An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges. Orogeny is the primary mechanism; the word "orogeny" comes from Ancient Greek. Although it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny; the formation of an orogen can be accomplished by the tectonic processes such as oceanic subduction or continental subduction convergence of two or more continents for collisional orogeny). Orogeny produces long arcuate structures, known as orogenic belts. Orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Although orogenic belts are associated with subduction zones, subduction tectonism may be ongoing or past processes.
The subducting tectonism would consume crust, thicken lithosphere, produce earthquake and volcanoes, build island arcs in many cases. Geologists attribute the arcuate structure to the rigidity of the descending plate, island arc cusps relate to tears in the descending lithosphere; these island arcs may be added to a continental margin during an accretionary orogeny. On the other hand, subduction zones may be reworked at a time due to lithospheric rifting, leading to amphibolite to granulite facies metamorphism of the thinned orogenic crust; the processes of orogeny can take tens of millions of years and build mountains from plains or from the seabed. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range and the buoyant upward forces exerted by the dense underlying mantle. Rock formations that undergo orogeny are deformed and undergo metamorphism. Orogenic processes may push buried rocks to the surface.
Sea-bottom and near-shore material may cover all of the orogenic area. If the orogeny is due to two continents colliding high mountains can result. An orogenic event may be studied: as a tectonic structural event, as a geographical event, as a chronological event. Orogenic events: cause distinctive structural phenomena related to tectonic activity affect rocks and crust in particular regions, happen within a specific period In general, there are two main types of orogens at convergent plate margins: accretionary orogens, which were produced by subduction of one oceanic plate beneath one continental plate to result in either continental arc magmatism or the accretion of island arc terranes to continental margins. An orogeny produces an orogen, but a range-foreland basin system is only produced on passive plate margins; the foreland basin forms ahead of the orogen due to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems.
The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are derived from the erosion of the uplifting rocks of the mountain range, although some sediments derive from the foreland; the fill of many such basins shows a change in time from deepwater marine through shallow water to continental sediments. Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including crustal deformation, crustal thickening, crustal thinning and crustal melting as well as magmatism and mineralization. What happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, how these properties change during orogenesis. In addition to orogeny, the orogen is subject to other processes, such as erosion; the sequence of repeated cycles of sedimentation and erosion, followed by burial and metamorphism, by crustal anatexis to form granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle.
For example, the Caledonian Orogeny refers to a series of tectonic events due to the continental collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana in the Early Paleozoic. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle. In summary, an orogeny is an episode of deformation and magmatism at convergent plate margins, during which many geological processes play a role at convergent plate margins; every orogeny has its own orogenic cycle, but composite orogenesis is common at convergent plate margins. Erosion represents a subsequent phase of the orogenic cycle. Erosion removes much of the mountains
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, soil, bones, or metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond ground optically flat, it is mounted on a glass slide and ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method involved using the Michel-Lévy interference colour chart. Quartz is used as the gauge to determine thickness as it is one of the most abundant minerals; when placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock forming minerals can be identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes.
The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene. Thin sections are prepared in order to investigate the optical properties of the minerals in the rock; this work helps to reveal the origin and evolution of the parent rock. A photograph of a rock in thin section is referred to as a photomicrograph. Under thin section, in plane polarized light, quartz is colorless with no cleavage, its habit is either equant or anhedral if it infills around other minerals as a cement. Under cross polarized light quartz displays low interference colors and is the defining mineral used to determine if the thin section is at standardized thickness of 30 microns as quartz will only display up to a pale yellow interference color and no further at that thickness, it is common in most rocks so it will be available to judge the thickness. In thin section, quartz grain provenance in a sedimentary rock can be estimated. In crossed polarized light, the quartz grain can go extinct all at once, called monocrystalline quartz, or in waves, called polycrystalline quartz.
The extinction in waves is called undulose extinction and indicates dislocation walls in mineral grains. Dislocation walls are where dislocations, intracrystalline deformation via movement of a dislocation front within a plane, organize themselves into planes of sufficient quantity, they change the crystallographic orientation across the walls, so for example in quartz, the two sides of the wall will have different extinction angles and thus result in undulose extinction. Since undulose extinction requires dislocation walls to have developed, these occur more at higher pressures and temperatures, quartz grains with undulose extinction indicate metamorphic rock provenance for that grain; those grains that are monocrystalline quartz are more to have been formed by igneous processes. Differing sources suggest the extent; some note the trend for immature sandstones to have less polycrystalline quartz grains compared to mature sandstones, which have grains that have passed through many sedimentary cycles.
Quartz grains derived from previous sedimentary sources are determined by looking for authigenic, or grown in place, overgrowths of silica cement over the grain. The above descriptions of quartz in thin section is enough to identify it. Minerals with similar appearance may include plagioclase, although it can be distinguished by the distinctive twinning in crossed polarized light and cleavage in plane polarized light, cordierite, although it can be distinguished by twinning or inclusions in the grain. However, for certainty, other distinguishing features of quartz include the fact that it is uniaxial, it has a positive optic sign, length-slow sign of elongation, zero degree extinction angle. Fine-grained rocks those containing minerals of high birefringence, such as calcite, are sometimes prepared as ultra-thin sections. An ordinary 30 μm thin section is prepared as described above but the slice of rock is attached to the glass slide using a soluble cement such as Canada balsam to allow both sides to be worked on.
The section is polished on both sides using a fine diamond paste until it has a thickness in the range of 2-12 μm. This technique has been used to study the microstructure of fine-grained carbonates such as the Lochseitenkalk mylonite in which the matrix grains are less than 5 μm in size; this method is sometimes used in the preparation of mineral and rock specimens for transmission electron microscopy and allows greater accuracy in comparing features using both optical and electron imaging. Ceramography: thin sections of ceramics Shelley, D. Optical Mineralogy, Second Edition. University of Canterbury, New Zealand. Thin sections of soils. Collection of Prof. Kubiëna
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, to understand the stress field that resulted in the observed strain and geometries; this understanding of the dynamics of the stress field can be linked to important events in the geologic past. The study of geologic structures has been of prime importance in economic geology, both petroleum geology and mining geology. Folded and faulted rock strata form traps that accumulate and concentrate fluids such as petroleum and natural gas. Faulted and structurally complex areas are notable as permeable zones for hydrothermal fluids, resulting in concentrated areas of base and precious metal ore deposits. Veins of minerals containing various metals occupy faults and fractures in structurally complex areas; these structurally fractured and faulted zones occur in association with intrusive igneous rocks.
They also occur around geologic reef complexes and collapse features such as ancient sinkholes. Deposits of gold, copper, lead and other metals, are located in structurally complex areas. Structural geology is a critical part of engineering geology, concerned with the physical and mechanical properties of natural rocks. Structural fabrics and defects such as faults, folds and joints are internal weaknesses of rocks which may affect the stability of human engineered structures such as dams, road cuts, open pit mines and underground mines or road tunnels. Geotechnical risk, including earthquake risk can only be investigated by inspecting a combination of structural geology and geomorphology. In addition, areas of karst landscapes which reside atop underground caverns, potential sinkholes, or other collapse features are of particular importance for these scientists. In addition, areas of steep slopes are potential landslide hazards. Environmental geologists and hydrogeologists need to apply the tenets of structural geology to understand how geologic sites impact groundwater flow and penetration.
For instance, a hydrogeologist may need to determine if seepage of toxic substances from waste dumps is occurring in a residential area or if salty water is seeping into an aquifer. Plate tectonics is a theory developed during the 1960s which describes the movement of continents by way of the separation and collision of crustal plates, it is in a sense structural geology on a planet scale, is used throughout structural geology as a framework to analyze and understand global and local scale features. Structural geologists use a variety of methods to measure rock geometries, reconstruct their deformational histories, estimate the stress field that resulted in that deformation. Primary data sets for structural geology are collected in the field. Structural geologists measure a variety of planar features, linear features; the inclination of a planar structure in geology is measured by dip. The strike is the line of intersection between the planar feature and a horizontal plane, taken according to the right hand convention, the dip is the magnitude of the inclination, below horizontal, at right angles to strike.
For example. Alternatively and dip direction may be used as this is absolute. Dip direction is measured in 360 degrees clockwise from North. For example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115. Note that this is the same as above; the term hade is used and is the deviation of a plane from vertical i.e.. Fold axis plunge is measured in dip direction; the orientation of a fold axial plane is dip and dip direction. Lineations are measured in terms of dip direction, if possible. Lineations occur expressed on a planar surface and can be difficult to measure directly. In this case, the lineation may be measured from the horizontal as a pitch upon the surface. Rake is measured by placing a protractor flat on the planar surface, with the flat edge horizontal and measuring the angle of the lineation clockwise from horizontal; the orientation of the lineation can be calculated from the rake and strike-dip information of the plane it was measured from, using a stereographic projection.
If a fault has lineations formed by movement on the plane, e.g.. It is easier to record strike and dip information of planar structures in dip/dip direction format as this will match all the other structural information you may be recording about folds, etc. although there is an advantage to using different formats that discriminate between planar and linear data. The convention for analysing structural geology is to identify the planar structures called planar fabrics because this implies a textural formation, the linear structures and, from analysis of these, unravel deformations. Planar structures a
A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb little energy prior to fracture those of high strength. Breaking is accompanied by a snapping sound. Brittle materials include some polymers, such as PMMA and polystyrene. Many steels become brittle depending on their composition and processing; when used in materials science, it is applied to materials that fail when there is little or no plastic deformation before failure. One proof is to match the broken halves, which should fit since no plastic deformation has occurred; when a material has reached the limit of its strength, it has the option of either deformation or fracture. A malleable metal can be made stronger by impeding the mechanisms of plastic deformation, but if this is taken to an extreme, fracture becomes the more outcome, the material can become brittle. Improving material toughness is therefore a balancing act; this principle generalizes to other classes of material.
Brittle materials, such as glass, are not difficult to toughen effectively. Most such techniques involve one of two mechanisms: to deflect or absorb the tip of a propagating crack, or to create controlled residual stresses so that cracks from certain predictable sources will be forced closed; the first principle is used in laminated glass where two sheets of glass are separated by an interlayer of polyvinyl butyral, which as a viscoelastic polymer absorbs the growing crack. The second method is used in pre-stressed concrete. A demonstration of glass toughening is provided by Prince Rupert's Drop. Brittle polymers can be toughened by using metal particles to initiate crazes when a sample is stressed, a good example being high-impact polystyrene or HIPS; the least brittle structural ceramics are silicon transformation-toughened zirconia. A different philosophy is used in composite materials, where brittle glass fibres, for example, are embedded in a ductile matrix such as polyester resin; when strained, cracks are formed at the glass–matrix interface, but so many are formed that much energy is absorbed and the material is thereby toughened.
The same principle is used in creating metal matrix composites. The brittle strength of a material can be increased by pressure; this happens as an example in the brittle-ductile transition zone at an approximate depth of 10 kilometres in the Earth's crust, at which rock becomes less to fracture, more to deform ductilely. Supersonic fracture is crack motion faster than the speed of sound in a brittle material; this phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart and IBM Almaden Research Center in San Jose, California. Izod impact strength test Charpy impact test Fractography Forensic engineering Ductility Strengthening mechanisms of materials Lewis, Peter Rhys. Forensic Materials Engineering: Case studies. CRC Press. ISBN 978-0-8493-1182-6. Rösler, Joachim. Mechanical behaviour of engineering materials: metals, ceramics and composites. Springer. ISBN 978-3-642-09252-7
En echelon veins
In structural geology, en échelon veins or "en échelon gash fractures" are structures within rock caused by noncoaxial shear. They appear as sets of short, planar, mineral-filled lenses within a body of a rock, they originate as tension fractures that are parallel to the major stress orientation, σ1, in a shear zone. They are subsequently filled by precipitation of a mineral quartz or calcite; as soon as they form, they begin to rotate in the shear zone. Subsequent growth of the fracture therefore causes the vein to take on a sigmoidal shape, they can be used to determine the incremental kinematics of the deformation history of the rock
Metamorphism is the change of minerals or geologic texture in pre-existing rocks, without the protolith melting into liquid magma. The change occurs due to heat and the introduction of chemically active fluids; the chemical components and crystal structures of the minerals making up the rock may change though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism occurs between diagenesis, melting; the geologists who study metamorphism are known as "metamorphic petrologists." To determine the processes underlying metamorphism, they rely on statistical mechanics and experimental petrology. Three types of metamorphism exist: contact and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism. Metamorphic rocks can change without melting. Heat causes atomic bonds to break, the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures, or enabling recrystallization.
When pressure is applied, somewhat flattened grains that orient in the same direction have a more stable configuration. The temperature lower limit on what is considered to be a metamorphic process is considered to be 100 – 200 °C; the upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is 700 – 900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not segregated from the refractory residue. Since the 1980s it has been recognized that rocks are dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900 – 1100 °C; the metamorphic process has to be over pressure of at least 100 mega pascals but below 300 mega pascals, the depth of 100 mega pascals varies depending on what type of rock is applying pressure. Regional or Barrovian metamorphism covers large areas of continental crust associated with mountain ranges those associated with convergent tectonic plates or the roots of eroded mountains.
Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism; these orogenic mountains are eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone produce regional metamorphic effects, characterized by paired metamorphic belts; the techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane. Contact metamorphism occurs around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock; the area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole.
Contact metamorphic rocks are known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are fine-grained. Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact; the size of the aureole depends on the heat of the intrusion, its size, the temperature difference with the wall rocks. Dikes have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have thick and well-developed contact metamorphism; the metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is related to the metamorphic temperatures of pelitic or aluminosilicate rocks and the minerals they form; the metamorphic grades of aureoles are sillimanite hornfels, pyroxene hornfels. Magmatic fluids coming from the intrusive rock may take part in the metamorphic reactions. An extensive addition of magmatic fluids can modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism.
If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest. A special type of contact metamorphism, associated with fossil fuel fires, is known as pyrometamorphism. Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition; the difference in composition between an existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic, circulating ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas
The mica group of sheet silicate minerals includes several related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, are similar in chemical composition; the nearly perfect cleavage, the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms. The word mica is derived from the Latin word mica, meaning a crumb, influenced by micare, to glitter. Chemically, micas can be given the general formula X2Y4–6Z8O204,in which X is K, Na, or Ca or less Ba, Rb, or Cs. Structurally, micas can be classed as trioctahedral. If the X ion is K or Na, the mica is a common mica, whereas if the X ion is Ca, the mica is classed as a brittle mica. Muscovite Common micas: Biotite Lepidolite Phlogopite ZinnwalditeBrittle micas: Clintonite Very fine-grained micas, which show more variation in ion and water content, are informally termed "clay micas", they include: Hydro-muscovite with H3O+ along with K in the X site.
Mica is distributed and occurs in igneous and sedimentary regimes. Large crystals of mica used for various applications are mined from granitic pegmatites; until the 19th century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, their price dropped when large reserves were found and mined in Africa and South America during the early 19th century; the largest documented single crystal of mica was found in Lacey Mine, Canada. Similar-sized crystals were found in Karelia, Russia; the British Geological Survey reported that as of 2005, Koderma district in Jharkhand state in India had the largest deposits of mica in the world. China was the top producer of mica with a third of the global share followed by the US, South Korea and Canada. Large deposits of sheet mica were mined in New England from the 19th century to the 1970s. Large mines existed in Connecticut, New Hampshire, Maine. Scrap and flake mica is produced all over the world. In 2010, the major producers were Russia, United States, South Korea and Canada.
The total global production was 350,000 t. Most sheet mica was produced in Russia. Flake mica comes from several sources: the metamorphic rock called schist as a byproduct of processing feldspar and kaolin resources, from placer deposits, from pegmatites. Sheet mica is less abundant than flake and scrap mica, is recovered from mining scrap and flake mica; the most important sources of sheet mica are pegmatite deposits. Sheet mica prices vary with grade and can range from less than $1 per kilogram for low-quality mica to more than $2,000 per kilogram for the highest quality; the mica group represents 37 phyllosilicate minerals that have a platy texture. The commercially important micas are muscovite and phlogopite, which are used in a variety of applications. Mica’s value is based on several of its unique physical properties; the crystalline structure of mica forms layers that can be split or delaminated into thin sheets causing foliation in rocks. These sheets are chemically inert, elastic, hydrophilic, lightweight, reflective, refractive and range in opacity from transparent to opaque.
Mica is stable when exposed to electricity, light and extreme temperatures. It has superior electrical properties as an insulator and as a dielectric, can support an electrostatic field while dissipating minimal energy in the form of heat. Muscovite, the principal mica used by the electrical industry, is used in capacitors that are ideal for high frequency and radio frequency. Phlogopite mica remains stable at higher temperatures and is used in applications in which a combination of high-heat stability and electrical properties is required. Muscovite and phlogopite are used in ground forms; the leading use of dry-ground mica in the US is in the joint compound for filling and finishing seams and blemishes in gypsum wallboard. The mica acts as a filler and extender, provides a smooth consistency, improves the workability of the compound, provides resistance to cracking. In 2008, joint compound accounted for 54% of dry-ground mica consumption. In the paint industry, ground mica is used as a pigment extender that facilitates suspension, reduces chalking, prevents shrinking and shearing of the paint film, increases the resistance of the paint film to water penetration and weathering and brightens the tone of colored pigments.
Mica promotes paint adhesion in aqueous and oleoresinous formulations. Consumption of dry-ground mica in paint, the second-ranked use, accounted for 22% of the dry-ground mica used in 2008. Ground mica is used in the well-drilling industry as an additive to drilling fluids; the coarsely ground mica flakes help prevent the loss of circulation by sealing po