Shock lamellae in a quartz grain. Shock also known as impact metamorphism is metamorphism resulting from meteor or other bolide impacts, or from a similar high-pressure shock event.
Shock metamorphism is the result of very high pressures and higher, but less extreme temperatures delivered relatively rapidly. Shock metamorphism produces planar deformation features, tektites, shatter cones, and quartz polymorphs. Shock metamorphism produces planar deformation features shock laminae , which are narrow planes of glassy material with distinct orientations found in silicate mineral grains.
Shocked quartz has planar deformation features. Shatter cone. Shatter cones are cone-shaped pieces of rock created by dynamic branching fractures caused by impacts. While not strictly a metamorphic structure, they are common around shock metamorphism.
Their diameter can range from microscopic to several meters. Fine-grained rocks with shatter cones show a distinctive horsetail pattern. Shock metamorphism can also produce index minerals , though they are typically only found via microscopic analysis.
The quartz polymorphs coesite and stishovite are indicative of impact metamorphism. As discussed in chapter 3, polymorphs are minerals with the same composition but different crystal structures. Tektites Shock metamorphism can also produce glass.
Tektites are gravel-size glass grains ejected during an impact event. They resemble volcanic glass but, unlike volcanic glass, tektites contain no water or phenocrysts , and have a different bulk and isotopic chemistry.
Tektites contain partially melted inclusions of shocked mineral grains. Although all are melt glasses, tektites are also chemically distinct from trinitite, which is produced from thermonuclear detonations , and fulgurites, which are produced by lightning strikes. All geologic glasses not derived from volcanoes can be called with the general term pseudotachylytes , a name which can also be applied to glasses created by faulting.
Barrow noticed and described the metamorphic sequence across a mountain belt showing regional metamorphism , now concluded to represent continental collision. The core The innermost chemical layer of the Earth, made chiefly of iron and nickel. Regional metamorphism occurs when temperatures and pressures are exerted on a rock over a large geographic area. This is often associated with mountain belts from converging continental tectonic plates.
Increasing metamorphic grade can be observed as one travels from the edge of a mountain belt into its high- grade core The innermost chemical layer of the Earth, made chiefly of iron and nickel.
Metamorphic facies are characterized by rock properties or assemblages groups of index minerals. Which metamorphic facies is associated with subduction zones? By analyzing facies on the Metamorphic PT diagram, blueschist is shown on the left at a low temperature but at a high pressure. The PT diagram indicates that high pressure and low temperature minerals are found at subduction zones.
When magma intrudes pre-existing country rock , the rock will be cooked by the magma. Contact metamorphism occurs when rocks are exposed to high temperatures and low pressures. For example, when a magma intrudes into pre-existing rocks, or a lava Liquid rock on the surface of the Earth.
Metamorphism is the process that changes existing rocks called protoliths into new rocks with new minerals and new textures. Increases in temperature and pressure are the main causes of metamorphism , with fluids adding important mobilization of materials. The primary way metamorphic rocks are identified is with texture.
Foliated textures come from platy minerals forming planes in a rock, while non-foliated metamorphic rocks have no internal fabric. Grade describes the amount of metamorphism in a rock, and facies are a set of minerals that can help guide an observer to an interpretation of the metamorphic history of a rock. Different tectonic or geologic environments cause metamorphism , including collisions, subduction , faulting , and even impacts from space. Use this quiz to check your comprehension of this chapter.
How does stress differ from strain? Stress is applied force; strain is the resulting change or deformation. How does burial metamorphism occur? Burial metamorphism occurs when sediments are buried in deep depositional basins where heat causes diagenesis to extend beyond compaction and cementation to actual changes in minerals. If you find a rock with distinct foliation , how was this rock metamorphosed? Directed stress is the likely cause of foliation. Tektites are gravel size grains of glass ejected during an impact event.
How can you easily distinguish between quartzite and marble using only what you normally carry with you on a hike? The simple test of rubbing them together will do. Quartz is harder than calcite and quartzite will scratch marble. By indicating the range of temperature and pressure at which they are stable, index minerals help geologists constrain the conditions of metamorphism.
Marble is the result of metamorphic recrystallization of limestone. What is the difference between heat and temperature? Heat is thermal energy; temperature is vibrational kinetic energy of atoms caused by heat. What does a phase diagram show? What is metamorphism? Metamorphism is the process by which previously existing rocks protoliths are changed in composition and texture by heat, pressure, and active fluids.
Peter Davis, Pacific Lutheran University KEY CONCEPTS Describe the temperature and pressure conditions of the metamorphic environment Identify and describe the three principal metamorphic agents Describe what recrystallization is and how it affects mineral crystals Explain what foliation is and how it results from directed pressure and recrystallization Explain the relationships among slate , phyllite , schist , and gneiss in terms of metamorphic grade Define index mineral Explain how metamorphic facies relate to plate tectonic processes Describe what a contact aureole is and how contact metamorphism affects surrounding rock Describe the role of hydrothermal metamorphism in forming mineral deposits and ore.
Valuable material in the Earth, typically used for metallic mineral resources. Foliation vs. Foliation is caused by metamorphism. Bedding is a result of sedimentary processes. They do not have to align. Source: Peter Davis. Garnet staurolite muscovite schist. Marble Source: Peter Davis. Baraboo Quartzite Source: Peter Davis.
Which of these has the largest mica. The mineral grains in schist especially the mica. Quartzite is foliated. Quartzite fizzes when in contact with acid.
Quartzite is darker in color. Quartzite is softer. Quartzite breaks across the grains. Minerals that have the same crystal structure but different chemical formulas. Those minerals that can have multiple different crystal habits depending on formation. When the same mineral appears in different metamorphic grades.
Minerals that have the same chemical formula but different crystal structures. When the same mineral appears in different metamorphic facies. The innermost chemical layer of the Earth, made chiefly of iron and nickel.
Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism. The metamorphic sequence across the Sierra Nevada Mountains when they were first discovered. The core. Increasing metamorphic grade can be observed as one travels from the edge of a mountain belt into its high- grade core. For example, when a magma intrudes into pre-existing rocks, or a lava. Chapter 6 Review Use this quiz to check your comprehension of this chapter.
Stress causes minerals to dissolve or recrystallize; strain causes minerals to grow. Stress is compression of minerals and rocks; strain is when minerals melt.
Stress is a very large force; strain is a lesser force. Stress is applied force; strain is the resulting deformation. Stress is a lesser force; strain is a very large force. Sediments are buried in deep depositional basins where heat causes diagenesis to extend to changes in minerals. Ocean crust minerals change when slabs are deeply buried in subduction zones. Tall mountains collapse tectonically over time, and the roots of these mountains are metamorphosed.
Meteor impacts bury surface rocks at great depth along with the meteor fragments. Continental collisions cause crunching and burial of rocks that were formerly near the surface. What is a tektite? Rounded quartz grains. Obsidian shattered by tectonic violence. Melted rock from the action of faulting. Glass shards in tephra. Quartzite is always much darker in color than marble. Quartzite breaks across the grains while marble breaks around the grains. Quartzite is foliated while marble is non-foliated.
Quartzite fizzes with acid while marble does not. Rub them together; quartzite will scratch marble. A fluid phase may introduce or remove chemical substances into or out of the rock during metamorphism, but in most metamorphic rock, most of the atoms in the protolith are be present in the metamorphic rock after metamorphism; the atoms will likely be rearranged into new mineral forms within the rock.
Therefore, not only does the protolith determine the initial chemistry of the metamorphic rock, most metamorphic rocks do not change their bulk overall chemical compositions very much during metamorphism. The fact that most metamorphic rocks retain most of their original atoms means that even if the rock was so thoroughly metamorphosed that it no longer looks at all like the protolith, the rock can be analyzed in terms of its bulk chemical composition to determine what type of rock the protolith was.
Temperature is another major factor of metamorphism. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes.
If rocks are buried within the Earth, the deeper they go, the higher the temperatures they experience. This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short.
Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. Tectonic processes are another way rocks can be moved deeper along the geotherm. Faulting and folding the rocks of the crust, can move rocks to much greater depth than simple burial can.
Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Pressure is a measure of the stress, the physical force, being applied to the surface of a material.
It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. The source of the pressure is the weight of all the rocks above.
Lithostatic pressure increases as depth within the Earth increases and is a uniform stress— the pressure applies equally in all directions on the rock. If pressure does not apply equally in all directions, differential stress occurs.
There are two types of differential stress. Normal stress compresses pushes together rock in one direction, the direction of maximum stress. At the same time, in a perpendicular direction, the rock undergoes tension stretching , in the direction of minimum stress.
Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the other side of the rock is being pushed in the opposite direction. Differential stress has a major influence on the the appearance of a metamorphic rock. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below.
Metamorphic minerals that grow under differential stress will have a preferred orientation if the minerals have atomic structures that tend to make them form either flat or elongate crystals.
This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any of these flat minerals are growing under normal stress, they will grow with their sheets oriented perpendicular to the direction of maximum compression.
This results in a rock that can be easily broken along the parallel mineral sheets. Such a rock is said to be foliated, or to have foliation. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Most commonly, if there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. Less commonly, it may be a carbon dioxide fluid or some other fluid.
The presence of a fluid phase is a major factor during metamorphism because it helps determine which metamorphic reactions will occur and how fast they will occur. The fluid phase can also influence the rate at which mineral crystals deform or change shape. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. If during metamorphism enough ions are introduced to or removed from the rock via the fluid to change the bulk chemical composition of the rock, the rock is said to have undergone metasomatism.
However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. Most metamorphism of rocks takes place slowly inside the Earth. Regional metamorphism takes place on a timescale of millions of years. Metamorphism usually involves slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that are breaking down in the given pressure and temperature conditions and migrate into new minerals that are stable in those conditions.
This type of chemical reaction takes a long time. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism.
This is not far beyond the conditions in which sediments get lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat like its protolith. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure.
Examples of low grade hydrous minerals include clay, serpentine, and chlorite. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and more equant in shape about the same diameter in all directions , thus standing out among the smaller, flatter, or more elongate minerals.
Micas tend to break down. New minerals such as hornblende will form, which is stable at higher temperatures. However, as metamorphic grade increases to even higher grade, all hydrous minerals, which includes hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.
Index minerals, which are indicators of metamorphic grade. In a given rock type, which starts with a particular chemical composition, lower-grade index minerals are replaced by higher-grade index minerals in a sequence of chemical reactions that proceeds as the rock undergoes prograde metamorphism. For example, in rocks made of metamorphosed shale, metamorphism may prograde through the following index minerals:.
Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock. A geologist maps and collects rock samples across the region and marks the geologic map with the location of each rock sample and the type of index mineral it contains. By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. The lines are known as isograds. Regional metamorphism occurs where large areas of rock are subjected to large amounts of differential stress for long intervals of time, conditions typically associated with mountain building.
Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Slate forms at the lowest metamorphic grade of the three. Schist, also derived from shales and mudrocks, is formed at much higher metamorphic grades than slate. Gneiss is a coarse grained, foliated, metamorphic rock rich in equidimensional grains of feldspar and quartz. Micas are sparse and are concentrated into thin, discontinuous layers. These mica-rich zones accentuate a strong foliation, easily seen as alternating bands or layers of darker- and lighter-colored minerals.
Gneiss generally forms under the highest metamorphic grades; however, many coarse grained schists form under equivalent metamorphic conditions.
Migmatites Fig. They form by partial melting under pressure-temperature conditions in the melting range for granitic compositions. Migmatites are streaky, layered rocks composed of alternating, dark-colored, residual minerals of the original parent rock and light-colored streaks and veins that crystallized from the melted granitic fraction. The high pressures generated by deep burial and elevated temperatures generated by deep burial and magma intrusion are associated with convergent boundaries Figs.
In oceanic-oceanic and oceanic-continent collisions, the oceanic slab and some of its sedimentary cover can be subducted to great depths, generating very high pressures and eventually, high temperatures. In continent-continent collisions, marine sedimentary strata in the closing marine basin between the converging continents are squeezed laterally, folded, thrust faulted, and overthickened. Continued shortening and thickening can generate the deep burial and elevated temperatures required for regional metamorphism.
In addition, the crystalline-rock crusts of one or both continents are shortened and thickened by thrust faults, resulting in even deeper burial for some rocks and strong uplift for others.
Partial melting at depth leads to magma generation and intrusion of granitic batholiths at mid to upper crustal levels.
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