1.
Africa
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Africa is the worlds second-largest and second-most-populous continent. At about 30.3 million km² including adjacent islands, it covers 6% of Earths total surface area and 20.4 % of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the human population. The continent includes Madagascar and various archipelagos and it contains 54 fully recognized sovereign states, nine territories and two de facto independent states with limited or no recognition. Africas population is the youngest amongst all the continents, the age in 2012 was 19.7. Algeria is Africas largest country by area, and Nigeria by population, afarensis, Homo erectus, H. habilis and H. ergaster – with the earliest Homo sapiens found in Ethiopia being dated to circa 200,000 years ago. Africa straddles the equator and encompasses numerous climate areas, it is the continent to stretch from the northern temperate to southern temperate zones. Africa hosts a diversity of ethnicities, cultures and languages. In the late 19th century European countries colonized most of Africa, Africa also varies greatly with regard to environments, economics, historical ties and government systems. However, most present states in Africa originate from a process of decolonization in the 20th century, afri was a Latin name used to refer to the inhabitants of Africa, which in its widest sense referred to all lands south of the Mediterranean. This name seems to have referred to a native Libyan tribe. The name is connected with Hebrew or Phoenician ʿafar dust. The same word may be found in the name of the Banu Ifran from Algeria and Tripolitania, under Roman rule, Carthage became the capital of the province of Africa Proconsularis, which also included the coastal part of modern Libya. The Latin suffix -ica can sometimes be used to denote a land, the later Muslim kingdom of Ifriqiya, modern-day Tunisia, also preserved a form of the name. According to the Romans, Africa lay to the west of Egypt, while Asia was used to refer to Anatolia, as Europeans came to understand the real extent of the continent, the idea of Africa expanded with their knowledge. 25,4, whose descendants, he claimed, had invaded Libya, isidore of Seville in Etymologiae XIV.5.2. Suggests Africa comes from the Latin aprica, meaning sunny, massey, in 1881, stated that Africa is derived from the Egyptian af-rui-ka, meaning to turn toward the opening of the Ka. The Ka is the double of every person and the opening of the Ka refers to a womb or birthplace
2.
Atlantic Ocean
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The Atlantic Ocean is the second largest of the worlds oceans with a total area of about 106,460,000 square kilometres. It covers approximately 20 percent of the Earths surface and about 29 percent of its surface area. It separates the Old World from the New World, the Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Eurasia and Africa to the east, and the Americas to the west. The Equatorial Counter Current subdivides it into the North Atlantic Ocean, in contrast, the term Atlantic originally referred specifically 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 years ago. The term Aethiopian Ocean, derived from Ancient Ethiopia, was applied to the Southern Atlantic as late as the mid-19th century, many Irish or British people refer to the United States and Canada as across the pond, and vice versa. The Black Atlantic refers to the role of ocean in shaping black peoples history. Irish migration to the US is meant when the term The Green Atlantic is used, the term Red Atlantic has been used in reference to the Marxian concept of an Atlantic working class, as well as to the Atlantic experience of indigenous Americans. Correspondingly, the extent and number of oceans and seas varies, the Atlantic Ocean is bounded on the west by North and South America. It connects to the Arctic Ocean through the Denmark Strait, Greenland Sea, Norwegian Sea, to the east, the boundaries of the ocean proper are Europe, the Strait of Gibraltar and 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 later maps it is bounded at the 60° parallel by the Southern Ocean, the Atlantic has irregular coasts indented by numerous bays, gulfs, and seas. 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 ocean and has a volume of 310,410,900 km3 or 23. 3%. 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 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 2000 m along most of its length, the MAR is a barrier for bottom water, but at these two transform faults deep water currents can pass from one side to the other
3.
Mediterranean Sea
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The sea is sometimes considered a part of the Atlantic Ocean, although it is usually identified as a separate body of water. The name Mediterranean is derived from the Latin mediterraneus, meaning inland or in the middle of land and it covers an approximate area of 2.5 million km2, but its connection to the Atlantic is only 14 km wide. The Strait of Gibraltar is a strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Gibraltar. In oceanography, it is called the Eurafrican Mediterranean Sea or the European Mediterranean Sea to distinguish it from mediterranean seas elsewhere. The Mediterranean Sea has a depth of 1,500 m. The sea is bordered on the north by Europe, the east by Asia and it is located between latitudes 30° and 46° N and longitudes 6° W and 36° E. Its west-east length, from the Strait of Gibraltar to the Gulf of Iskenderun, the seas average north-south length, from Croatia’s southern shore to Libya, is approximately 800 km. The Mediterranean Sea, including the Sea of Marmara, has an area of approximately 2,510,000 square km. The sea was an important route for merchants and travelers of ancient times that allowed for trade, the history of the Mediterranean region is crucial to understanding the origins and development of many modern societies. In addition, the Gaza Strip and the British Overseas Territories of Gibraltar and Akrotiri, the term Mediterranean derives from the Latin word mediterraneus, meaning amid the earth or between land, as it is between the continents of Africa, Asia and Europe. The Ancient Greek name Mesogeios, is similarly from μέσο, between + γη, land, earth) and it can be compared with the Ancient Greek name Mesopotamia, meaning between rivers. The Mediterranean Sea has historically had several names, for example, the Carthaginians called it the Syrian Sea and latter Romans commonly called it Mare Nostrum, and occasionally Mare Internum. Another name was the Sea of the Philistines, from the people inhabiting a large portion of its shores near the Israelites, the sea is also called the Great Sea in the General Prologue by Geoffrey Chaucer. In Ottoman Turkish, it has also been called Bahr-i Sefid, in Modern Hebrew, it has been called HaYam HaTikhon, the Middle Sea, reflecting the Seas name in ancient Greek, Latin, and modern languages in both Europe and the Middle East. Similarly, in Modern Arabic, it is known as al-Baḥr al-Mutawassiṭ, in Turkish, it is known as Akdeniz, the White Sea since among Turks the white colour represents the west. Several ancient civilisations were located around the Mediterranean shores, and were influenced by their proximity to the sea. It provided routes for trade, colonisation, and war, as well as food for numerous communities throughout the ages, due to the shared climate, geology, and access to the sea, cultures centered on the Mediterranean tended to have some extent of intertwined culture and history. Two of the most notable Mediterranean civilisations in classical antiquity were the Greek city states, later, when Augustus founded the Roman Empire, the Romans referred to the Mediterranean as Mare Nostrum
4.
Plate tectonics
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The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major plates, where the plates meet, their relative motion determines the type of boundary, convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually, tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading. In this way, the surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle, earlier theories, since disproven, proposed gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
5.
Equator
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The Equator usually refers to an imaginary line on the Earths surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere. The Equator is about 40,075 kilometres long, some 78. 7% lies across water and 21. 3% over land, other planets and astronomical bodies have equators similarly defined. Generally, an equator is the intersection of the surface of a sphere with the plane that is perpendicular to the spheres axis of rotation. The latitude of the Earths equator is by definition 0° of arc, the equator is the only line of latitude which is also a great circle — that is, one whose plane passes through the center of the globe. The plane of Earths equator when projected outwards to the celestial sphere defines the celestial equator, in the cycle of Earths seasons, the plane of the equator passes through the Sun twice per year, at the March and September equinoxes. To an observer on the Earth, the Sun appears to travel North or South over the equator at these times, light rays from the center of the Sun are perpendicular to the surface of the Earth at the point of solar noon on the Equator. Locations on the Equator experience the quickest sunrises and sunsets because the sun moves nearly perpendicular to the horizon for most of the year. The Earth bulges slightly at the Equator, the diameter of the Earth is 12,750 kilometres. Because the Earth spins to the east, spacecraft must also launch to the east to take advantage of this Earth-boost of speed, seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earths axis relative to the plane of revolution. During the year the northern and southern hemispheres are inclined toward or away from the sun according to Earths position in its orbit, the hemisphere inclined toward the sun receives more sunlight and is in summer, while the other hemisphere receives less sun and is in winter. At the equinoxes, the Earths axis is not tilted toward the sun, instead it is perpendicular to the sun meaning that the day is about 12 hours long, as is the night, across the whole of the Earth. Near the Equator there is distinction between summer, winter, autumn, or spring. The temperatures are usually high year-round—with the exception of high mountains in South America, the temperature at the Equator can plummet during rainstorms. In many tropical regions people identify two seasons, the wet season and the dry season, but many places close to the Equator are on the oceans or rainy throughout the year, the seasons can vary depending on elevation and proximity to an ocean. The Equator lies mostly on the three largest oceans, the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. The highest point on the Equator is at the elevation of 4,690 metres, at 0°0′0″N 77°59′31″W and this is slightly above the snow line, and is the only place on the Equator where snow lies on the ground. At the Equator the snow line is around 1,000 metres lower than on Mount Everest, the Equator traverses the land of 11 countries, it also passes through two island nations, though without making a landfall in either. Starting at the Prime Meridian and heading eastwards, the Equator passes through, Despite its name, however, its island of Annobón is 155 km south of the Equator, and the rest of the country lies to the north
6.
Prime meridian
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A prime meridian is a meridian in a geographical coordinate system at which longitude is defined to be 0°. Together, a meridian and its antimeridian form a great circle. This great circle divides the sphere, e. g. the Earth, if one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere. The most widely used modern meridian is the IERS Reference Meridian and it is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884. The notion of longitude was developed by the Greek Eratosthenes in Alexandria, and Hipparchus in Rhodes, but it was Ptolemy who first used a consistent meridian for a world map in his Geographia. The main point is to be comfortably west of the tip of Africa as negative numbers were not yet in use. His prime meridian corresponds to 18°40 west of Winchester today, at that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries. Ptolemys Geographia was first printed with maps at Bologna in 1477, but there was still a hope that a natural basis for a prime meridian existed. The Tordesillas line was settled at 370 leagues west of Cape Verde. This is shown in Diogo Ribeiros 1529 map, in 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuertaventura in the Canaries. His later maps used the Azores, following the magnetic hypothesis, but by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as is usual today and this practice was followed by navigators well into the 18th century. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 19°55 west of Paris, the geographer Delisle decided to round this off to 20°, so that it simply became the meridian of Paris disguised. In the early 18th century the battle was on to improve the determination of longitude at sea, between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich. Maskelynes tables not only made the lunar method practicable, they made the Greenwich meridian the universal reference point. In 1884, at the International Meridian Conference in Washington, D. C.22 countries voted to adopt the Greenwich meridian as the meridian of the world. The French argued for a line, mentioning the Azores and the Bering Strait. In October 1884 the Greenwich Meridian was selected by delegates to the International Meridian Conference held in Washington, united States to be the common zero of longitude and standard of time reckoning throughout the world
7.
Continent
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A continent is one of several very large landmasses on Earth. Generally identified by convention rather than any strict criteria, up to seven regions are regarded as continents. Ordered from largest in size to smallest, they are, Asia, Africa, North America, South America, Antarctica, Europe, in geology, areas of continental crust include regions covered with water. Islands are frequently grouped with a continent to divide all the worlds land into geopolitical regions. Under this scheme, most of the countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form a geopolitical region called Oceania. By convention, continents are understood to be large, continuous, discrete masses of land, many of the seven most commonly recognized continents identified by convention are not discrete landmasses separated completely by water. Earths major landmasses all have coasts on a single, continuous World Ocean, the most restricted meaning of continent is that of a continuous area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. From this perspective the edge of the shelf is the true edge of the continent. In this sense the islands of Great Britain and Ireland are part of Europe, while Australia, as a cultural construct, the concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered part of Europe and Madagascar part of Africa, extrapolating the concept to its extreme, some geographers group the Australasian continental plate with other islands in the Pacific into one continent called Oceania. This divides the land surface of Earth into continents or quasi-continents. The ideal criterion that each continent be a discrete landmass is commonly relaxed due to historical conventions, of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by ocean. Several continents are defined not as absolutely distinct bodies but as more or less discrete masses of land, Asia and Africa are joined by the Isthmus of Suez, and North and South America by the Isthmus of Panama. In both cases, there is no separation of these landmasses by water. Both these isthmuses are very narrow compared to the bulk off the landmasses they unite, North America and South America are treated as separate continents in the seven-continent model. However, they may also be viewed as a continent known as America or the Americas. This viewpoint was common in the United States until World War II and this remains the more common vision in Latin American countries, Spain, Portugal, France, Italy and Greece, where they are taught as a single continent. The criterion of a landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents, Europe and Asia
8.
Somali Plate
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The Somali Plate, or Somalian Plate, is a minor tectonic plate, which straddles the equator in the eastern hemisphere. The Somali Plate formed between 60 million years ago and 10 million years ago by the rifting of the African Plate along the East African Rift. Because the East Africa Rift separates the continent into two plates, the African Plate west of the rift is sometimes referred to as the Nubian Plate, the northern boundary is the Aden Ridge along the coast of Saudi Arabia. The eastern boundary is the Central Indian Ridge, the portion of which is also known as the Carlsberg Ridge. The southern boundary is the Southwest Indian Ridge, dating of volcanism in the Comoro Islands and northern Madagascar volcanoes indicates a rate of motion to the east over a hotspot of approximately 45 mm/yr. Emerick, C. M.1985, Age progressive volcanism in the Comores Archipelago and Northern Madagascar, corvallis, PhD Thesis, Abstract Chorowicz, Jean
9.
Rift
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In geology, a rift is a linear zone where the Earths crust and lithosphere are being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting, where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the area may contain volcanic rocks, and active volcanism is a part of many. Major rifts occur along the axis of most mid-ocean ridges. Failed rifts are the result of rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a junction where three converging rifts meet over a hotspot. Two of these evolve to the point of spreading, while the third ultimately fails. Most rifts consist of a series of segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault, segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in cases the polarity. Segment boundaries often have a complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are known as accommodation zones. Accommodation zones may be located where older crustal structures intersect the rift axis, in the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. At the onset of rifting, the part of the lithosphere starts to extend on a series of initially unconnected normal faults. In subaerial rifts, drainage at this stage is generally internal, as the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults, the longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage, during rifting, as the crust is thinned, the Earths surface subsides and the Moho becomes correspondingly raised
10.
East African Rift
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The East African Rift is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago, in the past, it was considered to be part of a larger Great Rift Valley that extended north to Asia Minor. As extension continues, lithospheric rupture will occur within 10 million years, the Somalian plate will break off, a series of distinct rift basins, the East African Rift System extends over thousands of kilometers. The EAR consists of two main branches, the Eastern Rift Valley includes the Main Ethiopian Rift, running eastward from the Afar Triple Junction, which continues south as the Kenyan Rift Valley. The Western Rift Valley includes the Albertine Rift, and farther south, to the north of the Afar Triple Junction, the rift follows one of two paths, west to the Red Sea Rift or east to the Aden Ridge in the Gulf of Aden. The EAR runs from the Afar Triple Junction in the Afar Triangle of Ethiopia through eastern Africa, the EAR transects through Ethiopia, Kenya, Uganda, Rwanda, Burundi, Zambia, Tanzania, Malawi and Mozambique. The Davie Ridge ranges between 30–120 km wide, with a west facing scarp along the half of its length that rises up to 2300 m above the sea floor. Its movement is concurrent with the EAR, over time, many theories have tried to clarify the evolution of the East African Rift. In 1972 it was proposed that the EAR was not caused by tectonic activity, others proposed an African superplume causing mantle deformation. However, the varying geochemical signatures of a suite of Ethiopian lavas suggest multiple plume sources, at least one of deep mantle origin, additionally, the subject of deep-rooted mantle plumes is still a matter of controversy, and therefore cannot be confirmed. The most recent and accepted view is the theory put forth in 2009, at that time it was suggested that lithospheric thinning generated volcanic activity, further increasing the magmatic processes at play such as intrusions and numerous small plumes. These processes further thin the lithosphere in saturated areas, forcing the thinning lithosphere to behave like a mid-ocean ridge, prior to rifting, enormous continental flood basalts erupted on the surface and uplift of the Ethiopian, Somalian, and East African plateaus occurred. The first stage of rifting of the EAR is characterized by rift localization, periods of extension alternated with times of relative inactivity. There was also the reactivation of a weakness in the crust, a suture zone of multiple cratons, displacement along large boundary faults. The second stage of rifting is characterized by the deactivation of large boundary faults, the development of internal fault segments, today, the narrow rift segments of the East African Rift system form zones of localized strain. These rifts are the result of the actions of numerous faults which are typical of all tectonic rift zones. The African continental crust is generally cool and strong, many cratons are found throughout the EAR, such as the Tanzania and Kaapvaal cratons. The cratons are thick, and have survived for billions of years with little tectonic activity and they are characterized by greenstone belts, tonalites, and other high-grade metamorphic lithologies
11.
Divergent boundary
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In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys, most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates apart to produce gaps which molten lava rises to fill. Current research indicates that complex convection within the Earths mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary. This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere beneath the area forming large flood basalt or lava flows. Each eruption occurs in only a part of the boundary at any one time. Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary, because of this, rocks closest to a boundary are younger than rocks further away on the same plate. At divergent boundaries, two plates move apart from other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface, the hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year. Divergent boundaries can create massive fault zones in the ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different and these are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center and it is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne geomagnetic surveys showed a pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine, the magnetic banding directly corresponds with the Earths polar reversals
12.
North American Plate
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The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, the Bahamas, extreme northeastern Russia, and parts of Iceland and the Azores. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia, the plate includes both continental and oceanic crust. The interior of the continental landmass includes an extensive granitic core called a craton. Along most of the edges of this craton are fragments of material called terranes. It is thought that much of North America west of the Rocky Mountains is composed of such terranes, the parallel Septentrional and Enriquillo-Plantain Garden faults, which run through the island of Hispaniola and bound the Gonâve Microplate, are also a part of the boundary. On the northerly boundary is a continuation of the Mid-Atlantic ridge called the Gakkel Ridge, the rest of the boundary in the far northwestern part of the plate extends into Siberia. On its western edge, the Farallon Plate has been subducting under the North American Plate since the Jurassic Period, the Juan de Fuca, Explorer, Gorda, Rivera, Cocos and Nazca plates are remnants of the Farallon Plate. The boundary along the Gulf of California is complex and it is generally accepted that a piece of the North American Plate was broken off and transported north as the East Pacific Rise propagated northward, creating the Gulf of California. A few hotspots are thought to exist below the North American Plate, the most notable hotspots are the Yellowstone, Raton, and Anahim hotspots. The Yellowstone and Anahim hotspots are thought to have first arrived during the Miocene period and are still geologically active, for the most part, the North American Plate moves in roughly a southwest direction away from the Mid-Atlantic Ridge. One recent study suggests that a convective current is propelling the plate. Geologic timeline of Western North America New Madrid Seismic Zone - an ancient intraplate fault zone within the North American Plate, notable as early as 1699 Notes
13.
South American Plate
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The South American Plate is a tectonic plate which includes the continent of South America and also a sizeable region of the Atlantic Ocean seabed extending eastward to the Mid-Atlantic Ridge. The easterly side is a divergent boundary with the African Plate forming the part of the Mid-Atlantic Ridge. The southerly side is a boundary with the Antarctic Plate. The westerly side is a convergent boundary with the subducting Nazca Plate, the northerly side is a boundary with the Caribbean Plate and the oceanic crust of the North American Plate. At the Chile Triple Junction in Taitato-Tres Montes Peninsula, an oceanic ridge — the Chile Rise — is subducting under the South American plate, the South American Plate is in motion. Parts of the plate boundaries consisting of alternations of relatively short transform fault, moving westward away from the Mid-Atlantic Ridge. The eastward-moving and more dense Nazca Plate is subducting under the edge of the South American Plate along the Pacific coast of the continent at a rate of 77 mm per year. This collision of plates is responsible for lifting the massive Andes Mountains and causing the volcanoes which are strewn throughout them
14.
Mid-Atlantic Ridge
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In the North Atlantic, it separates the Eurasian and North American Plates, whereas in the South Atlantic it separates the African and South American Plates. The ridge extends from a junction with the Gakkel Ridge northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic, although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level. The section of the ridge that includes the island of Iceland is also known as the Reykjanes Ridge, the ridge has an average spreading rate of about 2.5 cm per year. A ridge under the Atlantic Ocean was first inferred by Matthew Fontaine Maury in 1850, the ridge was discovered during the expedition of HMS Challenger in 1872. A team of scientists on board, led by Charles Wyville Thomson, the existence of such a ridge was confirmed by sonar in 1925 and was found to extend around the Cape of Good Hope into the Indian Ocean by the German Meteor expedition. Ewing, Heezen and Tharp discovered that the ridge is part of a 40, the ridge is central to the breakup of the hypothetical supercontinent of Pangaea that began some 180 million years ago. The Mid-Atlantic Ridge includes a deep valley that runs along the axis of the ridge along nearly its entire length. This rift marks the boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava. This trench, however, is not regarded as the boundary between the North and South American Plates, nor the Eurasian and African Plates and this bulge is thought to be caused by upward convective forces in the asthenosphere pushing the oceanic crust and lithosphere. This divergent boundary first formed in the Triassic period, when a series of three-armed grabens coalesced on the supercontinent Pangaea to form the ridge, usually, only two arms of any given three-armed graben become part of a divergent plate boundary. The failed arms are called aulacogens, and the aulacogens of the Mid-Atlantic Ridge eventually became many of the river valleys seen along the Americas. The Fundy Basin on the Atlantic coast of North America between New Brunswick and Nova Scotia in Canada is evidence of the ancestral Mid-Atlantic Ridge, atlantis Massif Central Atlantic Magmatic Province Fifteen-Twenty Fracture Zone Evans, Rachel. Plumbing Depths to Reach New Heights, Marie Tharp Explains Marine Geological Maps, the Library of Congress Information Bulletin. MAR-ECO, a Census of Marine Life project on life along the Mid-Atlantic Ridge
15.
Arabian Plate
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The Arabian Plate is a minor tectonic plate in the northern and eastern hemispheres. It is one of three continental plates that have been moving northward in recent geological history and colliding with the Eurasian Plate, the Arabian Plate consists mostly of the Arabian peninsula, it extends eastward at the Sinai and the Red Sea and northward to the Levant. The Arabian Plate was part of the African plate during much of the Phanerozoic Eon, the opening of the Red Sea rift led to extensive volcanic activity. There are large volcanic fields called the Older Harrats, such as Harrat Khaybar and Harrat Rahat, some activity still continues especially around Medina, and there are regular eruptions within the Red Sea. The collision between the Arabian Plate and Eurasia is pushing up the Zagros Mountains of Iran, because the Arabian Plate and Eurasian Plate collide, many cities are in danger such as those in southeastern Turkey. These dangers include earthquakes, tsunamis, and volcanoes, countries within the plate include Lebanon, Syria, Djibouti, Yemen, Oman, Qatar, Bahrain, Kuwait, Iraq, Saudi Arabia, Jordan, and the United Arab Emirates. Regions include parts of the Southern Denkalya Subregion, the Southeastern Anatolia Region, Awdal and the Khuzestan Province
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Eurasian Plate
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It also includes oceanic crust extending westward to the Mid-Atlantic Ridge and northward to the Gakkel Ridge. The eastern side is a boundary with the North American Plate to the north and a boundary with the Philippine Sea Plate to the south, and possibly with the Okhotsk Plate and the Amurian Plate. The southerly side is a boundary with the African Plate to the west, the Arabian Plate in the middle, the westerly side is a divergent boundary with the North American Plate forming the northernmost part of the Mid-Atlantic Ridge, which is straddled by Iceland. The geodynamics of central Asia is dominated by the interaction between the Eurasian and Indian Plates, in this area, many subplates or crust blocks have been recognized, which form the Central Asian and the East Asian transit zones
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Aegean Sea Plate
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The Aegean Sea Plate is a small tectonic plate located in the eastern Mediterranean Sea under southern Greece and far western Turkey. Its southern edge is a subduction zone south of Crete, where the African Plate is being swept under the Aegean Sea Plate, to the north is the Eurasian Plate, which is a divergent boundary responsible for the formation of the Gulf of Corinth. An updated digital model of plate boundaries
18.
Anatolian Plate
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The Anatolian Plate or the Turkish Plate is a continental tectonic plate comprising most of the Anatolia peninsula. To the east, the East Anatolian Fault, a left lateral transform fault, to the south and southwest is a convergent boundary with the African Plate. The northern edge is a boundary with the Eurasian Plate. Research indicates that the Anatolian Plate is rotating counterclockwise as it is being pushed west by the Arabian Plate, in some references, the Anatolian Plate is referred to as a block of continental crust still coupled to the Eurasian Plate. But studies of the North Anatolian Fault indicate that Anatolia is de-coupled from the Eurasian Plate and it is now being squeezed by the Arabian Plate from the east and forced toward the west as the Eurasian Plate to its north is blocking motion in that direction. The African Plate is subducting beneath the Anatolian Plate along the Cyprus and Hellenic Arcs offshore in the Mediterranean Sea
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Azores
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Its main industries are agriculture, dairy farming, livestock, fishing, and tourism, which is becoming the major service activity in the region. In addition, the government of the Azores employs a large percentage of the population directly or indirectly in the service, the main settlement of the Azores is Ponta Delgada. There are nine major Azorean islands and a cluster, in three main groups. These are Flores and Corvo, to the west, Graciosa, Terceira, São Jorge, Pico, and Faial in the centre, and São Miguel, Santa Maria, and they extend for more than 600 km and lie in a northwest-southeast direction. All the islands have volcanic origins, although some, such as Santa Maria, have had no recorded activity since the islands were settled, mount Pico, on the island of Pico, is the highest point in Portugal, at 2,351 m. The Azores are actually some of the tallest mountains on the planet, measured from their base at the bottom of the ocean to their peaks, which thrust high above the surface of the Atlantic. The climate of the Azores is very mild for such a location, being influenced by its distance to continents. Due to the influence, temperatures remain mild year-round. Daytime temperatures normally fluctuate between 16 °C and 25 °C depending on season, temperatures above 30 °C or below 3 °C are unknown in the major population centres. It is also generally wet and cloudy, the culture, dialect, cuisine, and traditions of the Azorean islands vary considerably, because these once-uninhabited and remote islands were settled sporadically over a span of two centuries. However, these kinds of structures have always used in the Azores to store cereals. Detailed examination and dating to authenticate the validity of these speculations is lacking and it is unclear whether these structures are natural or man-made and whether they predate the 15th-century Portuguese colonization of the Azores. Solid confirmation of a human presence in the archipelago has not yet been published. The islands were known in the century and parts of them appear in the Atlas Catalan. In 1427, a captain sailing for Henry the Navigator, possibly Gonçalo Velho, rediscovered the Azores, but this is not certain. In Thomas Ashes 1813 work, A History of the Azores, the author identified a Fleming, Joshua Vander Berg of Bruges and he stated that the Portuguese explored the area and claimed it for Portugal. Other stories note the discovery of the first islands by sailors in the service of Henry the Navigator, although there are few documents to support the claims. Although it is said that the archipelago received its name from the goshawk
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Craton
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A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earths two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by sedimentary rock. They have a thick crust and deep roots that extend as much as several hundred kilometres into the Earths mantle. The term craton is used to distinguish the stable portion of the continental crust from regions that are geologically active. Cratons can be described as shields, in which the basement rock crops out at the surface, the word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen, referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later authors shortened the term to kraton and then to craton. Cratons are subdivided geographically into geologic provinces, a geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt. Adjoining provinces may appear similar in structure but be considered due to differing histories. At that depth, craton roots extend into the asthenosphere, craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy, and a low intrinsic isopycnic density. This low density offsets density increases due to contraction and prevents the craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years, rock fragments carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites. These inclusions have densities consistent with composition and are composed of mantle material residual from high degrees of partial melt. Peridotite is strongly influenced by the inclusion of moisture, craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium, peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent the crystalline residues after extraction of melts of compositions like basalt, an associated class of inclusions called eclogites, consists of rocks corresponding compositionally to oceanic crust that has metamorphosed under deep mantle conditions. Isotopic studies reveal that many eclogite inclusions are samples of ancient oceanic crust subducted billions of years ago to depths exceeding 150 km into the deep kimberlite diamond areas and they remained fixed there within the drifting tectonic plates until carried to the surface by deep-rooted magmatic eruptions
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Mantle (geology)
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The mantle is a layer inside a terrestrial planet and some other rocky planetary bodies. For a mantle to form, the body must be large enough to have undergone the process of planetary differentiation by density. The mantle surrounds the planetary core, the mantle is surrounded by the crust. The terrestrial planets, the Moon, two of Jupiters moons and the asteroid Vesta each have a made of silicate rock. Interpretation of spacecraft data suggests that at least two moons of Jupiter, as well as Titan and Triton each have a mantle made of ice or other solid volatile substances. The interior of Earth, similar to the terrestrial planets, is divided into layers of different composition. The mantle is a layer between the crust and the outer core, Earths mantle is a silicate rocky shell with an average thickness of 2,886 kilometres. The mantle makes up about 84% of Earths volume and it is predominantly solid but in geological time it behaves as a very viscous fluid. The mantle encloses the hot core rich in iron and nickel, past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface. A thin crust, the part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick. In some places under the ocean the mantle is exposed on the surface of Earth. The mantle is divided into sections which are based upon results from seismology and these layers are the following, the upper mantle, the transition zone, the lower mantle, and anomalous core–mantle boundary with a variable thickness. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the shear velocity is reduced. Inge Lehmann discovered a seismic discontinuity at about 220 km depth, although this discontinuity has been found in other studies, the transition zone is an area of great complexity, it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous, the D layer at the core–mantle boundary separates the mantle from the core. A principal source of the heat that drives plate tectonics is the decay of uranium, thorium. The mantle differs substantially from the crust in its properties as the direct consequence of the difference in composition
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Supercontinent
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In geology, a supercontinent is the assembly of most or all of Earths continental blocks or cratons to form a single large landmass. However, the definition of a supercontinent can be ambiguous, Many earth scientists, such as P. F. Hoffman, use the term supercontinent to mean a clustering of all continents. This definition leaves room for interpretation when labeling a continental body and is easier to apply to Precambrian times, the landmass of Pangaea is the collective name describing all of these continental masses when they were most recently in proximity to one another. This would classify Pangaea as a supercontinent, according to the definition by Rogers and Santosh, a supercontinent does not exist today. Supercontinents have assembled and dispersed multiple times in the geologic past, the positions of continents have been accurately determined back to the early Jurassic. However, beyond 200 Ma, continental positions are less certain. The following table displays historical supercontinents, using a general definition, there are two contrasting models for supercontinent evolution through geological time. The first model theorizes that at least two separate supercontinents existed comprising Vaalbara and Kenorland, the Neoarchean supercontinent consisted of Superia and Sclavia. These parts of Neoarchean age broke off at ~2300 and 2090 Ma, Nuna continued to develop during the Mesoproterozoic, primarily by lateral accretion of juvenile arcs, and in ~1000 Ma Nuna collided with other land masses, forming Rodinia. Between ~800 and 700 Ma Rodinia broke apart, however, before completely breaking up, some fragments of Rodinia had already come together to form Gondwana by ~530 Ma. Pangaea formed by ~300 Ma through the collision of Gondwana, Laurentia, Baltica, during the intervening periods, the poles conform to a unified apparent polar wander path. The explanation for the duration of the Protopangea-Paleopangea supercontinent appears to be that Lid Tectonics prevailed during Precambrian times. Plate Tectonics as seen on the contemporary Earth became dominant only during the part of geological times. The Phanerozoic supercontinent Pangaea began to break up 180 Ma and is doing so today. Because Pangaea is the most recent of Earths supercontinents, it is the most well known, contributing to Pangaeas popularity in the classroom is the fact that its reconstruction is almost as simple as fitting the present continents bordering the Atlantic-type oceans like puzzle pieces. A supercontinent cycle is the break-up of one supercontinent and the development of another, supercontinent cycles are not the same as the Wilson cycle, which is the opening and closing of an individual oceanic basin. The Wilson cycle rarely synchronizes with the timing of a supercontinent cycle, however, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia
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Pangaea
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Pangaea or Pangea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from earlier continental units approximately 335 million years ago, in contrast to the present Earth and its distribution of continental mass, much of Pangaea was in the southern hemisphere and surrounded by a superocean, Panthalassa. Pangaea was the most recent supercontinent to have existed and the first to be reconstructed by geologists, the name Pangaea/Pangea is derived from Ancient Greek pan and Gaia. The name Pangea occurs in the 1920 edition of Die Entstehung der Kontinente und Ozeane, but only once, the forming of supercontinents and their breaking up appears to have been cyclical through Earths history. There may have many others before Pangaea. When Rodinia broke up, it split into three pieces, the supercontinent of Proto-Laurasia, the supercontinent of Proto-Gondwana, and the smaller Congo craton, Proto-Laurasia and Proto-Gondwana were separated by the Proto-Tethys Ocean. Next Proto-Laurasia itself split apart to form the continents of Laurentia, Siberia, Baltica moved to the east of Laurentia, and Siberia moved northeast of Laurentia. The splitting also created two new oceans, the Iapetus Ocean and Paleoasian Ocean, most of the above masses coalesced again to form the relatively short-lived supercontinent of Pannotia. This supercontinent included large amounts of land near the poles and, near the equator, Pannotia lasted until 540 Ma, near the beginning of the Cambrian period and then broke up, giving rise to the continents of Laurentia, Baltica, and the southern supercontinent of Gondwana. Baltica, Laurentia, and Avalonia all came together by the end of the Ordovician to form a supercontinent called Euramerica or Laurussia. The collision also resulted in the formation of the northern Appalachians, Siberia sat near Euramerica, with the Khanty Ocean between the two continents. While all this was happening, Gondwana drifted slowly towards the South Pole and this was the first step of the formation of Pangaea. The second step in the formation of Pangaea was the collision of Gondwana with Euramerica, by Silurian time,440 Ma, Baltica had already collided with Laurentia, forming Euramerica. Avalonia had not yet collided with Laurentia, but as Avalonia inched towards Laurentia, meanwhile, southern Europe broke off from Gondwana and began to move towards Euramerica across the newly formed Rheic Ocean. It collided with southern Baltica in the Devonian, though this microcontinent was an underwater plate, the Iapetus Oceans sister ocean, the Khanty Ocean, shrank as an island arc from Siberia collided with eastern Baltica. Behind this island arc was a new ocean, the Ural Ocean, by late Silurian time, North and South China split from Gondwana and started to head northward, shrinking the Proto-Tethys Ocean in their path and opening the new Paleo-Tethys Ocean to their south. In the Devonian Period, Gondwana itself headed towards Euramerica, causing the Rheic Ocean to shrink, South America moved northward to southern Euramerica, while the eastern portion of Gondwana headed toward the South Pole from the equator. North and South China were on independent continents, the Kazakhstania microcontinent had collided with Siberia
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Congo Craton
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The Congo Craton, covered by the Palaeozoic-to-recent Congo Basin, is an ancient Precambrian craton that with four others makes up the modern continent of Africa. These cratons were formed between about 3.6 and 2.0 billion years ago and have been stable since that time. All of these cratons are bounded by younger fold belts formed between 2.0 billion and 300 million years ago, the Congo Craton occupies a large part of central southern Africa, extending from the Kasai region of the DRC into Sudan and Angola. It forms parts of the countries of Gabon, Cameroon, a small portion extends into Zambia as well, where it is called the Bangweulu Block. Around 2.11 Ga, Congo was part of the minor supercontinent Atlantica, around 1.82 Ga, Congo was part of the major supercontinent Nuna. Around 1.31 Ga, Congo was an independent continent, around 1.071 Ga, Congo was part of the major supercontinent Rodinia. Around 990 Ma, Congo was an indepandent continent, around 600 Ma, Congo was part of the major supercontinent Pannotia. Around Cambrian, Congo was part of the minor supercontinent Gondwana, around Carboniferous, all major continents collide against each other for forming the major supercontinent Pangaea. Around Jurassic, Pangaea rifted into two supercontinents, Laurasia and Gondwana. Congo was part of the minor supercontinent Gondwana, around Cretaceous, Congo was an independent continent called Africa. Around Neogene, Congo, in the form of Africa, crashed into Eurasia, around 250 Ma from now, all continents may crash together, forming the major supercontinent Pangaea Ultima. Congo would be part of Pangaea Ultima, around 450-600 Ma from now, Pangaea Ultima will eventually rift apart. Congo will be an independent continent
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Tanzania Craton
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The Tanzania Craton is an old and stable part of the continental lithosphere in central Tanzania. Some of the rocks are over 3 billion years old, the Tanzania Craton forms the highest part of the East African Plateau. The craton is surrounded by Proterozoic mobile belts of various ages and grades of metamorphism and these include the Ubendian, Usagaran, Karagwe-Ankolean and Bukoban systems. The Mozambique Belt lies to the east, the craton divides the east and west branches of the East African Rift. The southern end of the Gregory Rift Valley terminates against the craton, the volcanic area of this rift covers the surface interface between the Mozambique orogenic fold belt and the Tanzania Craton. The craton is a composite of several different terranes of Archaean metasediments, the Dodoman is the oldest, and others include the Nyanzian and Kavirondian. Some of the belts are more than 3 billion years old. They were intruded by granites and migmatized in different events that date back to 2.9,2.7,2.4 and 1.85 billion years ago. The craton mainly consists of Archaean granitic complexes, but also includes rocks from the Dodoma System in the central area, gneisses, schists, quartzites, migmatites, amphibolites and granulite are also found. There was widespread intrusion of kimberlites in the Cretaceous Period, mostly in the part of the craton that lies south of Lake Victoria and this includes the Mwadui kimberlite pipe in the center of Tanzania
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West African Craton
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These land masses came together in the late Precambrian and early Palaeozoic eras to form the African continent. At one time, volcanic action around the rim of the craton may have contributed to a global warming event. The craton appears to have formed when three Archean cratons fused, Leo-Man-Ghana, Taoudeni and Reguibat, the first two docked around 2.1 Ga, and the Reguibat Craton docked with the craton around 2 Ga. The roots of the combined craton extend to a depth of over 300 km in the lithospheric mantle. The West African Craton stretches from the Little Atlas mountains of Morocco to the Gulf of Guinea, the oldest rocks were metamorphosed 2.9 to 2.5 billion years ago. In the Sahara they are covered by more recent sediments from the Phanerozoic Eon. Further south, younger volcanic and sedimentary rocks outcrop in Ghana, Ivory Coast, the Earth formed about 4.54 billion years ago. As it cooled the lithosphere, consisting of the crust and the rigid uppermost part of the mantle solidified, the lithosphere rides on the asthenosphere, which is also solid but can flow like a liquid on geological time scales. Proponents of the Snowball Earth theory claim that sometime before 650 million years ago the Earth was going through a cold period. The oceans were frozen to depths, and their snow covering reflected the heat from the sun through cloudless skies. Only simple forms of life could survive in such as deep oceanic hydrothermal vents. At the end of this period the edges of the West African craton became highly active, the thermal activity was caused by excessive mantle heat that had built up below the craton, insulated by the lithosphere. The volcanic eruptions created an effect on massive scale, melting the ice. The climate quickly switched to one much warmer than today, resulting in the Cambrian explosion of life forms. During its wanderings, at different times covered by ice sheets, forests, marshes or arid desert, in most places the original rocks are buried far below more recent volcanic and sedimentary deposits. The visible features are usually of comparatively recent origin, erosion has reduced the Little Atlas range so that it is today lower than the High Atlas range to the north. South of the mountains, the West African Craton is relatively flat, however, below the surface there are ancient sedimentary basins such as the Taoudeni basin that may contain large reserves of oil and gas. The southern region of the West African Craton is interrupted by the remains of weathered volcanoes that are younger than the craton itself, the boundaries of the West African Craton, with special reference to the basement of the Moroccan metacratonic Anti-Atlas belt
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Orogeny
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An Orogeny is an event that leads to a large structural deformation of the Earths lithosphere due to the interaction between tectonic plates. Orogens or orogenic belts develop when a plate is crumpled and is pushed upwards to form mountain ranges. Orogeny is the mechanism by which mountains are built on continents. The word orogeny comes from Ancient Greek, though it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny. Formation of an orogen is accomplished in part by the processes of subduction or convergence of two or more continents. Orogeny usually produces long arcuate structures, known as orogenic belts, generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with zones, which consume crust, produce volcanoes. Geologists attribute the arcuate structure to the rigidity of the descending plate and these island arcs may be added to a continent during an orogenic event. The processes of orogeny can take tens of millions of years, frequently, rock formations that undergo orogeny are severely deformed and undergo metamorphism. Orogenic processes may push deeply buried rocks to the surface, sea-bottom and near-shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, very high mountains can result, an orogenic event may be studied, as a tectonic structural event, as a geographical event, and as a chronological event. The foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the mountain belt. The basin migrates with the front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the basin are mainly derived from the erosion of the actively uplifting rocks of the mountain range. The fill of many such shows an change in time from deepwater marine through shallow water to continental sediments. Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including magmatism, metamorphism, crustal melting, what exactly happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis. In addition to orogeny, the orogen is subject to other processes, for example, the Caledonian Orogeny refers to the Silurian and Devonian events that resulted from the collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana. The Caledonian Orogen resulted from events and various others that are part of its peculiar orogenic cycle
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Tectonic plates
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The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major plates, where the plates meet, their relative motion determines the type of boundary, convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually, tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading. In this way, the surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle, earlier theories, since disproven, proposed gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
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Saharan Metacraton
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The Saharan Metacraton is a term used by some geologists to describe a large area of continental crust in the north-central part of Africa. The geology of the continent has only partially explored, and other names have been used to describe the general area that reflect different views of its nature. These include Nile Craton, Sahara Congo Craton, Eastern Saharan Craton and this last term is because the older rocks are almost completely covered by recent sediments and desert sands, making geological analysis difficult. The metacraton covers an area of about 5,000,000 km2 and it lies between the Tuareg shield to the west, the Congo craton to the south, the Arabian-Nubian Shield to the east, and the northern African continental margin. The southern boundary is not well-defined, but may be considered to run along the edge of the Oubangides orogenic belt in the southwest. It lies under southern Egypt and Libya and western Sudan, northern Kenya, Uganda and Congo, the craton may have formed during a period of accelerated crustal growth between 3000 Ma and 2000 Ma. The oldest rocks in Egypt date to about 2700 Ma, rocks in southern Libya and eastern Chad have been dated between 2900 Ma and 2600 Ma. Some geologists date a rifting episode to between 1200 Ma and 950 Ma, the Pan-African orogeny saw the cratons that had formed the Rodinia super-continent come together in a completely new arrangement into the Gondwana super-continent around 500 Ma. The Saharan craton may have been decratonized, perhaps oceanized, during the Pan-African event, at the end of this period the craton was extended and rifts appeared. By about 500 Ma the Saharan craton was sutured with the Congo craton during the assembly of Gondwana. Although Gondwana later split up, the core African Plate remained intact, perhaps the simplest view is that the craton is an assembly of microcontinental blocks with different origins that were swept together during the Pan African orogeny
30.
Sedimentary basin
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Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. Sedimentary basins occur in diverse geological settings usually associated with tectonic activity. Basins formed in different plate tectonic regimes vary in their preservation potential, on oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure, a number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins. Sedimentary basins form primarily in convergent, divergent and transform settings, convergent boundaries create foreland basins through tectonic compression of oceanic and continental crust during lithospheric flexure. Tectonic extension at divergent boundaries where continental rifting is occurring can create a nascent ocean basin leading to either an ocean or the failure of the rift zone. If the lithosphere is caused to stretch horizontally, by such as ridge-push or trench-pull. The lower, hotter part of the lithosphere will flow away from the main area being stretched, whilst the upper, cooler and more brittle crust will tend to fault. The combined effect of two mechanisms is for the Earths surface in the area of extension to subside, creating a geographical depression which is then often infilled with water and/or sediments. An example of a basin caused by lithospheric stretching is the North Sea – also an important location for significant hydrocarbon reserves, another such feature is the Basin and Range Province which covers most of the USA state of Nevada, forming a series of horst and graben structures. Another expression of lithospheric stretching results in the formation of basins with central ridges, The Red Sea is in fact an incipient ocean. The mouth of the Red Sea is also a triple junction where the Indian Ocean Ridge, Red Sea Rift. This is the place on the planet where such a triple junction in oceanic crust is exposed subaerially. The reason for this is twofold, due to a high thermal buoyancy of the junction, if a load is placed on the lithosphere, it will tend to flex in the manner of an elastic plate. The magnitude of the lithospheric flexure is a function of the load and the flexural rigidity of the lithosphere. Flexural rigidity is in itself, a function of the mineral composition, thermal regime. The nature of the load is varied, for instance, the Hawaiian Islands chain of volcanic edifices has sufficient mass to cause deflection in the lithosphere. Deformation of the lithosphere in the plane of the earth occurs as a result of near horizontal maximum and minimum principal stresses, the resulting zones of subsidence are known as strike-slip or pull-apart basins
31.
Tindouf Basin
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The Tindouf Basin is a major sedimentary basin in West Africa, to the south of the little Atlas region, Morocco. It stretches from west to east about 700km and covers about 100,000 km2, mostly in Algeria, in the Ordovician period the area was an embayment sloping down from the West African craton into the Tethys sea. It became a basin in the Late Carboniferous. The basin has a steep northern edge against the Anti Atlas, the basin is filled with up to 8 km of sediment from the Cambrian and Carboniferous ageas. These marine formations are overlain by a continental Cretaceous and Pliocene Hamada cover, the basin may have potential for oil and/or gas production, but has been largely unexplored
32.
Taoudeni basin
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The Taoudeni Basin is a major geological formation in West Africa named after the Taoudenni village in northern Mali. It covers large parts of the West African craton in Mauritania and it is of considerable interest due to its possible reserves of oil. The Taoudeni is the largest sedimentary basin in Northwest Africa, formed during the mid-late Proterozoic and it continued to subside until the mid-Paleozoic age, when Hercynian deformation and uplift occurred. It contains up to 6000 m of Late Precambrian and Paleozoic sediments, exploratory drilling since the 1980s has found indications of petroleum in the Late Precambrian, Silurian and Late Devonian layers. Sediments are thicker in the half of the basin. The government of Mali, one of the poorest countries in the world, is eager to create an oil industry, companies that have been exploring in the area include Baraka Petroleum, Sonatrach, Eni, Total S. A. Woodside and China National Petroleum Corporation. However, the location and harsh environment of the Sahara Desert would make extraction expensive. ICONS atlas, AFR - Taoudeni Basin
33.
Congo Basin
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The Congo Basin is the sedimentary basin of the Congo River, located in west equatorial Africa. The broad region is known simply as the Congo. The basin is a total of 3.7 million square kilometers and is home to some of the largest undisturbed stands of tropical rainforest on the planet, the basin ends where the river empties its load in the Gulf of Guinea on the Atlantic Ocean. The climate is tropical, with two rainy seasons including very high rainfalls, and high temperature year round. The basin is home to the western lowland gorilla. The basin was the watershed of the Congo River populated by pygmy peoples, belgium, France, and Portugal later established colonial control over the entire region by the late 19th century. Congo is a name for the equatorial Middle Africa that lies between the Gulf of Guinea and the African Great Lakes. It contains some of the largest tropical rainforests in the world and it is home to okapi, bonobo and the Congo peafowl, but is also an important source of African teak, used for building furniture and flooring. An estimated 40 million people depend on woodlands, surviving on traditional livelihoods. At a global level, Congos forests act as the planets second lung and they are a huge carbon sink, trapping carbon that could otherwise become carbon dioxide, the main cause of global warming. The Congo Basin holds roughly 8 percent of the worlds forest-based carbon and these forests also affect rainfall across the North Atlantic. In other words, these distant forests are crucial to the future of climate stability, a moratorium on logging in the Congo forest was agreed with the World Bank and the Democratic Republic of the Congo in May 2002. The World Bank agreed to provide $90 million of development aid to RDC with the proviso that the government did not issue any new concessions granting logging companies rights to exploit the forest, the deal also prohibited the renewal of existing concessions. Greenpeace is calling on the World Bank to think outside the box, if these woodlands are deforested, the carbon they trap will be released into the atmosphere. It says that 8% of the Earths forest-based carbon is stored in the RDCs forests, the government has written a new forestry code that requires companies to invest in local development and follow a sustainable, twenty-five-year cycle of rotational logging. In its current form, the Kyoto protocol does not reward so-called avoided deforestation - initiatives that protect forest from being cut down, but many climate scientists and policymakers hope that negotiations for Kyotos successor will include such measures. If this were the case, there could be an incentive for protecting forests. L’Île Mbiye in Kisangani is part of the Sustainable Forest Management in Africa Symposium project of forest ecosystem conservation conducted by Stellenbosch University, RDC is also looking to expand the area of forest under protection, for which it hopes to secure compensation through emerging markets for forest carbon
34.
Shear zone
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A shear zone is a very important structural discontinuity surface in the Earths crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones, intervening blocks stay relatively unaffected by the deformation. Due to the motion of the surrounding more rigid medium. Because the discontinuity surface usually passes through a wide depth-range, a variety of different rock types with their characteristic structures are produced. A shear zone is a zone of strong deformation surrounded by rocks with a state of finite strain. It is characterised by a length to width ratio of more than 5,1, Shear zones form a continuum of geological structures, ranging from brittle shear zones via brittle–ductile shear zones, ductile–brittle to ductile shear zones. Between these end-members, there are types of brittle–ductile and ductile–brittle shear zones that can combine these geometric features in different proportions. By passing through the transition the ductile response to deformation is starting to set in. This transition is not tied to a depth, but rather occurs over a certain depth range - the so-called alternating zone. The main reason for this is found in the usually heteromineral composition of rocks, thus differences in lithology, grain size, and preexisting fabrics determine a different rheological response. Yet other, purely physical factors, influence the depth as well, including, geothermal gradient. In Scholzs model for a quartzo-feldspathic crust, the transition starts at about 11 km depth with an ambient temperature of 300 °C. The underlying alternating zone then extends to roughly 16 km depth with a temperature of about 360 °C, below approximately 16 km depth, only ductile shear zones are found. The seismogenic zone, in which earthquakes nucleate, is tied to the brittle domain, below an intervening alternating zone, there is the plastosphere. In the seismogenic layer, which occurs below an upper stability transition related to an upper seismicity cutoff, the seismogenic layer then yields to the alternating zone at 11 km depth. Yet big earthquakes can rupture both up to the surface and well into the zone, sometimes even into the plastosphere. Shear zones are very important structures for unravelling the history of a specific terrane. Starting at the Earths surface, the rock types are usually encountered in a shear zone
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Central African Shear Zone
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The Central African Shear Zone is a wrench fault system extending in an ENE direction from the Gulf of Guinea through Cameroon into Sudan. The structure is not well understood, as of 2008, there was still no general agreement about how the individual shears along the lineament link up. The shear zone dates to at least 640 Ma, motion occurred along the zone during the break-up of Gondwanaland in the Jurassic and Cretaceous ages. Some of the faults in the zone were rejuvenated more than once before, the Pernambuco fault in Brazil is a continuation of the shear zone to the west. In Cameroon, the CASZ cuts across the Adamawa uplift, a post-Cretaeous formation, the Benue Trough lies to the north, and the Foumban Shear Zone to the south. Volcanic activity has occurred along most of the length of the Cameroon line from 130 Ma to the present, the lithosphere beneath the CASZ in this area is thinned in a relatively narrow belt, with the asthenosphere upwelling from a depth of about 190 km to about 120 km. The Mesozoic and Tertiary movements have produced elongated rift basins in central Cameroon, northern Central African Republic, the CASZ was formerly thought to extend eastward only to the Darfur region of western Sudan. It is now known to extend into central and eastern Sudan, in the Sudan, the shear zone may have acted as a structural barrier to development of deep Cretaceous-Terriary sedimentary basins in the north of the area. Objections to this theory are that the Bahr el Arab and Blue Nile rifts extend northwest beyond one proposed line for the shear zone, however, the alignment of the northwestern ends of the rifts in this areas supports the theory
36.
Afar Triangle
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The Afar Triangle is a geological depression caused by the Afar Triple Junction, which is part of the Great Rift Valley in East Africa. The Depression overlaps the borders of Eritrea, Djibouti and the entire Afar Region of Ethiopia, the Awash River is the main waterflow into the region, but it runs dry during the annual dry season, and ends as a chain of saline lakes. The northern part of the Afar Depression is also known as the Danakil Depression, the lowlands are affected by heat, drought, and minimal air circulation, and contain the hottest places of anywhere on Earth. Many important fossil localities exist in the Afar region, including the Middle Awash region and the sites of Hadar, Dikika and these sites have produced specimens of the earliest hominins and of human tool culture, as well as many fossils of various flora and fauna. Dallol in the Danakil Depression is one of the hottest places year-round anywhere on Earth, there is no rain for most of the year, the yearly rainfall averages range from 100 to 200 millimetres, with even less rain falling closer to the coast. The climate varies from around 25 °C during the short rainy season to 48 °C during the dry season. About 128 kilometres from the Red Sea the Awash ends in a chain of salt lakes, some 1,200 km2 of the Afar Depression is covered by salt deposits, and mining salt is a major source of income for many Afar groups. The Afar Depression biome is characterized as desert scrubland, vegetation is mostly confined to drought-resistant plants such as small trees, shrubs, and grasses. Wildlife includes many such as Grevys zebra, Soemmerings gazelle, beisa and, notably. Birds include the ostrich, the endemic Archers lark, the bird, Arabian and kori bustards, Abyssinian roller. In the southern part of the lies the Mille-Sardo Wildlife Reserve in Ethiopia. The Afar Triangle is a source of the earliest hominins. In 1994, near the Awash River in Ethiopia, Tim D. White found the then-oldest known human ancestor,4.4 million-year-old Ar. ramidus. A fossilized almost complete skeleton of a female hominin which he named Ardi, it took nearly 15 years to excavate, preserve. The Afar Depression is the product of a tectonic triple-rifts junction, where the ridges forming the Red Sea. The conjunction of three plates of Earths crust is near Lake Abbe. The Afar Depression is one of two places on Earth where a mid-ocean ridge can be studied on land, the other being Iceland. Within the Triangle, the Earths crust is slowly rifting apart at a rate of 1–2 centimetres per year each of the three rift zones forming the legs of the triple junction
37.
Triple junction
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A triple junction is the point where the boundaries of three tectonic plates meet. Of the many types of triple junction only a few are stable through time. The first scientific paper detailing the triple junction concept was published in 1969 by W. Jason Morgan, Dan McKenzie, the term had traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles, in plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point. One of these divergent plate boundaries fails and the two continue spreading to form an ocean. The NE-trending Benue Trough is the arm of this junction. In the years since, the term triple-junction has come to refer to any point where three tectonic plates meet, the junction of the Red Sea, the Gulf of Aden and the East African Rift centered in the Afar Triangle is the only Ridge-Ridge-Ridge triple junction above sea level. The Rodrigues Triple Point is a R-R-R triple junction in the southern Indian Ocean, where the African, the Indo-Australian, the Galapagos Triple Junction is an R-R-R triple junction where the Nazca, the Cocos, and the Pacific Plates meet. The East Pacific Rise extends north and south from this junction and this example is made more complex by the Galapagos Microplate which is a small separate plate on the rise just to the southeast of the triple junction. Chiapas coast off Tapachula where Guatemala, North America and Pacific join and this is pushed eastward by the Cocos plate. On the west coast of North America is another triple junction offshore of Cape Mendocino. To the south, the San Andreas Fault, a fault and transform plate boundary, separates the Pacific Plate. To the north lies the Cascadia subduction zone, where a section of the Juan de Fuca Plate called the Gorda Plate is being subducted under the North American Plate, another transform fault, the Mendocino Fault, runs along the boundary between the Pacific Plate and the Gorda Plate. Where the three intersect is the active, F-F-T Mendocino Triple Junction. The Amurian Plate, the Okhotsk Plate, and the Philippine Sea Plate meet in Japan near Mt. Fuji. The Azores Triple Junction is a triple junction where the boundaries of three tectonic plates intersect, the North American Plate, the Eurasian Plate and the African Plate. The Boso Triple Junction offshore Japan is a T-T-T triple junction between the Okhotsk Plate, Pacific Plate and Philippine Sea Plate, the North Sea is located at the extinct triple junction of three former continental plates of the Palaeozoic era, Avalonia, Laurentia and Baltica. The properties of triple junctions are most easily understood from the purely kinematic point of view where the plates are rigid, no knowledge of the Earths interior or the geological details of the crust are then needed
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Hypothesis
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A hypothesis is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot satisfactorily be explained with the scientific theories. Even though the hypothesis and theory are often used synonymously. A working hypothesis is a provisionally accepted hypothesis proposed for further research, P is the assumption in a What If question. Remember, the way that you prove an implication is by assuming the hypothesis, --Philip Wadler In its ancient usage, hypothesis referred to a summary of the plot of a classical drama. The English word hypothesis comes from the ancient Greek ὑπόθεσις word hupothesis, in Platos Meno, Socrates dissects virtue with a method used by mathematicians, that of investigating from a hypothesis. In this sense, hypothesis refers to an idea or to a convenient mathematical approach that simplifies cumbersome calculations. In common usage in the 21st century, a hypothesis refers to an idea whose merit requires evaluation. For proper evaluation, the framer of a hypothesis needs to define specifics in operational terms, a hypothesis requires more work by the researcher in order to either confirm or disprove it. In due course, a hypothesis may become part of a theory or occasionally may grow to become a theory itself. Normally, scientific hypotheses have the form of a mathematical model, in entrepreneurial science, a hypothesis is used to formulate provisional ideas within a business setting. The formulated hypothesis is then evaluated where either the hypothesis is proven to be true or false through a verifiability- or falsifiability-oriented Experiment, any useful hypothesis will enable predictions by reasoning. It might predict the outcome of an experiment in a setting or the observation of a phenomenon in nature. The prediction may also invoke statistics and only talk about probabilities, other philosophers of science have rejected the criterion of falsifiability or supplemented it with other criteria, such as verifiability or coherence. The scientific method involves experimentation, to test the ability of some hypothesis to adequately answer the question under investigation. In contrast, unfettered observation is not as likely to raise unexplained issues or open questions in science, a thought experiment might also be used to test the hypothesis as well. In framing a hypothesis, the investigator must not currently know the outcome of a test or that it remains reasonably under continuing investigation, only in such cases does the experiment, test or study potentially increase the probability of showing the truth of a hypothesis
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Mantle plume
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A mantle plume is a mechanism proposed in 1971 to explain volcanic regions of the Earth that were not thought to be explicable by the then-new theory of plate tectonics. Some such volcanic regions lie far from plate boundaries, for example. Others represent unusually large-volume volcanism, whether on plate boundaries, e. g. Iceland, a mantle plume is posited to exist where hot rock nucleates at the core-mantle boundary and rises through the Earths mantle becoming a diapir in the Earths crust. The currently active volcanic centers are known as hot spots, the hypothesis of mantle plumes from depth is not universally accepted as explaining all such volcanism. It has required progressive hypothesis-elaboration leading to variant propositions such as mini-plumes, another hypothesis for unusual volcanic regions is the Plate model. In 1971, geophysicist W. Jason Morgan proposed the hypothesis of mantle plumes, in this hypothesis, convection in the mantle transports heat from the core to the Earths surface in thermal diapirs. The latter type of convection is postulated to be independent of plate motions, the sizes and occurrence of mushroom mantle plumes can be predicted easily by transient instability theory developed by Tan and Thorpe. The theory predicts mushroom mantle plumes of about 2000 km diameter with a time of about 830 Myr for a core mantle heat flux of 20 mW/m2. The number of mantle plumes is predicted to be about 17, the plume hypothesis was studied using laboratory experiments conducted in small fluid-filled tanks in the early 1970s. Thermal or compositional fluid-dynamical plumes produced in that way were presented as models for the much larger postulated mantle plumes, the entire structure is considered to resemble a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the faster than the plume itself rises through its surroundings. In the late 1980s and early 1990s, experiments with thermal models showed that as the bulbous head expands it may entrain some of the adjacent mantle into the head. When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and it may then erupt onto the surface. Numerical modelling predicts that melting and eruption will take place several million years. These eruptions have been linked to flood basalts, although many of those erupt over much shorter time scales, flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean. As the overlying tectonic plate moves over this hot spot, the eruption of magma from the fixed conduit onto the surface is expected to form a chain of volcanoes that parallels plate motion, the Hawaiian Islands chain in the Pacific Ocean is the type example. The eruption of continental flood basalts is often associated with continental rifting and this has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins. In the context of the alternative Plate model, continental breakup is an integral to plate tectonics
40.
Subduction
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Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the rate of convergence being approximately two to eight centimeters per year along most plate boundaries. Plates include both oceanic crust and continental crust, stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere. That is, the lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that have a rate of volcanism, earthquakes. Subduction zones are sites of convective downwelling of Earths lithosphere, subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The descending slab, the plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of approximately twenty-five to forty-five degrees to Earths surface and this sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere is, on average, denser. At a depth of approximately 80–120 kilometers, the basalt of the oceanic crust is converted to a rock called eclogite. At that point, the density of the oceanic crust increases and provides additional negative buoyancy and it is at subduction zones that Earths lithosphere, oceanic crust, sedimentary layers and some trapped water are recycled into the deep mantle. Earth is so far the only planet where subduction is known to occur, subduction is the driving force behind plate tectonics, and without it, plate tectonics could not occur. Subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins, subduction zones burrow deeply but are imperfectly camouflaged, and geophysics and geochemistry can be used to study them. Not surprisingly, the shallowest portions of subduction zones are known best, subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at oceanic trenches and their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660-kilometer discontinuity. Subduction zones are defined by the array of earthquakes known as the Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths than elsewhere on Earth, such deep earthquakes may be driven by deep phase transformations, thermal runaway, the subducting basalt and sediment are normally rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the slab bends downward
41.
Oceanic crust
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Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. The crust overlies the solidified and uppermost layer of the mantle, the crust and the solid mantle layer together constitute oceanic lithosphere. Oceanic crust is the result of erupted mantle material originating from below the plate, cooled and in most instances and this occurs mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. It is primarily composed of rocks, or sima, which is rich in iron. Although a complete section of oceanic crust has not yet been drilled, Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. Layer 1 is on an average 0.4 km thick and it consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge. Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and it constitutes over two-thirds of oceanic crust volume with almost 5 km thickness. The most voluminous volcanic rocks of the floor are the mid-oceanic ridge basalts. These rocks have low concentrations of large ion lithophile elements, light rare earth elements, volatile elements, there can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland. Oceanic crust is continuously being created at mid-ocean ridges, as plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, the youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges. As the mantle rises it cools and melts, as the pressure decreases, the amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness, an example of this is the Gakkel Ridge under the Arctic Ocean. Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a depth, creating more melt. An example of this is Iceland which has crust of thickness ~20 km, the oceanic lithosphere subducts at what are known as convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another, in the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is more than 200 million years old. The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle, the oldest large scale oceanic crust is in the west Pacific and north-west Atlantic - both are about up to 180-200 million years old
42.
Continental crust
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The continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its composition is more felsic compared to the oceanic crust. The continental crust consists of layers, with a bulk composition that is intermediate to felsic. The average density of continental crust is about 2.7 g/cm3, less dense than the material that makes up the mantle. Continental crust is less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is thicker than oceanic crust. About 40% of Earths surface is occupied by continental crust. It makes up about 70% of the volume of Earths crust, because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. There is little evidence of continental crust prior to 3.5 Ga, all continental crust ultimately derives from the fractional differentiation of oceanic crust over many eons. This process has been and continues today primarily as a result of the associated with subduction. In contrast to the persistence of continental crust, the size, shape, different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle. There are currently about 7 billion cubic kilometers of continental crust, the relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted.01 Ga, whereas the oldest oceanic crust is from the Jurassic, Continental crust and the rock layers that lie on and within it are thus the best archive of Earths history. The height of mountain ranges is usually related to the thickness of crust and this results from the isostasy associated with orogeny. The crust is thickened by the compressive forces related to subduction or continental collision, the buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, the thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way are termed passive margins, igneous rock may also be underplated to the underside of the crust, i. e. adding to the crust by forming a layer immediately beneath it
