Plant Earth Ch 6: Metamorphism PDF

# Metamorphism: Alteration of Rocks by Temperature and Pressure ## During the Rock Cycle - Rocks may be subjected to temperatures and pressures great enough to cause changes in their mineralogy, texture, or chemical composition. - Cooking batter in a waffle iron not only heats up the batter but al...

# Metamorphism: Alteration of Rocks by Temperature and Pressure ## During the Rock Cycle - Rocks may be subjected to temperatures and pressures great enough to cause changes in their mineralogy, texture, or chemical composition. - Cooking batter in a waffle iron not only heats up the batter but also puts pressure on it, transforming it into a rigid solid. - Rocks are transformed as they encounter high temperatures and pressures deep in Earth's crust. - Tens of kilometers below the surface, temperatures and pressures are high enough to cause chemical reactions and recrystallization that transform rock without being high enough to melt it. - Increases in temperature and pressure, and changes in the chemical environment, can alter the mineral composition and crystalline texture of igneous and sedimentary rock, even though it remains solid all the while. - Metamorphic rocks have undergone changes in mineralogy, texture, chemical composition, or all three. - Most metamorphism is a dynamic process, not a static event. - Earth's internal heat engine drives the plate tectonic processes that push rocks formed at Earth's surface down great depths, subjecting them to high pressures as well as high temperatures. - The transformed rocks return to Earth's surface eventually, and that process is largely powered by weathering and erosion. ## Causes of Metamorphism - Sediments and sedimentary rocks are products of Earth’s surface environments. - Igneous rocks are products of the magmas that originate in the lower crust and mantle. - Metamorphic rocks are the products of processes acting on rocks at depths ranging from the upper to the lower crust. - When a rock is subjected to significant changes in temperature or pressure, it will undergo changes in its chemical composition, mineralogy and texture. - A limestone filled with fossils may be transformed into a white marble in which no trace of fossils remains. - The mineral and chemical composition of the rock may be unaltered, but its texture may have changed drastically. - Shale may become schist, in which the original bedding is obscured and the texture is dominated by large crystals of mica. ### The Role of Temperature - Heat can transform a rock’s chemical composition, mineralogy and texture by breaking chemical bonds and altering the existing crystal structures of the rock. - When rock is moved from Earth’s surface to its interior, where temperatures are higher, the rock adjusts to the new temperature. - Its atoms and ions recrystallize, linking up in new arrangements and creating new mineral assemblages. - The increase in temperature with increasing depth in Earth’s interior is called the geothermal gradient. - The geothermal gradient varies among plate tectonic settings, but on average it is about 30°C per kilometer of depth. - In areas where the continental lithosphere has been stretched and thinned, the geothermal gradient is steep. - In areas where the continental lithosphere is old and thick, the geothermal gradient is shallow. - Because different minerals crystallize and remain stable at different temperatures, we can use a rock’s mineral composition as a kind of geothermometer to gauge the temperature at which it formed. - As sedimentary rocks containing clay minerals are buried deeper and deeper, the clay minerals begin to recrystallize and form new minerals such as micas. - Plate tectonic processes such as subduction and continent-continent collision, which transport rocks and sediments into the hot depths of the crust, are the mechanisms that form most metamorphic rocks. - Limited metamorphism may occur where rocks are subjected to elevated temperatures near igneous intrusions. ### The Role of Pressure - Pressure, like temperature, changes a rock’s chemical composition, mineralogy, and texture. - Solid rock is subjected to two basic kinds of pressure, also called stress: - **Confining Pressure:** A general force applied equally in all directions, like the pressure a swimmer feels under water. - **Directed Pressure:** Force exerted in a particular direction. - The compressive force exerted where lithospheric plates converge is a form of directed pressure, and it results in deformation of the rocks near the plate boundary. - Heat reduces the strength of a rock, so directed pressure is likely to cause severe folding and other forms of ductile deformation, as well as metamorphism, where temperatures are high. - Rocks subjected to differential stress may be severely distorted. - The minerals in a rock under pressure may be compressed, elongated, or rotated to line up in a particular direction. - Directed pressure guides the shape and orientation of the new crystals formed as minerals recrystallize under the influence of both heat and pressure. - Marble owes its remarkable strength to this recrystallization process. - When limestone is heated to the very high temperatures that cause it to recrystallize, the original minerals and crystals become reoriented and tightly interlocked to form a very strong structure with no planes of weakness. - The pressure to which rock is subjected deep in Earth's crust is related to both the thickness and the density of the overlying rocks. - Minerals that are stable at the lower pressures near Earth's surface become unstable and recrystallize into new minerals under the increased pressures deep in Earth's crust. ### The Role of Fluids - Metamorphic processes can alter a rock's mineralogy by introducing or removing chemical components that are soluble in heated water. - Hydrothermal fluids accelerate metamorphic chemical reactions because they carry dissolved carbon dioxide as well as other chemical substances. - As hydrothermal solutions percolate up to the shallower parts of the crust, they react with the rocks they penetrate, changing their chemical and mineral compositions. - This kind of change in a rock's composition by fluid transport of chemical substances into or out of it is called metasomatism. - Many valuable deposits of copper, zinc, lead, and other metallic ores are formed by this kind of chemical substitution. ## Types of Metamorphism - Geologists can duplicate metamorphic conditions in the laboratory and determine the precise combinations of pressure, temperature, and parent rock composition under which particular transformations might take place. - But to understand when, where, and how such conditions came about in Earth's interior, we must categorize metamorphic rocks on the basis of their geologic settings. ### Regional Metamorphism - The most widespread type of metamorphism. - Takes place where both high temperatures and high pressures are imposed over large parts of the crust. - Occurs in volcanic mountain belts and in the cores of mountain chains produced by continent-continent collisions. - Zones of regional metamorphism are often linear features. - Often represent sites of former mountain chains that were eroded over millions of years. ### Contact Metamorphism - The heat from an igneous intrusion metamorphoses the rock immediately surrounding it. - Affects only a thin zone of country rock along the zone of contact. - Occurs especially at the margins of shallow intrusions. - Pressure effects are important only where the magma is intruded at great depths. - Contact metamorphism by volcanic deposits is limited to very thin zones because lavas cool quickly at Earth's surface. - Contact metamorphism may also affect xenoliths that are not completely melted. ### Seafloor Metamorphism - A form of metasomatism. - Often associated with mid-ocean ridges. - Hot basaltic lava at a seafloor spreading center heats infiltrating seawater, which starts to circulate through the newly forming oceanic crust by convection. - The increase in temperature promotes chemical reactions between the seawater and the rock, forming altered basalts. - Metasomatism results from percolation of high-temperature fluids that circulate near igneous intrusions. ### Other Types of Metamorphism - There are several other types of metamorphism that produce smaller amounts of metamorphic rock. - Several are extremely important in helping geologists understand conditions deep within Earth's crust. #### Burial Metamorphism - Transformations that occur as sedimentary rocks are gradually buried. - Diagenesis grades into burial metamorphism. - Occurs at depths of 6 to 10 km, where temperatures range between 100°C and 200°C and pressures are less than 3 kbar. #### High-Pressure and Ultra-High-Pressure Metamorphism - Metamorphic rocks formed by high-pressure metamorphism (at 8 to 12 kbar) and ultra-high-pressure metamorphism (at pressures greater than 28 kbar) are rarely exposed at Earth's surface. - These rocks are rare because they form at such great depths. - Most high-pressure metamorphic rocks form in subduction zones as sediments scraped from subducting oceanic crust are plunged to depths of over 30 km, where they experience pressures of up to 12 kbar. #### Shock Metamorphism - Occurs when a meteorite collides with Earth. - The energy represented by the meteorite’s mass and velocity is transformed into heat and shock waves that pass through the impacted country rock. - The country rock can be shattered and even partially melted to produce tektites. - Most large impacts have left no trace of a meteorite because these bodies are usually destroyed in the collision with Earth. - Earth’s dense atmosphere causes most meteorites to burn up before they strike its surface, so shock metamorphism is rare on Earth. - On the surface of the Moon, however, shock metamorphism is pervasive. ## Metamorphic Textures - Metamorphism imprints new textures on the rocks it alters. - Texture of a metamorphic rock is determined by the sizes, shapes, and arrangement of its constituent crystals. - Some metamorphic rock textures depend on the particular kinds of minerals formed under metamorphic conditions. - Variation in grain size is also important. - In general, grain size increases as metamorphic grade increases. - Each textural variety of metamorphic rock tells us something about the metamorphic process that created it. ### Foliation and Cleavage - The most prominent textural feature of regionally metamorphosed rocks is foliation. - A set of flat or wavy parallel cleavage planes produced by deformation of igneous and sedimentary rocks under directed pressure. - The foliation planes may cut through the bedding of the original sedimentary rock at any angle or be parallel to the bedding. - In general, as the grade of regional metamorphism increases, foliation becomes more pronounced. - A major cause of foliation is the formation of minerals with a platy crystal habit, chiefly the micas and chlorite. - The planes of all the platy crystals are aligned parallel to the foliation, an alignment called the preferred orientation of the minerals. - As platy minerals crystallize, their preferred orientation is usually perpendicular to the main direction of the forces squeezing the rock during metamorphism. - Crystals of preexisting minerals may contribute to the foliation, especially for sheet silicates, by rotating until they also lie parallel to the developing foliation plane. - The most familiar form of foliation is seen in slate, a common metamorphic rock. ### Foliated Rocks - Classified according to four main criteria: - metamorphic grade - grain size - type of foliation - banding - In general, foliation progresses from one texture to another with increasing metamorphic grade. - A shale may metamorphose first to a slate, then to a phyllite, then to a schist, then to a gneiss, and finally to a migmatite. ### Granoblastic Rocks - Nonfoliated metamorphic rocks composed mainly of crystals that grow in equant shapes such as cubes and spheres rather than in platy or elongate shapes. - These rocks result from metamorphic processes such as contact metamorphism, in which directed pressure is absent, so foliation does not occur. - Granoblastic rocks include hornfels, quartzite, marble, greenstone, amphibolite, and granulite. - All have a homogeneous granular texture. - **Hornfels:** A high-temperature contact metamorphic rock of uniform grain size that has undergone little or no deformation. - **Quartzite:** Very hard, white rocks derived from quartz-rich sandstones. - **Marble:** Metamorphic products of heat and pressure acting on limestones and dolomites. - **Greenstone:** Metamorphosed mafic volcanic rocks. - **Amphibolite:** Made up of amphibole and plagioclase feldspar. - **Granulite:** A high-grade metamorphic rock. ### Porphyroblasts - Newly formed metamorphic minerals may grow into large crystals surrounded by a much finer grained matrix of other minerals. - These large crystals are called porphyroblasts. ## Regional Metamorphism and ## Metamorphic Grade - Metamorphic rocks form under a wide range of conditions, and their mineralogies and textures are clues to the pressures and temperatures in the crust where and when they formed. - Geologists who study the formation of metamorphic rocks constantly seek to determine the intensity and character of metamorphism more precisely. - "Read" minerals as though they were pressure gauges and thermometers. - These techniques are best illustrated by their application to regional metamorphism. ### Mineral Isograds: Mapping ### Zones of Change - Different zones within a belt of regional metamorphism may be distinguished by index minerals. - Index minerals: Abundant minerals that each form under a limited range of temperatures and pressures. - A new mineral may appear when we move into a new zone with a higher metamorphic grade. - Using index minerals, we can make a map of the boundaries between metamorphic zones. - We can use the occurrences of index minerals to make a map of the boundaries between zones. - Geologists define these boundaries by drawing lines called isograds. - Isograds: Plot the transitions from one zone to the next. - Pattern of isograds tends to follow the deformation features (folds and faults) of a region. - An isograd based on a single index mineral, such as the chlorite isograd, provides a good approximate measure of metamorphic pressure and temperature. - To determine metamorphic pressure and temperature more precisely, geologists can examine a group of two or three minerals that crystallized together. ### Metamorphic Grade and Parent ### Rock Composition - The kind of metamorphic rock that results from a given grade of metamorphism depends partly on the mineralogy of the parent rock. - As the metamorphic grade increases, the mineral assemblages in the parent rock change. - In the regional metamorphism of a basalt, the lowest-grade rocks characteristically contain various zeolite minerals. - The zeolite minerals contain water within their crystal structure. - Zeolite minerals form at very low temperatures and pressures. - Overlapping with the zeolite grade is a higher grade of metamorphosed mafic volcanic rocks, the greenschists. - The granulites, coarse-grained rocks containing pyroxene, constitute the highest grade of metamorphosed mafic volcanic rocks. - Rocks of the greenschist, amphibolite, and granulite grades are also formed during metamorphism of sedimentary rocks such as shale. ### Metamorphic Facies - We can plot this information about the grades of the metamorphic rocks in a regional metamorphic belt-derived from parent rocks of many different chemical compositions- on a graph of temperature and pressure. - Metamorphic facies are groupings of rocks of various mineral compositions formed under particular conditions of temperature and pressure from different parent rocks. - By delineating metamorphic facies, we can be more specific about the grades of metamorphism observed in rocks. ## Plate Tectonics and ## Metamorphism - Different types of metamorphism are likely to occur in different plate tectonic settings. ### Continental Interiors - Contact metamorphism, burial metamorphism, and perhaps regional metamorphism occur at different levels in the crust. - Shock metamorphism is likely to be best preserved in continental interiors. ### Divergent Plate Boundaries - Seafloor metamorphism and contact metamorphism around plutons intruding into the oceanic crust occur at divergent plate boundaries. ### Convergent Plate Boundaries - Regional metamorphism, high-pressure and ultra-high-pressure metamorphism, and contact metamorphism occur. ### Transform Faults - In oceanic settings, seafloor metamorphism may occur. - In both oceanic and continental settings, we find extensive metamorphism caused by shearing forces along transform faults. ### Metamorphic Pressure- ### Temperature Paths - Metamorphic grade can inform us of the maximum pressure or temperature to which a metamorphic rock has been subjected, but it tells us nothing about where the rock encountered those conditions. - This history of changing temperature and pressure that is reflected in its texture and mineralogy is called a metamorphic pressure-temperature path, or P-T path. - The P-T path can be a sensitive recorder of many important factors that influence metamorphism. - Sources of heat: Change temperatures. - Rates of tectonic transport: Change pressures. - P-T paths are characteristic of particular plate tectonic settings. - To obtain a P-T path, geologists must analyze specific minerals from metamorphic rock samples in the laboratory. - Garnet: A common porphyroblast that serves as a sort of P-T path recording device. - The oldest part of a garnet crystal is its core, and the youngest is its outer edge, so the variation in its composition from core to edge will yield the history of the metamorphic conditions under which it formed. - Geologists can measure the chemical composition of a garnet porphyroblast in the laboratory and plot the corresponding pressure and temperature values as a P-T path. ### Ocean-Continent Convergence - A distinct metamorphic assemblage forms when oceanic lithosphere is subducted beneath a plate carrying a continent on its leading edge. - Thick sediments eroded from the continent rapidly fill the deep-sea trench that forms a flexural basin at the subduction zone. - The oceanic lithosphere stuffs the region below the inner wall of the trench with those sediments, as well as with shreds of ophiolite suites scraped off the descending plate. - Assemblages of this type are located in the forearc region of a subduction zone. - These rocks are all highly folded, intricately faulted, and metamorphosed. ### Continent-Continent Collision - Continental crust is buoyant, when a continent collides with another continent, both continents resist subduction and stay afloat on the mantle. - A wide zone of intense deformation develops at the convergent boundary where the continents grind together. - In the geologic record, it is called a suture. - The intense deformation results in a much-thickened continental crust. - The deep parts of the continental crust heat up and undergo varying grades of metamorphism, and melting may begin at the same time. - Erosion has stripped off the surface layers of the mountains. ### Exhumation: A Link Between the ### Plate Tectonic and Climate Systems - Plate tectonic theory provided a ready explanation for how metamorphic rocks could be produced by seafloor spreading, subduction, and continent-continent collision. - The study of PT paths provided a clearer picture of the specific tectonic mechanisms involved in the deep burial and metamorphism of rocks. - Geologists have been searching for exclusively tectonic mechanisms that could bring these rocks back to Earth's surface so quickly. - One popular idea is that mountains, having been built to great elevations during collisional crustal thickening, suddenly fail by gravitational collapse. - The saying "what goes up must go down" applies here, but with surprisingly fast results. ## Thought Questions 1. What is the significance of the "economic basement"? 2. What is a "schistosity"? 3. What is the difference between a greenschist and a blueschist? 4. In what type of geologic setting would you find eclogites? 5. What type of metamorphism would you find in a zone of plate extension? 6. What is the difference between a prograde and a retrograde path? 7. Why do geologists refer to minerals as "pressure gauges" and "thermometers" when conducting studies about metamorphic rocks? 8. How do the mineral compositions of metamorphic rocks change, and what are some examples? 9. How do geologists use metamorphic rocks to make inferences about the geologic history of an area? 10. What are some of the main differences between the metamorphic settings of high-pressure, low-temperature metamorphism (as found in subduction zones) and high-pressure, high-temperature metamorphism (as found in collision zones)? 11. What is the relationship between metamorphism and weathering and erosion? 12. In what ways are metamorphic rocks important for understanding the origins of Earth's resources?