A rotation is a circular movement of an object around a center of rotation. A three-dimensional object always rotates around a line called a rotation axis. If the axis passes through the center of mass, the body is said to rotate upon itself. A rotation about a point, e. g. the Earth about the Sun, is called a revolution or orbital revolution. The axis is called a pole, mathematically, a rotation is a rigid body movement which, unlike a translation, keeps a point fixed. This definition applies to rotations within both two and three dimensions All rigid body movements are rotations, translations, or combinations of the two, a rotation is simply a progressive radial orientation to a common point. That common point lies within the axis of that motion, the axis is 90 degrees perpendicular to the plane of the motion. If the axis of the rotation lies external of the body in question the body is said to orbit, there is no fundamental difference between a “rotation” and an “orbit” and or spin. The key distinction is simply where the axis of the rotation lies and this distinction can be demonstrated for both “rigid” and “non rigid” bodies.
If a rotation around a point or axis is followed by a rotation around the same point/axis. The reverse of a rotation is a rotation, the rotations around a point/axis form a group. However, a rotation around a point or axis and a rotation around a different point/axis may result in something other than a rotation, Rotations around the x, y and z axes are called principal rotations. Rotation around any axis can be performed by taking a rotation around the x axis, followed by a rotation around the y axis and that is to say, any spatial rotation can be decomposed into a combination of principal rotations. In flight dynamics, the rotations are known as yaw, pitch. This terminology is used in computer graphics. In astronomy, rotation is an observed phenomenon. Stars and similar bodies all spin around on their axes, the rotation rate of planets in the solar system was first measured by tracking visual features. Stellar rotation is measured through Doppler shift or by tracking active surface features and this rotation induces a centrifugal acceleration in the reference frame of the Earth which slightly counteracts the effect of gravity the closer one is to the equator
A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments of the rock before dynamic recrystallisation or cataclasis produced the groundmass. This means they are older than the groundmass and they were stronger pieces of the original rock, that could not as easily deform and were therefore not or hardly affected by recrystallisation. They may have been phenocrysts or porphyroblasts in the original rock, porphyroclasts are often confused with porphyroblasts. The latter are large crystals in a finer matrix, but they grew during, or after deformation took place. The timing of porphyroblast growth can be determined by examining the microstructure preserved within them as poikiloblasts, in strongly deformed rocks porphyroclasts are often rotated by the shear stress in the rock. Their shape can be used to determine the direction of the shear, where porphyroclasts have rims made of finer grained crystals, they are referred to as porphyroclast systems.
The geometries of porphyroclast systems can be used to determine the sense of shear within a shear zone
For example, when a solid vertical bar is supporting a weight, each particle in the bar pushes on the particles immediately below it. When a liquid is in a container under pressure, each particle gets pushed against by all the surrounding particles. The container walls and the pressure-inducing surface push against them in reaction and these macroscopic forces are actually the net result of a very large number of intermolecular forces and collisions between the particles in those molecules. Strain inside a material may arise by various mechanisms, such as stress as applied by external forces to the material or to its surface. Any strain of a material generates an internal elastic stress, analogous to the reaction force of a spring. In liquids and gases, only deformations that change the volume generate persistent elastic stress, however, if the deformation is gradually changing with time, even in fluids there will usually be some viscous stress, opposing that change. Elastic and viscous stresses are usually combined under the mechanical stress.
Significant stress may exist even when deformation is negligible or non-existent, stress may exist in the absence of external forces, such built-in stress is important, for example, in prestressed concrete and tempered glass. Stress may be imposed on a material without the application of net forces, for example by changes in temperature or chemical composition, stress that exceeds certain strength limits of the material will result in permanent deformation or even change its crystal structure and chemical composition. In some branches of engineering, the stress is occasionally used in a looser sense as a synonym of internal force. For example, in the analysis of trusses, it may refer to the total traction or compression force acting on a beam, since ancient times humans have been consciously aware of stress inside materials. Until the 17th century the understanding of stress was largely intuitive and empirical, with those tools, Augustin-Louis Cauchy was able to give the first rigorous and general mathematical model for stress in a homogeneous medium.
Cauchy observed that the force across a surface was a linear function of its normal vector, moreover. The understanding of stress in liquids started with Newton, who provided a formula for friction forces in parallel laminar flow. Stress is defined as the force across a small boundary per unit area of that boundary, following the basic premises of continuum mechanics, stress is a macroscopic concept. In a fluid at rest the force is perpendicular to the surface, in a solid, or in a flow of viscous liquid, the force F may not be perpendicular to S, hence the stress across a surface must be regarded a vector quantity, not a scalar. Moreover, the direction and magnitude depend on the orientation of S. Thus the stress state of the material must be described by a tensor, called the stress tensor, with respect to any chosen coordinate system, the Cauchy stress tensor can be represented as a symmetric matrix of 3×3 real numbers
Cataclasite is a type of cataclastic rock that is formed by fracturing and comminution during faulting. It is normally cohesive and non-foliated, consisting of angular clasts in a finer-grained matrix, there are many varieties of cataclasite, classified by the percentage of the volume formed from the matrix. Protocataclasite, protocataclasite is a type of cataclasite in which the matrix takes up less than 50% of the total volume, mesocataclasite is a type of cataclasite in which the matrix occupies between 50 and 90 percent of the total volume. Ultracataclasite, ultracataclasite is a type of cataclasite characterized by a matrix occupying greater than 90% of the total volume, foliated cataclasite, foliated cataclasite is a type of cataclasite with a significant content of clay. Grades through to fault gouge when the proportion of clay is greater than 70%, fault breccia, fault breccia is a medium to coarse-grained cataclasite. Cataclasite forms by the fracturing of mineral grains and aggregates.
From on deformation is accommodated by continued sliding and rolling of fragments, a deformation mechanism known as cataclastic flow
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have different mineralogical compositions, it is a classification based on the textural appearance of the rock. Mylonites are ductilely deformed rocks formed by the accumulation of large shear strain, in fault zones. Mechanical abrasion of grains by milling does not occur, although this was thought to be the process that formed mylonites. There are many different mechanisms that accommodate crystal-plastic deformation, in crustal rocks the most important processes are dislocation creep and diffusion creep. Dislocation generation acts to increase the energy of crystals. This process tends to organize dislocations into subgrain boundaries and this process, sometimes referred to as subgrain rotation recrystallization, acts to reduce the mean grain size. Volume and grain-boundary diffusion, the mechanisms in diffusion creep, become important at high temperatures.
Mylonites generally develop in ductile shear zones where high rates of strain are focused and they are the deep crustal counterparts to cataclastic brittle faults that create fault breccias. Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding, ultramylonites usually have undergone extreme grainsize reduction. In structural geology, ultramylonite is a kind of defined by modal percentage of matrix grains more than 90%. Ultramylonite is often hard, cherty to flinty in appearance and sometimes resemble pseudotachylite, in reverse, ultramylonite-like rocks are sometimes deformed pseudotachylyte. Mesomylonites have undergone an appreciable amount of reduction, and are defined by their modal percentage of matrix grains being between 50 and 90%. Protomylonites are mylonites which have experienced limited grainsize reduction, and are defined by their percentage of matrix grains being less than 50%. Because mylonitisation is incomplete in these rocks, relict grains and textures are apparent and they typically have a well-developed secondary shear fabric.
This is referred to as determining the shear sense and it is common practice to assume that the deformation is plane strain simple shear deformation. This type of strain field assumes that deformation occurs in a zone where displacement is parallel to the shear zone boundary. Furthermore, during deformation the incremental strain axis maintains a 45 degree angle to the zone boundary
A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale.
Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind.
Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
It may be either a detailed description of these characteristics or be a summary of the gross physical character of a rock. It is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping, in certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7. The naming of a lithology is based on the rock type, the three major rock types are sedimentary, metamorphic. Sedimentary rocks are classified by whether they are siliciclastic or carbonate. Siliciclastic sedimentary rocks are subcategorized based on their size distribution and the relative proportions of quartz, feldspar. Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock, the name of an igneous rock requires information on crystal size and mineralogy. This classification can often be performed with a QAPF diagram, metamorphic rock naming can be based on texture, metamorphic facies, and/or the locations in which they are found.
Naming based on texture and a pelite protolith can be used to slate and phyllite. Texture-based names are schist and gneiss and these textures, from slate to gneiss, define a continually-increasing extent of metamorphism. Metamorphic facies are defined by the fields in which particular minerals form. In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in the rock. In igneous rock, this is used to determine the rate at which the material cooled, large crystals typically indicate intrusive igneous rock, as metamorphic reactions progress, the grains in metamorphic rocks can often be broken down into smaller grains. In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock and these are used to determine which rock naming system to use. In the case of sandstones and conglomerates, which cover a range of grain sizes. Examples are pebble conglomerate and fine quartz arenite, in rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description.
The mineralogical composition of a rock is one of the ways in which it is classified. In general, igneous rocks can be categorized by increasing silica content as ultramafic, intermediate, or felsic, the fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main fabric is normally bedding and the scale
Quartz is the second most abundant mineral in Earths continental crust, behind feldspar. There are many different varieties of quartz, several of which are semi-precious gemstones, since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia. The word quartz is derived from the German word Quarz and its Middle High German ancestor twarc, the Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning icy cold, because some philosophers apparently believed the mineral to be a form of supercooled ice. Today, the rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end, well-formed crystals typically form in a bed that has unconstrained growth into a void, usually the crystals are attached at the other end to a matrix and only one termination pyramid is present.
However, doubly terminated crystals do occur where they develop freely without attachment, a quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward. α-quartz crystallizes in the crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the system, space group P6222 and P6422. These space groups are truly chiral, both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks. The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is an identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, common colored varieties include citrine, rose quartz, smoky quartz, milky quartz, and others.
The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties, the cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, heliotrope, amethyst is a form of quartz that ranges from a bright to dark or dull purple color. The worlds largest deposits of amethysts can be found in Brazil, Uruguay, France, sometimes amethyst and citrine are found growing in the same crystal. It is referred to as ametrine, an amethyst is formed when there is iron in the area where it was formed
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earths interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth near the surface in most of the world, strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets. A line tracing the gradient through the body is called a geotherm on Earth. On the Moon it is called a selenotherm, the Earths internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, and possibly heat from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, at the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa. Temperature within the Earth increases with depth, the heat content of the Earth is 1031 joules. Much of the heat is created by decay of radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from decay of elements mainly located in the mantle.
Heat of impact and compression released during the formation of the Earth by accretion of in-falling meteorites. Heat released as abundant heavy metals descended to the Earths core, latent heat released as the liquid outer core crystallizes at the inner core boundary. Heat may be generated by tidal force on the Earth as it rotates, since rock cannot flow as readily as water it compresses and distorts, in Earths continental crust, the decay of natural radioactive isotopes has had significant involvement in the origin of geothermal heat. The continental crust is abundant in lower density minerals but contains significant concentrations of heavier lithophilic minerals such as uranium, because of this, it holds the largest global reservoir of radioactive elements found in the Earth. Especially in layers closer to Earths surface, naturally occurring isotopes are enriched in the granite and these high levels of radioactive elements are largely excluded from the Earths mantle due to their inability to substitute in mantle minerals and consequent enrichment in partial melts.
The mantle is made up of high density minerals with high contents of atoms that have relatively small atomic radii such as magnesium, titanium. Heat flows constantly from its sources within the Earth to the surface, total heat loss from the Earth is estimated at 44.2 TW. Mean heat flow is 65 mW/m2 over continental crust and 101 mW/m2 over oceanic crust and this is 0.087 watt/square meter on average, but is much more concentrated in areas where thermal energy is transported toward the crust by convection such as along mid-ocean ridges and mantle plumes. The Earths crust effectively acts as an insulating blanket which must be pierced by fluid conduits in order to release the heat underneath. More of the heat in the Earth is lost through plate tectonics, the final major mode of heat loss is by conduction through the lithosphere, the majority of which occurs in the oceans due to the crust there being much thinner and younger than under the continents
Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe.
It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
Pseudotachylite or Pseudotachylyte is a cohesive glassy or very fine-grained rock that occurs as veins and often contains inclusions of wall-rock fragments. Pseudotachylite is typically dark in color, and is glassy in appearance and it was named after its appearance resembling the basaltic glass, tachylyte. Typically, the glass is completely devitrified into very fine-grained material with radial, the glass may contain crystals with quench textures that formed via crystallization from the melt. Chemical composition of pseudotachylyte generally reflects the bulk chemistry. Pseudotachylyte may form via frictional melting of faults, in large-scale landslides, many researchers often define the rock as one formed via the melting. However, the original description/definition by Shand did not include interpretation about its generation and it is found either along fault surfaces, as the matrix to a fault breccia, or as veins injected into the walls of the fault. In most cases, researchers search for and describe good evidence that the pseudotachylite formed by melting of the wall rocks during rapid fault movement associated with a seismic event.
This has caused them to be termed fossil earthquakes, the thickness of the pseudotachylite zone is indicative of the magnitude of the associated displacement and the general magnitude of the paleoseismic event. Some pseudotachylites have been interpreted as forming by comminution rather than melting and they have a similar occurrence to melt-derived pseudotachylites but lack clear indications of a melt origin. Pseudotachylite is associated with impact structures, in an impact event, melting forms as part of the shock metamorphic effects. Pseudotachylite veins associated with impacts are much larger than those associated with faults, impact-generated veins form by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. The first described pseudotachylyte is of this type, chapter 4, Pseudotachylitic breccias, other breccias and veins. Structural analysis of impact-related deformation in the rocks of the Vredefort Dome.
School of Geosciences, University of the Witwatersrand, South Africa