Miscellany

Jai Ganesh · 2020-12-16 00:35:44

854) Fire triangle

The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires.

The triangle illustrates the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen). A fire naturally occurs when the elements are present and combined in the right mixture, meaning that fire is actually an event rather than a thing. A fire can be prevented or extinguished by removing any one of the elements in the fire triangle. For example, covering a fire with a fire blanket removes the oxygen part of the triangle and can extinguish a fire. In large fires where firefighters are called in, decreasing the amount of oxygen is not usually an option because there is no effective way to make that happen in an extended area.

Fire tetrahedron

The fire tetrahedron represents the addition of a component in the chemical chain reaction, to the three already present in the fire triangle. Once a fire has started, the resulting exothermic chain reaction sustains the fire and allows it to continue until or unless at least one of the elements of the fire is blocked. Foam can be used to deny the fire the oxygen it needs. Water can be used to lower the temperature of the fuel below the ignition point or to remove or disperse the fuel. Halon can be used to remove free radicals and create a barrier of inert gas in a direct attack on the chemical reaction responsible for the fire.

Combustion is the chemical reaction that feeds a fire more heat and allows it to continue. When the fire involves burning metals like lithium, magnesium, titanium, etc. (known as a class-D fire), it becomes even more important to consider the energy release. The metals react faster with water than with oxygen and thereby more energy is released. Putting water on such a fire results in the fire getting hotter or even exploding. Carbon dioxide extinguishers are ineffective against certain metals such as titanium. Therefore, inert agents (e.g. dry sand) must be used to break the chain reaction of metallic combustion.

In the same way, as soon as one of the four elements of the tetrahedron is removed, combustion stops.

Oxidizer

The oxidizer is the other reactant of the chemical reaction. In most cases, it is the ambient air, and in particular one of its components, oxygen (O2). By depriving a fire of air, it can be extinguished; for example, when covering the flame of a small candle with an empty glass, fire stops; to the contrary, if air is blown over a wood fire with bellows, the fire is activated by the introduction of more air .

Some chemicals, such as fluorine gas, perchlorate salts such as ammonium perchlorate, or chlorine trifluoride, act as oxidisers, sometimes more powerful ones than oxygen itself. A fire based on a reaction with these oxidisers can be very difficult to put out until the oxidiser is exhausted; that leg of the fire triangle cannot be broken by normal means (i.e., depriving it of air will not smother it).

In certain cases such as some explosives, the oxidizer and combustible are the same (e.g., nitroglycerin, an unstable molecule that has oxidizing parts in the same molecule as the oxidizeable parts).

Reaction is initiated by an activating energy, in most cases, it is heat. Several examples include friction, as in case of matches, heating an electrical wire, a flame (propagation of fire), or a spark (from a lighter or from any starting electrical device). There are also many other ways to bring sufficient activation energy including electricity, radiation, and pressure, all of which will lead to a temperature rise. In most cases, heat production enables self-sustainability of the reaction, and enables a chain reaction to grow. The temperature at which a liquid produces sufficient vapor to get a flammable mix with self-sustainable combustion is called its flash-point.

Extinction of the fire

To stop a combustion reaction, one of the three elements of the fire-triangle has to be removed.

Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by the application of a substance which reduces the amount of heat available to the fire reaction. This is often water, which absorbs heat for phase change from water to steam. Introducing sufficient quantities and types of powder or gas in the flame reduces the amount of heat available for the fire reaction in the same manner. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source.

Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics, such as controlled burns. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat.

Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion process slows. Oxygen can be denied to a fire using a carbon dioxide fire extinguisher, a fire blanket or water.

Role of water in fire-fighting

Water can have two different roles. In the case of a solid combustible, the solid fuel produces pyrolyzing products under the influence of heat, commonly radiation. This process is halted by the application of water, since water is more easily evaporated than the fuel is pyrolyzed. Thereby energy is removed from the fuel surface and it is cooled and the pyrolysis is stopped, removing the fuel supply to the flames. In fire fighting, this is referred to as surface cooling.

In the gas phase, i.e. in the flames or in the smoke, the combustible can not be separated from the oxidizer, and the only possible action consists of cooling down. In this case, water droplets are evaporated in the gas phase, thereby lowering the temperature and adding water vapour making the gas mixture non combustible. This requires droplets of a size less than about 0.2 mm. In fire fighting, this is referred to as gas cooling or smoke cooling.

Cases also exist where the ignition factor is not the activation energy. For example, a smoke explosion is a very violent combustion of unburned gases contained in the smoke created by a sudden fresh air input (oxidizer input). The interval in which an air/gas mix can burn is limited by the explosive limits of the air. This interval can be very small (kerosene) or large (acetylene).

Water cannot be used on certain type of fires:
• Fires where live electricity is present – as water conducts electricity it presents an electrocution hazard.
• Hydrocarbon fires – as it will only spread the fire because of the difference in density/hydrophobicity. For example, adding water to a fire with an oil source will cause the oil to spread, since oil and water do not mix.
• Metal fires – as these fires produce huge amounts of energy (up to 7.550 calories/kg for aluminium) and water can also create violent chemical reactions with burning metal (possibly even serving as an additional oxidizing agent).

Since these reactions are well understood, it has been possible to create specific water-additives which will allow:
• A better heat absorption with a higher density than water.
• Carrying free radical catchers on the fire.
• Carrying foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release.
• Carrying specific reactives which will react and change the nature of the burning material.

Water-additives are generally designed to be effective on several categories of fires (class A + class B or even class A + class B + class F), meaning a better global performance and usability of a single extinguisher on many different types of fires (or fires that involve several different classes of materials).

Multi-scale fire triangles for wildland fires

In the context of wildland fire, the fire triangle can be scaled up to apply to understand fire spread over landscapes (scales of days and several km) and recurrence of fire over time (scales of decades and hundreds of km). Thus, while heat is important to ignite a flame, topography is important for aiding fire spread especially by preheating upslope fuels, and ignitions sources are important to help explain recurrence on longer time scales. Similarly, while oxygen is relevant to sustain a flame, weather and associated winds feed oxygen into a spreading fire, and the longer-term pattern of weather is summarized as climate. Lastly, fuels is the term to describe what burns in a single flame to the range of materials burnt in a spreading wildfire, but fuels vary over larger space and time scales in what is called vegetation.

At the smallest scale, the combustion fire triangle, individual fuel particles are ignited once at a critical temperature, and fire transfers energy to the nearest surroundings. Combustion events range on the scale of several seconds to a couple of days and their effects are monitored at the quadrant scale. The largest scale, in contrast, describes the fire regime concept. Global climate change drives many of the factors involved in the 'wildfire' and the 'fire regime' triangles. For example, with respect to the fire regime, a particular vegetation type will support a characteristic fire in terms of recurrence, intensity, seasonality and biological effects; a change in vegetation type will have implications for a changing fire regime.

Jai Ganesh · 2020-12-17 00:37:55

855) Rift valley

Rift valley, any elongated trough formed by the subsidence of a segment of the Earth’s crust between dip-slip, or normal, faults. Such a fault is a fracture in the terrestrial surface in which the rock material on the upper side of the fault plane has been displaced downward relative to the rock below the fault. A rift valley constitutes a type of tectonic valley and, as such, differs from river and glacial valleys, which are produced by erosional forces.

A brief treatment of rift valleys follows.

Rift valleys are usually narrow and long, some measuring hundreds of kilometres in length. Their floors are relatively flat, owing in large part to volcanic deposition and marine or lacustrine sedimentation. The sides of rift valleys drop steeply away in the form of steps and terraces. At their margins, the walls of the valleys may rise hundreds of metres.

Rift valleys are found both on the continents and on the floor of ocean basins. In terms of the theory of plate tectonics, they occur in divergence zones, belts where two of the various lithospheric plates that make up the Earth’s surface are separating. Numerous submarine rift valleys have been discovered along the crests of the large ridges that run throughout the Earth’s oceans. These ridges are centres of seafloor spreading: areas where magma from the mantle is welling up, cooling to form new oceanic crust, and is moving away from the crests in either direction.

The distribution of rift valleys on the continents is irregular and relatively sparse, but they seem to occur at sites of incipient plate spreading. Many have volcanic cones on their floors or contain deep lakes. The most extensive of the continental rift valleys are those of the East African Rift System, which extend northward to the Red Sea and eastward into the Indian Ocean. Other notable examples include the Baikal Rift Valley (Russia) and the Rhine Rift Valley (Germany).

A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. Rift valleys are found both on land and at the bottom of the ocean, where they are created by the process of seafloor spreading. Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion.

Tectonic plates are huge, rocky slabs of Earth's lithosphere—its crust and upper mantle. Tectonic plates are constantly in motion—shifting against each other in fault zones, falling beneath one another in a process called subduction, crashing against one another at convergent plate boundaries, and tearing apart from each other at divergent plate boundaries.

Many rift valleys are part of “triple junctions,” a type of divergent boundary where three tectonic plates meet at about 120° angles. Two arms of the triple junction can split to form an entire ocean. The third, “failed rift” or aulacogen, may become a rift valley. The Atlantic Ocean, for instance, is a result of a triple junction that started in what is now the Gulf of Guinea on the west coast of Africa. Two arms of a triple junction on the supercontinent Pangaea “opened” the ocean, while the aulacogen formed the rift valley known as the Benue Trough through what is now southern Nigeria.

Rift valleys can also form at transform faults, where tectonic plates are grinding past each other. The Salton Trough, which stretches through the states of California (United States) and Baja California (Mexico), is a rift valley created in part by the San Andreas Fault. The San Andreas is a transform fault that marks the roughly northward movement of the Pacific plate and the roughly southern movement of the North American plate.

Mid-Ocean Ridges

Many of Earth’s deepest rift valleys are found underwater, dividing long mountain ranges called mid-ocean ridges. As tectonic plates move away from one another at mid-ocean ridges, molten rock from the mantle may well up and harden as it contacts the frigid sea, forming new oceanic crust at the bottom of the rift valley.

In the northern Mid-Atlantic Ridge, the North American plate and the Eurasian plate are splitting apart at a rate of about 2.5 centimeters (1 inch) per year. Over millions of years, the Mid-Atlantic Ridge has formed rift valleys as wide as 15 kilometers (9 miles).

In the Pacific Ocean, the East Pacific Rise has created rift valleys where the Pacific plate is separating from the North American plate, Cocos plate, Nazca plate, and Antarctic plate. Like many underwater rift valleys, the East Pacific Rise is dotted with hydrothermal vents. Geologic activity beneath the underwater rift valley creates these vents, which spew super-heated water and vent fluids into the ocean.

Continental Rift Valleys

Very few active rift valleys are found on continental lithosphere. The East African Rift, the Baikal Rift Valley, the West Antarctic Rift, and the Rio Grande Rift are Earth’s major active continental rift valleys. The East African Rift is part of the “Great Rift Valley” system discussed below.

The Baikal Rift Valley (sometimes called the Baikal Rift Zone) cuts through 2,000 kilometers (1,200 miles) of Siberia, in eastern Russia. The Baikal Rift Valley is formed by a divergent plate boundary, where the Amur plate is slowly tearing itself away from the Eurasian plate, and has been doing so for about 25 million years. The Amur plate is moving eastward at a rate of about 4 to 5 millimeters (.16 to .2 inch) a year.

The West Antarctic Rift is a series of smaller rifts that roughly separate the two regions of Earth’s southernmost continent, West Antarctica and East Antarctica. The West Antarctic Rift is one of the most difficult rift valleys to study, because it lies beneath the massive Antarctic Ice Sheet, which can be more than 2 kilometers (1.2 miles) thick.

The Rio Grande Rift is a series of rift valleys along faults in the Southwestern United States. The Rio Grande Rift separates the Colorado Plateau, which is generally moving in a clockwise direction, from the older part (craton) of the North American plate. The Rio Grande Rift stretches from central Colorado to the Mexican state of Chihuahua.

Great Rift Valley

The most well-known rift valley on Earth is probably the so-called "Great Rift Valley System" which stretches from the Middle East in the north to Mozambique in the south. The area is geologically active, and features volcanoes, hot springs, geysers, and frequent earthquakes.

Today, however, the Great Rift Valley exists as a cultural concept, not a scientific one. All of the rift valleys in the “system” are connected, but not part of a single unit.

The northern part of the system is the Jordan Rift Valley. The Jordan Rift Valley stretches from the Golan Heights, near Israel’s border with Syria and Lebanon, to the Dead Sea, to the Gulf of Aqaba—an inlet of the Red Sea that separates the Sinai Peninsula from the Arabian Peninsula.

Associated with the Jordan Rift Valley to the south is the Red Sea Rift. Millions of years ago, the Arabian Peninsula was connected to Africa. Seafloor spreading caused the Arabian and African plates to rift apart. The Indian Ocean flooded the rift valley between the continents, creating the Red Sea. Today, Africa and Asia are connected by the triangle of the Sinai Peninsula. Eventually, the Red Sea Rift will separate Africa and Asia entirely and connect the Mediterranean and Red Seas.

East African Rift

South of the Red Sea Rift lies the massive, complex East African Rift. Throughout the East African Rift, the continent of Africa is splitting in two. The African plate, sometimes called the Nubian plate, carries most of the continent, while the smaller Somali plate carries Horn of Africa.

The two major rift valley systems of the East African Rift are the Gregory Rift and the Western Rift. These rift valleys are dotted by volcanoes: Erta Ale, Ethiopia; Mount Kenya, Kenya (an extinct stratovolcano); Ol Doinyo Lengai, Tanzania; Mount Kilimanjaro, Tanzania (a dormant stratovolcano); and Mount Nyiragongo, Democratic Republic of Congo.

The Gregory Rift stretches from the Red Sea and the Arabian Sea to as far south as Mount Kilimanjaro. One of the most important features of the Gregory Rift is the Afar Triple Junction, found where the Horn of Africa straddles the Red Sea and the Gulf of Aden in the Arabian Sea. At the Afar Triple Junction, the Arabian plate, Nubian plate and Somali plate are all tearing away from each other.

Two arms of the Afar Triple Junction continue to widen in the process of seafloor spreading—the arm extending into the Red Sea and the arm extending into the Gulf of Aden. As these rifts continue, the narrow valley created by the Gregory Rift (the arm of the Afar Triple Junction located above sea level) may sink low enough that the Arabian Sea will flood it. Separated from Africa by this new strait, Horn of Africa (sitting on the Somali plate) would become a continental island, like Madagascar or New Zealand.

The Western Rift, also called the Albertine Rift, includes many of the African Great Lakes. The Western Rift is one of the most biodiverse regions in Africa, featuring a narrow corridor of highland forests, snow-capped mountains, savannas, and chains of lakes and wetlands.

Rift Lakes

Rift lakes, formed as freshwater floods rift valleys, often mark rift valley systems. More than a billion years ago, for instance, the North American plate began a rifting process. A triple junction formed in the middle of the young continent, and deep rift valley developed. Freshwater drained and collected in this rift valley, creating a lake. After millions of years, however, the rift failed. The continent remained intact and the rift’s arms failed to open up a new ocean. Today, the remains of that ancient rift lake, Lake Superior, rest atop one of the oldest and deepest rift valleys in the world.

Lake Baikal, the rift lake over the Baikal Rift Valley in Siberia, is the deepest and oldest freshwater lake in the world. The deepest parts of Lake Baikal are 1,642 meters (5,387 feet), and are getting deeper every year. In addition, over the past 25 million years, layers of soft sediment have accumulated on the lakebed. The actual floor of the rift valley is more than 5 kilometers (3 miles) deep. Lake Baikal also has the largest volume of liquid freshwater in the world—a staggering 23,615 cubic kilometers (5,700 cubic miles).

The Dead Sea is a rift lake in the Jordan Rift Valley. Although the Dead Sea is not the world's deepest lake, the deep Jordan Rift makes it the lowest land elevation on Earth. The surface of the Dead Sea is 429 meters (1,407 feet) below sea level, and the lake’s depth is another 304 meters (997 feet). Unlike Lake Baikal, however, the Dead Sea is not a true rift lake as it was not formed entirely by the rift beneath it. The so-called Dead Sea Transform is a geologically complex area, where tectonic plates interact in many ways.

The most famous rift lakes in the world may be the series of narrow, deep rift valleys in the East African Rift known simply as the Rift Valley lakes. The Rift Valley lakes, stretching from Ethiopia to Malawi, are sites of amazing biodiversity. They include freshwater lakes, similar to Lake Baikal, as well as saltwater “soda lakes” similar to the Dead Sea.

Lake Tanganyika, whose long shores are shared by Burundi, Democratic Republic of Congo, Tanzania, and Zambia, is the largest of the Rift Valley lakes. Lake Tanganyika is the world’s second-deepest and second-biggest (by volume of freshwater) lake in the world. Only Lake Baikal is deeper and holds more water. Like many freshwater Rift Valley lakes, Lake Tanganyika is home to hundreds of endemic species of cichlid fish.

Lake Natron, Tanzania, is one of the shallow, alkali-rich soda lakes of the East African Rift. Its dazzling red color is not a product of the region’s rocky geology, but the pink salt-loving bacteria that live in the briny water.

Jai Ganesh · 2020-12-18 01:17:46

856. Tectonic landform

Tectonic landform, any of the relief features that are produced chiefly by uplift or subsidence of the Earth’s crust or by upward magmatic movements. They include mountains, plateaus, and rift valleys.

Whereas erosion shapes landforms, their origins lie in tectonic processes that build the major structures of the Earth. The word tectonic is derived from the Greek word tekton, which means “builder.” Tectonic processes build landforms mainly by causing the uplift or subsidence of rock material—blocks, layers, or slices of the Earth’s crust, molten lavas, and even large masses that include the entire crust and uppermost part of the planet’s mantle. In some areas, these processes create and maintain high elevations such as mountains and plateaus. In others, they produce topographic depressions, as exemplified by Death Valley in the western United States, the Dead Sea in the Middle East, or the Turfan Depression in western China. Virtually all areas below sea level have been formed by tectonic processes.

Mountain ranges and plateaus result either from the uplift of the Earth’s surface or from the emplacement of volcanic rock onto the surface. Many mountain ranges consist of chains of volcanoes that are made up of rocks derived from depths of tens of kilometres below the surface. Some plateaus are created by huge outpourings of lavas over vast areas. In addition, the intrusion of molten rock into the crust from below can raise the surface. Many other mountain ranges have been formed by the overthrusting of one terrain or block of crust over an adjacent one, which is another mechanism that uplifts the surface. Similarly, the folding of rocks at the surface creates the ridges and valleys that define some mountain chains. These processes of overthrusting (or underthrusting) and folding result from horizontal forces that cause crustal shortening (in its horizontal dimension) and crustal thickening. Finally, heating and thermal expansion of the outer 100 to 200 kilometres of the Earth can uplift broad areas into either mountain ranges or plateaus.

Similarly, tectonic valleys, basins, and depressions of smaller size can form by the reverse of two of the processes mentioned above. Crustal extension (in its horizontal dimension) and crustal thinning occur where two blocks of crust move apart; a valley or basin forms between such blocks where the intervening segment of crust has been thinned and its top surface subsides. Likewise, subsidence of the Earth’s surface can occur by the cooling and the thermal contraction of the outer 100 kilometres of the planet. Plateaus and entire mountain ranges can subside by this mechanism to form large basins in some areas.

Virtually all large-scale landforms are the result of both tectonic processes that built the large differences in elevation and erosional processes that sculpted the relief of such areas into their individual shapes. Thus, it might be said that tectonic processes built the Alps, but erosional processes gave the Matterhorn its unique profile. In all cases, erosion acts to reduce differences in elevation, but when the rate of erosion is not too rapid, landforms created by tectonic processes can persist for hundreds of millions of years after the processes have ceased to operate.

Jai Ganesh · 2020-12-19 00:32:48

857) Plateau

Plateau, extensive area of flat upland usually bounded by an escarpment (i.e., steep slope) on all sides but sometimes enclosed by mountains. The essential criteria for plateaus are low relative relief and some altitude.

Although plateaus stand at higher elevation than surrounding terrain, they differ from mountain ranges in that they are remarkably flat. Some plateaus, like the Altiplano in southern Peru and western Bolivia, are integral parts of mountain belts. Others, such as the Colorado Plateau (across which the Colorado River has cut the Grand Canyon), were produced by processes very different from those that built neighbouring mountain ranges. Some plateaus—for example, the Deccan plateau of central India—occur far from mountain ranges. The differences among plateaus can be ascribed to the different geologic processes that have created them.

Geomorphic Characteristics

The high flat surface that defines a plateau can continue for hundreds or even thousands of kilometres, as in the case of the Plateau of Tibet. In spite of the paucity of roads, one can drive over most of that plateau, where elevations exceed 4,500 metres (about 14,760 feet), and encounter less relief than in some major cities of the world (e.g., San Francisco or Rio de Janeiro). Although ranges of hills and mountains rise above the rest of the plateau, their topography too is rather gentle.

Plateaus dissected (eroded) by rivers have remarkably uniform maximum elevations, but their surfaces can be interrupted by deep canyons. In the case of some regions described as plateaus, the surface is so dissected that one does not see any flat terrain. Instead, such a plateau is defined by a uniform elevation of the highest ridges and mountains. The eastern part of the Plateau of Tibet, which constitutes the headwaters of many of the great rivers of Asia (e.g., Huang He, Yangtze, Mekong, Salween, and Irrawaddy), is dissected into deep canyons separated by narrow, steep ridges; the high uniform elevation that characterizes plateaus is only barely discernible in that area.

Formative Processes

The formation of a plateau requires one of the same three types of tectonic processes that create mountain ranges—volcanism, crustal shortening (by the thrusting of one block or slice of crust over another or by the folding of layers of rock), and thermal expansion. The simplest of these is thermal expansion of the lithosphere (or the replacement of cold mantle lithosphere by hot asthenosphere).

When the lithosphere underlying a broad area is heated rapidly—e.g., by an upwelling of hot material in the underlying asthenosphere—the consequent warming and thermal expansion of the uppermost mantle causes an uplift of the overlying surface. If the uplifted surface had originally been low and without prominent relief, it is likely to remain relatively flat when uplifted to a relatively uniform elevation. The high plateaus of East Africa and Ethiopia were formed that way. As in parts of Africa, plateaus of that sort can be associated with volcanism and with rift valleys, but those features are not universal. Most of the high plateau in East Africa that holds Lake Victoria does not contain volcanic rock and is cut only by small, minor rift valleys.

Where the uplifted surface lay at a low elevation for a very long time and was covered by resistant sedimentary rock, the flatness of the plateau can be particularly marked. The rock underlying the Colorado Plateau has undergone only very mild deformation since about 1.7 billion years ago during Precambrian time (4.6 billion to 541 million years ago), and layers of very resistant limestone and sandstone deposited during the Paleozoic Era (541 million to 252 million years ago) form its top surface in many areas. The warming of the underlying lithosphere in late Cenozoic time (66 million years ago to the present) caused that area to rise to its present elevation, and those erosion-resistant formations dating to Paleozoic time define the surfaces that make the remarkably flat horizons at the Grand Canyon.

The great heights of some plateaus, such as the Plateau of Tibet or the Altiplano, are due to crustal shortening. The geologic structure of plateaus of that kind is entirely different from that of the Colorado Plateau, for instance. Crustal shortening, which thickens the crust as described above, has created high mountains along what are now the margins of such plateaus. In most mountain ranges, streams and rivers transport eroded material from the mountains to the neighbouring plains. When drainage is internal and streams and rivers deposit their debris in the valleys between mountains, however, a plateau can form. The surface of this sort of plateau is defined by very flat, broad valleys surrounded by eroded hills and mountains. The rocks that make up the mountains and the basement of the valleys are often strongly deformed, but the young sediment deposited in the valleys usually lies flat. Those plateaus generally survive erosion only in dry climates where erosion is slow. In many cases, the valleys, or basins, are occupied by flat dry lake beds (playas). Thus, plateaus built by crustal shortening are really mountain ranges buried in their own debris.

A third type of plateau can form where extensive lava flows (called flood basalts or traps) and volcanic ash bury preexisting terrain, as exemplified by the Columbia Plateau in the northwestern United States. The volcanism involved in such situations is commonly associated with hot spots. The lavas and ash are generally carried long distances from their sources, so that the topography is not dominated by volcanoes or volcanic centres. The thickness of the volcanic rock can be tens to even hundreds of metres, and the top surface of flood basalts is typically very flat but often with sharply incised canyons and valleys.

The separation of plateaus into the above three types is not always easy, because two or even all three of the processes involved frequently operate simultaneously. For instance, where the uppermost mantle is particularly hot, volcanism is common. The Ethiopian Plateau, on which Precambrian rocks crop out, stands high because the underlying lithosphere has been heated, but Cenozoic volcanic rocks cover much of the plateau, especially those areas that are the flattest. Although the scale is different, there are active volcanoes and young lavas covering some broad basins on the northern part of the Plateau of Tibet. All three processes—thermal expansion, crustal shortening, and volcanism—may have contributed to the high, flat elevation of at least part of that plateau.

Geographic Distribution

Plateaus of one type or another can be found on most continents. Those caused by thermal expansion of the lithosphere are usually associated with hot spots. The Yellowstone Plateau in the United States, the Massif Central in France, and the Ethiopian Plateau in Africa are prominent examples. Most hot spots are associated with the upwelling of hot material in the asthenosphere, and the hot upwelling not only heats the overlying lithosphere and melts holes through it to produce volcanoes but also uplifts the lithosphere. The relationship of such plateaus to hot spots ensures both a wide distribution of plateaus and an absence of belts of plateaus or of interrelated plateaus.

Some plateaus, like the Colorado Plateau, the Ordos Plateau in northern China, or the East African Highlands, do not seem to be related to hot spots or to vigorous upwelling in the asthenosphere but appear to be underlain by unusually hot material. The reason for localized heating beneath such areas is poorly understood, and thus an explanation for the distribution of plateaus of that type is not known.

Plateaus that were formed by crustal shortening and internal drainage lie within major mountain belts and generally in arid climates. They can be found in North Africa, Turkey, Iran, and Tibet, where the African, Arabian, and Indian continental masses have collided with the Eurasian continent. The Altiplano lies between the Cordillera Occidental composed of volcanoes and the Cordillera Oriental beneath which the Brazilian shield is being thrust. All those areas have undergone crustal shortening during Cenozoic time (66 million years ago to the present), and in each case the surface of the plateau includes both strongly deformed pre-Cenozoic rocks and very young flat-lying sediment.

There are some plateaus whose origin is not known. Those of the Iberian Peninsula and north-central Mexico exhibit a topography that is largely high and relatively flat. Crustal shortening clearly occurred in Mexico during the Late Cretaceous and Early Cenozoic (between 100.5 million and 23 million years ago) and in some parts of Spain during the Cenozoic, but the high elevations in either case do not seem to be supported by thick crust. Those areas are probably underlain by a hot uppermost mantle, but proof of that is still lacking.

Volcanic plateaus are commonly associated with eruptions that occurred during the Cenozoic or Mesozoic. Eruptions on the scale needed to produce volcanic plateaus are rare, and none seems to have taken place in recent time. The volcanic eruptions that produce lava plateaus tend to be associated with hot spots. For example, the basalts of the Deccan Traps, which cover the Deccan plateau in India, were erupted 60–65 million years ago when India lay in the Southern Hemisphere, probably over the same hot spot that presently underlies the volcanic island of Réunion. The Serra Geral basalts that cap a plateau of the same name on the Atlantic coast of Brazil were erupted some 135 million years ago, before Africa and South America separated from each other and when the future continental margins overlay the hot spot now beneath the volcanic island of Tristan da Cunha in the South Atlantic Ocean. In North America the Columbia River basalts may have been ejected over the same hot spot that underlies the Yellowstone area today. Lava plateaus of the scale of those three are not common features on Earth.

Jai Ganesh · 2020-12-20 01:09:10

858) Continent

Continent, one of the larger continuous masses of land, namely, Asia, Africa, North America, South America, Antarctica, Europe, and Australia, listed in order of size. (Europe and Asia are sometimes considered a single continent, Eurasia.)

There is great variation in the sizes of continents; Asia is more than five times as large as Australia. The largest island in the world, Greenland, is only about one-fourth the size of Australia. The continents differ sharply in their degree of compactness. Africa has the most regular coastline and, consequently, the lowest ratio of coastline to total area. Europe is the most irregular and indented and has by far the highest ratio of coastline to total area.

The continents are not distributed evenly over the surface of the globe. If a hemisphere map centred in northwestern Europe is drawn, most of the world’s land area can be seen to lie within that hemisphere. More than two-thirds of the Earth’s land surface lies north of the Equator, and all the continents except Antarctica are wedge shaped, wider in the north than they are in the south.

The distribution of the continental platforms and ocean basins on the surface of the globe and the distribution of the major landform features have long been among the most intriguing problems for scientific investigation and theorizing. Among the many hypotheses that have been offered as explanation are: (1) the tetrahedral (four-faced) theory, in which a cooling earth assumes the shape of a tetrahedron by spherical collapse; (2) the accretion theory, in which younger rocks attached to older shield areas became buckled to form the landforms; (3) the continental-drift theory, in which an ancient floating continent drifted apart; and (4) the convection-current theory, in which convection currents in the Earth’s interior dragged the crust to cause folding and mountain making.

Geological and seismological evidence accumulated in the 20th century indicates that the continental platforms do “float” on a crust of heavier material that forms a layer completely enveloping the Earth. Each continent has one of the so-called shield areas that formed 2 billion to 4 billion years ago and is the core of the continent to which the remainder (most of the continent) has been added. Even the rocks of the extremely old shield areas are older in the centre and younger toward the margins, indicating that this process of accumulation started early. In North America the whole northeast quarter of the continent, called the Canadian, or Laurentian, Shield, is characterized by the ancient rocks of what might be called the original continent. In Europe the shield area underlies the eastern Scandinavian peninsula and Finland. The Guiana Highlands of South America are the core of that continent. Much of eastern Siberia is underlain by the ancient rocks, as are western Australia and southern Africa.

Jai Ganesh · 2020-12-21 00:46:03

859) Atmospheric science

Atmospheric science, interdisciplinary field of study that combines the components of physics and chemistry that focus on the structure and dynamics of Earth’s atmosphere. Mathematical tools, such as differential equations and vector analysis, and computer systems are used to evaluate the physical and chemical relations that describe the workings of the atmosphere.

The atmospheric sciences are traditionally divided into three topical areas—meteorology (the study and forecasting of weather), climatology (the study of long-term atmospheric patterns and their influences), and aeronomy (the study of the physics and chemistry of the upper atmosphere). In meteorology, the focus of study concerns day-to-day and hour-to-hour changes in weather within the lower stratosphere and troposphere. Climatology, on the other hand, concentrates more on longer time periods ranging from a single month to millions of years and attempts to describe the interaction of the atmosphere with the oceans, lakes, land, and glaciers. For example, of the three topical areas, climatology would be the best equipped to provide a farmer with the most likely date of the first frost in the autumn. The focus of aeronomy is on the atmosphere from the stratosphere outward. This field also considers the role the atmosphere plays in the propagation of electromagnetic communications, such as shortwave radio transmissions.

Within these three major topical areas, the broad nature of the atmospheric sciences has spawned practitioners who specialize in several distinct subfields. Scientists who investigate the physics associated with atmospheric flow are called dynamic meteorologists or simply dynamicists. When the investigation procedure involves the application of large computer models of atmospheric structure and dynamics, the scientists are called numerical modelers. Scientists and technicians who specifically investigate procedures of weather forecasting are called synoptic meteorologists, while those who investigate the physical mechanisms associated with the growth of cloud droplets and ice crystals and related precipitation processes are called cloud physicists. Researchers who study atmospheric optical effects are referred to as physical meteorologists, while individuals who investigate the dynamics and observations of climate are called climatologists or climate scientists. Paleoclimatologists are researchers who concentrate on ancient climate patterns. Scientists who investigate atmospheric structure and dynamics within the boundary layer (the layer of the atmosphere closest to Earth’s surface) are referred to as boundary layer meteorologists or micrometeorologists.

Jai Ganesh · 2020-12-22 00:20:23

860) Human migration

Human migration, the permanent change of residence by an individual or group; it excludes such movements as nomadism, migrant labour, commuting, and tourism, all of which are transitory in nature.

A brief treatment of human migration follows.

Migrations fall into several broad categories. First, internal and international migration may be distinguished. Within any country there are movements of individuals and families from one area to another (for example, from rural areas to the cities), and this is distinct from movements from one country to another. Second, migration may be voluntary or forced. Most voluntary migration, whether internal or external, is undertaken in search of better economic opportunities or housing. Forced migrations usually involve people who have been expelled by governments during war or other political upheavals or who have been forcibly transported as slaves or prisoners. Intermediate between these two categories are the voluntary migrations of refugees fleeing war, famine, or natural disasters.

Human migrations within recorded history have transformed the entire aspect of lands and continents and the racial, ethnic, and linguistic composition of their populations. The map of Europe, for example, is the product of several major early migrations involving the Germanic peoples, the Slavs, and the Turks, among others. And in the course of 400 years—from the late 16th through the 20th century—the Americas, Australia, Oceania, the northern half of Asia, and parts of Africa were colonized by European migrants. The overseas migration of Europeans during this period totaled about 60 million people.

The largest migration in history was the so-called Great Atlantic Migration from Europe to North America, the first major wave of which began in the 1840s with mass movements from Ireland and Germany. In the 1880s a second and larger wave developed from eastern and southern Europe; between 1880 and 1910 some 17 million Europeans entered the United States. The total number of Europeans reaching the United States amounted to 37 million between 1820 and 1980.

From 1801 to 1914 about 7.5 million migrants moved from European to Asiatic Russia (i.e., Siberia), and between World Wars I and II about 6 million more, not counting innumerable deportees to Soviet labour camps, voluntarily migrated there. Since World War II the largest voluntary migrations have involved groups from developing countries moving to the industrialized nations. Some 13 million migrants became permanent residents of western Europe from the 1960s through the ’80s, and more than 10 million permanent immigrants were admitted legally to the United States in that same period, with illegal immigration adding several millions more.

Slave migrations and mass expulsions also have been part of human history for millennia. The largest slave migrations were probably those compelled by European slave traders operating in Africa from the 16th to the 19th century; perhaps 20 million slaves were consigned to the Americas, though substantial numbers died in the appalling conditions of the Atlantic passage. The largest mass expulsions have probably been those imposed by Nazi Germany, which deported 7–8 million persons during World War II (1939–45), and by the Soviet Union, which forcibly expelled 9–10 million ethnic Germans from eastern Europe into Germany in the closing year of the war and afterwards. Some 14 million people fled in one direction or another during the partition of British India into India and Pakistan in the late 1940s. The largest migrations in the second half of the 20th century have consisted of refugees fleeing war, such as the estimated 3–4 million people who fled Afghanistan in the 1980s.

The dominant trend in internal migration during the 20th century has been the movement from rural to urban areas. As a consequence, urban growth since World War II has been very rapid in much of the world, particularly in developing countries.

Jai Ganesh · 2020-12-23 00:43:45

861) Oceanography

Oceanography, scientific discipline concerned with all aspects of the world’s oceans and seas, including their physical and chemical properties, their origin and geologic framework, and the life forms that inhabit the marine environment.

A brief treatment of oceanography follows.

Traditionally, oceanography has been divided into four separate but related branches: physical oceanography, chemical oceanography, marine geology, and marine ecology. Physical oceanography deals with the properties of seawater (temperature, density, pressure, and so on), its movement (waves, currents, and tides), and the interactions between the ocean waters and the atmosphere. Chemical oceanography has to do with the composition of seawater and the biogeochemical cycles that affect it. Marine geology focuses on the structure, features, and evolution of the ocean basins. Marine ecology, also called biological oceanography, involves the study of the plants and animals of the sea, including life cycles and food production.

Oceanography is the sum of these several branches. Oceanographic research entails the sampling of seawater and marine life for close study, the remote sensing of oceanic processes with aircraft and Earth-orbiting satellites, and the exploration of the seafloor by means of deep-sea drilling and seismic profiling of the terrestrial crust below the ocean bottom. Greater knowledge of the world’s oceans enables scientists to more accurately predict, for example, long-term weather and climatic changes and also leads to more efficient exploitation of the Earth’s resources. Oceanography also is vital to understanding the effect of pollutants on ocean waters and to the preservation of the quality of the oceans’ waters in the face of increasing human demands made on them.

Jai Ganesh · 2020-12-24 00:26:20

862) Ecosystem

Ecosystem, the complex of living organisms, their physical environment, and all their interrelationships in a particular unit of space.

A brief treatment of ecosystems follows.

An ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem.

The fundamental source of energy in almost all ecosystems is radiant energy from the Sun. The energy of sunlight is used by the ecosystem’s autotrophic, or self-sustaining, organisms. Consisting largely of green vegetation, these organisms are capable of photosynthesis—i.e., they can use the energy of sunlight to convert carbon dioxide and water into simple, energy-rich carbohydrates. The autotrophs use the energy stored within the simple carbohydrates to produce the more complex organic compounds, such as proteins, lipids, and starches, that maintain the organisms’ life processes. The autotrophic segment of the ecosystem is commonly referred to as the producer level.

Organic matter generated by autotrophs directly or indirectly sustains heterotrophic organisms. Heterotrophs are the consumers of the ecosystem; they cannot make their own food. They use, rearrange, and ultimately decompose the complex organic materials built up by the autotrophs. All animals and fungi are heterotrophs, as are most bacteria and many other microorganisms.

Together, the autotrophs and heterotrophs form various trophic (feeding) levels in the ecosystem: the producer level, composed of those organisms that make their own food; the primary consumer level, composed of those organisms that feed on producers; the secondary consumer level, composed of those organisms that feed on primary consumers; and so on. The movement of organic matter and energy from the producer level through various consumer levels makes up a food chain. For example, a typical food chain in a grassland might be grass (producer) → mouse (primary consumer) → snake (secondary consumer) → hawk (tertiary consumer). Actually, in many cases the food chains of the ecosystem overlap and interconnect, forming what ecologists call a food web. The final link in all food chains is made up of decomposers, those heterotrophs that break down dead organisms and organic wastes. A food chain in which the primary consumer feeds on living plants is called a grazing pathway; that in which the primary consumer feeds on dead plant matter is known as a detritus pathway. Both pathways are important in accounting for the energy budget of the ecosystem.

Jai Ganesh · 2020-12-25 00:18:00

863) Hydrography

Hydrography, the art and science of compiling and producing charts, or maps, of water-covered areas of Earth’s surface. A brief treatment of hydrography follows.

The terms hydrography and hydrographer are based on an analogy with geography and geographer and date from the mid-16th century. Hydrography ordinarily denotes only the study of ocean depths and of the directions and intensities of ocean currents. Other facets—such as temperature profiles or mineral content—are covered by the sciences of hydrology and oceanography.

The British navy appointed its first hydrographer in 1795, and the United States established a naval observatory and hydrographical office in 1854. Since then many maritime nations have established hydrographic offices to furnish mariners with nautical charts and other publications necessary for navigation of their territorial waters and the oceans of the world. Hydrographic survey information is exchanged through the International Hydrographic Organization, chartered in 1970 under the auspices of the United Nations.

The earliest navigators sailed from headland to headland by always keeping the coastline within sight. Navigators did not require charts until the advent of the magnetic compass in 1187 made it possible to proceed directly from one port to another across open water. Early charts were hand-drawn and very expensive. They were based entirely on magnetic directions and on map projections that assumed a degree of longitude equal to a degree of latitude. The assumption was not significant in the Mediterranean, but it caused serious distortions in maps drawn of areas at higher latitudes.

Interest in the charting of the oceans away from seacoasts developed in the second half of the 19th century. A feature of marine science since the 1950s has been increasingly detailed bathymetric (water-depth measurement) surveys of selected portions of the seafloor.

A hydrographic survey consists of two operations: determining the horizontal coordinates of points on the surface of the body of water (position fixing) and determining the water’s depth at those points.

The scale of a hydrographic chart expresses the relationship between a given distance on the chart and the actual distance it represents on Earth’s surface. Hydrographic charts are constructed on widely different scales; they range from ocean sailing charts drawn to a small scale of 1:5,000,000 (where 1 inch on the map represents 79 miles; or 1 cm = 50 km) to harbour charts, which are drawn to a scale of 1:50,000 (1 inch to 0.8 mile) or larger. Virtually all navigational charts, except for those made of the polar regions, represent Earth’s surface by the ordinary Mercator projection.

Jai Ganesh · 2020-12-26 01:02:43

864) Avalanche

Avalanche, a mass of material moving rapidly down a slope. An avalanche is typically triggered when material on a slope breaks loose from its surroundings; this material then quickly collects and carries additional material down the slope. There are various kinds of avalanches, including rock avalanches (which consist of large segments of shattered rock), ice avalanches (which typically occur in the vicinity of a glacier), and debris avalanches (which contain a variety of unconsolidated materials, such as loose stones and soil). Snow avalanches, the subject of the remainder of this article, constitute a relatively common phenomenon in many mountainous areas.

The size of a snow avalanche can range from a small shifting of loose snow (called sluffing) to the displacement of enormous slabs of snow. In a slab avalanche, the mass of descending snow may reach a speed of 130 km (80 miles) per hour and is capable of destroying forests and small villages in its path. Avalanches kill about 150 people a year in North America and Europe. Most of those killed are backcountry skiers, climbers, snowshoers, and snowmobilers who accidentally trigger an avalanche and become buried in the snow. The number of North American fatalities has risen with the increasing popularity of winter sports. Avalanches also have been triggered intentionally in warfare to kill enemy troops. In World War I, during fighting in the Alps on the Austrian-Italian front in December 1916, more than 10,000 troops were killed in a single day by avalanches triggered by artillery fired onto slopes of unstable snow.

Avalanche Conditions

The occurrence of an avalanche depends on the interaction of mountainous terrain, weather conditions, snowpack conditions, and a trigger. Slab avalanches typically occur on slopes of 30 to 50 degrees. On slopes that are less steep, there is generally insufficient gravitational force to overcome frictional resistance and cause the displacement of a snow slab; on steeper slopes snow tends to sluff off. However, slab avalanches do occur on steeper slopes in climates with dense, wet snowfall. An important feature of terrain that can lead to an avalanche is the lack of objects that serve to anchor the snow, such as trees. Slab avalanches will not occur on slopes with sufficiently dense tree cover, which is about 1,000 conifer trees per hectare (400 per acre) on steep slopes and about half that density on gentler slopes. Other objects that can anchor the snow are large exposed rock outcroppings and rocks that are large enough to stick up through the snow cover. The probability of avalanches may be increased or decreased by several other terrain features, such as slope shape, a slope’s exposure to sun and wind, and elevation.

Certain types of weather lead directly to dangerous avalanche conditions—that is, to a high risk that an avalanche will occur. Slab avalanches are commonly associated with heavy snowfall and strong wind. With heavy snowfall, weaknesses in the existing snowpack may become overloaded, and the snow may fall so quickly that the new snow is unable to bond to the snow beneath it. Strong wind tends to break down the snow into ice crystals that readily bond together into a slab, and it also transports snow onto the lee sides of ridges and gullies, where wind-loaded snow leads to more frequent avalanching. Other meteorological conditions that can quickly lead to dangerous avalanche conditions are rapidly rising air temperature and rainfall on existing snow cover.

A snowpack consists of layers of snow, each formed at different times. Once the snow is on the ground, the ice crystals undergo physical changes that differentiate the layers deeper in the snowpack from those on top. These changes can weaken a layer underlying a cohesive slab of snow and thereby help set up a slab avalanche.

Once the conditions for an avalanche exist, a trigger simply applies sufficient force to release it. Natural triggers include new snowfall, wind-deposited snow, and a falling cornice (an overhanging mass of windblown ice or snow extending from a ridge). Other triggers include skiers, snowmobilers, snowboarders, and explosive blasts. Contrary to popular belief, noises such as yelling, yodeling, or the sound of a snowmobile will not trigger avalanches. Research has shown that only the loudest sonic booms under the most sensitive avalanche conditions might be able to trigger a slide.

Prediction And Protective Measures

In order to reduce fatalities and to protect villages and roads, people attempt to predict and prevent avalanches. Accurate avalanche prediction requires an experienced avalanche forecaster who often works both in the field to gather snowpack information and in the office with sophisticated tools such as remotely accessed weather data, detailed historical weather and avalanche databases, weather models, and avalanche-forecasting models. Avalanche forecasters combine their historical knowledge of past conditions with their knowledge of the affected terrain, current weather, and current snowpack conditions to predict when and where avalanches are most likely to occur. Such forecasting work typically takes place along mountain highways, adjacent to potentially affected villages, at ski areas, and in terrain heavily used for backcountry skiing and snowmobiling.

In addition to predicting avalanches, people employ a variety of techniques to reduce avalanche danger. Explosives are used to trigger avalanches on potentially unstable slopes so that the avalanches will occur when people are not endangered. Such avalanche control is particularly effective for ski areas and highway corridors. In some areas prone to avalanches, particularly near villages and fixed structures, devices such as avalanche rakes (large reinforced fencing) are used on slopes to hold snow in place, and diversion structures such as dams or wedges are used at the base of the slope to stop, split, or deflect the snow in an avalanche. Though expensive, these defensive measures are common throughout the Alps, where numerous villages are found in areas known for dangerous avalanches.

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Jai Ganesh · 2020-12-27 00:28:27

865) Dislocation
(joint damage)

Dislocation, in physiology and medicine, displacement of the bones forming a joint, with consequent disruption of tissues.

Dislocations are caused by stresses forceful enough to overcome the resistance of the ligaments, muscles, and capsule that hold the joint in place. A dislocation is called simple when the joint surfaces are not exposed to the air; it is called compound when the joint surfaces are exposed by the destruction of overlying skin or by the end of a bone piercing the skin.

A congenital dislocation is present at birth as the result of defective formation of the joint. A recurrent, or habitual, dislocation (repeated dislocation of the same joint) may be the result of improper healing of an old injury or may be natural, as in “double joints,” common in fingers and toes, which are the result of loose ligamentation. A pathological dislocation occurs as the result of a disease, such as Marfan’s syndrome, which weakens the capsule and ligaments about the joint.

Symptoms of dislocation include pain and tenderness at the site, a sensation of grating or grinding on attempting to use the part, and inability to use the part. Common signs are deformed appearance of the joint, swelling of surrounding tissue, and discoloration of the overlying skin. X-ray examination is useful to indicate the extent of the injury. Simple dislocations are treated by returning the bones to their normal position (reduction) by manipulation or occasionally by traction. The joint is then kept immobile until healing is complete. Recurrent and congenital dislocations are special problems that usually require surgical reconstruction of the joint.

Overview

A dislocation is an injury to a joint — a place where two or more bones come together — in which the ends of your bones are forced from their normal positions. This painful injury temporarily deforms and immobilizes your joint.

Dislocation is most common in shoulders and fingers. Other sites include elbows, knees and hips. If you suspect a dislocation, seek prompt medical attention to return your bones to their proper positions.

When treated properly, most dislocations return to normal function after several weeks of rest and rehabilitation. However, some joints, such as your shoulder, may have an increased risk of repeat dislocation.

Symptoms

A dislocated joint can be:
• Visibly deformed or out of place
• Swollen or discolored
• Intensely painful
• Immovable

When to see a doctor

It can be difficult to tell a broken bone from a dislocated bone. For either type of injury, get medical help right away. If possible, ice the joint and keep it immobile while you're waiting to be seen.

Causes

Dislocations can occur in contact sports, such as football and hockey, and in sports in which falls are common, such as downhill skiing, gymnastics and volleyball.
Basketball players and football players also commonly dislocate joints in their fingers and hands by accidentally striking the ball, the ground or another player.

A hard blow to a joint during a motor vehicle accident and landing on an outstretched arm during a fall are other common causes.

Risk factors

Risk factors for a joint dislocation include:
• Susceptibility to falls. Falling increases your chances of a dislocated joint if you use your arms to brace for impact or if you land forcefully on a body part, such as your hip or shoulder.
• Heredity. Some people are born with ligaments that are looser and more prone to injury than those of other people.
• Sports participation. Many dislocations occur during high-impact or contact sports, such as gymnastics, wrestling, basketball and football.
• Motor vehicle accidents. These are the most common cause of hip dislocations, especially for people not wearing a seat belt.

Complications

Complications of a joint dislocation can include:
• Tearing of the muscles, ligaments and tendons that reinforce the injured joint
• Nerve or blood vessel damage in or around your joint
• Susceptibility to reinjury if you have a severe dislocation or repeated dislocations
• Development of arthritis in the affected joint as you age

Stretching or tearing of ligaments or tendons that support your injured joint or damage to nerves or blood vessels surrounding the joint might require surgery to repair these tissues.

Prevention

To help prevent a dislocation:
• Take precautions to avoid falls. Get your eyes checked regularly. Ask your doctor or pharmacist if any of the drugs you take might make you dizzy. Be sure your home is well-lighted and that you remove any potential tripping hazards from the areas where you walk.
• Play safely. Wear the suggested protective gear when you play contact sports.
• Avoid recurrence. Once you've dislocated a joint, you might be more susceptible to future dislocations. To avoid recurrence, do strength and stability exercises as recommended by your doctor or physical therapist to improve joint support.

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Jai Ganesh · 2020-12-28 00:26:54

866) Wound

Wound, a break in the continuity of any bodily tissue due to violence, where violence is understood to encompass any action of external agency, including, for example, surgery. Within this general definition many subdivisions are possible, taking into account and grouping together the various forms of violence or tissue damage.

The most important distinction is between open and closed wounds. Open wounds are those in which the protective body surface (the skin or mucous membranes) has been broken, permitting the entry of foreign material into the tissues. In closed wounds, by contrast, the damaged tissues are not exposed to the exterior, and the process of repair can take place without the interference that contamination brings, in greater or lesser degree. Further divisions may be made on the basis of the mode of production of the wound.

Closed Wounds

The degree of injury sustained from a direct blow depends upon the force of the blow and its direction. Obviously the degree of damage increases with increasing force. The effects of direction are equally important, although not so readily appreciated. For example, a hammerblow to the side of the head may severely bruise the scalp or, delivered with equal force but directed in a slightly different way, may cause extensive damage to the base of the skull. Anatomic and physiologic factors may also affect the degree of injury. Thus, a fall on an outstretched hand may have extremely different effects on a child, a young adult, and an elderly person.

A relatively slight blow may damage the skin and underlying soft tissues, as shown by bruising, or contusion, which results from the infiltration of blood into the tissues from ruptured small vessels and by swelling caused by the passage of fluid through the walls of damaged capillaries. As a rule, the hemorrhage ceases abruptly, the blood and fluid are absorbed within a few days, and the part is restored to normal. When larger vessels are injured, much more blood escapes, and it collects in the tissues and forms a mass called a hematoma.

A direct, forceful blow may damage any of the underlying tissues; blood vessels, nerves, muscles, bones, joints, or the internal organs may be affected. Damage to the deeper tissues may result from the direct impact of the blow upon a tissue, as in the fracturing of a skull by a hammer or, more commonly, from the transmission of the force of impact through the body to a relatively weak point. Thus, a fall on an outstretched hand may injure the flesh and bones of the hand itself, but a common result is a break at some other site in the arm through which the force is transmitted—at the scaphoid bone in the wrist, at the radius in the forearm just above the wrist, at the elbow, or at the shoulder—the breaking point being determined by the direction of force and the anatomy of the individual.

Other common forms of indirect injury result from twisting, as occurs when a person’s foot becomes caught and he or she twists upon it, suffering, if the force is great enough, a sprained or broken ankle or a broken leg or hip; from bending; or from deceleration, a form of injury frequently encountered in automobile and aircraft accidents, where one part of the body is fixed while another is relatively mobile, giving rise, in abrupt stops, to a displacement of the mobile parts, commonly called whiplash.

Open Wounds

When the skin—or, in the case of injuries of the base of the skull or the sinuses, the mucous membrane—is broken, a wound is exposed to additional hazards, since the tissues may be invaded by foreign material such as bacteria, dirt, and fragments of clothing, which may give rise to serious local or general complications from infection. Furthermore, if the break in the skin is large, the resulting exposure of the wounded tissues to the drying and cooling effects of the air may increase the damage caused by the wounding agent itself.

A needle or a sharp knife that passes through the tissues with ease, dividing them cleanly or separating them, will produce relatively little damage except to those tissues directly in its course, and, indeed, unless an important structure is injured, the wounds caused are seldom serious. On the other hand, a bomb fragment, irregular and jagged, will, as it churns and rips through the soft tissues, produce extensive damage for a considerable distance in all directions. Likewise, the injury caused by crushing is frequently serious.

Skin, being sturdy and elastic and well supplied with blood, tolerates injury well and recovers quickly. The subcutaneous fatty tissues are more delicate and more easily deprived of their blood supply. Muscle, likewise, is sensitive to the damaging effect of shrapnel, being readily torn and unable to survive diminished blood supply for any appreciable time. Muscle, when damaged, is particularly prone to infection.

An injury to bone in an open wound is always serious, for any broken fragment detached from its blood supply will not survive if infection occurs, and it will remain as a foreign body in the wound to cause further complications. Even if the bone is cleanly broken and there are no loose fragments, infection may enter the raw surfaces of the fracture with disastrous results.

Clearly the seriousness of a wound is greatly increased if there is injury to a joint, a nerve, a major blood vessel, or an internal organ.

Contamination of a wound may occur at the moment of wounding or at any time thereafter until healing is complete. The effects of various nonbacterial contaminants vary considerably. In general, the critical factor for nonbacterial contaminants is the extent of the contamination. In the case of bacterial contaminants, the type of contaminant is of greater importance. Infection caused by virulent bacteria nourished by dead tissue and organic foreign material in the wound may take several forms, of which the three most important are: gas gangrene, the most dreaded, arising almost exclusively in damaged muscle tissue and spreading with alarming rapidity to cause death if unchecked by surgical or medical treatment; infections caused by organisms such as Streptococcus and Staphylococcus and the coliform bacteria, in which the local production of pus is a prominent feature accompanying a general reaction that may be severe; and tetanus, an often fatal infection that becomes evident some days after the wound has occurred, frequently without any marked local manifestations but characterized by generalized muscle spasms.

The final healing of a wound is the result of a series of complex biological events taking place over a long period. Viewed in the simplest way, in an untreated but uncomplicated wound, as from a clean knife cut, the process is as follows: When tissues are cut, the edges of the wound separate, apparently pulled apart by the elasticity of the skin. Blood from the severed blood vessel fills the cavity of the wound and overflows its edges. The blood clots and eventually the surface of the clot dries out and becomes hard, forming a scab. During the first 24 hours the scab shrinks, drawing the edges of the wound closer together. If the scab sloughs off or is removed after about a week, a layer of reddish granulation tissue will be seen to have covered the cut edges of the subcutaneous tissue. Gradually a pearly, grayish, thin membrane extends out from the skin edge; eventually it covers the whole surface. The actual area of the wound, meanwhile, is steadily reduced by a process of contraction; finally, there is no raw surface to be seen.

The thin linear scar that forms is at first red and raised above the level of the surrounding skin but gradually fades until it is considerably paler than the surrounding skin. For many weeks after the scar forms, this process of contracture continues as is shown by the gradual shortening of the wound. Wounds that cross normal “skin lines” tend, after several months, to widen and become depressed below the level of the surrounding skin. Scars do not tan in sunlight, and they produce neither hair nor sweat, all evidences of the failure of the skin to return to full function.

Microscopically one can observe in the clot the whole process of the development of fibrin that causes the clot to contract, the arrival of the white blood cells and the macrophages that digest the debris in the wound, and the growth of blood capillaries followed by the growth inward of fibrous tissue migrating from the cells on the margin of the wound. The fibres arising from these cells can be identified and seen to increase, eventually filling the wound cavity with a network of interlacing threads of the protein collagen that, influenced by lines of tension, finally range themselves in firm bands. Meanwhile, the surface of the wound is being covered by a process of enlargement and flattening and by multiplication of the preexisting skin cells at the edge of the wound. These covering, or epithelial, cells start very early to spread down into the wound, clearing a way for themselves beneath the scab, perhaps by the production of an enzyme that dissolves the deeper layers of the crust. Eventually the proliferating epithelial sheets from the two sides of the wound coalesce to heal the wound superficially.

Jai Ganesh · 2020-12-29 00:18:03

867) Turkey

Turkey, either of two species of birds classified as members of either the family Phasianidae or Meleagrididae (order Galliformes). The best known is the common turkey (Meleagris gallopavo), a native game bird of North America that has been widely domesticated for the table. The other species is Agriocharis (or Meleagris) ocellata, the ocellated turkey. For unrelated but similar birds, see bustard (Australian turkey), megapode (brush turkey), and snakebird (water turkey).

Domestication of the common turkey was probably begun by the Indians of pre-Columbian Mexico. The birds were first taken to Spain about 1519, and from Spain they spread throughout Europe, reaching England in 1541. When the bird became popular in England, the name turkey-math, formerly used for the guinea fowl of Islamic (or “Turkish”) lands, was transferred to it. English colonists then introduced European-bred strains of the turkey to eastern North America in the 17th century. Turkeys were mainly bred for their beautifully coloured plumage until about 1935, after which the breeding emphasis changed to their meat qualities.

Races of the common turkey found today in Mexico and in the southeastern and southwestern United States differ slightly in feather markings and in rump colour, but all are basically dark, with iridescent bronze and green plumage. Adult males have a naked, heavily carunculated (bumpy) head that is normally bright red in colour but turns to white overlaid with bright blue when the birds are excited. Other distinguishing features of the common turkey are a long red fleshy ornament (called a snood) that grows from the forehead over the bill; a fleshy wattle growing from the throat; a tuft of coarse, black, hairlike feathers (known as a beard) projecting from the breast; and more or less prominent leg spurs. The male turkey, or gobbler, or tom, may be 130 cm (50 inches) long and weigh 10 kg (22 pounds), though average weight is less. Female turkeys, or hens, generally weigh only half as much as the males and have less warty heads than do the males. Domesticated strains of the common turkey, developed for their fine-tasting flesh, may be much heavier.

Roast turkey in many European countries has long been a customary Christmas dish. In the United States the bird is especially associated with the holiday of Thanksgiving. Turkey production has thus tended to be seasonal, though in the United States and some other countries, ready-to-cook lean, boned turkey is available in rolls any time of the year.

The wild turkey prefers woodlands near water. It eats seeds, insects, and an occasional frog or lizard. When alarmed, it may run rapidly to cover. It can fly strongly only for short distances (about 0.4 km, or 0.25 mile). Formerly diminished under hunting pressure, M. gallopavo has come back well under various state game management programs in the United States.

In courtship display, the male spreads his tail, droops his wings and shakes the quills audibly, retracts his head, struts about, and utters rapid gobbling sounds. He assembles a harem, and each hen lays 8–15 brownish spotted eggs in a hollow in the ground. The young (poults) hatch in 28 days.

The ocellated turkey, of Central America, is smaller than M. gallopavo. It has a blue head with reddish yellow bumps, bright-tipped feathers, almost peacocklike, and, in addition to the long bill wattle, a yellow-tipped knob on the crown. It has never been domesticated.

Jai Ganesh · 2020-12-30 00:28:49

868) Umbrella

Umbrella, a portable, hand-held device that is used for protection against rain and sunlight. The modern umbrella consists of a circular fabric or plastic screen stretched over hinged ribs that radiate from a central pole. The hinged ribs permit the screen to be opened and closed so that the umbrella can be carried with ease when not in use.

Umbrellas in ancient Egypt, Mesopotamia, China, and India were used to protect important persons from the sun. They were often large and held by bearers, and they served as marks of honour and authority for the wearer. The ancient Greeks helped introduce umbrellas into Europe as sunshades, and the Romans used them to protect against rain. The use of umbrellas disappeared in Europe during the Middle Ages but had reappeared in Italy by the late 16th century, where they were regarded as marks of distinction for the pope and clergy. By the 17th century the use of the umbrella had spread to France, and by the 18th century umbrellas were common throughout Europe. A small, dainty umbrella used for shading women’s faces from the sun became known as a parasol and was a standard element of fashionable women’s outdoor attire in the 18th and 19th centuries. The traditional construction of umbrellas using cane ribs was replaced in the 1850s by modern umbrellas using a very light but strong steel frame. Men in the West began carrying umbrellas for personal use in the mid-19th century. Men’s umbrellas were generally black, but in the 20th century men’s as well as women’s umbrellas were made in a variety of bright and colourful designs.

Jai Ganesh · 2020-12-31 12:30:40

869) Glove

Glove, covering for the hand with separate sections for the fingers and thumb, sometimes extending over the wrist or part of the arm. Fingerless gloves, called mitts in colonial America, have five holes through which the fingers and thumb extend.

Well-formed linen gloves with a drawstring closure at the wrist were found in the tomb of the Egyptian king Tutankhamen (14th century BC). Ancient Greek and Latin literature contain many allusions to gloves. Medieval European nobles, patricians, and prelates wore gloves made of fabric or leather, often richly jeweled and embroidered. By the 14th century, gloves were worn generally by upper-class men; but not until the 16th century did Catherine de Médicis, queen consort of Henry II of France, set the fashion for women. At the turn of the 17th century, women’s gloves of soft kidskin were introduced.

The ancient art of glove making became an industry in 1834, when Xavier Jouvin of Grenoble, France, invented the cutting die that made possible a glove of precise fit. The kid glove has retained supremacy as the aristocrat of gloves, but other kinds of leather are also utilized in modern glove manufacture, including capeskin, cabretta, pigskin, buckskin, reindeer skin, and lambskin, also called doeskin.

There are usually eight components of a leather glove: palm and back (one piece), thumb, three fourchettes (slender pieces of leather that form the sides of the fingers), and three quirks, or diamond-shaped pieces inserted at the bottom between the fingers. In cutting gloves, a single trank, or rectangular piece of leather the size of the glove, may be cut by hand to a desired pattern with shears; or a number of tranks may be cut simultaneously by a weighted, sharp steel die. The glove is closed by stitching up along the outside to the tip of the little finger; then the thumbs, quirks, and fourchettes are set in and sewed with great care. Although some sewing is done by hand, most is by machine and closely resembles hand stitching. The completed glove is dampened, tailored on an electrically heated metal model hand, and buffed.

Fabric gloves of antiquity were made of woven material, but modern fabric gloves are knit. Silk was the favoured material before World War II, but the glove industry now relies on cotton and man-made fibres such as rayon and nylon. Glove-sized squares of finished fabric are arranged face-to-face so that the left and right hands are cut out together by the knife-sharp glove die, which is forced through the built-up layers of fabric. Gores, triangular pieces of fabric, are cut separately and attached between the fingers when the cutout glove is folded over and stitched together. Thumbs are also cut separately and attached. The fingers are given a tubular shape by seaming. Fabric gloves are tailored on electrically heated metal hands, as are leather gloves.

Gloves of wool, synthetic fibres, and cotton yarns can be knit by machine with or without seams; and their colours, designs, patterns, and stitch variations rival those of gloves knit by hand. Seamed, or wrought, gloves are first machine knit as flat selvage pieces of fabric, folded so that complementing parts fall together, and then stitched. Seamless gloves also may be knit entirely on such a flat machine, or the cuff and palm may be knit on a circular machine and then the stitches carefully transferred to a flat fingering machine.

Protective gloves have been developed for special uses. Thin rubber or latex gloves are used by medical and laboratory personnel. Heavy rubber gloves are used by electrical workers. Asbestos gloves protect against burns, as do gloves of heavy, twisted loop pile similar to terry cloth. Canton flannel gloves treated with polyvinyl provide plastic-coated work gloves that are heat resistant, impermeable to most fluids, and proof against acids, alkalies, industrial oils, greases, and other chemicals. Lead-impregnated gloves may be used in order to shield the hands from X-rays.

Participants in many sports wear gloves for protection or to improve grip. In baseball, players in the field wear one large padded leather glove, or mitt, to protect the fielder’s hand and to facilitate catching the ball via a pocket woven between the glove’s thumb and index finger. Hockey players and cricket batsmen wear oversized padded gloves on both hands to cushion them from the hard, fast-moving projectiles used in these sports. Thin gloves made of leather or synthetic material are used in a variety of sports, such as gridiron football, golf, and cycling, to enhance the wearer’s grip.

Jai Ganesh · 2021-01-01 01:26:00

870) Sweater

Sweater, outer garment, usually knitted or crocheted, that is worn on the upper part of the body, either pulled over the head or buttoned down the front or back. Although hand knitting of wool had been practiced for about 2,000 years, it was not until the 15th century that the first knitted shirts or tunics were produced on the English Channel islands of Guernsey and Jersey; hence the English name jersey. The knitted garments were made by the wives of fishermen and sailors from natural wool, which, by retaining its oil, protected against the cold even when damp. The use of the jersey spread throughout Europe, especially among workingmen. In the 1890s it was adopted by athletes in the United States and called a sweater.

The first sweaters were heavy, dark blue pullovers, worn before and after athletic contests to protect against cold. By the 1920s designers such as Jeanne Lanvin and Gabrielle (“Coco”) Chanel introduced sweaters into their collections. Throughout the 20th century, sweaters in a variety of designs, knitted from natural and synthetic fibres, were worn by men, women, and children.

Jai Ganesh · 2021-01-02 00:46:49

871) Button

Button, usually disklike piece of solid material having holes or a shank through which it is sewed to one side of an article of clothing and used to fasten or close the garment by passing through a loop or hole in the other side. Purely decorative, nonutilitarian buttons are also frequently used on clothing.

In medieval Europe, garments were laced together or fastened with brooches or clasps and points, until buttonholes were invented in the 13th century. Then buttons became so prominent that in some places sumptuary laws were passed putting limits on their use.

By the 14th century buttons were worn as ornaments and fastenings from the elbow to the wrist and from the neckline to the waist. The wearing of gold, silver, and ivory buttons was an indication of wealth and rank. Expensive buttons were also made of copper and its alloys. The metalsmith frequently embellished such buttons with insets of ivory, tortoiseshell, and jewels. More commonly, buttons were made of bone or wood. Button forms of these materials were also used as foundations for fabric-covered buttons. Thread buttons were made by wrapping the thread over a wire ring.

In the 18th century luxury metals and ivory largely replaced fabric, although embroidered buttons in designs to complement particular garments were popular. Pewter, the familiar metal of the age, was used to make molded or stamped-out buttons, but these were scorned by the wealthy. Cast brass buttons, particularly calamine brass, with ornamental and distinguishing designs, also became popular on both military and civilian dress.

In the middle of the 18th century, Matthew Boulton, the English manufacturer and partner of James Watt, introduced the bright, costly, cut-steel button, which was made by attaching polished steel facets to a steel blank. In France the facets of the cut-steel button were elaborated by openwork designs. During the first quarter of the 19th century, a less costly stamped steel button was made in an openwork pattern. Brass buttons that were gilded by dipping in an amalgam of mercury and gold also became popular.

The two-shell metal button was introduced about the same time as the stamped-steel type by B. Sanders, a Danish manufacturer in England. The two shells, thin metal disks enclosing a small piece of cloth or pasteboard, were crimped together on the edges. Sanders also originated the canvas shank. By 1830 fabric-covered buttons were being made mechanically. Also coming into use were animal horns and hoofs, which could be made malleable by heating and then could be cut, dyed, and molded.

Buttons were also made of ceramics and glass. Porcelain buttons became a French specialty; they were decorated by hand painting or by transfer printing designs using coloured inks. Bohemia, in the present-day Czech Republic, produced most of the coloured glass used in button manufacture.

In Japan, ceramic buttons, hand painted in traditional motifs, were developed. Buttons with an intricately carved thickness of vermilion lacquer on a wooden base became a Chinese specialty, and decorated and lacquered papier-mâché buttons became popular in Europe in the late 1800s.

The use of the pearly shells of sea mollusks in button making increased with the mechanization of production. Shell was separated into its component layers by treatment with a nitric acid solution, and blanks were cut out by tubular saws. Holes were bored in the blanks for sewing, and an engraved decoration was mechanically applied. At first only seashell was used, but in the 1890s the American manufacturer John F. Boepple began to use the less iridescent but abundant freshwater mussel shells found along the Mississippi River and its tributaries.

In the 20th century, buttons became primarily utilitarian, not decorative, and in many applications were supplanted by the zipper. Buttons began to be made of plastics such as cellulose, polystyrene, and polyvinyl resins; designs tended to be abstract or geometric. Mass-production machines produce molded buttons either by compressing powdered plastics or by injection—forcing liquid plastic into individual molds through small openings.

Some old buttons are considered valuable and are collected for their art and workmanship. The place, date, and name of the maker are usually marked on their backs.

Jai Ganesh · 2021-01-03 00:26:01

872) Aorta

The aorta is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes oxygenated blood to all parts of the body through the systemic circulation.

Function

The aorta supplies all of the systemic circulation, which means that the entire body, except for the respiratory zone of the lung, receives its blood from the aorta. Broadly speaking, branches from the ascending aorta supply the heart; branches from the aortic arch supply the head, neck, and arms; branches from the thoracic descending aorta supply the chest (excluding the heart and the respiratory zone of the lung); and branches from the abdominal aorta supply the abdomen. The pelvis and legs get their blood from the common iliac arteries.

General

Aorta, in vertebrates and some invertebrates, the blood vessel (or vessels) carrying blood from the heart to all the organs and other structures of the body.

At the opening from the left ventricle into the aorta is a three-part valve that prevents backflow of blood from the aorta into the heart. The aorta emerges from the heart as the ascending aorta, turns to the left and arches over the heart (the aortic arch), and passes downward as the descending aorta. The left and right coronary arteries branch from the ascending aorta to supply the heart muscle. The three main arteries branch from the aortic arch and give rise to further branches that supply oxygenated blood to the head, neck, upper limbs, and upper part of the body. The descending aorta runs down through the posterior centre of the trunk past the heart, lungs, and esophagus, through an opening in the diaphragm, and into the abdominal cavity.

In the chest the aorta, as it descends, gives off branches to (1) the pericardium, the sac that encloses the heart, (2) the connective tissues of the lungs, (3) the bronchi, which carry air from the windpipe into the lungs, (4) the esophagus, (5) part of the diaphragm, and (6) the chest wall.

In the abdominal cavity the aorta gives off a number of branches, which form an extensive network supplying blood to the stomach, liver, pancreas, spleen, small and large intestines, kidneys, reproductive glands, and other organs. At the level of the fourth lumbar vertebra, which is about even with the top of the hip bones, the aorta divides into the right and left common iliac arteries, the principal arteries to the legs.

Jai Ganesh · 2021-01-04 00:31:02

873) Antacid

Antacid, also spelled antiacid, any substance, such as sodium bicarbonate, magnesium hydroxide, calcium carbonate, or aluminum hydroxide, used to counteract or neutralize gastric acids and relieve the discomfort caused by gastric acidity. Indigestion, gastritis, and several forms of ulcers are alleviated by the use of antacids.

Numerous nonprescription liquid antacids reduce the gastric acidity of indigestion or gastritis for up to three hours after a single dose. Of the many liquid antacids available, those consisting of either magnesium or aluminum alkalinizing agents are preferable to antacids containing calcium salts, which have been shown to lead to a secondary increase in gastric acidity. Antacids should be taken when gastric acidity is most likely to be increasing—namely, between one and three hours after each meal and at bedtime. Although they are more convenient, antacid tablets are not nearly so effective as liquid forms. Because magnesium-containing antacids tend to have a laxative effect if used regularly and aluminum-containing antacids tend to constipate, many patients prefer to alternate doses of the two types.

In the treatment of ulcers, acid secretion can be reduced by several agents that block the action of hormones on the acid-secreting parietal cells of the stomach. Histamine receptor antagonists such as famotidine, ranitidine (Zantac), and cimetidine (Tagamet) block histamine receptors on the parietal cells and are effective for about 12 hours. The most potent acid-inhibitory drugs are the proton pump inhibitors such as omeprazole, lansoprazole, and rabeprazole that block the final pathway of acid secretion and are effective for 15 to 17 hours. Histamine receptor antagonists and proton pump inhibitors, when given in conjunction with medication to eradicate Helicobacter pylori, are highly successful in healing duodenal ulcers.

In addition to mild laxative or constipating effects, some antacids, such as those that are positively charged, may cause the blood to become alkaline, leading to metabolic alkalosis in severe cases. Some antacids may affect the absorption of other drugs by binding with them in the gastrointestinal tract.

Jai Ganesh · 2021-01-05 00:41:21

874) Pet (animal)

Pet, any animal kept by human beings as a source of companionship and pleasure.

While a pet is generally kept for the pleasure that it can give to its owner, often, especially with horses, dogs, and cats, as well as with some other animals, this pleasure appears to be mutual. Thus, pet keeping can be described as a symbiotic relationship, one that benefits both animals and human beings. As the keeping of pets has been practiced from prehistoric times to the present and as pets are found in nearly every culture and society, pet keeping apparently satisfies a deep, universal human need.

The history of pets is intertwined with the process of animal domestication, and it is likely that the dog, as the first domesticated species, was also the first pet. Perhaps the initial steps toward domestication were taken largely through the widespread human practice of making pets of captured young wild animals. Eventually, a working relationship developed between the dogs and their human captors. The dog was swifter, had stronger jaws, and was better at tracking prey; therefore, it could be of great use in hunting and guarding duties. From human beings, on the other hand, the dogs were assured of a constant supply of food as well as warmth from the fire. There is indirect evidence that the dog may have been domesticated and kept as a pet since Paleolithic times, as can be surmised from the paintings and carvings that archaeologists have found in ancient campsites and tombs. In Mesopotamia, dogs that look remarkably like the present-day mastiff were shown participating in a lion hunt. Domestic pets were often depicted in the scenes of family life in ancient Egypt; hunting dogs of the greyhound or saluki type accompany their master to the chase, and lap dogs frequently sit under the chair of their master or mistress.

Next to the dog, horses and cats are the animals most intimately associated with human beings. Surprisingly, both these animal groups were domesticated rather late in human history. There is no evidence that horses were domesticated in Paleolithic or Mesolithic times, but by about 2000 BCE horses used in chariot battles were an established phenomenon throughout the Middle East. It seems that riding astride horses was a practice developed a few centuries later (see horsemanship). The cat too does not seem to have been domesticated as a pet until the New Kingdom period (about the 16th century BCE) in Egypt. This is all the more strange as the ancient Egyptians had tamed many types of animals, such as lions, hyenas, monkeys, the Nile goose, and dogs, since the Old Kingdom period. But once cats were finally domesticated, their popularity was enormous. Gradually, the cat became one of the most universally worshiped animals.

As has been noted, the primary bond distinguishing a pet-and-owner relationship is affection. As useful as many of these animals are, what differentiates a pet from other economically useful livestock is the degree of contact between the animals and human beings. Often, this relationship has been unabashedly sentimentalized in myth, art, and literature. The affection between Alexander the Great and his favourite horse, Bucephalus, has become legendary, while in the modern age the popularity of such canine motion-picture stars as Rin Tin Tin and Lassie is further evidence of the importance placed on the relationship between owner and pet.

The pet-and-owner relationship, however, is not only founded on companionship; since the earliest period of domestication, pets have fulfilled practical, economic ends. Catching other animals to feed their human masters is one of the most fundamental uses of pets, and not only dogs have served in this capacity but cats, hyenas, and lions have also been used for hunting. The aristocratic, rather arcane sport of falconry made use of the natural talent of hawks to aid in hunting game birds. Pets have also been used for the purpose of guarding—either other livestock, the home or territory of their owners, or the owners themselves. Any pet that has a sharp sense of smell or hearing and that makes a loud noise when aroused can be used as a guard, although dogs are the best-known examples. It is thought that the Nile goose, a favourite household pet of the ancient Egyptians, may have served such a purpose. The herding and guarding of livestock is another practical use of pets, in particular the dog. Over the centuries, many specialized breeds of dog have been developed to suit this purpose.

Often, pets have been used as a source of food when other sources become scarce. This has been the case with dogs throughout their history of domestication in both the Old World and the New World. Guinea pigs, domesticated as pets in the New World, also assured a stable food supply.

Pets have also been used to eliminate animal pests. Finally, pets themselves have become a self-perpetuating industry, bred for a variety of purposes, including their value as breeding animals. Pets that are bred for aesthetic purposes may have full-fledged show careers. Other pets may be bred for racing or other competitive sports, around which sizable industries have been built.

Animals kept as pets can be classified according to the type of premises or habitat they usually occupy. Dogs, cats, and birds such as canaries and parakeets are kept as household pets. Other birds, such as jays, magpies, and members of the crow family, are kept in aviaries. When kept as pets, reptiles and amphibians frequently require special conditions of heat and moisture. For this reason, they are best kept in glassed enclosures called vivaria. The most common vivarium pets are snakes, lizards, turtles, frogs, and toads. Many people keep fish as aquarium pets. Fishes constitute a completely separate section of the pet world, and an international industry exists for catching, breeding, transporting, and supplying stock. Hutch, or cage, pets can be kept indoors or outdoors under protected conditions. These pets include rabbits, guinea pigs, rats, mice, hamsters, gerbils, and, recently, chinchillas. Paddock pets are those that must be stabled outdoors and include such animals as horses, ponies, donkeys, and mules. Several kinds of insects are also kept as pets. These include walking-stick insects (kept in simple containers at room temperature) and ants (kept in artificial nests).

Of increasing concern is the sale of exotic pets (e.g., jaguars, alligators, ocelots, monkeys, apes, kinkajous, etc.). Rarely are the owners of such pets able to provide the basic nutritional or habitat needs of these animals; most of the animals soon die or are sent to a zoo. Furthermore, in order to obtain the young, which are considered most desirable as pets, many adults of the wild species are killed, seriously depleting populations already endangered. Several countries have passed laws to prohibit the importation of endangered species as pets, but an active black market flourishes.

Jai Ganesh · 2021-01-06 00:40:55

875) Greeting card

Greeting card, an illustrated message that expresses, either seriously or humorously, affection, good will, gratitude, sympathy, or other sentiments. Greeting cards are usually sent by mail in observance of a special day or event and can be divided into two general classifications: seasonal and everyday. Seasonal cards include those for Christmas, Valentine’s Day, Mother’s Day, Father’s Day, Easter, graduation, Halloween, and St. Patrick’s Day. Everyday cards include those commemorating birthdays, anniversaries, or births; cards of condolence, congratulations, or friendship; as well as get-well cards, gift cards, bon voyage cards, and thank you cards.

Modern greeting cards are usually of stiff paper or cardboard, but some are made of cloth, leather, celluloid, vellum, metal, or even wood, clay, cork, or other materials. Size is determined by common usage, the availability of suitable envelopes, ease of mailing, and the system of grading according to price and quality. Extreme exceptions include an inscribed grain of rice presented in 1929 as a Christmas greeting to the prince of Wales and a Christmas card sent to Pres. Calvin Coolidge in 1924 that was 21 by 33 inches (53 by 84 cm). The imprinted messages on cards may vary in length from a brief word or two to 100 words or more in prose or verse.

Early Greeting Cards

The exchange of illustrated greetings among friends dates from ancient times. In Egypt the new year was celebrated by the exchange of symbolic presents, such as scent bottles and scarabs inscribed au ab nab (“all good luck”). The Romans exchanged strenae, originally branches of laurel or olive, frequently coated with gold leaf. Symbols of seasonal good will, such as a Roman lamp impressed with the figure of Victory surrounded by strenae, were inscribed with Anno novo faustum felix tibi sit (“May the new year be happy and lucky for you”). The acknowledgment of the new year with exchanges of good will continued in Europe through the early days of Christianity.

In the 15th century, master wood engravers produced inscribed prints which had the same intent as the modern Christmas and New Year’s cards. One of these, by Master E.S., shows the Christ Child with a halo before a cross and holding a scroll on which appears Ein guot selig ior (“A good and happy year”). During the 18th and early 19th centuries, copperplate engravers produced prints and calendars for the new year, and greetings by organizations, merchants, and tradesmen were common.

The valentine is also regarded as a forerunner of the greeting card. Its history is related to pre-Christian Rome when boys drew the names of girls from a love urn on the feast of the Lupercalia (February 15). The custom was introduced to England by the Romans and continued through the Christian era. In order to adapt the practice to Christianity, the church transferred it to the feast of St. Valentine.

The paper valentine with inscribed sentiment dates from the 16th century, and the first printed valentine may have been the frontispiece of A Valentine Writer, a book of verses that offered assistance to the inarticulate and was issued as early as 1669. By 1800 hand-painted copperplates by such artists as Francesco Bartolozzi were in demand. These were followed by woodcuts and lithographs, all in quarto size, some further embellished with an embossed frame. With the introduction of penny postage and envelopes in England in 1840, the exchange of valentines increased, and the use of lace paper, delicately ornamented, became popular. In the U.S., crude woodcut valentines were produced by Robert H. Elton and Thomas W. Strong of New York but gave way to the lace paper delicacies imported from England. The less expensive creations of Esther Howland of Worcester, Massachusetts, first appeared in 1850.

Recognized as the first Christmas card is one designed in England by John Callcott Horsley in 1843 for his friend Sir Henry Cole. An edition of 1,000 copies was placed on sale at Felix Summerly’s Home Treasury Office in London (Felix Summerly was a pseudonym of Cole’s). It was printed by lithography on stiff cardboard, 5 1/8 by 3 1/4 inches (13 by 8 cm), in dark sepia and was hand coloured. The design shows a family party in progress, beneath which is the greeting, “A Merry Christmas and a Happy New Year to You.” Inside panels, formed by a rustic trellis, are representations of Christmas charity. A similar card was designed by W.M. Egley and produced as an etching in 1848. While this card is more elaborate, its design suggests a relationship to the Cole-Horsley card. The same may be said of a U.S. Christmas card of the same period designed by R.H. Pease of Albany, New York, which bore the inscription, “Pease’s Great Variety Store in the Temple of Fancy.” Sentiment cards (approximately 3 by 1 1/2 inches) were also exchanged and collected in the U.S. from 1830 to the Civil War period. Many have survived, among them an “expanding heart” Christmas present or greeting card dating from about 1850, which may be one of the first American Christmas cards.

Growth Of The Greeting Card Industry

Greeting card production in commercial quantities started in 1860, the first offerings being valentines with applied Christmas ornaments and verses. These were followed by embossed or lithographed letter sheets and envelopes in multiple colours with matching cards. The latter were actually visiting cards with holiday sentiments, similar to the sentiment cards so popular earlier in the U.S. Visiting cards, which date in Europe from the 16th century, had long been used to carry messages of affection, respect, or condolence. A card with its corner bent gave the comforting assurance of personal interest. Early commercial greeting cards bore illuminations copied from manuscripts in the British Museum. There were also small cards with embossed frames, similar to the visiting cards but bearing illustrations of robins and children. These were issued in sets of six and were collected and mounted in albums or scrapbooks. The cards also appeared in booklet form, attached to make a strip.

The colourful printed card ran a parallel course with the lace paper valentine. One of the English publishers, Marcus Ward & Co., employed Kate Greenaway as a designer. Her productions, in sets of two to six, were used for more than one occasion, the same designs being frequently used for Christmas, New Year’s Day, St. Valentine’s Day, birthday, and everyday cards. Some of the designs appeared as book illustrations, and others were used in annual four-subject calendars.

Louis Prang of Boston is called the “father of the American Christmas card.” He started with sets of album cards (flora, birds, animals) and continued with Civil War scenes by Winslow Homer. He also printed advertising and visiting cards with floral designs and in 1875 added seasonal greetings. These were an immediate success. Prang cards were among the best in the market and were much admired abroad. He instituted design competitions in 1880, a practice continued later in England by Raphael Tuck and in the U.S. with Hallmark Cards Art awards. Prang’s business flourished until 1895, when a decline of greeting card production in England and the U.S. resulted from overwhelming competition from European printers whose product was so inexpensive that it could not be ignored. Cards were delivered in blank form and sentiments were applied by local printers, the same design being used for several purposes. From 1900 to World War I the greeting card business was practically a German monopoly.

The U.S.-made greeting card reasserted itself about 1910 and was given enormous impetus by World War I with its resultant increase in transiency, a situation that was repeated during World War II. In the intervening years the custom of exchanging cards on both seasonal and everyday occasions became firmly established in the United States. U.S. greeting card manufacturers, chief among them Hallmark, assumed world leadership during this period and brought many innovations to the design and manufacture of cards in the realm of novelties, animation, three-dimensional effects and visual and sound effects. Hallmark also transformed the industry by packaging inexpensive cards with custom-sized envelopes, essentially ushering out the era of the postcard.

The exchange of greeting cards in the United States is on a scale far beyond that in any other country, and, according to industry estimates in the early 21st century, more than three-fourths of greeting cards were being purchased by women. A shopper engaged in the selection of a greeting card could have as many as 1,000 different cards from which to choose, commemorating almost any possible occasion. Fine art from old masters appeared alongside Snoopy and Mickey Mouse, and written sentiments ran the gamut from Shakespearean sonnets to the bawdiest of low humour. Despite the growing use of e-cards and social media to celebrate special occasions and holidays, the industry displayed a surprising resilience. In the 2010s, U.S. households annually purchased about seven billion cards, with retail sales topping $7.5 billion. Christmas cards accounted for the overwhelming majority of seasonal cards, while birthday cards topped the everyday card market.

Jai Ganesh · 2021-01-07 00:30:43

876) Goldfish

Goldfish, (Carassius auratus), ornamental aquarium and pond fish of the carp family (Cyprinidae) native to East Asia but introduced into many other areas. The goldfish resembles the carp (Cyprinus carpio) but differs from its relative in having no mouth barbels. It was domesticated by the Chinese at least as early as the Sung dynasty (960–1279).

The goldfish is naturally greenish-brown or gray. The species, however, is variable, and numerous abnormalities occur. A deviant fish may be black, spotted, golden, white, or white with silver; it may have a trilobed tail fin or protruding eyes; or it may lack the normally long dorsal fin. Centuries of selecting out and breeding such abnormal specimens have produced over 125 breeds of goldfish, including the common, pet-shop comet; the veiltail, with a three-lobed, flowing tail; the lionhead, with a swollen “hood” on the head; and the celestial, with protruding, upward-directed eyes.

The goldfish is omnivorous, feeding on plants and small animals. In captivity it does well on small crustaceans, and the diet may be supplemented with chopped mosquito larvae, cereal, and other foods.

Spawning occurs in spring or summer. As the season approaches, colours become brighter, the belly of the female enlarges, and the male may develop pinhead-sized tubercles on the gill covers, back, and pectoral fins. The eggs stick to water plants until hatching about a week later. Pet goldfish have been known to live 25 years; the average life span, however, is much shorter.

Escaping from park and garden pools, the goldfish has become naturalized in many areas of the eastern United States. Upon resuming wild life, it reverts to its original colour and may grow from an aquarium size of about 5–10 centimetres (2–4 inches) to a length of up to 30 centimetres.

Jai Ganesh · 2021-01-08 00:44:07

877) Veterinary medicine

Veterinary medicine, also called veterinary science, medical specialty concerned with the prevention, control, diagnosis, and treatment of diseases affecting the health of domestic and wild animals and with the prevention of transmission of animal diseases to people. Veterinarians ensure a safe food supply for people by monitoring and maintaining the health of food-producing animals.

Persons serving as doctors to animals have existed since the earliest recorded times, and veterinary practice was already established as a specialty as early as 2000 BCE in Babylonia and Egypt. The ancient Greeks had a class of physicians who were called “horse-doctors,” and the Latin term for the specialty, veterinarius (“pertaining to beast of burden”), came to denote the field in modern times. Today veterinarians serve worldwide in private and corporate clinical practice, academic programs, private industry, government service, public health, and military services. They often are supported in their work by other veterinary medicine professionals, such as veterinary nurses and veterinary technicians.

Veterinary medicine has made many important contributions to animal and human health. Included are dramatic reductions in animal sources of human exposure to tuberculosis and brucellosis. Safe and effective vaccines have been developed for prevention of many companion (pet) animal diseases—e.g., canine distemper and feline distemper (panleukopenia). The vaccine developed for control of Marek’s disease in chickens was the first anticancer vaccine. Veterinarians developed surgical techniques, such as hip-joint replacement and organ transplants, that were later applied successfully to people.

A major challenge to veterinary medicine is adequately attending to the diversity of animal species. Veterinarians address the health needs of domestic animals, including cats, dogs, chickens, horses, cows, sheep, pigs, and goats; wildlife; zoo animals; pet birds; and ornamental fish. The sizes of animals that are treated vary from newborn hamsters to adult elephants, as do their economic values, which range from the undefinable value of pet animal companionship to the high monetary value of a winning racehorse. Medicating this variety of tame and wild animals requires special knowledge and skills.

On the basis of recognition by the World Health Organization (WHO) or the government of a country, there are about 450 veterinary degree programs worldwide. The level of veterinary training varies greatly among the various countries, and only about one-third of these programs designate the degree awarded as a doctor’s degree. Professional training of veterinarians is commonly divided into two phases. The first, or basic science, phase consists of classroom study and laboratory work in the preclinical sciences, including the fields of anatomy, physiology, pathology, pharmacology, toxicology, nutrition, microbiology, and public health. The second phase focuses on the clinical sciences and includes classroom study of infectious and noninfectious diseases, diagnostic and clinical pathology, obstetrics, radiology, anesthesiology, surgery, and practice management and hands-on clinical experience in the college’s veterinary teaching hospital. The clinical experience gives students the opportunity to treat sick animals, perform surgery, and communicate with animal owners. Student activities in the clinical setting are conducted under the supervision of graduate veterinarians on the faculty. Several important opportunities for additional training are available to graduate veterinarians. Internship (one-year) and residency (two-to-five-year) programs enable veterinarians to gain clinical proficiency in one or two medical specialties. Graduate veterinarians can also pursue advanced degree programs. Usually the field of advanced study is medically oriented, but some seek advanced degrees in areas such as business.

Most clinical-practice veterinarians treat only companion animals and usually within the practice’s clinic, or animal hospital. A small proportion treat only food-producing animals or horses, most often by traveling to the location of the animal in a vehicle equipped for veterinary services in the field. Most of the remainder in clinical practice are in mixed practices, which deal with both small animals and large domestic animals such as cattle or horses. Some small-animal practices offer services for special species such as ornamental fish, caged birds, and reptiles. Some practices may limit work to a specific medical area such as surgery, dentistry, dermatology, or ophthalmology. Corporate-owned animal hospitals have increased in number and are often combined with a retail outlet for pet supplies.

Veterinarians in academia administer the basic and clinical science programs of veterinary colleges. In addition, they conduct basic and clinical research, the latter of which may involve application of new instrumentation technologies for diagnosis and treatment of animal diseases. Included are echocardiography, laser lithotripsy, endoscopy, nuclear scintigraphy, ultrasonography, computed tomography (CT) scans, and magnetic resonance imaging (MRI).

Veterinary medicine intersects with private industry in such areas as marketing of animal-health products, monitoring of animal health in large commercial animal-production programs, and biomedical research. Veterinary specialists in industry work in the fields of toxicology, laboratory animal medicine, pathology, molecular biology, and genetic engineering. Pharmaceutical companies employ veterinarians in the development, safety testing, and clinical evaluation of drugs, chemicals, and biological products such as antibiotics and vaccines for animals and people.

National and local governments employ veterinarians in those agencies charged with public health, protection of the environment, agricultural research, food and drug safety, food-animal inspection, the health of imported animals, and the humane treatment of animals. Veterinarians working in public-health programs, for example, evaluate the safety of food-processing plants, restaurants, and water supplies. They also monitor and help control animal and human disease outbreaks. The increased threat of bioterrorism has given veterinarians vital roles in the protection of the food supply for animals and people and in early detection of use of zoonotic organisms as weapons. Veterinarians also work in aerospace; e.g., they have been scientific advisers on animal use in the U.S. space program and have been members of U.S. space shuttle crews. Veterinarians in military service perform biomedical research, care for military dogs, and protect troops through food-inspection and communicable-disease monitoring-and-control programs.

Jai Ganesh · 2021-01-09 00:43:43

878) Livestock

Livestock, farm animals, with the exception of poultry. In Western countries the category encompasses primarily cattle, sheep, pigs, goats, horses, donkeys, and mules; other animals, such as buffalo, oxen, llamas, or camels, may predominate in the agriculture of other areas.

A brief treatment of livestock follows.

Cattle (genus Bos) make up the largest livestock group worldwide. Among those prominent in beef production are Hereford, Shorthorn, and Angus. The chief dairy cattle breeds are Holstein-Friesian, Brown Swiss, Ayrshire, Jersey, and Guernsey. Cattle feed primarily on pasture by grazing, but in modern farming their diet is ordinarily supplemented with prepared animal feeds. Cattle are sometimes used as draft animals, particularly in small-scale farming and in less developed regions.

Sheep (genus Ovis) were among the first animals to be domesticated, perhaps as early as 10,000 BCE. Some 200 breeds are recognized. Closely related to goats, sheep are raised primarily for the fleece or wool of their coats, for meat (mutton and lamb), and, to a lesser degree, for milk. Like cattle, sheep graze for their food, eating both short, fine grasses and coarse, brushy weeds.

Pigs, or domestic swine (family Suidae), have been raised for their meat (pork) since ancient times. There are more than 300 breeds worldwide. In the United States, the term hog is used for swine weighing more than 54 kg (120 pounds), and the animals, regardless of breed, are classified for marketing purposes as lard, bacon, or pork types, the lard types being the heaviest. Corn is usually the basic feed for pigs, although wheat, sorghum, oats, and barley are often included in their diet.

Goats (genus Capra) are raised for their milk and its by-products and for meat, hides, and wool. The numerous breeds comprise three major groups: the prick-eared (e.g., Swiss); the eastern (e.g., Nubian); and the wool (e.g., Angora [mohair] and Cashmere). Goats eat pasture grass, alfalfa or other hays, and feeds made from grain.

Horses (Equus caballus), first intensively domesticated in Central Asia, are bred not only as livestock but also for riding, show, and racing. As livestock, horses are used for farm work or for riding, the latter especially on large cattle ranches. The numerous breeds may be classified according to place of origin (e.g., Clydesdale, Arabian), by their principal use (e.g., riding, draft), or by outward appearance (light, heavy, pony). Horses feed on grass and other pasture growths, and their diets are usually supplemented with hays, grain (primarily oats), and other nutritive feeds.

Donkeys (Equus asinus), also called maths, and mules, the hybrids formed by crossbreeding a male donkey and a female horse, are used as work animals on many farms. Sure-footed and strong, they are often employed as saddle mounts as well.

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