhaps most common in low-grade to moderate-grade metamorphic rocks with a high mica content, such as slate or phyllite where crystalline structure of platy or sheet silicate minerals have been reoriented by metamorphic processes in an alignment perpendicular to the principle vector direction of stress caused by compression.
slaty cleavage—rock cleavage formed by the reorientation and alignment of clay and mica minerals in fine-grained sedimentary rocks by compaction (high confining pressure) that results in a narrow splitting properties of slate. The splitting (slaty cleavage) forms perpendicular to the principle vector direction of stress in the metamorphic setting.
schistosity—the type of rock foliation that characterizes schist, resulting from the parallel arrangement of coarse-grained platy minerals, such as mica, chlorite, and talc.
gneissic texture—a coarsely foliated texture in which the minerals have been segregated into discontinuous bands, each of which is dominated by one or two minerals, giving the rock a striped appearance.
migmatite—rock intermediate between metamorphic igneous character. Migmatite shows irregular banding and much recrystallization, typical of metamorphic gneisses that have taken on igneous character through partial melting.
mylonite—a fine-grained metamorphic rock, typically banded, resulting from the grinding or crushing of other rocks in a fault zone.
breccia—a rock consisting of angular rock fragments cemented together.
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| foliation |
foliation illustrated |
rock cleavage |
slaty cleavage |
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| schistosity |
gneissic texture |
migmatite |
mylonite |
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| breccia |
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6. Illustrate Important Metamorphic Minerals
Many common silicate and non-silicate minerals found in intrusive igneous and sedimentary rocks can occur in metamorphic rocks including quartz, feldspars, micas (biotite and muscovite), amphibole, pyroxene, calcite, dolomite, magnetite. However, other minerals are unique to metamorphic settings.
chlorite—a dark green mineral consisting of a basic hydrated aluminosilicate of magnesium and iron, often formed by metamorphic alteration (low grade or retrograde greenschist metamorphic facies).
epidote—A yellow-green crystalline mineral, common in metamorphic rocks, consisting of a hydroxyl silicate of calcium, aluminum, and iron.
garnet—most commonly a deep red vitreous silicate mineral found in high-grade metamorphic rocks, sometimes found in gemstone quality crystals.
actinolite—a green mineral of the amphibole group with bladed or platy crystals; calcium magnesium iron silicate. Sometimes forms schist.
serpentine minerals—Any of a group of greenish, brownish, or spotted hydrated magnesium silicate minerals [such as Mg3Si2O5(OH)4 ] mined as a source of magnesium and asbestos
crysotile asbestos—A fibrous variety of serpentine group minerals. Also called white asbestos.
talc—a fine-grained white, greenish, or gray mineral, Mg3Si4O10(OH)2, having a soft soapy feel. |
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| metamorphic minerals |
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7. List the most common metamorphic rocks and how they are classified.
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| Metamorphic rocks and their related host rocks |
argillite—a metamorphic rock, intermediate between shale and slate, that does not possess true slaty cleavage.
slate—a fine-grained gray, green, or bluish metamorphic rock easily split into smooth, flat plates along cleavage planes.
phyllite—a fine-grained metamorphic rock with a well-developed laminar structure, intermediate between slate and schist.
metasandstone—sandstone that has undergone partial metamorphism, but retains enough characteristics to show that it was derived from.
quartzite—a rock formed from the metamorphism of quartz sandstone or quartz-rich volcanic ash.
metaconglomerate—conglomerate that has been partially metamorphosed, retaining some of the distinct character of original gravel clasts, although they may be stretched or recrystallized.
greenstone—a metamorphic rock derived from any basic igneous rocks (typically altered basalt) colored green by the minerals chlorite, hornblende, or epidote.
schist—any medium-grained to coarse-grained metamorphic rocks composed of laminated, often flaky parallel layers of chiefly micaceous minerals.
gneiss—a metamorphic rock with a banded or foliated structure, typically coarse-grained (crystalline) and consisting mainly of feldspar, quartz, and mica.
ecologite—A high-grade metamorphic rock containing granular minerals, typically red garnet mixed with grains of pyroxene, quartz, and feldspars.
marble—a crystalline metamorphic rock composed primarily of calcium carbonate (CaCO3). A product of metamorphism of limestone.
metachert—extremely hard, brecciated, and recrystallized chert that still retains its microcrystalline texture.
serpentinite—a metamorphic rock consisting almost entirely of minerals in the serpentine group. Serpentinite forms from the alteration of mafic silicate materials, such as olivine and pyroxene, during metamorphism. An ultramafic rock consisting almost wholly of serpentine-group minerals (such as antigorite and chrysotile) derived from the alteration of peridotite. Accessory chlorite, magnetite, and talc may be present. The State rock of California. |
Pathways to the formation of selected metamorphic rocks
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| greenstone and schist |
marble |
quartzite |
slate and schist |
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| gneiss |
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8. Describe the role of metamorphism in the formation of oil, gas, and coal.
Many processes take place in the processes that change dead organic matter into energy resources like coal, oil and gas. These changes take place as sediments containing organic material undergoes increasing burial—increasing heat and pressure through geologic time. The changes that take place are called "organic maturation" and are similar to the processes that take place when food is cooked in an oven. Rocks go through organic maturation, going through stages, even to the point of being "over toasted or "burned"—causing organic materials to break down, driving off their volatile components (including methane, CO2, ammonia, water) leaving behind pure carbon residues.
trophic respiration—processes in organisms that result in release of energy related to the consumption of substances that go through a chemical changes. These processes may result in the excretion of substances that can alter and/or accumulate in the environment.
organic maturation—the gradual metamorphic processes that take place over time, involving burial and geothermal heating, that convert organic remain preserved in sediments into petroleum (oil, gas, and tar) or coal (conversion of plant material to peat, lignite, subbituminous coal, and anthracite coal, in increasing order of maturation.
source rock—sedimentary rocks rich in organic residues that with enough heat and time releases their volatile components, allowing them to migrate to other locations, including the surface.
reservoir—a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Oil and gas float on water.
petroleum—a natural flammable liquid mixture of hydrocarbons that is present in certain rock strata and can be extracted and refined to produce fuels including gasoline, kerosene, diesel oil, or chemically converted to other materials, such as plastics, and other petroleum-based byproducts.
oil—the liquid component of petroleum (as opposed to gas or asphalt or tar). Petroleum is the derivative of the metamorphism of organic-rich sedimentary rocks rich in volatile components, especially lipids.
gas—hydrocarbons that are in gaseous state under normal atmospheric pressures.
peat—an accumulation of partially decayed vegetation matter that has a brown, soil-like character typical of boggy, acid ground or swampy setting.
lignite—an organic deposit of soft brownish coal preserving traces of plant structure, intermediate between peat and bituminous coal.
bituminous coal—soft black coal with a high volatile content, and typically burns with a smoky yellow flame.
anthracite—a hard, metamorphic variety of coal, having a low volatile content. Typically burns very hot and clean relative to other varieties of coal.
graphite—a high-grade metamorphic mineral composed of pure carbon, possibly formed from the metamorphic decay of hydrocarbons. |
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| trophic respiration produces organic residues |
organic maturation illustrated |
oil and gas form by organic maturation |
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| Formation of petroleum and coal from organic residues |
Formation of coal from plant material |
Traditional oil well and gas-shale fracking |
Petroleum products |
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Quiz Questions
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