Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray depending on their mineralogy; the word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar; the term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic.
A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids; the extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive and tough; these properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
Overfishing is the removal of a species of fish from a body of water at a rate that the species cannot replenish in time, resulting in those species either becoming depleted or underpopulated in that given area. Overfishing has been present for centuries. Overfishing can occur in water bodies of any sizes, such as ponds, lakes or oceans, can result in resource depletion, reduced biological growth rates and low biomass levels. Sustained overfishing can lead to critical depensation, where the fish population is no longer able to sustain itself; some forms of overfishing, such as the overfishing of sharks, has led to the upset of entire marine ecosystems. The ability of a fishery to recover from overfishing depends on whether the ecosystem's conditions are suitable for the recovery. Dramatic changes in species composition can result in an ecosystem shift, where other equilibrium energy flows involve species compositions different from those, present before the depletion of the original fish stock.
For example, once trout have been overfished, carp might take over in a way that makes it impossible for the trout to re-establish a breeding population. Overfishing has stripped many fisheries around the world of their stocks; the United Nations Food and Agriculture Organization estimated in a 2018 report that 33.1% of world fish stocks are subject to overfishing. Significant overfishing has been observed in pre-industrial times. In particular, the overfishing of the western Atlantic Ocean from the earliest days of European colonisation of the Americas has been well documented. Daniel Pauly, a fisheries scientist known for pioneering work on the human impacts on global fisheries, has commented: It is as though we use our military to fight the animals in the ocean. We are winning this war to exterminate them, and to see this destruction happen, for nothing – for no reason –, a bit frustrating. Strangely enough, these effects are all reversible, all the animals that have disappeared would reappear, all the animals that were small would grow, all the relationships that you can't see any more would re-establish themselves, the system would re-emerge.
Examples of overfishing exist in areas such as the North Sea, the Grand Banks of Newfoundland and the East China Sea. In these locations, overfishing has not only proved disastrous to fish stocks but to the fishing communities relying on the harvest. Like other extractive industries such as forestry and hunting, fisheries are susceptible to economic interaction between ownership or stewardship and sustainability, otherwise known as the tragedy of the commons; the Peruvian coastal anchovy fisheries crashed in the 1970s after overfishing and an El Niño season depleted anchovies from its waters. Anchovies were a major natural resource in Peru. However, the following five years saw the Peruvian fleet's catch amount to only about 4 million tons; this was a major loss to Peru's economy. The collapse of the cod fishery off Newfoundland, the 1992 decision by Canada to impose an indefinite moratorium on the Grand Banks, is a dramatic example of the consequences of overfishing; the sole fisheries in the Irish Sea, the west English Channel, other locations have become overfished to the point of virtual collapse, according to the UK government's official Biodiversity Action Plan.
The United Kingdom has created elements within this plan to attempt to restore this fishery, but the expanding global human population and the expanding demand for fish has reached a point where demand for food threatens the stability of these fisheries, if not the species' survival. Many deep sea fish are at risk, such as orange sablefish; the deep sea is completely dark, near freezing and has little food. Deep sea fish grow because of limited food, have slow metabolisms, low reproductive rates, many don't reach breeding maturity for 30 to 40 years. A fillet of orange roughy at the store is at least 50 years old. Most deep sea fish are in international waters. Most of these fish are caught by deep trawlers near seamounts, where they congregate because of food. Flash freezing allows the trawlers to work for days at a time, modern fishfinders target the fish with ease. Blue walleye became extinct in the Great Lakes in the 1980s; until the middle of the 20th century, it was a commercially valuable fish, with about a half million tonnes being landed during the period from about 1880 to the late 1950s, when the populations collapsed through a combination of overfishing, anthropogenic eutrophication, competition with the introduced rainbow smelt.
The World Wide Fund for Nature and the Zoological Society of London jointly issued their "Living Blue Planet Report" on 16 September 2015 which states that there was a dramatic fall of 74% in worldwide stocks of the important scombridae fish such as mackerel and bonitos between 1970 and 2010, the global overall "population sizes of mammals, reptiles and fish fell by half on average in just 40 years." Several countries are now managing their fisheries. Examples include New Zealand; the United States has turned many of its fisheries around from being in a depleted state. According to a 2008 UN report, the world's fishing fleets are losing US$50 billion each year through depleted stocks and poor fisheries management; the report, produced jointly by the World Bank and the UN Food and Agriculture Organization, asserts that half the world's fishing fleet could be scrapped with no change in catch. In addition, the biomass of global fish stocks have been allowed to run down to the point where it is no longer possible to cat
Guinea the Republic of Guinea, is a west-coastal country in West Africa. Known as French Guinea, the modern country is sometimes referred to as Guinea-Conakry in order to distinguish it from other countries with "Guinea" in the name and the eponymous region, such as Guinea-Bissau and Equatorial Guinea. Guinea has an area of 245,860 square kilometres; the sovereign state of Guinea is a republic with a president, directly elected by the people and is head of state and head of government. The unicameral Guinean National Assembly is the legislative body of the country, its members are directly elected by the people; the judicial branch is led by the Guinea Supreme Court, the highest and final court of appeal in the country. The country is named after the Guinea region. Guinea is a traditional name for the region of Africa, it ends at the Sahel. The English term Guinea comes directly from the Portuguese word Guiné, which emerged in the mid-15th century to refer to the lands inhabited by the Guineus, a generic term for the black African peoples below the Senegal River, as opposed to the'tawny' Zenaga Berbers, above it, whom they called Azenegues or Moors.
Guinea is a predominantly Islamic country, with Muslims representing 85 percent of the population. Guinea's people belong to twenty-four ethnic groups. French, the official language of Guinea, is the main language of communication in schools, in government administration, the media, but more than twenty-four indigenous languages are spoken. Guinea's economy is dependent on agriculture and mineral production, it is the world's second largest producer of bauxite, has rich deposits of diamonds and gold. The country was at the core of the 2014 Ebola outbreak. Human rights in Guinea remain a controversial issue. In 2011 the United States government claimed that torture by security forces, abuse of women and children were ongoing abuses of human rights; the land, now Guinea belonged to a series of African empires until France colonized it in the 1890s, made it part of French West Africa. Guinea declared its independence from France on 2 October 1958. From independence until the presidential election of 2010, Guinea was governed by a number of autocratic rulers.
For the origin of the name "Guinea" see Guinea § Etymology. What is now Guinea was on the fringes of the major West African empires; the earliest, the Ghana Empire, grew on trade but fell after repeated incursions of the Almoravids. It was in this period; the Sosso kingdom flourished in the resulting void but the Mali Empire came to prominence when Soundiata Kéïta defeated the Sosso ruler Soumangourou Kanté at the Battle of Kirina in c. 1235. The Mali Empire was ruled by Mansa, the most famous being Kankou Moussa, who made a famous hajj to Mecca in 1324. Shortly after his reign the Mali Empire began to decline and was supplanted by its vassal states in the 15th century; the most successful of these was the Songhai Empire, which expanded its power from about 1460 and surpassed the Mali Empire in both territory and wealth. It continued to prosper until a civil war over succession followed the death of Askia Daoud in 1582; the weakened empire fell to invaders from Morocco at the Battle of Tondibi just three years later.
The Moroccans proved unable to rule the kingdom however, it split into many small kingdoms. After the fall of the major West African empires, various kingdoms existed in. Fulani Muslims migrated to Futa Jallon in Central Guinea and established an Islamic state from 1735 to 1898 with a written constitution and alternate rulers; the Wassoulou or Wassulu empire was a short-lived empire, led by Samori Toure in the predominantly Malinké area of what is now upper Guinea and southwestern Mali. It moved to Ivory Coast before being conquered by the French; the slave trade came to the coastal region of Guinea with European traders in the 16th century. Slaves were exported to work elsewhere in the triangular trade. Guinea's colonial period began with French military penetration into the area in the mid-19th century. French domination was assured by the defeat in 1898 of the armies of Samori Touré, Mansa of the Ouassoulou state and leader of Malinké descent, which gave France control of what today is Guinea and adjacent areas.
France negotiated Guinea's present boundaries in the late 19th and early 20th centuries with the British for Sierra Leone, the Portuguese for their Guinea colony, Liberia. Under the French, the country formed the Territory of Guinea within French West Africa, administered by a governor general resident in Dakar. Lieutenant governors administered the individual colonies, including Guinea. In 1958, the French Fourth Republic collapsed due to political instability and its failures in dealing with its colonies Indochina and Algeria; the founding of a Fifth Republic was supported by the French people, while French President Charles de Gaulle made it clear on 8 August 1958 that France's colonies were to be given a stark choice between more autonomy in a new French Community or immediate independence in the referendum to be held on 28 September 1958. The other colonies chose the former but Guinea—under the leadership of Ahmed Sékou Touré whose Democratic Party of Guinea had won 56 of 60 seats in 1957 territorial elections — voted overwhelmingly for independence.
The French withdrew and
Guinea-Bissau the Republic of Guinea-Bissau, is a country in West Africa that covers 36,125 square kilometres with an estimated population of 1,815,698. Guinea-Bissau was once part of the kingdom of Gabu, as well as part of the Mali Empire. Parts of this kingdom persisted until the 18th century, while a few others were under some rule by the Portuguese Empire since the 16th century. In the 19th century, it was colonized as Portuguese Guinea. Upon independence, declared in 1973 and recognised in 1974, the name of its capital, was added to the country's name to prevent confusion with Guinea. Guinea-Bissau has a history of political instability since independence, no elected president has served a full five-year term. Only 14% of the population speaks noncreolized Portuguese, established as both the official and national language. Portuguese exists in creole continuum with Crioulo, a Portuguese creole spoken by half the population and an larger number speaks it as second tongue; the remainder speak a variety of native African languages.
There are diverse religions in Guinea-Bissau with no one religion having a majority. The CIA World Factbook states there are about 40% Muslims, 22% Christians, 15% Animists and 18% unspecified or other; the country's per-capita gross domestic product is one of the lowest in the world. The sovereign state of Guinea-Bissau is a member of the United Nations, African Union, Economic Community of West African States, Organisation of Islamic Cooperation, Community of Portuguese Language Countries, La Francophonie and the South Atlantic Peace and Cooperation Zone, was a member of the now-defunct Latin Union. Guinea-Bissau was once part of the kingdom of part of the Mali Empire. Other parts of the territory in the current country were considered by the Portuguese as part of their empire. Portuguese Guinea was known as the Slave Coast, as it was a major area for the exportation of African slaves by Europeans to the western hemisphere. Early reports of Europeans reaching this area include those of the Venetian Alvise Cadamosto's voyage of 1455, the 1479–1480 voyage by Flemish-French trader Eustache de la Fosse, Diogo Cão.
In the 1480s this Portuguese explorer reached the Congo River and the lands of Bakongo, setting up the foundations of modern Angola, some 4200 km down the African coast from Guinea-Bissau. Although the rivers and coast of this area were among the first places colonized by the Portuguese, who set up trading posts in the 16th century, they did not explore the interior until the 19th century; the local African rulers in Guinea, some of whom prospered from the slave trade, controlled the inland trade and did not allow the Europeans into the interior. They kept them in the fortified coastal settlements. African communities that fought back against slave traders distrusted European adventurers and would-be settlers; the Portuguese in Guinea were restricted to the ports of Bissau and Cacheu. A small number of European settlers established isolated farms along Bissau's inland rivers. For a brief period in the 1790s, the British tried to establish a rival foothold on an offshore island, at Bolama, but by the 19th century the Portuguese were sufficiently secure in Bissau to regard the neighbouring coastline as their own special territory up north in part of present South Senegal.
An armed rebellion, begun in 1956 by the African Party for the Independence of Guinea and Cape Verde under the leadership of Amílcar Cabral consolidated its hold on the Portuguese Guinea. Unlike guerrilla movements in other Portuguese colonies, the PAIGC extended its military control over large portions of the territory, aided by the jungle-like terrain, its reached borderlines with neighbouring allies, large quantities of arms from Cuba, the Soviet Union, left-leaning African countries. Cuba agreed to supply artillery experts and technicians; the PAIGC managed to acquire a significant anti-aircraft capability in order to defend itself against aerial attack. By 1973, the PAIGC was in control of many parts of Guinea, although the movement suffered a setback in January 1973 when Cabral was assassinated. Independence was unilaterally declared on 24 September 1973. Recognition became universal following 25 April 1974 socialist-inspired military coup in Portugal, which overthrew Lisbon's Estado Novo regime.
Luís Cabral, brother of Amílcar and co-founder of PAIGC, was appointed the first President of Guinea-Bissau. Following independence, the PAIGC killed thousands of local Guinean soldiers who had fought alongside the Portuguese Army against the guerrillas; some escaped to settle in other African nations. One of the massacres occurred in the town of Bissorã. In 1980 the PAIGC acknowledged in its newspaper Nó Pintcha that many Guinean soldiers had been executed and buried in unmarked collective graves in the woods of Cumerá, Mansabá; the country was controlled by a revolutionary council until 1984. The first multi-party elections were held in 1994. An army uprising in May 1998 led to the Guinea-Bissau Civil War and the president's ousting in June 1999. Elections were held again in 2000, Kumba Ialá was elected president. In September 2003, a military coup was conducted; the military arrested Ialá on the charge of being "unable to solve the problems". After being delayed several times, legislative elections were held in March 2004.
A mutiny of military factions in October 2004 resulted in the death of the head of the armed forces and caused widespread unrest. In June 2005
Deforestation, clearcutting or clearing is the removal of a forest or stand of trees from land, converted to a non-forest use. Deforestation can involve conversion of forest land to ranches, or urban use; the most concentrated deforestation occurs in tropical rainforests. About 31% of Earth's land surface is covered by forests. Deforestation can occur for several reasons: trees can be cut down to be used for building or sold as fuel, while cleared land can be used as pasture for livestock and plantation; the removal of trees without sufficient reforestation has resulted in habitat damage, biodiversity loss, aridity. It has adverse impacts on biosequestration of atmospheric carbon dioxide. Deforestation has been used in war to deprive the enemy of vital resources and cover for its forces. Modern examples of this were the use of Agent Orange by the British military in Malaya during the Malayan Emergency and by the United States military in Vietnam during the Vietnam War; as of 2005, net deforestation rates had ceased to increase in countries with a per capita GDP of at least US$4,600.
Deforested regions incur significant adverse soil erosion and degrade into wasteland. Disregard of ascribed value, lax forest management, deficient environmental laws are some of the factors that lead to large-scale deforestation. In many countries, deforestation–both occurring and human-induced–is an ongoing issue. Deforestation causes extinction, changes to climatic conditions and displacement of populations, as observed by current conditions and in the past through the fossil record. More than half of all plant and land animal species in the world live in tropical forests. Between 2000 and 2012, 2.3 million square kilometres of forests around the world were cut down. As a result of deforestation, only 6.2 million square kilometres remain of the original 16 million square kilometres of tropical rainforest that covered the Earth. An area the size of a football pitch is cleared from the Amazon rainforest every minute, with 136 million acres of rainforest cleared for animal agriculture overall.
According to the United Nations Framework Convention on Climate Change secretariat, the overwhelming direct cause of deforestation is agriculture. Subsistence farming is responsible for 48% of deforestation. Experts do not agree on whether industrial logging is an important contributor to global deforestation; some argue that poor people are more to clear forest because they have no alternatives, others that the poor lack the ability to pay for the materials and labour needed to clear forest. One study found that population increases due to high fertility rates were a primary driver of tropical deforestation in only 8% of cases. Other causes of contemporary deforestation may include corruption of government institutions, the inequitable distribution of wealth and power, population growth and overpopulation, urbanization. Globalization is viewed as another root cause of deforestation, though there are cases in which the impacts of globalization have promoted localized forest recovery. In 2000 the United Nations Food and Agriculture Organization found that "the role of population dynamics in a local setting may vary from decisive to negligible", that deforestation can result from "a combination of population pressure and stagnating economic and technological conditions".
The degradation of forest ecosystems has been traced to economic incentives that make forest conversion appear more profitable than forest conservation. Many important forest functions have no markets, hence, no economic value, apparent to the forests' owners or the communities that rely on forests for their well-being. From the perspective of the developing world, the benefits of forest as carbon sinks or biodiversity reserves go to richer developed nations and there is insufficient compensation for these services. Developing countries feel that some countries in the developed world, such as the United States of America, cut down their forests centuries ago and benefited economically from this deforestation, that it is hypocritical to deny developing countries the same opportunities, i.e. that the poor shouldn't have to bear the cost of preservation when the rich created the problem. Some commentators have noted a shift in the drivers of deforestation over the past 30 years. Whereas deforestation was driven by subsistence activities and government-sponsored development projects like transmigration in countries like Indonesia and colonization in Latin America, Java, so on, during the late 19th century and the earlier half of the 20th century, by the 1990s the majority of deforestation was caused by industrial factors, including extractive industries, large-scale cattle ranching, extensive agriculture.
Since 2001, commodity-driven deforestation, more to be permanent, has accounted for about a quarter of all forest disturbance, this loss has been concentrated in South America and Southeast Asia. Deforestation is shaping climate and geography. Deforestation is a contributor to global warming, is cited as one of the major causes of the enhanced greenhouse effect. Tropical deforestation is responsible for 20% of world greenhouse gas emissions. According to the Intergovernmental Panel on Climate Change deforestation in tropical areas, could account for up to one-third of total anthropogenic carbon d
The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers. It covers 20 percent of the Earth's surface and about 29 percent of its water surface area, it separates the "Old World" from the "New World". The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, the Americas to the west; as one component of the interconnected global ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, the Southern Ocean in the south. The Equatorial Counter Current subdivides it into the North Atlantic Ocean and the South Atlantic Ocean at about 8°N. Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office; the oldest known mentions of an "Atlantic" sea come from Stesichorus around mid-sixth century BC: Atlantikoi pelágei and in The Histories of Herodotus around 450 BC: Atlantis thalassa where the name refers to "the sea beyond the pillars of Heracles", said to be part of the sea that surrounds all land.
Thus, on one hand, the name refers to Atlas, the Titan in Greek mythology, who supported the heavens and who appeared as a frontispiece in Medieval maps and lent his name to modern atlases. On the other hand, to early Greek sailors and in Ancient Greek mythological literature such as the Iliad and the Odyssey, this all-encompassing ocean was instead known as Oceanus, the gigantic river that encircled the world. In contrast, the term "Atlantic" referred to the Atlas Mountains in Morocco and the sea off the Strait of Gibraltar and the North African coast; the Greek word thalassa has been reused by scientists for the huge Panthalassa ocean that surrounded the supercontinent Pangaea hundreds of millions of years ago. The term "Aethiopian Ocean", derived from Ancient Ethiopia, was applied to the Southern Atlantic as late as the mid-19th century. During the Age of Discovery, the Atlantic was known to English cartographers as the Great Western Ocean; the term The Pond is used by British and American speakers in context to the Atlantic Ocean, as a form of meiosis, or sarcastic understatement.
The term dates to as early as 1640, first appearing in print in pamphlet released during the reign of Charles I, reproduced in 1869 in Nehemiah Wallington's Historical Notices of Events Occurring Chiefly in The Reign of Charles I, where "great Pond" is used in reference to the Atlantic Ocean by Francis Windebank, Charles I's Secretary of State. The International Hydrographic Organization defined the limits of the oceans and seas in 1953, but some of these definitions have been revised since and some are not used by various authorities and countries, see for example the CIA World Factbook. Correspondingly, the extent and number of oceans and seas varies; the Atlantic Ocean is bounded on the west by South America. It connects to the Arctic Ocean through the Denmark Strait, Greenland Sea, Norwegian Sea and Barents Sea. To the east, the boundaries of the ocean proper are Europe: the Strait of Africa. In the southeast, the Atlantic merges into the Indian Ocean; the 20° East meridian, running south from Cape Agulhas to Antarctica defines its border.
In the 1953 definition it extends south to Antarctica, while in maps it is bounded at the 60° parallel by the Southern Ocean. The Atlantic has irregular coasts indented by numerous bays and seas; these include the Baltic Sea, Black Sea, Caribbean Sea, Davis Strait, Denmark Strait, part of the Drake Passage, Gulf of Mexico, Labrador Sea, Mediterranean Sea, North Sea, Norwegian Sea all of the Scotia Sea, other tributary water bodies. Including these marginal seas the coast line of the Atlantic measures 111,866 km compared to 135,663 km for the Pacific. Including its marginal seas, the Atlantic covers an area of 106,460,000 km2 or 23.5% of the global ocean and has a volume of 310,410,900 km3 or 23.3% of the total volume of the earth's oceans. Excluding its marginal seas, the Atlantic covers 81,760,000 km2 and has a volume of 305,811,900 km3; the North Atlantic covers 41,490,000 km2 and the South Atlantic 40,270,000 km2. The average depth is 3,646 m and the maximum depth, the Milwaukee Deep in the Puerto Rico Trench, is 8,486 m.
The bathymetry of the Atlantic is dominated by a submarine mountain range called the Mid-Atlantic Ridge. It runs from 87°N or 300 km south of the North Pole to the subantarctic Bouvet Island at 42°S; the MAR divides the Atlantic longitudinally into two halves, in each of which a series of basins are delimited by secondary, transverse ridges. The MAR reaches above 2,000 m along most of its length, but is interrupted by larger transform faults at two places: the Romanche Trench near the Equator and the Gibbs Fracture Zone at 53°N; the MAR is a barrier for bottom water, but at these two transform faults deep water currents can pass from one side to the othe
Desertification is a type of land degradation in which a dry area of land becomes a desert losing its bodies of water as well as vegetation and wildlife. It is caused by a variety of factors, such as through climate change and through the overexploitation of soil through human activity; when deserts appear automatically over the natural course of a planet's life cycle it can be called a natural phenomenon. Desertification is a significant global ecological and environmental problem with far reaching consequences on socio-economic and political conditions. Considerable controversy exists over the proper definition of the term "desertification" for which Helmut Geist has identified more than 100 formal definitions; the most accepted of these is that of the Princeton University Dictionary which defines it as "the process of fertile land transforming into desert as a result of deforestation, drought or improper/inappropriate agriculture". Desertification has been neatly defined in the text of the United Nations Convention to Combat Desertification as "land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities."Another major contribution to the controversy comes from the sub-grouping of types of desertification.
Spanning from the vague yet shortsighted view as the "man-made-desert" to the broader yet less focused type as the "Non-pattern-Desert". The earliest known discussion of the topic arose soon after the French colonization of West Africa, when the Comité d'Etudes commissioned a study on desséchement progressif to explore the prehistoric expansion of the Sahara Desert; the world's most noted deserts have been formed by natural processes interacting over long intervals of time. During most of these times, deserts have shrunk independent of human activities. Paleodeserts are large sand seas now inactive because they are stabilized by vegetation, some extending beyond the present margins of core deserts, such as the Sahara, the largest hot desert. Desertification has played a significant role in human history, contributing to the collapse of several large empires, such as Carthage and the Roman Empire, as well as causing displacement of local populations. Historical evidence shows that the serious and extensive land deterioration occurring several centuries ago in arid regions had three epicenters: the Mediterranean, the Mesopotamian Valley, the Loess Plateau of China, where population was dense.
Drylands occupy 40–41% of Earth’s land area and are home to more than 2 billion people. It has been estimated that some 10–20% of drylands are degraded, the total area affected by desertification being between 6 and 12 million square kilometres, that about 1–6% of the inhabitants of drylands live in desertified areas, that a billion people are under threat from further desertification; as of 1998, the then-current degree of southward expansion of the Sahara was not well known, due to a lack of recent, measurable expansion of the desert into the Sahel at the time. The impact of global warming and human activities are presented in the Sahel. In this area, the level of desertification is high compared to other areas in the world. All areas situated in the eastern part of Africa are characterized by a dry climate, hot temperatures, low rainfall. So, droughts are the rule in the Sahel region; some studies have shown that Africa has lost 650,000 km² of its productive agricultural land over the past 50 years.
The propagation of desertification in this area is considerable. Some statistics have shown that since 1900 the Sahara has expanded by 250 km to the south over a stretch of land from west to east 6,000 km long; the survey, done by the research institute for development, had demonstrated that this means dryness is spreading fast in the Sahelian countries. 70% of the arid area has deteriorated and water resources have disappeared, leading to soil degradation. The loss of topsoil means that plants cannot take root and can be uprooted by torrential water or strong winds; the United Nations Convention says that about six million Sahelian citizens would have to give up the desertified zones of sub-Saharan Africa for North Africa and Europe between 1997 and 2020. Another major area, being impacted by desertification is the Gobi Desert; the Gobi desert is the fastest moving desert on Earth. This has destroyed many villages in its path. Photos show that the Gobi Desert has expanded to the point the entire nation of Croatia could fit inside its area.
This is causing a major problem for the people of China. They will soon have to deal with the desert. Although the Gobi Desert itself is still a distance away from Beijing, reports from field studies state there are large sand dunes forming only 70 km outside the city; as the desertification takes place, the landscape may progress through different stages and continuously transform in appearance. On sloped terrain, desertification can create larger empty spaces over a large strip of land, a phenomenon known as "Brousse tigrée". A mathematical model of this phenomenon proposed by C. Klausmeier attributes this patterning to dynamics in plant-water interaction. One outcome of this observation suggests an optimal plan